This thesis investigated the neuromechanical response of the human foot to changes in surface and was motivated by a series of works that highlighted the potential for the intrinsic foot muscles to tune movement in response to perturbations in foot-surface interaction. A review of the literature published prior to this thesis (Chapter Two) revealed that, owing to anatomically imprecise modelling of the ankle joint, no evidence existed as to the role of the foot in movement adaptations to changes in surface, despite its known contribution to storing, returning, generating, and dissipating mechanical work in a range of other tasks. A number of experiments were devised to highlight the impact of previous modelling approaches and to provide an understanding of normal foot function through changes in surface properties.

Chapter Four tested the hypothesis that increased drive to the intrinsic foot muscles with decreasing surface stiffness would stiffen the foot in line with the changes seen elsewhere in the lower limb (to preserve centre of mass motion). While midfoot compression was reduced with decreasing surface stiffness, so were active contributions from the intrinsic foot muscles and mechanical work contributions from the foot and ankle. This was not expected. Participants hopping at an imposed frequency tuned their foot mechanics to harness stored energy while preserving motion, with activations instead linked to mechanical work demands. Humans’ preference to minimise work and harness stored energy when interacting with an elastic surface was highlighted in Chapter Five where participants balanced the costs associated with generating muscular work and producing force, and in Chapter Seven where participants used stored energy to increase speed without incurring additional work. Chapter Six aimed to determine the role of the intrinsic muscles in generating mechanical work to replace that dissipated by a damped surface. Participants activated their intrinsic muscles more to perform more work on a damped surface, emphasising the important contribution of the foot in generating mechanical work.

This thesis emphasises that appreciating foot function should be a fundamental consideration in understanding how humans control movement and highlights an important energy-saving mechanism that may be incorporated into the design of footwear and assistive devices to restore and/or enhance normal function.