Fixed patterns of rapid postural responses among leg muscles during stance

Summary The aim of this study has been to present firmer evidence that during stance functionally related postural muscles in the legs are activated according to fixed patterns. The importance of fixed patterns of activation for stabilization, balance, and movement control has received considerable theoretical and experimental attention. With regard to postural adjustment in humans, however, evidence for fixed activation patterns has been circumstantial only. Previous studies could not rule out the possibility that fixed patterns were caused by the mechanical coupling of rotatory movements among the joints of the body.This study has shown that in subjects employing FSR adjustments during stance activation patterns among leg muscles at FSR latency (functional stretch response, 100–120 msec) are preprogrammed prior to a response and are, on the average, fixed, independent of the associated motions among the ankle, knee, and hip joints. The identical fixed patterns were produced by sway rotation about the ankle joints and by direct rotation of the ankles. A pattern was characterized as fixed when, during a 1 hr session, the ratios of estimated force between pairs of functionally related leg muscles remained constant. In addition, the sequence of activation among muscles was fixed and followed a course beginning at the ankle muscle and proceeding proximally.The discussion of these results considered the functional implications of fixed contractile patterns during stance posture control.