Influence of prior knowledge on automatic and voluntary postural adjustments in healthy and hemiplegic subjects

The purpose of this study was to analyze the effects of prior instruction on automatic postural responses and voluntary postural sways measured about the ankle joint. Ten subjects with right hemiplegia resulting from a cerebrovascular accident (mean age = 56 +/- 14 years) and 5 healthy subjects (mean age = 47 +/- 6 years) stood symmetrically on a movable force platform. During platform-induced sway, the support surface was translated horizontally to induce antero-posterior body sway about the ankle joints. Surface electromyographs were obtained from the tibialis anterior, quadriceps femoris, gastrocnemius, and hamstring muscles bilaterally. Prior knowledge appeared to have no significant influence on healthy subjects' ability to execute postural adjustments more rapidly during AP displacements. Hemiplegic subjects exhibited longer and more variable latencies in the paretic limb than in the nonparetic limb during voluntary AP weight shifts. When hemiplegic subjects had prior knowledge of the platform's movement (time and direction), latencies were significantly shorter in the paretic limb and could be as brief as those seen in the nonparetic limb. The results showed that prior knowledge may be an important treatment consideration for patients with muscle timing disorders.