Publications by category
Journal articles
Wade L, Lichtwark G, Farris D (In Press). Joint and muscle-tendon coordination strategies during submaximal jumping. Journal of Applied Physiology
Birch J, Kelly L, Cresswell A, Dixon S, Farris D (In Press). Neuromechanical adaptations of foot function to changes in surface stiffness during hopping. Journal of Applied Physiology
Riddick R, Farris D, Cresswell A, Kuo A, Kelly L (In Press). Stepping onto the unknown: reflexes of the foot and ankle while stepping with perturbed perceptions of terrain. Journal of the Royal Society Interface
Farris DJ, Harris DJ, Rice HM, Campbell J, Weare A, Risius D, Armstrong N, Rayson MP (2023). A systematic literature review of evidence for the use of assistive exoskeletons in defence and security use cases.
Ergonomics,
66(1), 61-87.
Abstract:
A systematic literature review of evidence for the use of assistive exoskeletons in defence and security use cases.
Advances in assistive exoskeleton technology, and a boom in related scientific literature, prompted a need to review the potential use of exoskeletons in defence and security. A systematic review examined the evidence for successful augmentation of human performance in activities deemed most relevant to military tasks. Categories of activities were determined a priori through literature scoping and Human Factors workshops with military stakeholders. Workshops identified promising opportunities and risks for integration of exoskeletons into military use cases. The review revealed promising evidence for exoskeletons' capacity to assist with load carriage, manual lifting, and working with tools. However, the review also revealed significant gaps in exoskeleton capabilities and likely performance levels required in the use case scenarios. Consequently, it was recommended that a future roadmap for introducing exoskeletons to military environments requires development of performance criteria for exoskeletons that can be used to implement a human-centred approach to research and development.
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Author URL.
Farris D (2023). Musculoskeletal simulations to examine the effects of accentuated eccentric loading (AEL) on jump height. PeerJ – the Journal of Life & Environmental Sciences
Smith R, Lichtwark G, Farris D, Kelly L (2022). Examining the intrinsic foot muscles' capacity to modulate plantar flexor gearing and ankle joint contributions to propulsion in vertical jumping.
J Sport Health SciAbstract:
Examining the intrinsic foot muscles' capacity to modulate plantar flexor gearing and ankle joint contributions to propulsion in vertical jumping.
BACKGROUND: During human locomotion, a sufficiently stiff foot allows the ankle plantar flexors to generate large propulsive powers. Increasing foot stiffness (e.g. via a carbon plate) increases the ankle's external moment arm in relation to the internal moment arm (i.e. increasing gear ratio), reduces plantar flexor muscles' shortening velocity, and enhances muscle force production. In contrast, when activation of the foot's intrinsic muscles is impaired, there is a reduction in foot and ankle work and metatarsophalangeal joint stiffness. We speculated that the reduced capacity to actively control metatarsophalangeal joint stiffness may impair the gearing function of the foot at the ankle. METHODS: We used a tibial nerve block to examine the direct effects of the intrinsic foot muscles on ankle joint kinetics, in vivo medial gastrocnemius' musculotendinous dynamics, and ankle gear ratio on 14 participants during maximal vertical jumping. RESULTS: Under the nerve block, the internal ankle plantar flexion moment decreased (p = 0.004) alongside a reduction in external moment arm length (p = 0.021) and ankle joint gear ratio (p = 0.049) when compared to the non-blocked condition. Although medial gastrocnemius muscle-tendon unit and fascicle velocity were not different between conditions, the Achilles tendon was shorter during propulsion in the nerve block condition (p < 0.001). CONCLUSION: in addition to their known role of regulating the energetic function of the foot, our data indicate that the intrinsic foot muscles also act to optimize ankle joint torque production and leverage during the propulsion phase of vertical jumping.
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Birch JV, Farris DJ, Riddick R, Cresswell AG, Dixon SJ, Kelly LA (2022). Neuromechanical adaptations of foot function when hopping on a damped surface.
J Appl Physiol (1985),
133(6), 1302-1308.
Abstract:
Neuromechanical adaptations of foot function when hopping on a damped surface.
To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.
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Wade L, Birch J, Farris DJ (2022). Walking with increasing acceleration is achieved by tuning ankle torque onset timing and rate of torque development.
Journal of the Royal Society Interface,
19(191).
Abstract:
Walking with increasing acceleration is achieved by tuning ankle torque onset timing and rate of torque development
Understanding the mechanics of torque production about the ankle during accelerative gait is key to designing effective clinical and rehabilitation practices, along with developing functional robotics and wearable assistive technologies. We aimed to explore how torque and work about the ankle is produced as walking acceleration increases from 0 to 100% maximal acceleration. We hypothesized that as acceleration increased, greater work about the ankle would not be solely due to ramping up plantar flexor torque, and instead would be a product of adjustments to relative timing of ankle torque and angular displacement. Fifteen healthy participants performed walking without acceleration (constant speed), as well as low, moderate and maximal accelerations, while motion capture and ground reaction force data were recorded. We employed vector coding in a novel application to overcome limitations of previously employed evaluation methods. As walking acceleration increased, there was reduced negative work and increased positive work about the ankle. Furthermore, early stance dorsiflexion had reducing plantar flexor torque due to delayed plantar flexor torque onset as acceleration increased, while mid-stance ankle plantar flexor torque was substantially increased with minimal ankle dorsiflexion, irrespective of acceleration magnitude. Assistive devices need to account for these changes during accelerative walking to facilitate functional gait.
Abstract.
Smith R, Lichtwark G, Farris D, Kelly LA (2021). Examining the intrinsic foot muscles’ capacity to modulate plantar flexor gearing. Footwear Science, 13(S1), S87-S89.
Riddick RC, Farris DJ, Brown NAT, Kelly LA (2021). Stiffening the human foot with a biomimetic exotendon.
Scientific Reports,
11(1).
Abstract:
Stiffening the human foot with a biomimetic exotendon
AbstractShoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Recent footwear design developments have shown that this approach may also produce metabolic energy savings. Here we test an alternative approach to modify the foot–ground interface by adding additional stiffness in parallel to the plantar aponeurosis, targeting the windlass mechanism. Stiffening the windlass mechanism by about 9% led to decreases in peak activation of the ankle plantarflexors soleus (~ 5%, p < 0.001) and medial gastrocnemius (~ 4%, p < 0.001), as well as a ~ 6% decrease in positive ankle work (p < 0.001) during fixed-frequency bilateral hopping (2.33 Hz). These results suggest that stiffening the foot may reduce cost in dynamic tasks primarily by reducing the effort required to plantarflex the ankle, since peak activation of the intrinsic foot muscle abductor hallucis was unchanged (p = 0.31). Because the novel exotendon design does not operate via the compression or bending of a bulky midsole, the device is light (55 g) and its profile is low enough that it can be worn within an existing shoe.
Abstract.
Farris D, Birch J, Kelly L (2020). Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism. Journal of the Royal Society Interface, 17 (168)
Nuckols RW, Takahashi KZ, Farris DJ, Mizrachi S, Riemer R, Sawicki GS (2020). Mechanics of walking and running up and downhill: a joint-level perspective to guide design of lower-limb exoskeletons. PLOS ONE, 15(8), e0231996-e0231996.
Wade L, Lichtwark GA, Farris DJ (2019). Comparisons of laboratory‐based methods to calculate jump height and improvements to the field‐based flight‐time method. Scandinavian Journal of Medicine & Science in Sports, 30(1), 31-37.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2019). The Effect of Cadence on the Mechanics and Energetics of Constant Power Cycling.
Medicine and Science in Sports and Exercise,
51(5), 941-950.
Abstract:
The Effect of Cadence on the Mechanics and Energetics of Constant Power Cycling
At a constant power output, cyclists prefer to use a higher cadence than those that minimize metabolic cost. The neuromuscular mechanism underpinning the preferred higher cadence remains unclear. Purpose the aim of this study was to investigate the effect of cadence on joint level work and vastus lateralis (VL) fascicle mechanics while cycling at a constant, submaximal, power output. We hypothesized that preferred cycling cadence would enhance the power capacity of the VL muscle when compared with a more economical cadence. Furthermore, we predicted that the most economical cadence would coincide with minimal total electromyographic activity from the leg muscles. Methods: Metabolic cost, lower-limb kinematics, joint level work, VL fascicle mechanics, and muscle activation of the VL, rectus femoris, biceps femoris, gastrocnemius medialis, and soleus muscles were measured during cycling at a constant power output of 2.5 W·kg -1 and cadences of 40, 60, 80, and 100 rpm. A preferred condition was also performed where cadence feedback was hidden from the participant. Results: Metabolic cost was lowest at 60 rpm, but the mean preferred cadence was 81 rpm. The distribution of joint work remained constant across cadences, with the majority of positive work being performed at the knee. The preferred cadence coincided with the highest VL power capacity, without a significant penalty to efficiency, based on fascicle shortening velocity. Conclusions: Cycling at a higher cadence is preferred to ensure that the muscle's ability to produce positive power remains high. Further investigations are required to examine what feedback mechanism could be responsible for the optimization of this motor pattern.
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Riddick R, Farris DJ, Kelly LA (2019). The foot is more than a spring: human foot muscles perform work to adapt to the energetic requirements of locomotion.
JOURNAL OF THE ROYAL SOCIETY INTERFACE,
16(150).
Author URL.
Farris DJ, Kelly L, Cresswell A, Lichtwark G (2019). The functional importance of human foot muscles for bipedal locomotion. Proceedings of the National Academy of Sciences, 116(5), 1645-1650.
Wade L, Lichtwark G, Farris DJ (2019). The influences of added mass on muscle activation and contractile mechanics during submaximal and maximal countermovement jumping in humans. Journal of Experimental Biology, 222
Kelly L, Farris DJ, Cresswell A, Lichtwark G (2018). Intrinsic foot muscles contribute to elastic energy storage and return in the human foot. Journal of Applied Physiology
Lichtwark G, Farris DJ, Chen X, Hodges P, Delp S (2018). Microendoscopy reveals positive correlation in multiscale length changes and variable sarcomere lengths across different regions of human muscle. Journal of Applied Physiology, 125, 1812-1820.
Wade L, Lichtwark G, Farris DJ (2018). Movement Strategies for Countermovement Jumping are Potentially Influenced by Elastic Energy Stored and Released from Tendons.
Scientific Reports,
8(1).
