Design of the musculoskeletal leg CARL based on the physiology of mono-articular and biarticular muscles in the human leg

In a lower extremity musculoskeletal leg, the actuation kinematics define the interaction of the actuators with each other and the environment. Design of such a kinematic chain is challenging due to the existence of the redundant biarticular actuators which simultaneously act on two joints, generating a parallel mechanism. Actuator kinematics is mainly dependent on the moment arm profile of the actuation system. It is a common practice to select a constant moment arm value for robotic actuation system; nevertheless, biological muscles feature a distinctive nonlinear moment arm profile that has been ignored in the design of the musculoskeletal robots. In this paper, we propose a design paradigm for compliant robotic leg based on the direct replication of the human leg anatomy. The resulting mechanical system should (a) demonstrate a similar moment arm profile as in leg musculature, (b) exhibit expected physiological behavior of the muscles, (c) provide insight into the interaction of the actuators and possible improvement in the efficiency of the movements. We provide a comprehensive analysis of the moment arm profile of the leg musculature. The actuator kinematics of the designed leg is validated by comparing the contraction velocities of the muscles and actuators. The biological characteristics of the actuators are analyzed using the jump experiment data conducted on the previous version of the leg. The major physiological characteristics of the biarticular muscles, ligamentous action, and distal power transfer, is successfully demonstrated by the robotic leg. Our analysis demonstrates that the proposed structural design of the actuation system can improve the mechanical efficiency of this particular jump experiment up to compared to the leg without actuator redundancy. Compared to the previous version of the leg, by only modifying the moment arm profiles, we can achieve an efficiency improvement of approximately .