Feedforward postural adjustments associated with a goal-directed reach produced by infants in the sitting and standing positions.

Abstract

Two mechanisms of postural control, feedback and feedforward, are implicated for the maintenance of independent sitting and standing postures. More specifically, feedback mechanisms encompass all postural reactions produced to maintain equilibrium in response to an unexpected perturbation. Perturbations induced by a voluntary movement are controlled by feedforward mechanisms. The purpose of this study is to characterize feedforward postural adjustments produced by infants in the sitting and standing positions. Twenty-two infants between the ages of 5 to 18 months were tested using a goal-directed reach paradigm. Infants were separated in two behavioral groups, sitters and creepers. Surface electromyography, center of pressure displacement and video data were collected. Sitters and creepers showed no consistent patterns of postural muscle activation or differences in postural muscle onset latencies. Compared to sitters, creepers showed a greater proportion of trials with more than one postural muscle activated (p < .05) and reached more slowly (p < .05) in the sitting position. A descriptive analysis of the data revealed a large number of trials with tonic postural muscle activity in both the sitters and creepers group. In the creepers group, however, more phasic postural muscle activity was noted. It is possible that infants initially use tonic postural muscle activity to "stiffen" the body and maintain their balance. As they mature and gain experience in the sitting and standing positions, infants develop more phasic postural muscle activity. The reduction in reaching speed in the creepers group may be an early movement strategy used to minimize the self-induced perturbation caused by goal-directed reaching.