Fig. 1

The two main roles of push-off (a) and the expected effect of passive versus active knee flexion initiation (b). The main part of knee flexion, from 5-40deg, occurs passively after LLTD⁵. This study compares PKFI and AKFI to influence energy flow into the RB and TL by indirectly manipulating SAPF timing. This study aims to contribute to the debate on the primary function of ankle push-off in the human swing leg catapult mechanism¹⁶. (a) During a step-to-step transition, push-off plays two main roles: it redirects the center of mass velocity (\(\Delta {\textbf{v}}_\textrm{CoM}\)) from one inverted pendulum arc to the next, and it increases the velocity of the TL. The drawing was inspired by Fig. 1 in Ref.¹⁷ and Fig. 4 in Ref.⁴. (b) The AKFI experiment was designed with earlier knee flexion onset than the PKFI experiment to alter the SAPF timing. This results in an earlier SAPF in the AKFI experiment, prolonging the positive ankle power (\(\textrm{P}_\textrm{ank, pos}\)) period until LLTD. Before LLTD, the TL bears the whole body weight, while the loading leg (LL) is in swing. The filled part under the positive ankle power curve represents the predicted ankle energy mainly accelerating the RB. Due to the predicted SAPF delay in the PKFI experiment, lower ankle energy could accelerate the RB in the PKFI than in the AKFI experiment. TL: Trailing Leg, LL: Leading Leg, RB: Remaining Body, CoM: Center of Mass, vmin: time of minimum vertical velocity of the CoM (start of Step-to-Step transition period), vmax: second vertical velocity peak of the CoM after vmin (end of Step-to-Step transition period), SAPF: Start of Ankle Plantar Flexion, LLTD: Leading Leg Touch-Down, TO: Toe-Off, AKFI: Active Knee Flexion Initiation, PKFI: Passive Knee Flexion Initiation.