To understand malfunction of postural control system for human bipedal standing, we studied falls from a rocking-platform and a thin beam by motion analysis and electromyography. Five male subjects (age, 27+/-6y) stood on a rocking-platform (radius, 90mm ; height 100mm) and on a beam (width, 40mm ; height 100mm) with eyes open and closed. The falling process was divided into three phases. In the first phase the subjects successfully stood on the tested platforms (the standing phase). The second phase was a critical phase, in which the subjects made a maximal effort to prevent fall (the critical phase). In the last phase, protective stepping was induced (the falling phase). In the critical phase, the subjects took characteristic postures for forward and backward falls. Forward falls occurred from a posture with the hip ventriflexed and the ankle extended. Backward falls occurred from the inverse. Correspondingly the ankle extensor, triceps surae and flexor, tibialis anterior was strongly activated in the critical phases for forward and backward falls, respectively. Directional difference of muscle activity pattern was also observed at the onset of the falling phase. Forward protective stepping began with burst activity of the quadriceps of the swing limb, while backward one began with bursts of the same muscle of the support limb. Biceps femoris displayed the inverse pattern. In the falling phase, following such initial bursts, short bursts of tibialis anterior occurred in the swing limb for both direction of falling. The flexor activity was alternated by rather long bursts of triceps surae, during which the swing limb came to contact with the floor. It may be suggested that the brain can adaptively change the motor output of the central pattern generator depending on falling.

DESCENTE SPORTS SCIENCE Vol.21/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE