Voluntary and reflex control of the biceps brachii muscle in spastic-athetotic patients.

A cross-correlation technique of analysis was used to measure the transmission characteristics of tonic stretch reflex (TSR) pathways in spastic-athetoid subjects sustaining a voluntary contraction in the biceps brachii muscle. A comparison was made with the transmission characteristics of normal subjects measured by the same technique. It was found that gain and phase characteristics of spastic patients did not display the large resonant peaks present in normals. It is proposed that the resonant peaks in the TSR transmission of normal subjects were caused by long loop pathways. The absence of these peaks in the spastic patients supports the hypothesis that short-circuiting of long loop pathways by hyperactive spinal reflexes is part of the mechanism of spasticity.