Abstract:
Movement Strategies for Countermovement Jumping are Potentially Influenced by Elastic Energy Stored and Released from Tendons
The preferred movement strategies that humans choose to produce work for movement are not fully understood. Previous studies have demonstrated an important contribution of elastic energy stored within the Achilles tendon (AT) during jumping. This study aimed to alter energy available for storage in the AT to examine changes in how jumpers distribute work among lower limb joints. Participants (n = 16) performed maximal and sub-maximal jumps under two paradigms, matched for increasing total work output by manipulating jump height or adding body mass. Motion capture and ground reaction force data were combined in an inverse dynamics analysis to compute ankle, knee and hip joint kinetics. Results demonstrated higher peak moments about the ankle joint with added body mass (+26 Nm), likely resulting in additional energy storage in the AT. Work at the ankle joint increased proportionally with added mass, maintaining a constant contribution (~64%) to total work that was not matched with increasing jump height (-14%). This implies greater energy storage and return by the AT with added mass but not with increased height. When total work during jumping is constant but energy stored in tendons is not, humans prioritise the use of stored elastic energy over muscle work.
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KELLY LA, FARRIS DJ, LICHTWARK GA, CRESSWELL AG (2018). The Influence of Foot-Strike Technique on the Neuromechanical Function of the Foot. Medicine & Science in Sports & Exercise, 50(1), 98-108.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2018). The effect of muscle-tendon unit vs. fascicle analyses on vastus lateralis force-generating capacity during constant power output cycling with variable cadence.
Journal of Applied Physiology,
124(4), 993-1002.
Abstract:
The effect of muscle-tendon unit vs. fascicle analyses on vastus lateralis force-generating capacity during constant power output cycling with variable cadence
The maximum force-generating capacity of a muscle is dependent on the lengths and velocities of its contractile apparatus. Muscle-tendon unit (MTU) length changes can be estimated from joint kinematics; however, contractile element length changes are more difficult to predict during dynamic contractions. The aim of this study was to compare vastus lateralis (VL) MTU and fascicle level force-length and forcevelocity relationships, and dynamic muscle function while cycling at a constant submaximal power output (2.5 W/kg) with different cadences. We hypothesized that manipulating cadence at a constant power output would not affect VL MTU shortening, but significantly affect VL fascicle shortening. Furthermore, these differences would affect the predicted force capacity of the muscle. Using an isokinetic dynamometer and B-mode ultrasound (US), we determined the forcelength and force-velocity properties of the VL MTU and its fascicles. In addition, three-dimensional kinematics and kinetics of the lower limb, as well as US images of VL fascicles were collected during submaximal cycling at cadences of 40, 60, 80, and 100 rotations per minute. Ultrasound measures revealed a significant increase in fascicle shortening as cadence decreased (84% increase across all conditions, P < 0.01), whereas there were no significant differences in MTU lengths across any of the cycling conditions (maximum of 6%). The MTU analysis resulted in greater predicted force capacity across all conditions relative to the force-velocity relationship (P < 0.01). These results reinforce the need to determine muscle mechanics in terms of separate contractile element and connective tissue length changes during isokinetic contractions, as well as dynamic movements like cycling. NEW & NOTEWORTHY We demonstrate that vastus lateralis (VL) muscle tendon unit (MTU) length changes do not adequately reflect the underlying fascicle mechanics during cycling. When examined across different pedaling cadence conditions, the force-generating potential measured only at the level of MTU (or joint) overestimated the maximum force capacity of VL compared with analysis using fascicle level data.
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Kelly LA, Cresswell AG, Farris DJ (2018). The energetic behaviour of the human foot across a range of running speeds.
Scientific Reports,
8(1).
Abstract:
The energetic behaviour of the human foot across a range of running speeds
The human foot contains passive elastic tissues that have spring-like qualities, storing and returning mechanical energy and other tissues that behave as dampers, dissipating energy. Additionally the intrinsic and extrinsic foot muscles have the capacity to act as dampers and motors, dissipating and generating mechanical energy. It remains unknown as to how the contribution of all passive and active tissues combine to produce the overall energetic function of the foot during running. Therefore, the aim of this study was to determine if the foot behaves globally as an active spring-damper during running. Fourteen participants ran on a force-instrumented treadmill at 2.2 ms-1, 3.3 ms-1 and 4.4 ms-1, while foot segment motion was collected simultaneously with kinetic measurements. A unified deformable segment model was applied to quantify the instantaneous power of the foot segment during ground contact and mechanical work was calculated by integrating the foot power data. At all running speeds, the foot absorbed energy from early stance through to mid-stance and subsequently returned/generated a proportion of this energy in late stance. The magnitude of negative work performed increased with running speed, while the magnitude of positive work remained relatively constant across all running speeds. The proportion of energy dissipated relative to that absorbed (foot dissipation-ratio) was always greater than zero and increased with running speed, suggesting that the foot behaves as a viscous spring-damper.
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Mayfield D, Farris D, Cresswell A, Lichtwark G (2017). Effect of muscle force during stretch on eccentric contraction-induced muscle damage. Journal of Science and Medicine in Sport, 20, 56-56.
Farris DJ, Raiteri BJ (2017). Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans.
Journal of Experimental BiologyAbstract:
Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans
Humans and other cursorial mammals have distal leg muscles with high in-series compliance that aid locomotor economy. This muscle-tendon design is considered sub-optimal for injecting net positive mechanical work. However, humans change speed frequently when walking and any acceleration requires net positive ankle work. The present study unveiled how the muscle-tendon interaction of human ankle plantar flexors are adjusted and integrated with body mechanics to provide net positive work during accelerative walking. We found that for accelerative walking, a greater amount of active plantar flexor fascicle shortening early in the stance phase occurred and was transitioned through series elastic tissue stretch and recoil. Reorientation of the leg during early stance for acceleration allowed the ankle and whole soleus muscle-tendon complex to remain isometric while its fascicles actively shortened, stretching in-series elastic tissues for subsequent recoil and net positive joint work. This muscle-tendon behaviour is fundamentally different to constant speed walking, where the ankle and soleus muscle-tendon complex undergo a period of negative work to store energy in series elastic tissues before subsequent recoil, minimising net joint work. Muscles with high in-series compliance can therefore contribute to net positive work for accelerative walking and here we show a mechanism for how in human ankle muscles.
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Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2017). In vivo fascicle length measurements via B-mode ultrasound imaging with single vs dual transducer arrangements. Journal of Biomechanics, 64, 240-244.
Brennan S, Cresswell A, Farris D, Lichtwark G (2017). Mechanical and energetic determinants of optimal cycling cadence. Journal of Science and Medicine in Sport, 20, e72-e72.
Farris DJ, Raiteri BJ (2017). Modulation of leg joint function to produce emulated acceleration during walking and running in humans.
Royal Society Open Science,
4(3), 160901-160901.
Abstract:
Modulation of leg joint function to produce emulated acceleration during walking and running in humans
Understanding how humans adapt gait mechanics for a wide variety of locomotor tasks is important for inspiring the design of robotic, prosthetic and wearable assistive devices. We aimed to elicit the mechanical adjustments made to leg joint functions that are required to generate accelerative walking and running, using metrics with direct relevance to device design. Twelve healthy male participants completed constant speed (CS) walking and running and emulated acceleration (ACC) trials on an instrumented treadmill. External force and motion capture data were combined in an inverse dynamics analysis. Ankle, knee and hip joint mechanics were described and compared using angles, moments, powers and normalized functional indexes that described each joint as relatively more: spring, motor, damper or strut-like. To accelerate using a walking gait, the ankle joint was switched from predominantly spring-like to motor-like, while the hip joint was maintained as a motor, with an increase in hip motor-like function. Accelerating while running involved no change in the primary function of any leg joint, but involved high levels of spring and motor-like function at the hip and ankle joints. Mechanical adjustments for ACC walking were achieved primarily via altered limb positioning, but ACC running needed greater joint moments.
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Lichtwark G, Farris D, Chen X, Hodges P, Sanchez G, Delp S (2017). The variable relationship between sarcomere number and fascicle length when measured in vivo in human lower limb muscle. Journal of Science and Medicine in Sport, 20, 56-56.
Farris DJ (2016). Emulating constant acceleration locomotion mechanics on a treadmill.
Journal of Biomechanics,
49(5), 653-658.
Abstract:
Emulating constant acceleration locomotion mechanics on a treadmill
Locomotion on an accelerating treadmill belt is not dynamically similar to overground acceleration. The purpose of this study was to test if providing an external force to compensate for inertial forces during locomotion on an accelerating treadmill belt could induce locomotor dynamics similar to real accelerations. Nine males (mean±sd age=26±4 years, mass=81±9 kg, height=1.8±0.05 m) began walking and transitioned to running on an accelerating instrumented treadmill belt at three accelerations (0.27 m s-2, 0.42 m s-2, 0.76 m s-2). Half the trials were typical treadmill locomotion (TT) and half were emulated acceleration (EA), where elastic tubing harnessed to the participant provided a horizontal force equal to mass multiplied by acceleration. Net mechanical work (WCOM) and ground reaction force impulses (IGRF) were calculated for individual steps and a linear regression was performed with these experimental measures as independent variables and theoretically derived values of work and impulse as predictor variables. For EA, linear fits were significant for WCOM (y=1.19x+10.5, P
Abstract.
Kelly LA, Lichtwark GA, Farris DJ, Cresswell A (2016). Shoes alter the spring-like function of the human foot during running.
Journal of the Royal Society Interface,
13(119), 20160174-20160174.
Abstract:
Shoes alter the spring-like function of the human foot during running
The capacity to store and return energy in legs and feet that behave like springs is crucial to human running economy. Recent comparisons of shod and barefoot running have led to suggestions that modern running shoes may actually impede leg and foot-spring function by reducing the contributions from the leg and foot musculature. Here we examined the effect of running shoes on foot longitudinal arch (LA) motion and activation of the intrinsic foot muscles. Participants ran on a force-instrumented treadmill with and without running shoes. We recorded foot kinematics and muscle activation of the intrinsic foot muscles using intramuscular electromyography. In contrast to previous assertions, we observed an increase in both the peak (flexor digitorum brevis +60%) and total stance muscle activation (flexor digitorum brevis +70% and abductor hallucis +53%) of the intrinsic foot muscles when running with shoes. Increased intrinsic muscle activation corresponded with a reduction in LA compression (−25%). We confirm that running shoes do indeed influence the mechanical function of the foot. However, our findings suggest that these mechanical adjustments are likely to have occurred as a result of increased neuromuscular output, rather than impaired control as previously speculated. We propose a theoretical model for foot–shoe interaction to explain these novel findings.
Abstract.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2016). The effect of cadence on the muscle-tendon mechanics of the gastrocnemius muscle during walking. Scandinavian Journal of Medicine & Science in Sports, 27(3), 289-298.
Farris DJ, Lichtwark GA, Brown NAT, Cresswell AG (2016). The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo.
Journal of Experimental Biology,
219(4), 528-534.
