A postural perturbation is a sudden change in conditions that displaces the body posture away from equilibrium. These perturbations could consist of physiological, informational and mechanical perturbations. Mechanical postural perturbations can be applied to any body part such as push to the trunk, head or limbs. The most common experimental approach is to perturb the support surface, which displaces the base of support under the body’s center of mass. These support-surface perturbations are similar to a slip, trip, surface irregularity, or acceleration or deceleration of a moving surface such as standing in a bus in which an individual is balancing. In the literature, several studies have examined postural responses to support surface perturbation. Different types of perturbation (translation and rotation) should result in different response organizations. Most of the studies emphasized on normal subjects, elderly or patients with vestibular dysfunction. Few studies had investigated the problems of postural reactions in patients with hemiparesis by using single method of measurement. This study investigated the postural reactions to support surface perturbation in patients with hemiparesis by using electromyography and kinematic measurements. The results showed that there was an asymmetric pattern existed in hemiplegic limbs. The latency of muscle activity of distal agonist in affected leg was longer than in sound leg, and the sound leg’s onset latency was also longer than that in the normal subject’s leg. The activation sequence of agonist in the affected leg exhibited a wide range of variation and co-contraction of the distal and proximal agonist. When wearing the ankle-foot-orthosis, the muscle activation of the lower limbs was tend to be more symmetry, there was no significant different distal agonist latency between the affected and sound legs.
