Fig. 4. Hypothesis and underlying mechanism.

(A) Hypothesized best timing of a Sinusoidal Force condition. The plot shows the mean net aiding force (solid line) and the bipedal sum of the parallel GRF (dashed line) from the participants in the Sinusoidal Force condition where the net aiding force matches closest to the bipedal propulsion timing, plotted versus step time. (B) Best timing result. Condition with the highest reduction in metabolic rate. (C) Explanation based on COM velocity effect. Plot showing the mean COM velocity of all participants of the conditions with the hypothesized best timing and the highest metabolic rate reduction. (D) Simple pendulum model. Stick figures show the higher COM acceleration at the beginning of the step and lower COM velocity at the end of the step in the condition with early peak timing compared to the condition with late peak timing. The pendulum model predicts that the higher initial acceleration and lower final velocity will result in lower positive and negative leg work rates required to redirect the COM. (n = 10).