Abstract:
The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo
Humans utilise elastic tendons of lower limb muscles to store and return energy during walking, running and jumping. Anuran and insect species use skeletal structures and/or dynamics in conjunction with similarly compliant structures to amplify muscle power output during jumping. We sought to examine whether human jumpers use similar mechanisms to aid elastic energy usage in the plantar flexor muscles during maximal vertical jumping. Ten male athletes performed maximal vertical squat jumps. Three-dimensional motion capture and a musculoskeletal model were used to determine lower limb kinematics that were combined with ground reaction force data in an inverse dynamics analysis. B-mode ultrasound imaging of the lateral gastrocnemius (GAS) and soleus (SOL) muscles was used to measure muscle fascicle lengths and pennation angles during jumping. Our results highlighted that both GAS and SOL utilised stretch and recoil of their series elastic elements (SEEs) in a catapultlike fashion, which likely serves to maximise ankle joint power. The resistance of supporting of body weight allowed initial stretch of both GAS and SOL SEEs. A proximal-to-distal sequence of joint moments and decreasing effective mechanical advantage early in the extension phase of the jumping movement were observed. This facilitated a further stretch of the SEE of the biarticular GAS and delayed recoil of the SOL SEE. However, effective mechanical advantage did not increase late in the jump to aid recoil of elastic tissues.
Abstract.
Farris DJ, Lichtwark GA (2016). UltraTrack: Software for semi-automated tracking of muscle fascicles in sequences of B-mode ultrasound images.
Computer Methods and Programs in Biomedicine,
128, 111-118.
Abstract:
UltraTrack: Software for semi-automated tracking of muscle fascicles in sequences of B-mode ultrasound images
Background: Dynamic measurements of human muscle fascicle length from sequences of B-mode ultrasound images have become increasingly prevalent in biomedical research. Manual digitisation of these images is time consuming and algorithms for automating the process have been developed. Here we present a freely available software implementation of a previously validated algorithm for semi-automated tracking of muscle fascicle length in dynamic ultrasound image recordings, "UltraTrack". Methods: UltraTrack implements an affine extension to an optic flow algorithm to track movement of the muscle fascicle end-points throughout dynamically recorded sequences of images. The underlying algorithm has been previously described and its reliability tested, but here we present the software implementation with features for: tracking multiple fascicles in multiple muscles simultaneously; correcting temporal drift in measurements; manually adjusting tracking results; saving and re-loading of tracking results and loading a range of file formats. Results: Two example runs of the software are presented detailing the tracking of fascicles from several lower limb muscles during a squatting and walking activity. Conclusion: We have presented a software implementation of a validated fascicle-tracking algorithm and made the source code and standalone versions freely available for download.
Abstract.
Farris DJ, Lichtwark GA, Brown NAT, Cresswell AG (2015). Deconstructing the power resistance relationship for squats: a joint-level analysis. Scandinavian Journal of Medicine & Science in Sports, 26(7), 774-781.
Robertson BD, Farris DJ, Sawicki GS (2015). Erratum: More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics (Bioinspiration and Biomimetics (2014) 9 (046018)). Bioinspiration and Biomimetics, 10(1).
Mahon CE, Farris DJ, Sawicki GS, Lewek MD (2015). Individual limb mechanical analysis of gait following stroke.
Journal of Biomechanics,
48(6), 984-989.
Abstract:
Individual limb mechanical analysis of gait following stroke
The step-to-step transition of walking requires significant mechanical and metabolic energy to redirect the center of mass. Inter-limb mechanical asymmetries during the step-to-step transition may increase overall energy demands and require compensation during single-support. The purpose of this study was to compare individual limb mechanical gait asymmetries during the step-to-step transitions, single-support and over a complete stride between two groups of individuals following stroke stratified by gait speed (≥0.8. m/s or
Abstract.
Farris DJ, Hampton A, Lewek MD, Sawicki GS (2015). Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints.
Journal of NeuroEngineering and Rehabilitation,
12(1).
Abstract:
Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints
Background: Previous reports of the mechanics and energetics of post-stroke hemiparetic walking have either not combined estimates of mechanical and metabolic energy or computed external mechanical work based on the limited combined limbs method. Here we present a comparison of the mechanics and energetics of hemiparetic and unimpaired walking at a matched speed. Methods: Mechanical work done on the body centre of mass (COM) was computed by the individual limbs method and work done at individual leg joints was computed with an inverse dynamics analysis. Both estimates were converted to average powers and related to simultaneous estimates of net metabolic power, determined via indirect calorimetry. Efficiency of positive work was calculated as the ratio of average positive mechanical power P+ to net metabolic power. Results: Total P+ was 20% greater for the hemiparetic group (H) than for the unimpaired control group (C) (0.49 vs. 0.41 W · kg-1). The greater P+ was partly attributed to the paretic limb of hemiparetic walkers not providing appropriately timed push-off P+ in the step-to-step transition. This led to compensatory non-paretic limb hip and kneeP+ which resulted in greater total mechanical work. Efficiency of positive work was not different between H and C. Conclusions: Increased work, not decreased efficiency, explains the greater metabolic cost of hemiparetic walking post-stroke. Our results highlighted the need to target improving paretic ankle push-off via therapy or assistive technology in order to reduce the metabolic cost of hemiparetic walking.
Abstract.
Farris D (2014). Does unloading muscles save you energy? Paradoxical effects of spring-loaded ankle exoskeletons on plantar flexor muscle mechanics and energetics. Journal of Science and Medicine in Sport, 18, e129-e130.
Lichtwark G, Farris D, Kelly L, Brown N (2014). Lower limb biomechanics and muscle function. Journal of Science and Medicine in Sport, 18, e128-e128.
Robertson BD, Farris DJ, Sawicki GS (2014). More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics.
Bioinspiration and Biomimetics,
9(4).
Abstract:
More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics
Development P of robotic exoskeletons to assist/enhance human locomotor performance involves lengthy prototyping, testing, and analysis. This process is further convoluted by variability in limb/body morphology and preferred gait patterns between individuals. In an attempt to expedite this process, and establish a physiological basis for actuator prescription, we developed a simple, predictive model of human neuromechanical adaptation to a passive elastic exoskeleton applied at the ankle joint during a functional task. We modeled the human triceps surae-Achilles tendon muscle tendon unit (MTU) as a single Hill-type muscle, or contractile element (CE), and series tendon, or series elastic element (SEE). This modeled system was placed under gravitational load and underwent cyclic stimulation at a regular frequency (i.e. hopping) with and without exoskeleton (Exo) assistance. We explored the effect that both Exo stiffness (kExo) and muscle activation (Astim) had on combined MTU and Exo (MTU+ Exo), MTU, and CE/SEE mechanics and energetics. Model accuracy was verifi ed via qualitative and quantitative comparisons between modeled and prior experimental outcomes. We demonstrated that reduced Astim can be traded for increased kExo to maintain consistent MTU+ Exo mechanics (i.e. average positive power (P¯mech+) output) from an unassisted condition (i.e. kExo = 0 kN · m-1). For these regions of parameter space, our model predicted a reduction in MTU force, SEE energy cycling, and metabolic rate (P¯met), as well as constant CE P¯mech+ output compared to unassisted conditions. This agreed with previous experimental observations, demonstrating our model's predictive ability. Model predictions also provided insight into mechanisms of metabolic cost minimization, and/or enhanced mechanical performance, and we concluded that both of these outcomes cannot be achieved simultaneously, and that one must come at the detriment of the other in a spring-assisted compliant MTU.
Abstract.
Farris DJ, Hicks JL, Delp SL, Sawicki GS (2014). Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping.
Journal of Experimental Biology,
217(22), 4018-4028.
Abstract:
Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping
Experiments have shown that elastic ankle exoskeletons can be used to reduce ankle joint and plantar-flexor muscle loading when hopping in place and, in turn, reduce metabolic energy consumption. However, recent experimental work has shown that such exoskeletons cause less favourable soleus (SO) muscle-tendon mechanics than is observed during normal hopping, which might limit the capacity of the exoskeleton to reduce energy consumption. To directly link plantar-flexor mechanics and energy consumption when hopping in exoskeletons, we used a musculoskeletal model of the human leg and a model of muscle energetics in simulations of muscle-tendon dynamics during hopping with and without elastic ankle exoskeletons. Simulations were driven by experimental electromyograms, joint kinematics and exoskeleton torque taken from previously published data. The data were from seven males who hopped at 2.5 Hz with and without elastic ankle exoskeletons. The energetics model showed that the total rate of metabolic energy consumption by ankle muscles was not significantly reduced by an ankle exoskeleton. This was despite large reductions in plantar-flexor force production (40-50%). The lack of larger metabolic reductions with exoskeletons was attributed to increases in plantar-flexor muscle fibre velocities and a shift to less favourable muscle fibre lengths during active force production. This limited the capacity for plantarflexors to reduce activation and energy consumption when hopping with exoskeleton assistance.
Abstract.
Farris DJ, Trewartha G, McGuigan MP, Lichtwark GA (2013). Differential strain patterns of the human Achilles tendon determined in vivo with freehand three-dimensional ultrasound imaging.
Journal of Experimental Biology,
216(4), 594-600.
Abstract:
Differential strain patterns of the human Achilles tendon determined in vivo with freehand three-dimensional ultrasound imaging
The human Achilles tendon (AT) has often been considered to act as a single elastic structure in series with the muscles of the triceps surae. As such it has been commonly modelled as a Hookean spring of uniform stiffness. However, the free AT and the proximal AT have distinctly different structures that lend themselves to different elastic properties. This study aimed to use threedimensional freehand ultrasound imaging to determine whether the proximal AT and the free AT exhibit different elastic behaviour during sub-maximal, fixed-end contractions of the triceps surae. Six male and five female participants (mean ± s.d. age=27±5'years) performed fixed position contractions of the plantar-flexors on an isokinetic dynamometer at 50% of their maximum voluntary contraction in this position. Freehand three-dimensional ultrasound imaging was used to reconstruct the free-tendon and proximal AT at rest and during contraction. The free-tendon exhibited significantly (P=0.03) greater longitudinal strain (5.2±1.7%) than the proximal AT (2.6±2.0%). The lesser longitudinal strain of the proximal AT was linked to the fact that it exhibited considerable transverse (orthogonal to the longitudinal direction) strains (5.0±4%). The transverse strain of the proximal AT is likely due to the triceps surae muscles bulging upon contraction, and thus the level of bulging may influence the elastic behaviour of the proximal AT. This might have implications for the understanding of triceps surae muscle-tendon interaction during locomotion, tendon injury mechanics and previous measurements of AT elastic properties. © 2013. Published by the Company of Biologists Ltd.
Abstract.
Farris DJ, Robertson BD, Sawicki GS (2013). Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping.
Journal of Applied Physiology,
115(5), 579-585.
