We investigated how the central nervous system distributes joint torque among mono- and bi-articular muscles. Various combinations of knee and hip joint torques (Tk and Th, respectively) were realized by requesting seven subjects to pull the cable attached to the ankle joint in various directions. The muscle activation level of major lower limb muscles was evaluated by the surface electromyogram. The activation level of each muscle was able to be uniquely described as a linear combination of Tk and Th, indicating that each muscle has “preferred direction” (PD) on joint torque coordinate (Tk, Th) along which its activation level increases most steeply. The PD of bi-articular muscle (e.g., Rectus Femoris: -37.8 degree, Biceps Femoris Long Head: 154.3 degree) agreed well with the torque direction which it can generate (eigen-vector direction). On the other hand, the PD of mono-articular muscle (e.g., Gluteus Maximus: 38.9 degree, Vastus Medialis:16.4 degree) did not agree with the direction of its eigen-vector which is parallel to either Tk or Th axis. This result was unexpected because it indicates that mono-articular muscle is maximally activated when it accompanies the torque around the other joint. We demonstrate that a simple principle can explain these experimental results: the activation level of biarticular muscle is proportional to the orthogonal projection of torque vector (Tk, Th) to its eigen-vector, and the remaining part of torque vector is compensated by the activation of mono-articular muscles.

DESCENTE SPORTS SCIENCE Vol.23/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE