The ankle plantar flexors work within these geometric constraints to achieve favourable operating points on the force-length 6 and force-velocity profiles 7, 8, 9 and generate a burst of positive power during push-off. This gearing mechanism afforded by the foot influences not only muscle-tendon leverage, but also muscle-tendon force-generating capacity through modulation of fibre shortening velocity 4, 5. At each step during walking and running, forces underneath the foot propagate from heel to toe 1, 2, 3, creating a continuously changing ratio between these lever arms, or “gear ratio” 4. For example, during push-off from the ground, the foot functions as a lever with lever arms for input forces (ankle plantar flexors) and output forces (those originating from the ground). Recent evidence suggests that the foot and ankle embody fundamental structure-function relationships at play during human locomotion. This increased metabolic cost is likely due to the added force demand on the plantar flexors, as walking on a more rigid foot/shoe surface compromises the plantar flexors’ mechanical advantage. Despite this shift in force-velocity behaviour, the whole-body metabolic cost during walking increased with added foot stiffness (p < 0.001). Added foot stiffness also altered soleus muscle behaviour, leading to greater peak force (p < 0.001) and reduced fascicle shortening speed (p < 0.001). Added stiffness decreased energy dissipation at the foot (p < 0.001) and increased the gear ratio (i.e., ratio of ground reaction force and plantar flexor muscle lever arms) (p < 0.001). We investigated this foot-ankle interplay during walking by adding stiffness to the foot through shoes and insoles and characterized the resulting changes in in vivo soleus muscle-tendon mechanics using ultrasonography. At the same time, deformation of the foot may dissipate some of the mechanical energy generated by the plantar flexors during push-off. The structure of the foot defines the input and output lever arms that influences the force-generating capacity of the ankle plantar flexors during push-off.
Previous studies of human locomotion indicate that foot and ankle structures can interact in complex ways.