Supplementary Materials

The tables S1 to S4 in the supplementary material show the results of the sensitivity analysis on the net work of the Sol and gastrocnemii at the ankle joint as well as the net work of the gastrocnemii at the knee joint, which represents the contribution of the biarticular mechanisms to the increase of net ankle work. Different force potentials and ratios of the GM EMG-activity affect the absolute values in all three parameters. Nevertheless, in all combinations, the contribution of the Sol to the net mechanical work at the ankle joint was higher than the gastrocnemii. Both the energy production of the monoarticular Sol and the contribution of the biarticular mechanisms (i.e., in increased net ankle work) increased with walking speed. Thus, the sensitivity analysis clearly demonstrates that the main findings and conclusions generally hold true even under the assumption of varying force-length-velocity potentials and activation of the individual muscles. <br> The video clip illustrates a graphical animation of the right leg during the maximal walking speed of an individual. The vastus lateralis and the gastrocnemii muscles are colored in red, and the soleus in the blue spectrum. Lighter red and blue colors show lower muscle activity, and darker red and blue colors show higher muscle activity. From 47% to 72% of the stance phase, the knee joint extends, and the ankle joint is plantarflexing. This simultaneous knee extension and ankle plantar flexion initiate the energy transfer from the knee extensor to the ankle plantar flexor muscles.