Reconciling how positive foot power drives the arch recoil mechanism

The foot interfaces with the ground during locomotion, absorbing and returning energy through deformation and recoil of the longitudinal arch. Energy from 'arch recoil' is often assumed to propel the centre of mass (COM) forward through lifting of the talus. However, recent work has shown that arch recoil lowers and posteriorly tilts the talus, lowering and posteriorly translating the COM. These two motions caused by arch recoil-posterior tilt slowing the rotation of the talus and linear actuation shortening the arch, lowering the talus-seem counterintuitive when the foot is producing positive power. How do they contribute to foot power, and how can positive power coincide with the slowing of a body? To address this, we combined biplanar videoradiography with a first-principles model of the foot. We restricted the rotation and linear actuation of the talus caused by arch recoil, then analysed the outcomes using a decomposition of the unified deformable power model. We found that restricting linear actuation did not affect foot power, while limiting rotation substantially reduced it. This shows that positive foot power during propulsion is linked with posteriorly tilting the talus rather than direct COM propulsion. These findings demonstrate how positive foot power slows the talus, potentially orienting the talus for optimal ankle positioning during propulsion.