Neuromechanical adaptations of foot function to changes in surface stiffness during hopping.

Humans choose work-minimizing movement strategies when interacting with compliant surfaces. Our ankles are credited with stiffening our lower limbs and maintaining the excursion of our body's center of mass on a range of surface stiffnesses. We may also be able to stiffen our feet through an active contribution from our plantar intrinsic muscles (PIMs) on such surfaces. However, traditional modeling of the ankle joint has masked this contribution. We compared foot and ankle mechanics and muscle activation on low, medium, and high stiffness surfaces during bilateral hopping using a traditional and anatomical ankle model. The traditional ankle model overestimated work and underestimated stiffness compared with the anatomical model. Hopping on a low stiffness surface resulted in less longitudinal arch compression with respect to the high stiffness surface. However, because midfoot torque was also reduced, midfoot stiffness remained unchanged. We observed lower activation of the PIMs, soleus, and tibialis anterior on the low and medium stiffness conditions, which paralleled the pattern we saw in the work performed by the foot and ankle. Rather than performing unnecessary work, participants altered their landing posture to harness the energy stored by the sprung surface in the low and medium conditions. These findings highlight our preference to minimize mechanical work when transitioning to compliant surfaces and highlight the importance of considering the foot as an active, multiarticular, part of the human leg.NEW & NOTEWORTHY When seeking to understand how humans adapt their movement to changes in substrate, the role of the human foot has been neglected. Using multi-segment foot modeling, we highlight the importance of adaptable foot mechanics in adjusting to surfaces of different compliance. We also show, via electromyography, that the adaptations are under active muscular control.