Abstract:
Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping
Farris DJ, Robertson BD, Sawicki GS. Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping. J Appl Physiol 115: 579-585, 2013. First published June 20, 2013; doi:10.1152/japplphysiol.00253.2013.- Inspired by elastic energy storage and return in tendons of human leg muscle-tendon units (MTU), exoskeletons often place a spring in parallel with an MTU to assist the MTU. However, this might perturb the normally efficient MTU mechanics and actually increase active muscle mechanical work. This study tested the effects of elastic parallel assistance on MTU mechanics. Participants hopped with and without spring-loaded ankle exoskeletons that assisted plantar flexion. An inverse dynamics analysis, combined with in vivo ultrasound imaging of soleus fascicles and surface electromyography, was used to determine muscle-tendon mechanics and activations. Whole body net metabolic power was obtained from indirect calorimetry. When hopping with spring-loaded exoskeletons, soleus activation was reduced (30-70%) and so was the magnitude of soleus force (peak force reduced by 30%) and the average rate of soleus force generation (by 50%). Although forces were lower, average positive fascicle power remained unchanged, owing to increased fascicle excursion (+4-5 mm). Net metabolic power was reduced with exoskeleton assistance (19%). These findings highlighted that parallel assistance to a muscle with appreciable series elasticity may have some negative consequences, and that the metabolic cost associated with generating force may be more pronounced than the cost of doing work for these muscles. Copyright © 2013 the American Physiological Society.
Abstract.
Farris DJ, Sawicki GS (2013). Erratum: Linking the mechanics and energetics of hopping with elastic ankle exoskeletons (Journal of Applied Physiology (2012) 113 (1862-1872) DOI: 10.1152/japplphysiol.00802.2012). Journal of Applied Physiology, 115(2).
Farris DJ, Sawicki GS (2012). Human medial gastrocnemius force-velocity behavior shifts with locomotion speed and gait.
Proceedings of the National Academy of Sciences of the United States of America,
109(3), 977-982.
Abstract:
Human medial gastrocnemius force-velocity behavior shifts with locomotion speed and gait
Humans walk and run over a wide range of speeds with remarkable efficiency. For steady locomotion, moving at different speeds requires the muscle-tendon units of the leg to modulate the amount of mechanical power the limb absorbs and outputs in each step. How individual muscles adapt their behavior to modulate limb power output has been examined using computer simulation and animal models, but has not been studied in vivo in humans. In this study, we used a combination of ultrasound imaging and motion analysis to examine how medial gastrocnemius (MG) muscle-tendon unit behavior is adjusted to meet the varying mechanical demands of different locomotor speeds during walking and running in humans. The results highlighted key differences in MG fascicle-shortening velocity with both locomotor speed and gait. Fascicle-shortening velocity at the time of peak muscle force production increased with walking speed, impairing the ability of the muscle to produce high peak forces. Switching to a running gait at 2.0 m·s -1 caused fascicle shortening at the time of peak force production to shift to much slower velocities. This velocity shift facilitated a large increase in peak muscle force and an increase in MG power output. MG fascicle velocity may be a key factor that limits the speeds humans choose to walk at, and may explain the transition from walking to running. This finding is consistent with previous modeling studies.
Abstract.
Farris DJ, Sawicki GS (2012). Linking the mechanics and energetics of hopping with elastic ankle exoskeletons.
Journal of Applied Physiology,
113(12), 1862-1872.
Abstract:
Linking the mechanics and energetics of hopping with elastic ankle exoskeletons
The springlike mechanics of the human leg during bouncing gaits has inspired the design of passive assistive devices that use springs to aid locomotion. The purpose of this study was to test whether a passive spring-loaded ankle exoskeleton could reduce the mechanical and energetic demands of bilateral hopping on the musculoskeletal system. Joint level kinematics and kinetics were collected with electromyographic and metabolic energy consumption data for seven participants hopping at four frequencies (2.2, 2.5, 2.8, and 3.2 Hz). Hopping was performed without an exoskeleton; with an springless exoskeleton; and with a spring-loaded exoskeleton. Spring-loaded ankle exoskeletons reduced plantar flexor muscle activity and the biological contribution to ankle joint moment (15-25%) and average positive power (20-40%). They also facilitated reductions in metabolic power (15-20%) across frequencies from 2.2 to 2.8 Hz compared with hopping with a springless exoskeleton. Reductions in metabolic power compared with hopping with no exoskeleton were restricted to hopping at 2.5 Hz only (12%). These results highlighted the importance of reducing the rate of muscular force production and work to achieve metabolic reductions. They also highlighted the importance of assisting muscles acting at the knee joint. Exoskeleton designs may need to be tuned to optimize exoskeleton mass, spring stiffness, and spring slack length to achieve greater metabolic reductions. © 2012 the American Physiological Society.
Abstract.
Farris DJ, Trewartha G, McGuigan MP (2012). The effects of a 30-min run on the mechanics of the human Achilles tendon.
European Journal of Applied Physiology,
112(2), 653-660.
Abstract:
The effects of a 30-min run on the mechanics of the human Achilles tendon
Tendinous structures often exhibit reduced stiffness following repeated loading via static muscular contractions. The purpose of this study was to determine if human Achilles tendon (AT) stiffness is affected by the repeated loading experienced during running and if this affects normal muscle-tendon interaction. Twelve male participants (mean ± SD: age 27 ± 5 years, height 1.79 ± 0.06 m, mass 78.6 ± 8.4 kg) completed a 30 min run at 12 kmph on a treadmill. AT properties were determined before and after the run during a series of one-legged hops. During hopping and running, AT length data were acquired from a combination of ultrasound imaging (50 Hz) and kinematic data (200 Hz). AT force was estimated from inverse dynamics during hopping and AT stiffness was computed from plots of AT force and length. AT stiffness was not significantly different post run (pre 163 ± 41 N mm -1, post 147 ± 52 N mm -1, P > 0.05) and peak AT strain during the stance phase of running (calculated relative to AT length during standing) was similar at different time points during the run (3.5 ± 1.8% at 1 min, 3.2 ± 1.8% at 15 min and 3.8 ± 2% at 30 min). It was concluded that the loading experienced during a single bout of running does not affect the stiffness of the AT and that the properties of the AT are stable during locomotion. This may have implications for muscle fascicle behaviour and Achilles tendon injury mechanisms. © 2011 Springer-Verlag.
Abstract.
Farris DJ, Buckeridge E, Trewartha G, McGuigan MP (2012). The effects of orthotic heel lifts on achilles tendon force and strain during running.
Journal of Applied Biomechanics,
28(5), 511-519.
Abstract:
The effects of orthotic heel lifts on achilles tendon force and strain during running
This study assessed the effects of orthotic heel lifts on Achilles tendon (AT) force and strain during running. Ten females ran barefoot over a force plate in three conditions: no heel lifts (NHL), with 12 mm heel lifts (12HL) and with 18 mm heel lifts (18HL). Kinematics for the right lower limb were collected (200 Hz). AT force was calculated from inverse dynamics. AT strain was determined from kinematics and ultrasound images of medial gastrocnemius (50 Hz). Peak AT strain was less for 18HL (5.5 ± 4.4%) than for NHL (7.4 ± 4.2%) (p =. 029, effect size [ES] = 0.44) but not for 12HL (5.8 ± 4.8%) versus NHL (ES = 0.35). Peak AT force was significantly (p =. 024, ES = 0.42) less for 18HL (2382 ± 717 N) than for NHL (2710 ± 830 N) but not for 12HL (2538 ± 823 N, ES = 0.21). The 18HL reduced ankle dorsiflexion but not flexion-extension ankle moments and increased the AT moment arm compared with NHL. Thus, 18HL reduced force and strain on the AT during running via a reduction in dorsiflexion, which lengthened the AT moment arm. Therefore, heel lifts could be used to reduce AT loading and strain during the rehabilitation of AT injuries. © 2012 Human Kinetics, Inc.
Abstract.
Farris DJ, Sawicki GS (2012). The mechanics and energetics of human walking and running: a joint level perspective.
Journal of the Royal Society Interface,
9(66), 110-118.
Abstract:
The mechanics and energetics of human walking and running: a joint level perspective
Humans walk and run at a range of speeds. While steady locomotion at a given speed requires no net mechanical work, moving faster does demand both more positive and negative mechanical work per stride. Is this increased demand met by increasing power output at all lower limb joints or just some of them? Does running rely on different joints for power output than walking? How does this contribute to the metabolic cost of locomotion? This study examined the effects of walking and running speed on lower limb joint mechanics and metabolic cost of transport in humans. Kinematic and kinetic data for 10 participants were collected for a range of walking (0.75, 1.25, 1.75, 2.0 ms-1) and running (2.0, 2.25, 2.75, 3.25 ms-1) speeds. Net metabolic power was measured by indirect calorimetry. Within each gait, there was no difference in the proportion of power contributed by each joint (hip, knee, ankle) to total power across speeds. Changing from walking to running resulted in a significant ( p = 0.02) shift in power production from the hip to the ankle which may explain the higher efficiency of running at speeds above 2.0 ms-1 and shed light on a potential mechanism behind the walk-run transition. © 2011 the Royal Society.
Abstract.
Lichtwark G, Farris D, Newsham-West R (2011). Achilles tendon (3D): Do the mechanical properties of tendon change in response to exercise?. Journal of Science and Medicine in Sport, 14, e9-e10.
Farris DJ, Trewartha G, Polly McGuigan M (2011). Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?.
Journal of Biomechanics,
44(5), 822-826.
Abstract:
Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?
Chronic tendinopathy of the human Achilles tendon (AT) is common but its injury mechanism is not fully understood. It has been hypothesised that heat energy losses from the AT during running could explain the degeneration of AT material seen with injury. A mathematical model of AT temperature distribution was used to predict what temperatures the core of the AT could reach during running. This model required input values for mechanical properties of the AT (stiffness, hysteresis, cross-sectional area (CSA), strain during running) which were determined using a combination of ultrasound imaging, kinematic and kinetic data. AT length data were obtained during hopping and treadmill running (12kmph) using ultrasound images of the medial gastrocnemius (50Hz) and kinematic data (200Hz). AT force data were calculated from inverse dynamics during hopping and combined with AT length data to compute AT stiffness and hysteresis. AT strain was computed from AT length data during treadmill running. AT CSA was measured on transverse ultrasound scans of the AT. Mean±sd tendon properties were: stiffness=176±41Nmm -1, hysteresis=17±12%, strain during running=3.5±1.8% and CSA=42±8mm 2. These values were input into the model of AT core temperature and this was predicted to reach at least 41°C during running. Such temperatures were deemed to be conservative estimates but still sufficient for tendon hyperthermia to be a potential cause of tendon injury. © 2011 Elsevier Ltd.
Abstract.
Farris D, Buckeridge E, Trewartha G, McGuigan P (2008). The effects of orthotic heel lifts on Achilles tendon force and strain during running. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 150(3), S82-S82.
