Effect of optokinetic stimulation on human balance recovery in unexpected forward fall

We examined whether optokinetic stimulation (OKS) affects balance recovery in an unexpected forward fall. Ten healthy male subjects participated in the study. Each was held in an initial, leaning-forward position supported by a cable connected to a strong magnet which enable unpredicted release. Instruction was given to move against forward fall by taking steps. To evaluate the relation between OKS velocities and balance recovery, ten stages of OKS velocities ranging from -100 (upward) degree/sec to +100 (downward) degree/sec were presented randomly. Balance recovery against forward fall was characterized by the reaction time, heel off, maximum vertical push, and heel contact. The latencies of these events decreased as the downward velocity of OKS increased; the latencies increased with the increment in the upward velocity of OKS. Changes in the latencies of the parameters took place in the early phase, in which the parameters of balance recovery depended little on the proprioceptive afferents from the lower limbs. This suggests that vection induced by the OKS affected the condition of the motor program which controls the initiation of the motor response. In other words, information from the visual system may modulate the condition of the motor program before the onset of movement.