Task-Dependent Spinal Inhibition in Spastic Hemiplegia

Abstract

In the spastic hemiplegia, which so frequently develops following stroke (Aho et al. 1980), voluntary control of the upper limb is impaired, leading to severe handicap in the performance of activities of daily living. The upper limb is held in a typical posture of retraction of the shoulder, medial rotation of the humerus, pronation of the forearm and flexion of the elbow, wrist and finger joints (Twitchell 1951; Bobath 1990; Carr and Shepherd 1990). In this posture increased tone is observed in several superficial muscles, particularly pectoralis major, biceps and the forearm flexors (Bobath 1990; Brunnstrom 1970; De Souza et al. 1980; Johnstone 1987). Attempts to make voluntary movements of the upper limb out of this posture are hampered by an inability to inhibit the increased tone of these muscles (e.g., Chan 1986; Musa 1986). This deficit of voluntary movement control illustrates a disorder of Sherrington’s principle of spinal reciprocal inhibition, in which activation of agonist muscles is normally accompanied by centrally generated inhibition, of antagonists (Sherrington 1906). In fact, when movement is initiated voluntarily, reciprocal inhibition of antagonist muscles occurs in advance of the activation of the agonist muscles (Symoyama and Tanaka 1974).