Publications by year
In Press
Wade L, Lichtwark G, Farris D (In Press). Joint and muscle-tendon coordination strategies during submaximal jumping. Journal of Applied Physiology
Birch J, Kelly L, Cresswell A, Dixon S, Farris D (In Press). Neuromechanical adaptations of foot function to changes in surface stiffness during hopping. Journal of Applied Physiology
Riddick R, Farris D, Cresswell A, Kuo A, Kelly L (In Press). Stepping onto the unknown: reflexes of the foot and ankle while stepping with perturbed perceptions of terrain. Journal of the Royal Society Interface
2023
Farris DJ, Harris DJ, Rice HM, Campbell J, Weare A, Risius D, Armstrong N, Rayson MP (2023). A systematic literature review of evidence for the use of assistive exoskeletons in defence and security use cases.
Ergonomics,
66(1), 61-87.
Abstract:
A systematic literature review of evidence for the use of assistive exoskeletons in defence and security use cases.
Advances in assistive exoskeleton technology, and a boom in related scientific literature, prompted a need to review the potential use of exoskeletons in defence and security. A systematic review examined the evidence for successful augmentation of human performance in activities deemed most relevant to military tasks. Categories of activities were determined a priori through literature scoping and Human Factors workshops with military stakeholders. Workshops identified promising opportunities and risks for integration of exoskeletons into military use cases. The review revealed promising evidence for exoskeletons' capacity to assist with load carriage, manual lifting, and working with tools. However, the review also revealed significant gaps in exoskeleton capabilities and likely performance levels required in the use case scenarios. Consequently, it was recommended that a future roadmap for introducing exoskeletons to military environments requires development of performance criteria for exoskeletons that can be used to implement a human-centred approach to research and development.
Abstract.
Author URL.
Farris D (2023). Musculoskeletal simulations to examine the effects of accentuated eccentric loading (AEL) on jump height. PeerJ – the Journal of Life & Environmental Sciences
2022
Smith R, Lichtwark G, Farris D, Kelly L (2022). Examining the intrinsic foot muscles' capacity to modulate plantar flexor gearing and ankle joint contributions to propulsion in vertical jumping.
J Sport Health SciAbstract:
Examining the intrinsic foot muscles' capacity to modulate plantar flexor gearing and ankle joint contributions to propulsion in vertical jumping.
BACKGROUND: During human locomotion, a sufficiently stiff foot allows the ankle plantar flexors to generate large propulsive powers. Increasing foot stiffness (e.g. via a carbon plate) increases the ankle's external moment arm in relation to the internal moment arm (i.e. increasing gear ratio), reduces plantar flexor muscles' shortening velocity, and enhances muscle force production. In contrast, when activation of the foot's intrinsic muscles is impaired, there is a reduction in foot and ankle work and metatarsophalangeal joint stiffness. We speculated that the reduced capacity to actively control metatarsophalangeal joint stiffness may impair the gearing function of the foot at the ankle. METHODS: We used a tibial nerve block to examine the direct effects of the intrinsic foot muscles on ankle joint kinetics, in vivo medial gastrocnemius' musculotendinous dynamics, and ankle gear ratio on 14 participants during maximal vertical jumping. RESULTS: Under the nerve block, the internal ankle plantar flexion moment decreased (p = 0.004) alongside a reduction in external moment arm length (p = 0.021) and ankle joint gear ratio (p = 0.049) when compared to the non-blocked condition. Although medial gastrocnemius muscle-tendon unit and fascicle velocity were not different between conditions, the Achilles tendon was shorter during propulsion in the nerve block condition (p < 0.001). CONCLUSION: in addition to their known role of regulating the energetic function of the foot, our data indicate that the intrinsic foot muscles also act to optimize ankle joint torque production and leverage during the propulsion phase of vertical jumping.
Abstract.
Author URL.
Birch JV, Farris DJ, Riddick R, Cresswell AG, Dixon SJ, Kelly LA (2022). Neuromechanical adaptations of foot function when hopping on a damped surface.
J Appl Physiol (1985),
133(6), 1302-1308.
Abstract:
Neuromechanical adaptations of foot function when hopping on a damped surface.
To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.
Abstract.
Author URL.
Wade L, Birch J, Farris DJ (2022). Walking with increasing acceleration is achieved by tuning ankle torque onset timing and rate of torque development.
Journal of the Royal Society Interface,
19(191).
Abstract:
Walking with increasing acceleration is achieved by tuning ankle torque onset timing and rate of torque development
Understanding the mechanics of torque production about the ankle during accelerative gait is key to designing effective clinical and rehabilitation practices, along with developing functional robotics and wearable assistive technologies. We aimed to explore how torque and work about the ankle is produced as walking acceleration increases from 0 to 100% maximal acceleration. We hypothesized that as acceleration increased, greater work about the ankle would not be solely due to ramping up plantar flexor torque, and instead would be a product of adjustments to relative timing of ankle torque and angular displacement. Fifteen healthy participants performed walking without acceleration (constant speed), as well as low, moderate and maximal accelerations, while motion capture and ground reaction force data were recorded. We employed vector coding in a novel application to overcome limitations of previously employed evaluation methods. As walking acceleration increased, there was reduced negative work and increased positive work about the ankle. Furthermore, early stance dorsiflexion had reducing plantar flexor torque due to delayed plantar flexor torque onset as acceleration increased, while mid-stance ankle plantar flexor torque was substantially increased with minimal ankle dorsiflexion, irrespective of acceleration magnitude. Assistive devices need to account for these changes during accelerative walking to facilitate functional gait.
Abstract.
2021
Smith R, Lichtwark G, Farris D, Kelly LA (2021). Examining the intrinsic foot muscles’ capacity to modulate plantar flexor gearing. Footwear Science, 13(S1), S87-S89.
Riddick RC, Farris DJ, Brown NAT, Kelly LA (2021). Stiffening the human foot with a biomimetic exotendon.
Scientific Reports,
11(1).
Abstract:
Stiffening the human foot with a biomimetic exotendon
AbstractShoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Recent footwear design developments have shown that this approach may also produce metabolic energy savings. Here we test an alternative approach to modify the foot–ground interface by adding additional stiffness in parallel to the plantar aponeurosis, targeting the windlass mechanism. Stiffening the windlass mechanism by about 9% led to decreases in peak activation of the ankle plantarflexors soleus (~ 5%, p < 0.001) and medial gastrocnemius (~ 4%, p < 0.001), as well as a ~ 6% decrease in positive ankle work (p < 0.001) during fixed-frequency bilateral hopping (2.33 Hz). These results suggest that stiffening the foot may reduce cost in dynamic tasks primarily by reducing the effort required to plantarflex the ankle, since peak activation of the intrinsic foot muscle abductor hallucis was unchanged (p = 0.31). Because the novel exotendon design does not operate via the compression or bending of a bulky midsole, the device is light (55 g) and its profile is low enough that it can be worn within an existing shoe.
Abstract.
2020
Farris D, Birch J, Kelly L (2020). Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism. Journal of the Royal Society Interface, 17 (168)
Nuckols RW, Takahashi KZ, Farris DJ, Mizrachi S, Riemer R, Sawicki GS (2020). Mechanics of walking and running up and downhill: a joint-level perspective to guide design of lower-limb exoskeletons. PLOS ONE, 15(8), e0231996-e0231996.
2019
Wade L, Lichtwark GA, Farris DJ (2019). Comparisons of laboratory‐based methods to calculate jump height and improvements to the field‐based flight‐time method. Scandinavian Journal of Medicine & Science in Sports, 30(1), 31-37.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2019). The Effect of Cadence on the Mechanics and Energetics of Constant Power Cycling.
Medicine and Science in Sports and Exercise,
51(5), 941-950.
Abstract:
The Effect of Cadence on the Mechanics and Energetics of Constant Power Cycling
At a constant power output, cyclists prefer to use a higher cadence than those that minimize metabolic cost. The neuromuscular mechanism underpinning the preferred higher cadence remains unclear. Purpose the aim of this study was to investigate the effect of cadence on joint level work and vastus lateralis (VL) fascicle mechanics while cycling at a constant, submaximal, power output. We hypothesized that preferred cycling cadence would enhance the power capacity of the VL muscle when compared with a more economical cadence. Furthermore, we predicted that the most economical cadence would coincide with minimal total electromyographic activity from the leg muscles. Methods: Metabolic cost, lower-limb kinematics, joint level work, VL fascicle mechanics, and muscle activation of the VL, rectus femoris, biceps femoris, gastrocnemius medialis, and soleus muscles were measured during cycling at a constant power output of 2.5 W·kg -1 and cadences of 40, 60, 80, and 100 rpm. A preferred condition was also performed where cadence feedback was hidden from the participant. Results: Metabolic cost was lowest at 60 rpm, but the mean preferred cadence was 81 rpm. The distribution of joint work remained constant across cadences, with the majority of positive work being performed at the knee. The preferred cadence coincided with the highest VL power capacity, without a significant penalty to efficiency, based on fascicle shortening velocity. Conclusions: Cycling at a higher cadence is preferred to ensure that the muscle's ability to produce positive power remains high. Further investigations are required to examine what feedback mechanism could be responsible for the optimization of this motor pattern.
Abstract.
Riddick R, Farris DJ, Kelly LA (2019). The foot is more than a spring: human foot muscles perform work to adapt to the energetic requirements of locomotion.
JOURNAL OF THE ROYAL SOCIETY INTERFACE,
16(150).
Author URL.
Farris DJ, Kelly L, Cresswell A, Lichtwark G (2019). The functional importance of human foot muscles for bipedal locomotion. Proceedings of the National Academy of Sciences, 116(5), 1645-1650.
Wade L, Lichtwark G, Farris DJ (2019). The influences of added mass on muscle activation and contractile mechanics during submaximal and maximal countermovement jumping in humans. Journal of Experimental Biology, 222
2018
Kelly L, Farris DJ, Cresswell A, Lichtwark G (2018). Intrinsic foot muscles contribute to elastic energy storage and return in the human foot. Journal of Applied Physiology
Lichtwark G, Farris DJ, Chen X, Hodges P, Delp S (2018). Microendoscopy reveals positive correlation in multiscale length changes and variable sarcomere lengths across different regions of human muscle. Journal of Applied Physiology, 125, 1812-1820.
Wade L, Lichtwark G, Farris DJ (2018). Movement Strategies for Countermovement Jumping are Potentially Influenced by Elastic Energy Stored and Released from Tendons.
Scientific Reports,
8(1).
