Rotational foot placement specifies the lever arm of the ground reaction force during the push-off phase of walking initiation

The lever arm of the ground reaction force (GRF) about the talocrural joint axis is a functionally important indicator of the nature of foot loading. Walking initiation experiments (ten subjects; age, 23–29 years) were completed to demonstrate that rotational foot placement is a possible strategy to specify the lever arm. Externally-rotated foot placement resulted in larger lever arms during push-off. A computer simulation of push-off revealed that a decreased lever arm reduces the plantarflexion moment necessary to maintain a constant forward velocity, while increasing the required plantarflexion velocity. Shortening of the foot thus diminishes the muscular force demand but also requires high muscle fiber shortening velocities that may limit the force generating capacity of plantarflexors. Decreased plantarflexion moment and slow walking previously noted in partial-foot amputees may result from shortened lever arms in this manner.