Muscle architecture and function in humans.

The present study focused on architectural factors which are considered to influence the linkage of muscle fiber and joint actions. By means of real-time ultrasonography we can observe clearly and noninvasively in vivo the movement of fascicle and aponeurosis in human muscle and measure directly the changes in pennation angle and length of fascicle during muscle contraction. During dorsi and plantar flexion without load the movement of tendinous tissue in human tibialis anterior muscle (TA) appeared to synchronize with the displacement of the ankle joint, indicating that the muscle tendon complexes are stiff relative to the applied force, which is fairly small in the case of foot shaking motion. On the other hand, when the ankle joint was fixed and the TA contracted 'statically' the ultrasonic echo from deep aponeurosis in the TA was observed to move proximally, indicating the elastic component (i.e. mainly tendinous tissue) was stretched significantly by the contraction force of muscle. In the case of the kneejoint, a length of fascicle in vastus lateralis decreased by 18% with the extension of the knee passively from a 100 degrees flexed position. When the knee extensors contracted 'statically' the fascicle length decreased at every joint angles and its magnitude was greater (30%) when the knee was closer to full extension than (5%) at the flexed positions. The present results clearly show that the architecture of actively contracting muscle fibers differ considerably than that which occurs when movement is passively induced. The use of cadaver data in the study of architecture and modeling of muscle functions would result in inaccurate, and in some cases even erroneous results.