Function of Antagonistic Pair of the Bi-articular Muscles in the Femoral Region During Heel Contact

To examine how heel contact provides trunk stabilization, we propose a simple model of muscle fiber arrangement for the femoral region of the human leg. The model comprises three parts: (i) an antagonistic pair of mono-articular muscles at the hip joint, (ii) an antagonistic pair of mono-articular muscles at the knee joint, and (iii) a two-joint link model incorporating an antagonistic pair of biarticular muscles that is simultaneously involved in hip and knee joint movement. On the basis of this simplified model, leg movement was analyzed using electromyography (EMG) and robotic kinematic. EMG analysis shows that during heel contact, electrical discharges occur simultaneously in the pair of antagonistic bi-articular muscles in the femoral-region; the rectus femoris and hamstrings. The robotic kinematic analysis shows that this simultaneous electrical discharge is necessary for controlling not only output forces at the heel as the point of contact, but also the stiffness ellipse. These results clearly indicate that the heel contact point is a major contributor to postural stability at time of contact.