Abstract:
Movement Strategies for Countermovement Jumping are Potentially Influenced by Elastic Energy Stored and Released from Tendons
The preferred movement strategies that humans choose to produce work for movement are not fully understood. Previous studies have demonstrated an important contribution of elastic energy stored within the Achilles tendon (AT) during jumping. This study aimed to alter energy available for storage in the AT to examine changes in how jumpers distribute work among lower limb joints. Participants (n = 16) performed maximal and sub-maximal jumps under two paradigms, matched for increasing total work output by manipulating jump height or adding body mass. Motion capture and ground reaction force data were combined in an inverse dynamics analysis to compute ankle, knee and hip joint kinetics. Results demonstrated higher peak moments about the ankle joint with added body mass (+26 Nm), likely resulting in additional energy storage in the AT. Work at the ankle joint increased proportionally with added mass, maintaining a constant contribution (~64%) to total work that was not matched with increasing jump height (-14%). This implies greater energy storage and return by the AT with added mass but not with increased height. When total work during jumping is constant but energy stored in tendons is not, humans prioritise the use of stored elastic energy over muscle work.
Abstract.
KELLY LA, FARRIS DJ, LICHTWARK GA, CRESSWELL AG (2018). The Influence of Foot-Strike Technique on the Neuromechanical Function of the Foot. Medicine & Science in Sports & Exercise, 50(1), 98-108.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2018). The effect of muscle-tendon unit vs. fascicle analyses on vastus lateralis force-generating capacity during constant power output cycling with variable cadence.
Journal of Applied Physiology,
124(4), 993-1002.
Abstract:
The effect of muscle-tendon unit vs. fascicle analyses on vastus lateralis force-generating capacity during constant power output cycling with variable cadence
The maximum force-generating capacity of a muscle is dependent on the lengths and velocities of its contractile apparatus. Muscle-tendon unit (MTU) length changes can be estimated from joint kinematics; however, contractile element length changes are more difficult to predict during dynamic contractions. The aim of this study was to compare vastus lateralis (VL) MTU and fascicle level force-length and forcevelocity relationships, and dynamic muscle function while cycling at a constant submaximal power output (2.5 W/kg) with different cadences. We hypothesized that manipulating cadence at a constant power output would not affect VL MTU shortening, but significantly affect VL fascicle shortening. Furthermore, these differences would affect the predicted force capacity of the muscle. Using an isokinetic dynamometer and B-mode ultrasound (US), we determined the forcelength and force-velocity properties of the VL MTU and its fascicles. In addition, three-dimensional kinematics and kinetics of the lower limb, as well as US images of VL fascicles were collected during submaximal cycling at cadences of 40, 60, 80, and 100 rotations per minute. Ultrasound measures revealed a significant increase in fascicle shortening as cadence decreased (84% increase across all conditions, P < 0.01), whereas there were no significant differences in MTU lengths across any of the cycling conditions (maximum of 6%). The MTU analysis resulted in greater predicted force capacity across all conditions relative to the force-velocity relationship (P < 0.01). These results reinforce the need to determine muscle mechanics in terms of separate contractile element and connective tissue length changes during isokinetic contractions, as well as dynamic movements like cycling. NEW & NOTEWORTHY We demonstrate that vastus lateralis (VL) muscle tendon unit (MTU) length changes do not adequately reflect the underlying fascicle mechanics during cycling. When examined across different pedaling cadence conditions, the force-generating potential measured only at the level of MTU (or joint) overestimated the maximum force capacity of VL compared with analysis using fascicle level data.
Abstract.
Kelly LA, Cresswell AG, Farris DJ (2018). The energetic behaviour of the human foot across a range of running speeds.
Scientific Reports,
8(1).
Abstract:
The energetic behaviour of the human foot across a range of running speeds
The human foot contains passive elastic tissues that have spring-like qualities, storing and returning mechanical energy and other tissues that behave as dampers, dissipating energy. Additionally the intrinsic and extrinsic foot muscles have the capacity to act as dampers and motors, dissipating and generating mechanical energy. It remains unknown as to how the contribution of all passive and active tissues combine to produce the overall energetic function of the foot during running. Therefore, the aim of this study was to determine if the foot behaves globally as an active spring-damper during running. Fourteen participants ran on a force-instrumented treadmill at 2.2 ms-1, 3.3 ms-1 and 4.4 ms-1, while foot segment motion was collected simultaneously with kinetic measurements. A unified deformable segment model was applied to quantify the instantaneous power of the foot segment during ground contact and mechanical work was calculated by integrating the foot power data. At all running speeds, the foot absorbed energy from early stance through to mid-stance and subsequently returned/generated a proportion of this energy in late stance. The magnitude of negative work performed increased with running speed, while the magnitude of positive work remained relatively constant across all running speeds. The proportion of energy dissipated relative to that absorbed (foot dissipation-ratio) was always greater than zero and increased with running speed, suggesting that the foot behaves as a viscous spring-damper.
Abstract.
2017
Mayfield D, Farris D, Cresswell A, Lichtwark G (2017). Effect of muscle force during stretch on eccentric contraction-induced muscle damage. Journal of Science and Medicine in Sport, 20, 56-56.
Farris DJ, Raiteri BJ (2017). Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans.
Journal of Experimental BiologyAbstract:
Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans
Humans and other cursorial mammals have distal leg muscles with high in-series compliance that aid locomotor economy. This muscle-tendon design is considered sub-optimal for injecting net positive mechanical work. However, humans change speed frequently when walking and any acceleration requires net positive ankle work. The present study unveiled how the muscle-tendon interaction of human ankle plantar flexors are adjusted and integrated with body mechanics to provide net positive work during accelerative walking. We found that for accelerative walking, a greater amount of active plantar flexor fascicle shortening early in the stance phase occurred and was transitioned through series elastic tissue stretch and recoil. Reorientation of the leg during early stance for acceleration allowed the ankle and whole soleus muscle-tendon complex to remain isometric while its fascicles actively shortened, stretching in-series elastic tissues for subsequent recoil and net positive joint work. This muscle-tendon behaviour is fundamentally different to constant speed walking, where the ankle and soleus muscle-tendon complex undergo a period of negative work to store energy in series elastic tissues before subsequent recoil, minimising net joint work. Muscles with high in-series compliance can therefore contribute to net positive work for accelerative walking and here we show a mechanism for how in human ankle muscles.
Abstract.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2017). In vivo fascicle length measurements via B-mode ultrasound imaging with single vs dual transducer arrangements. Journal of Biomechanics, 64, 240-244.
Brennan S, Cresswell A, Farris D, Lichtwark G (2017). Mechanical and energetic determinants of optimal cycling cadence. Journal of Science and Medicine in Sport, 20, e72-e72.
Farris DJ, Raiteri BJ (2017). Modulation of leg joint function to produce emulated acceleration during walking and running in humans.
Royal Society Open Science,
4(3), 160901-160901.
Abstract:
Modulation of leg joint function to produce emulated acceleration during walking and running in humans
Understanding how humans adapt gait mechanics for a wide variety of locomotor tasks is important for inspiring the design of robotic, prosthetic and wearable assistive devices. We aimed to elicit the mechanical adjustments made to leg joint functions that are required to generate accelerative walking and running, using metrics with direct relevance to device design. Twelve healthy male participants completed constant speed (CS) walking and running and emulated acceleration (ACC) trials on an instrumented treadmill. External force and motion capture data were combined in an inverse dynamics analysis. Ankle, knee and hip joint mechanics were described and compared using angles, moments, powers and normalized functional indexes that described each joint as relatively more: spring, motor, damper or strut-like. To accelerate using a walking gait, the ankle joint was switched from predominantly spring-like to motor-like, while the hip joint was maintained as a motor, with an increase in hip motor-like function. Accelerating while running involved no change in the primary function of any leg joint, but involved high levels of spring and motor-like function at the hip and ankle joints. Mechanical adjustments for ACC walking were achieved primarily via altered limb positioning, but ACC running needed greater joint moments.
Abstract.
Lichtwark G, Farris D, Chen X, Hodges P, Sanchez G, Delp S (2017). The variable relationship between sarcomere number and fascicle length when measured in vivo in human lower limb muscle. Journal of Science and Medicine in Sport, 20, 56-56.
2016
Farris DJ (2016). Emulating constant acceleration locomotion mechanics on a treadmill.
Journal of Biomechanics,
49(5), 653-658.
Abstract:
Emulating constant acceleration locomotion mechanics on a treadmill
Locomotion on an accelerating treadmill belt is not dynamically similar to overground acceleration. The purpose of this study was to test if providing an external force to compensate for inertial forces during locomotion on an accelerating treadmill belt could induce locomotor dynamics similar to real accelerations. Nine males (mean±sd age=26±4 years, mass=81±9 kg, height=1.8±0.05 m) began walking and transitioned to running on an accelerating instrumented treadmill belt at three accelerations (0.27 m s-2, 0.42 m s-2, 0.76 m s-2). Half the trials were typical treadmill locomotion (TT) and half were emulated acceleration (EA), where elastic tubing harnessed to the participant provided a horizontal force equal to mass multiplied by acceleration. Net mechanical work (WCOM) and ground reaction force impulses (IGRF) were calculated for individual steps and a linear regression was performed with these experimental measures as independent variables and theoretically derived values of work and impulse as predictor variables. For EA, linear fits were significant for WCOM (y=1.19x+10.5, P
Abstract.
Kelly LA, Lichtwark GA, Farris DJ, Cresswell A (2016). Shoes alter the spring-like function of the human foot during running.
Journal of the Royal Society Interface,
13(119), 20160174-20160174.
Abstract:
Shoes alter the spring-like function of the human foot during running
The capacity to store and return energy in legs and feet that behave like springs is crucial to human running economy. Recent comparisons of shod and barefoot running have led to suggestions that modern running shoes may actually impede leg and foot-spring function by reducing the contributions from the leg and foot musculature. Here we examined the effect of running shoes on foot longitudinal arch (LA) motion and activation of the intrinsic foot muscles. Participants ran on a force-instrumented treadmill with and without running shoes. We recorded foot kinematics and muscle activation of the intrinsic foot muscles using intramuscular electromyography. In contrast to previous assertions, we observed an increase in both the peak (flexor digitorum brevis +60%) and total stance muscle activation (flexor digitorum brevis +70% and abductor hallucis +53%) of the intrinsic foot muscles when running with shoes. Increased intrinsic muscle activation corresponded with a reduction in LA compression (−25%). We confirm that running shoes do indeed influence the mechanical function of the foot. However, our findings suggest that these mechanical adjustments are likely to have occurred as a result of increased neuromuscular output, rather than impaired control as previously speculated. We propose a theoretical model for foot–shoe interaction to explain these novel findings.
Abstract.
Brennan SF, Cresswell AG, Farris DJ, Lichtwark GA (2016). The effect of cadence on the muscle-tendon mechanics of the gastrocnemius muscle during walking. Scandinavian Journal of Medicine & Science in Sports, 27(3), 289-298.
Farris DJ, Lichtwark GA, Brown NAT, Cresswell AG (2016). The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo.
Journal of Experimental Biology,
219(4), 528-534.
