Task-specific modulation of anticipatory postural adjustments in individuals with hemiparesis

Objectives: To study adaptation of anticipatory postural adjustments (APAs) in paretic and non-paretic muscles of individuals with hemiparesis to changes in the direction of the self-initiated perturbation and additional manual support. Methods: Electrical activity of leg and trunk muscles on both sides of the body and ground reaction forces were recorded in 10 patients with hemiparesis and a group of matched control subjects. Subjects released a standard load, held in the hand of the extended relatively unimpaired arm. The load was released either in front of the body or to the side, with or without the impaired arm touching an external stable surface. Results: APAs were reduced in individuals with hemiparesis, especially on the paretic side. In paretic muscles, the modulation of APAs with the direction of the perturbation was decreased or showed atypical patterns. Also the effects of touch were decreased in patients. Center of pressure displacements shortly after load release were similar in control subjects and patients. Conclusions: The results suggest that the ability of individuals with hemiparesis to prepare for a self-initiated predictable perturbation is reduced and that they may use alternative strategies of postural stabilization.