Vastus and Gastrocnemius improve hopping efficiency and joints synchronicity at different frequencies: a robotic study

The lower limb morphology of biological locomotors is abundant in muscle-tendon units. Yet, not much is known about how these actuation units contribute to the output performance and energy economy of movements. In this work, we investigate the functionality of four of the important lower limb muscles - Vastus, Popliteus, Soleus, and Gastrocnemius - in a hopping task at different frequencies (1.5-3.5 Hz). These muscles are implemented as pneumatic artificial muscles (PAMs) on the EPA-Hopper-II robot, which is a human-sized 3-segmented leg co-actuated by electrical motors and PAMs. A bioinspired reflex-based Force Modulated Control (FMC) is also implemented on the robot to achieve hopping at different frequencies. The results show that the Vastus contributes the most to energy-efficient hopping at low to mid frequencies. The biarticular Gastrocnemius also helps increase efficiency at low frequencies. Further, it is found that the Gastrocnemius synchronizes the knee-ankle motion and mitigates lateral knee motion. The outcomes of this work add further evidence to hypotheses regarding human lower-limb actuation and proper recruitment of muscles for building more efficient robots.