Interaction between voluntary contraction and tonic stretch reflex transmission in normal and spastic patients.

Transmission characteristics of tonic stretch reflex (TSR) pathways in both normal and spastic patients have been measured during different levels of sustained voluntary contraction of the biceps brachii muscle. A cross-correlation technique of analysis was used to separate reflex responses from the total electromyographic activity. TSR transmission in normal subjects was observed to change with the level of voluntary contraction; sensitivity to stretch increased approximately three-fold as the subject stiffened the arm by simultaneously contracting flexor and extensor muscles. In contrast, TSR transmission did not alter during voluntary contraction in spastic patients. It is proposed that in spastic patients supraspinal modulation of reflex transmission is impaired because hypersensitive spinal reflexes short-circuit long loop pathways.