Coordination of the thigh muscles in static leg extension.

Activation patterns of the four thigh muscles (rectus femoris, vastus medialis, vastus lateralis and semimembranosus) were investigated in static leg extension through the entire range of extension movement in three testing postures: sitting, recumbent and supine. Special interest was focused on the role of the two-joint muscles. Five male students performed both maximal and submaximal efforts. The highest force values were observed when the knee was almost extended (150 degrees), while the agonists were the most active in flexed positions. Maximum peak activity of all the agonist muscles occurred throughout a rather wide movement range of the knee joint (80 degrees-150 degrees) in the sitting posture, but the more extended postures tended by lengthening the two-joint muscle of rectus femoris to reduce the range of peak activity of the agonists. During the latter half of the knee extension the activity of the agonists was greatly reduced (p less than .01) and a fully extended knee joint always resulted in silence on the part of the rectus femoris. In that position the vasti muscles together with the semimembranosus formed a force couple for stabilizing the knee joint. The semimembranosus, the two-joint muscle on the antagonistic side, increased activation toward the end of knee extension. This exceptional activation of the antagonist muscle during static agonist effort is likely to be due to the different role of the semimembranosus which tried as an agonist to extend the hip joint in the sitting posture. This phenomenon is supported by the low maximum peak activity of the rectus femoris.(ABSTRACT TRUNCATED AT 250 WORDS)