Abstract:
The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo
Humans utilise elastic tendons of lower limb muscles to store and return energy during walking, running and jumping. Anuran and insect species use skeletal structures and/or dynamics in conjunction with similarly compliant structures to amplify muscle power output during jumping. We sought to examine whether human jumpers use similar mechanisms to aid elastic energy usage in the plantar flexor muscles during maximal vertical jumping. Ten male athletes performed maximal vertical squat jumps. Three-dimensional motion capture and a musculoskeletal model were used to determine lower limb kinematics that were combined with ground reaction force data in an inverse dynamics analysis. B-mode ultrasound imaging of the lateral gastrocnemius (GAS) and soleus (SOL) muscles was used to measure muscle fascicle lengths and pennation angles during jumping. Our results highlighted that both GAS and SOL utilised stretch and recoil of their series elastic elements (SEEs) in a catapultlike fashion, which likely serves to maximise ankle joint power. The resistance of supporting of body weight allowed initial stretch of both GAS and SOL SEEs. A proximal-to-distal sequence of joint moments and decreasing effective mechanical advantage early in the extension phase of the jumping movement were observed. This facilitated a further stretch of the SEE of the biarticular GAS and delayed recoil of the SOL SEE. However, effective mechanical advantage did not increase late in the jump to aid recoil of elastic tissues.
Abstract.
Farris DJ, Lichtwark GA (2016). UltraTrack: Software for semi-automated tracking of muscle fascicles in sequences of B-mode ultrasound images.
Computer Methods and Programs in Biomedicine,
128, 111-118.
Abstract:
UltraTrack: Software for semi-automated tracking of muscle fascicles in sequences of B-mode ultrasound images
Background: Dynamic measurements of human muscle fascicle length from sequences of B-mode ultrasound images have become increasingly prevalent in biomedical research. Manual digitisation of these images is time consuming and algorithms for automating the process have been developed. Here we present a freely available software implementation of a previously validated algorithm for semi-automated tracking of muscle fascicle length in dynamic ultrasound image recordings, "UltraTrack". Methods: UltraTrack implements an affine extension to an optic flow algorithm to track movement of the muscle fascicle end-points throughout dynamically recorded sequences of images. The underlying algorithm has been previously described and its reliability tested, but here we present the software implementation with features for: tracking multiple fascicles in multiple muscles simultaneously; correcting temporal drift in measurements; manually adjusting tracking results; saving and re-loading of tracking results and loading a range of file formats. Results: Two example runs of the software are presented detailing the tracking of fascicles from several lower limb muscles during a squatting and walking activity. Conclusion: We have presented a software implementation of a validated fascicle-tracking algorithm and made the source code and standalone versions freely available for download.
Abstract.
2015
Farris DJ, Lichtwark GA, Brown NAT, Cresswell AG (2015). Deconstructing the power resistance relationship for squats: a joint-level analysis. Scandinavian Journal of Medicine & Science in Sports, 26(7), 774-781.
Robertson BD, Farris DJ, Sawicki GS (2015). Erratum: More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics (Bioinspiration and Biomimetics (2014) 9 (046018)). Bioinspiration and Biomimetics, 10(1).
Mahon CE, Farris DJ, Sawicki GS, Lewek MD (2015). Individual limb mechanical analysis of gait following stroke.
Journal of Biomechanics,
48(6), 984-989.
Abstract:
Individual limb mechanical analysis of gait following stroke
The step-to-step transition of walking requires significant mechanical and metabolic energy to redirect the center of mass. Inter-limb mechanical asymmetries during the step-to-step transition may increase overall energy demands and require compensation during single-support. The purpose of this study was to compare individual limb mechanical gait asymmetries during the step-to-step transitions, single-support and over a complete stride between two groups of individuals following stroke stratified by gait speed (≥0.8. m/s or
Abstract.
Farris DJ, Hampton A, Lewek MD, Sawicki GS (2015). Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints.
Journal of NeuroEngineering and Rehabilitation,
12(1).
Abstract:
Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints
Background: Previous reports of the mechanics and energetics of post-stroke hemiparetic walking have either not combined estimates of mechanical and metabolic energy or computed external mechanical work based on the limited combined limbs method. Here we present a comparison of the mechanics and energetics of hemiparetic and unimpaired walking at a matched speed. Methods: Mechanical work done on the body centre of mass (COM) was computed by the individual limbs method and work done at individual leg joints was computed with an inverse dynamics analysis. Both estimates were converted to average powers and related to simultaneous estimates of net metabolic power, determined via indirect calorimetry. Efficiency of positive work was calculated as the ratio of average positive mechanical power P+ to net metabolic power. Results: Total P+ was 20% greater for the hemiparetic group (H) than for the unimpaired control group (C) (0.49 vs. 0.41 W · kg-1). The greater P+ was partly attributed to the paretic limb of hemiparetic walkers not providing appropriately timed push-off P+ in the step-to-step transition. This led to compensatory non-paretic limb hip and kneeP+ which resulted in greater total mechanical work. Efficiency of positive work was not different between H and C. Conclusions: Increased work, not decreased efficiency, explains the greater metabolic cost of hemiparetic walking post-stroke. Our results highlighted the need to target improving paretic ankle push-off via therapy or assistive technology in order to reduce the metabolic cost of hemiparetic walking.
Abstract.
2014
Farris D (2014). Does unloading muscles save you energy? Paradoxical effects of spring-loaded ankle exoskeletons on plantar flexor muscle mechanics and energetics. Journal of Science and Medicine in Sport, 18, e129-e130.
Lichtwark G, Farris D, Kelly L, Brown N (2014). Lower limb biomechanics and muscle function. Journal of Science and Medicine in Sport, 18, e128-e128.
Robertson BD, Farris DJ, Sawicki GS (2014). More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics.
Bioinspiration and Biomimetics,
9(4).
Abstract:
More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics
Development P of robotic exoskeletons to assist/enhance human locomotor performance involves lengthy prototyping, testing, and analysis. This process is further convoluted by variability in limb/body morphology and preferred gait patterns between individuals. In an attempt to expedite this process, and establish a physiological basis for actuator prescription, we developed a simple, predictive model of human neuromechanical adaptation to a passive elastic exoskeleton applied at the ankle joint during a functional task. We modeled the human triceps surae-Achilles tendon muscle tendon unit (MTU) as a single Hill-type muscle, or contractile element (CE), and series tendon, or series elastic element (SEE). This modeled system was placed under gravitational load and underwent cyclic stimulation at a regular frequency (i.e. hopping) with and without exoskeleton (Exo) assistance. We explored the effect that both Exo stiffness (kExo) and muscle activation (Astim) had on combined MTU and Exo (MTU+ Exo), MTU, and CE/SEE mechanics and energetics. Model accuracy was verifi ed via qualitative and quantitative comparisons between modeled and prior experimental outcomes. We demonstrated that reduced Astim can be traded for increased kExo to maintain consistent MTU+ Exo mechanics (i.e. average positive power (P¯mech+) output) from an unassisted condition (i.e. kExo = 0 kN · m-1). For these regions of parameter space, our model predicted a reduction in MTU force, SEE energy cycling, and metabolic rate (P¯met), as well as constant CE P¯mech+ output compared to unassisted conditions. This agreed with previous experimental observations, demonstrating our model's predictive ability. Model predictions also provided insight into mechanisms of metabolic cost minimization, and/or enhanced mechanical performance, and we concluded that both of these outcomes cannot be achieved simultaneously, and that one must come at the detriment of the other in a spring-assisted compliant MTU.
Abstract.
Farris DJ, Hicks JL, Delp SL, Sawicki GS (2014). Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping.
Journal of Experimental Biology,
217(22), 4018-4028.
Abstract:
Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping
Experiments have shown that elastic ankle exoskeletons can be used to reduce ankle joint and plantar-flexor muscle loading when hopping in place and, in turn, reduce metabolic energy consumption. However, recent experimental work has shown that such exoskeletons cause less favourable soleus (SO) muscle-tendon mechanics than is observed during normal hopping, which might limit the capacity of the exoskeleton to reduce energy consumption. To directly link plantar-flexor mechanics and energy consumption when hopping in exoskeletons, we used a musculoskeletal model of the human leg and a model of muscle energetics in simulations of muscle-tendon dynamics during hopping with and without elastic ankle exoskeletons. Simulations were driven by experimental electromyograms, joint kinematics and exoskeleton torque taken from previously published data. The data were from seven males who hopped at 2.5 Hz with and without elastic ankle exoskeletons. The energetics model showed that the total rate of metabolic energy consumption by ankle muscles was not significantly reduced by an ankle exoskeleton. This was despite large reductions in plantar-flexor force production (40-50%). The lack of larger metabolic reductions with exoskeletons was attributed to increases in plantar-flexor muscle fibre velocities and a shift to less favourable muscle fibre lengths during active force production. This limited the capacity for plantarflexors to reduce activation and energy consumption when hopping with exoskeleton assistance.
Abstract.
2013
Farris DJ, Trewartha G, McGuigan MP, Lichtwark GA (2013). Differential strain patterns of the human Achilles tendon determined in vivo with freehand three-dimensional ultrasound imaging.
Journal of Experimental Biology,
216(4), 594-600.
Abstract:
Differential strain patterns of the human Achilles tendon determined in vivo with freehand three-dimensional ultrasound imaging
The human Achilles tendon (AT) has often been considered to act as a single elastic structure in series with the muscles of the triceps surae. As such it has been commonly modelled as a Hookean spring of uniform stiffness. However, the free AT and the proximal AT have distinctly different structures that lend themselves to different elastic properties. This study aimed to use threedimensional freehand ultrasound imaging to determine whether the proximal AT and the free AT exhibit different elastic behaviour during sub-maximal, fixed-end contractions of the triceps surae. Six male and five female participants (mean ± s.d. age=27±5'years) performed fixed position contractions of the plantar-flexors on an isokinetic dynamometer at 50% of their maximum voluntary contraction in this position. Freehand three-dimensional ultrasound imaging was used to reconstruct the free-tendon and proximal AT at rest and during contraction. The free-tendon exhibited significantly (P=0.03) greater longitudinal strain (5.2±1.7%) than the proximal AT (2.6±2.0%). The lesser longitudinal strain of the proximal AT was linked to the fact that it exhibited considerable transverse (orthogonal to the longitudinal direction) strains (5.0±4%). The transverse strain of the proximal AT is likely due to the triceps surae muscles bulging upon contraction, and thus the level of bulging may influence the elastic behaviour of the proximal AT. This might have implications for the understanding of triceps surae muscle-tendon interaction during locomotion, tendon injury mechanics and previous measurements of AT elastic properties. © 2013. Published by the Company of Biologists Ltd.
Abstract.
Farris DJ, Robertson BD, Sawicki GS (2013). Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping.
Journal of Applied Physiology,
115(5), 579-585.
