Spastic cocontraction in hemiparesis: effects of botulinum toxin.

INTRODUCTION: In this study of spastic hemiparesis we evaluated cocontraction during sustained agonist/antagonist efforts, before and after botulinum toxin (BoNT) injection in 1 agonist. METHODS: Nineteen hemiparetic subjects performed maximal isometric elbow flexion/extension efforts with the elbow at 100° (extensors stretched). Using flexor and extensor surface electromyography we calculated agonist recruitment/cocontraction indices from 500-ms peak voluntary agonist recruitment, before and 1 month after onabotulinumtoxinA injection (160 U) into biceps brachii. RESULTS: Before injection, agonist recruitment and cocontraction indices were higher in extensors than flexors [0.74 ± 0.15 vs. 0.59 ± 0.10 (P < 0.01) and 0.43 ± 0.25 vs. 0.25 ± 0.13 (P < 0.05), respectively]. Biceps injection decreased extensor cocontraction index (-35%, P < 0.05) while increasing flexor agonist recruitment and cocontraction indices. CONCLUSIONS: In spastic hemiparesis, stretch may facilitate agonist recruitment and spastic cocontraction. In the non-injected antagonist, cocontraction may be reduced by enhanced reciprocal inhibition from a more relaxed, and therefore stretched, agonist, or through decreased recurrent inhibition from the injected muscle.