The biarticularity of the gastrocnemii muscles provides relevant mechanisms for managing drop-like gait perturbations in humans.

This study investigated the ankle-to-knee and knee-to-ankle joint energy transfer via the biarticular gastrocnemii muscles during unpredictable and adapted drop-like gait perturbations to understand how biarticular mechanisms of the gastrocnemii contribute to the mechanical work performed by the Achilles tendon (AT) force at the ankle joint. This was done by measuring AT elongation and quantifying AT force as an indicator of triceps surae muscle forces, as well as the body kinematics and electromyographic activity of the soleus, gastrocnemius medialis and gastrocnemius lateralis muscles, in 17 participants. Biarticular mechanisms contributed significantly to both the negative and positive mechanical work performed by the AT force at the ankle joint during both types of drop-like perturbations, constituting 17% to 26% of this mechanical work. In particular, during the initial stance phase of unpredictable, drop-like perturbations, a significant proportion of energy (26% of the negative mechanical work done at the ankle joint) was transferred from the ankle to the knee joint via the biarticular gastrocnemii muscles. More importantly, the rate of this energy transfer was elevated during the unpredictable perturbations, when beneficial stability control mechanisms based on prediction are unavailable, compared with adapted ones. Finally, our findings imply that elastic tissues contribute significantly to managing drop-like perturbations, including energy storage and recoil in the AT and potential for elastic energy exchange in the vasti tendons during the energy transfer phases. These findings could inform the design of prevention treatments and bioengineering approaches, especially for improving stability control in uneven terrain.