Contralateral and long latency effects of human biceps brachii stretch reflex conditioning

Results from previous studies on monkeys and human subjects have demonstrated that the biceps brachii spinal stretch reflex (SSR) can be operantly conditioned. The extent to which conditioning paradigms influence contralateral SSRs or longer latency responses in the same limb has not been examined. Nine subjects were given 10 training sessions to either increase or decrease the size of their biceps brachii SSR. Group changes were compared to the mean of six baseline (control) sessions. Both groups showed progressive SSR changes over the training sessions. Up-trained subjects increased their SSR responses by an average of 135.3% above baseline, with the last three sessions showing a 237.5% increase, while down-trained subjects reduced their average SSR responses by 43.4%, with a 52.7% reduction over the last three sessions. ipsilateral longer latency responses showed average changes of 68.9% and -68.7% for up- and down-trainers, respectively. As in the case of SSRs, these responses changed progressively over sessions, with a 131.5% increase seen in the last three up-training sessions and an 82.4% reduction over the same period for down-trainers. Correlation coefficients between SSR and longer latency responses were high (R = 0.90, up-trainers; R = 0.87, down-trainers). Contralateral SSR and longer latency responses, measured in the absence of feedback and at least 10 min after ipsilateral conditioning, showed directional changes that were similar to the trained side, but their magnitudes were not as profound. Collectively, these data suggest that unilateral SSR conditioning affects spinal circuits controlling contralateral SSRs and influences longer latency responses.