Abstract:
Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping
Farris DJ, Robertson BD, Sawicki GS. Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping. J Appl Physiol 115: 579-585, 2013. First published June 20, 2013; doi:10.1152/japplphysiol.00253.2013.- Inspired by elastic energy storage and return in tendons of human leg muscle-tendon units (MTU), exoskeletons often place a spring in parallel with an MTU to assist the MTU. However, this might perturb the normally efficient MTU mechanics and actually increase active muscle mechanical work. This study tested the effects of elastic parallel assistance on MTU mechanics. Participants hopped with and without spring-loaded ankle exoskeletons that assisted plantar flexion. An inverse dynamics analysis, combined with in vivo ultrasound imaging of soleus fascicles and surface electromyography, was used to determine muscle-tendon mechanics and activations. Whole body net metabolic power was obtained from indirect calorimetry. When hopping with spring-loaded exoskeletons, soleus activation was reduced (30-70%) and so was the magnitude of soleus force (peak force reduced by 30%) and the average rate of soleus force generation (by 50%). Although forces were lower, average positive fascicle power remained unchanged, owing to increased fascicle excursion (+4-5 mm). Net metabolic power was reduced with exoskeleton assistance (19%). These findings highlighted that parallel assistance to a muscle with appreciable series elasticity may have some negative consequences, and that the metabolic cost associated with generating force may be more pronounced than the cost of doing work for these muscles. Copyright © 2013 the American Physiological Society.
Abstract.
Farris DJ, Sawicki GS (2013). Erratum: Linking the mechanics and energetics of hopping with elastic ankle exoskeletons (Journal of Applied Physiology (2012) 113 (1862-1872) DOI: 10.1152/japplphysiol.00802.2012). Journal of Applied Physiology, 115(2).
2012
Farris DJ, Sawicki GS (2012). Human medial gastrocnemius force-velocity behavior shifts with locomotion speed and gait.
Proceedings of the National Academy of Sciences of the United States of America,
109(3), 977-982.
Abstract:
Human medial gastrocnemius force-velocity behavior shifts with locomotion speed and gait
Humans walk and run over a wide range of speeds with remarkable efficiency. For steady locomotion, moving at different speeds requires the muscle-tendon units of the leg to modulate the amount of mechanical power the limb absorbs and outputs in each step. How individual muscles adapt their behavior to modulate limb power output has been examined using computer simulation and animal models, but has not been studied in vivo in humans. In this study, we used a combination of ultrasound imaging and motion analysis to examine how medial gastrocnemius (MG) muscle-tendon unit behavior is adjusted to meet the varying mechanical demands of different locomotor speeds during walking and running in humans. The results highlighted key differences in MG fascicle-shortening velocity with both locomotor speed and gait. Fascicle-shortening velocity at the time of peak muscle force production increased with walking speed, impairing the ability of the muscle to produce high peak forces. Switching to a running gait at 2.0 m·s -1 caused fascicle shortening at the time of peak force production to shift to much slower velocities. This velocity shift facilitated a large increase in peak muscle force and an increase in MG power output. MG fascicle velocity may be a key factor that limits the speeds humans choose to walk at, and may explain the transition from walking to running. This finding is consistent with previous modeling studies.
Abstract.
Farris DJ, Sawicki GS (2012). Linking the mechanics and energetics of hopping with elastic ankle exoskeletons.
Journal of Applied Physiology,
113(12), 1862-1872.
Abstract:
Linking the mechanics and energetics of hopping with elastic ankle exoskeletons
The springlike mechanics of the human leg during bouncing gaits has inspired the design of passive assistive devices that use springs to aid locomotion. The purpose of this study was to test whether a passive spring-loaded ankle exoskeleton could reduce the mechanical and energetic demands of bilateral hopping on the musculoskeletal system. Joint level kinematics and kinetics were collected with electromyographic and metabolic energy consumption data for seven participants hopping at four frequencies (2.2, 2.5, 2.8, and 3.2 Hz). Hopping was performed without an exoskeleton; with an springless exoskeleton; and with a spring-loaded exoskeleton. Spring-loaded ankle exoskeletons reduced plantar flexor muscle activity and the biological contribution to ankle joint moment (15-25%) and average positive power (20-40%). They also facilitated reductions in metabolic power (15-20%) across frequencies from 2.2 to 2.8 Hz compared with hopping with a springless exoskeleton. Reductions in metabolic power compared with hopping with no exoskeleton were restricted to hopping at 2.5 Hz only (12%). These results highlighted the importance of reducing the rate of muscular force production and work to achieve metabolic reductions. They also highlighted the importance of assisting muscles acting at the knee joint. Exoskeleton designs may need to be tuned to optimize exoskeleton mass, spring stiffness, and spring slack length to achieve greater metabolic reductions. © 2012 the American Physiological Society.
Abstract.
Farris DJ, Trewartha G, McGuigan MP (2012). The effects of a 30-min run on the mechanics of the human Achilles tendon.
European Journal of Applied Physiology,
112(2), 653-660.
Abstract:
The effects of a 30-min run on the mechanics of the human Achilles tendon
Tendinous structures often exhibit reduced stiffness following repeated loading via static muscular contractions. The purpose of this study was to determine if human Achilles tendon (AT) stiffness is affected by the repeated loading experienced during running and if this affects normal muscle-tendon interaction. Twelve male participants (mean ± SD: age 27 ± 5 years, height 1.79 ± 0.06 m, mass 78.6 ± 8.4 kg) completed a 30 min run at 12 kmph on a treadmill. AT properties were determined before and after the run during a series of one-legged hops. During hopping and running, AT length data were acquired from a combination of ultrasound imaging (50 Hz) and kinematic data (200 Hz). AT force was estimated from inverse dynamics during hopping and AT stiffness was computed from plots of AT force and length. AT stiffness was not significantly different post run (pre 163 ± 41 N mm -1, post 147 ± 52 N mm -1, P > 0.05) and peak AT strain during the stance phase of running (calculated relative to AT length during standing) was similar at different time points during the run (3.5 ± 1.8% at 1 min, 3.2 ± 1.8% at 15 min and 3.8 ± 2% at 30 min). It was concluded that the loading experienced during a single bout of running does not affect the stiffness of the AT and that the properties of the AT are stable during locomotion. This may have implications for muscle fascicle behaviour and Achilles tendon injury mechanisms. © 2011 Springer-Verlag.
Abstract.
Farris DJ, Buckeridge E, Trewartha G, McGuigan MP (2012). The effects of orthotic heel lifts on achilles tendon force and strain during running.
Journal of Applied Biomechanics,
28(5), 511-519.
Abstract:
The effects of orthotic heel lifts on achilles tendon force and strain during running
This study assessed the effects of orthotic heel lifts on Achilles tendon (AT) force and strain during running. Ten females ran barefoot over a force plate in three conditions: no heel lifts (NHL), with 12 mm heel lifts (12HL) and with 18 mm heel lifts (18HL). Kinematics for the right lower limb were collected (200 Hz). AT force was calculated from inverse dynamics. AT strain was determined from kinematics and ultrasound images of medial gastrocnemius (50 Hz). Peak AT strain was less for 18HL (5.5 ± 4.4%) than for NHL (7.4 ± 4.2%) (p =. 029, effect size [ES] = 0.44) but not for 12HL (5.8 ± 4.8%) versus NHL (ES = 0.35). Peak AT force was significantly (p =. 024, ES = 0.42) less for 18HL (2382 ± 717 N) than for NHL (2710 ± 830 N) but not for 12HL (2538 ± 823 N, ES = 0.21). The 18HL reduced ankle dorsiflexion but not flexion-extension ankle moments and increased the AT moment arm compared with NHL. Thus, 18HL reduced force and strain on the AT during running via a reduction in dorsiflexion, which lengthened the AT moment arm. Therefore, heel lifts could be used to reduce AT loading and strain during the rehabilitation of AT injuries. © 2012 Human Kinetics, Inc.
Abstract.
Farris DJ, Sawicki GS (2012). The mechanics and energetics of human walking and running: a joint level perspective.
Journal of the Royal Society Interface,
9(66), 110-118.
Abstract:
The mechanics and energetics of human walking and running: a joint level perspective
Humans walk and run at a range of speeds. While steady locomotion at a given speed requires no net mechanical work, moving faster does demand both more positive and negative mechanical work per stride. Is this increased demand met by increasing power output at all lower limb joints or just some of them? Does running rely on different joints for power output than walking? How does this contribute to the metabolic cost of locomotion? This study examined the effects of walking and running speed on lower limb joint mechanics and metabolic cost of transport in humans. Kinematic and kinetic data for 10 participants were collected for a range of walking (0.75, 1.25, 1.75, 2.0 ms-1) and running (2.0, 2.25, 2.75, 3.25 ms-1) speeds. Net metabolic power was measured by indirect calorimetry. Within each gait, there was no difference in the proportion of power contributed by each joint (hip, knee, ankle) to total power across speeds. Changing from walking to running resulted in a significant ( p = 0.02) shift in power production from the hip to the ankle which may explain the higher efficiency of running at speeds above 2.0 ms-1 and shed light on a potential mechanism behind the walk-run transition. © 2011 the Royal Society.
Abstract.
2011
Lichtwark G, Farris D, Newsham-West R (2011). Achilles tendon (3D): Do the mechanical properties of tendon change in response to exercise?. Journal of Science and Medicine in Sport, 14, e9-e10.
Farris DJ, Trewartha G, Polly McGuigan M (2011). Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?.
Journal of Biomechanics,
44(5), 822-826.
Abstract:
Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?
Chronic tendinopathy of the human Achilles tendon (AT) is common but its injury mechanism is not fully understood. It has been hypothesised that heat energy losses from the AT during running could explain the degeneration of AT material seen with injury. A mathematical model of AT temperature distribution was used to predict what temperatures the core of the AT could reach during running. This model required input values for mechanical properties of the AT (stiffness, hysteresis, cross-sectional area (CSA), strain during running) which were determined using a combination of ultrasound imaging, kinematic and kinetic data. AT length data were obtained during hopping and treadmill running (12kmph) using ultrasound images of the medial gastrocnemius (50Hz) and kinematic data (200Hz). AT force data were calculated from inverse dynamics during hopping and combined with AT length data to compute AT stiffness and hysteresis. AT strain was computed from AT length data during treadmill running. AT CSA was measured on transverse ultrasound scans of the AT. Mean±sd tendon properties were: stiffness=176±41Nmm -1, hysteresis=17±12%, strain during running=3.5±1.8% and CSA=42±8mm 2. These values were input into the model of AT core temperature and this was predicted to reach at least 41°C during running. Such temperatures were deemed to be conservative estimates but still sufficient for tendon hyperthermia to be a potential cause of tendon injury. © 2011 Elsevier Ltd.
Abstract.
2008
Farris D, Buckeridge E, Trewartha G, McGuigan P (2008). The effects of orthotic heel lifts on Achilles tendon force and strain during running. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 150(3), S82-S82.
