REDUCING HUMAN BICEPS BRACHII SPINAL STRETCH REFLEX MAGNITUDE

1. Nine subjects received 6 baseline and 24 training sessions, each co nsisting of 250 elbow joint stretch perturbations into extension. The training sessions were designed to reduce the subjects' biceps brachii spinal stretch reflexes (SSR). Changes in longer-latency responses an d short-latency brachioradialis responses were also monitored. Backgro und electromyogram activity was recorded from the lateral head of tric eps brachii during the biceps sampling intervals. These data were comp ared with those from 12 control subjects who received equal numbers of stretches over an extended baseline (i.e., without application of a t raining paradigm). 2. Training subjects reduced their mean biceps SSR responses by 24%, whereas control subjects increased their responses b y 12%. When changes in activity were grouped by sets of six consecutiv e sessions following the baseline interval, a group-by-time interactio n was observed. Training subjects showed significant reductions from b aseline after the first training set. The increased magnitude of bicep s SSR for control subjects was significantly larger than baseline in s ets 2-5. After the first training interval, all subsequent differences between groups were statistically significant. 3. The brachioradialis showed greater response variability, but these responses paralleled t hose seen in biceps brachii. The training group reduced their brachior adialis responses by 18%, whereas the control group increased their re sponses by 12%. Background activity recorded over the lateral head of triceps brachii during the biceps brachii SSR window became smaller in both groups. 4. Comparison of data between control subjects and subje cts who had undergone biceps SSR up-training in previous studies sugge sts that the small increase seen among the present control subjects wa s probably not the result of a direct training effect. 5. Baseline dif ferences in longer-latency response onset time were seen between group s. Over extended baseline sessions, longer-latency responses showed on ly a 1% later onset, representing a change of 0.3 ms, in the control g roup. A 1.3% later onset, equaling a change of 1.1 ms, was observed ov er conditioning ses sions in the training group. When analyzed by sets following baseline, neither group showed significant within- or betwe en-group changes over time. 6. The magnitude of the longer-latency bic eps brachii responses showed 7% and 37% reductions for the control and training groups, respectively. No difference in set averages was seen within groups, but a significant difference was measured over sets 2- 4 between groups. Reductions in the magnitude of longer-latency respon ses in training subjects were more obvious in later training sessions. 7. These data suggest that 1) the biceps brachii SSR can be reduced i n human subjects; 2) concurrent changes are often observed in the syne rgist brachioradialis; and 3) the reduced biceps SSR magnitude is not linked to a compensatory increase in antagonist muscle activity. Reduc tions in the magnitude of longer-latency biceps brachii activity seen in parallel with the decreased SSR may imply that some degree of supra spinal processing is required to achieve this task.