Operant conditioning of spinal stretch reflexes in patients with spinal cord injuries

Hyperactive spinal stretch reflexes (SSRs) often occur with spinal cord injuries (SCI). These altered SSRs may impair movement. Recent studies in monkeys and human subjects have indicated that the magnitude of SSRs can be modulated using operant conditioning. The purpose of this study was to determine whether hyperactive biceps brachii SSRs could be operantly conditioned downward. Seventeen chronic (> 1 year postlesion) spinal cord-injured patients participated. Subjects were trained to keep biceps background (prestretch) electromyographic (EMG) activity and elbow angle at predetermined levels prior to having the elbow rapidly extended by a torque motor to elicit the biceps SSR. All subjects participated in six baseline sessions over a 2-week period. Then, subjects were randomly assigned to either control or training groups for the next 24 sessions over an 8-week period. By the end of the study, training subjects had significantly reduced biceps SSRs (t test, P < 0.001), while control subjects SSRs were not significantly reduced (t test, P > 0.05). The reduced SSRs persisted for up to 4 months following cessation of training. The results of this study support the hypothesis that hyperactive SSRs can be operantly conditioned downward in SCI patients.