Biarticular gastrocnemii muscles increase their joint energy transfer potential at high running speeds.

The involvement of energy transfer mechanisms between the ankle and the knee joint by the biarticular gastrocnemii muscles at high running speeds is currently unknown. During running at seven speeds (3.0-8.5 m s), the ankle and knee joint kinematics as well as the electromyographic activity of the gastrocnemius medialis and lateralis were captured. By means of the ankle-knee joint coupling angles, we determined the energy transfer potential between the two joints as the fraction of contact time where the joint angles are in-phase. At speeds above 6.0 m s, the ankle-to-knee joint energy transfer potential during the first part of stance and the knee-to-ankle energy transfer potential during the second part of stance were increased by 37% and 12%, respectively. This was accompanied by a 2.8-fold and 2.0-fold increase of the gastrocnemii muscle activation. The findings demonstrate a speed-dependent modification of the ankle-knee joint coordination towards an in-phase pattern in combination with an increase in muscle activation, which enhances the possibility of energy transfer between the two joints by the biarticular gastrocnemii muscles. An increased energy transfer from the knee to the ankle joint is probably necessary to increase the power output at the ankle joint, required for the highest running speeds.