Patterning of muscle activity in static knee extension

Activity patterning of the three agonist muscles (rectus femoris, vastus medialis, vastus lateralis) and one antagonist muscle (semimembranosus) was investigated during static knee extension. Male physical education students performed maximal and submaximal exertions in two test postures with different hip and knee positions corresponding to the postures used in our previous leg extension study. The character of the force curves was found in both postures to be convex with maximum peak force at 120 degrees. The supine posture changed the length of the two-joint muscles so as to produce an ordinary type of force curve. In the recumbent posture the efficient angles of the hip and knee joints did not match, thereby causing more plateau-like maximum peak force. All the agonists worked as a group and were highly activated throughout the entire range of the extension movement. The influence of postural variation also on the activity patterning could be seen in the latter half of the knee extension movement. The recumbent posture decreased whereas the supine posture maintained or tended even to increase the agonist activity. The difference is possibly due to the changing length of the two-joint muscles (rectus femoris and semimembranosus). Though the IEMG-force ratio of the agonist muscles was always nonlinear, the increased curvilinear relationship of the rectus femoris in the recumbent posture fits in with the view that in such conditions the central control system attempts to extend the hip joint. The results indicated that the two-joint muscles work like a single-joint muscle but, at the same time, control muscle coordination in single-joint knee extension movement.