Forward and Backward Axial Movements: Two Modes of Central Control

Abstract

Axial upper trunk movements are accompanied by hip and knee movements in the opposite direction, resulting in a close adjustement of the projection onto the ground of the center of gravity (CG). Two patterns of muscle activation were observed during axial movements. The first is characterized by the sequential activation of first the leg muscles and secondly the prime mover. This pattern can be observed with forward axial upper trunk movements. The second pattern of muscle activation consists of the nearly synchronous onset of both the leg muscles and the prime mover EMG. This pattern can be observed with backward upper trunk movements. The first hypothesis concerning the two different patterns is that they may result from the asymmetry of the knee joint, which is able to flex in association with backward movements, but not to hyperextend in association with forward movements. This hypothesis was tested by casting the knee joints in two subjects. It was found that in a subject performing backward upper trunk movements no change in the pattern of EMG activation appeared after casting. It is suggested that the synchronous mode observed with backward movements is less efficient in terms of motor performance whereas the sequential mode observed with forward movements is more efficient. The shift from the synchronous to the sequential mode might result from training which occurs with forward and not with backward movements: forward movements are performed every day from childhood onwards; whereas backward movements are little used in everyday life.