Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (~2°) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5–95% MVC, monitored via surface electromyography (EMG). Both narrow band (4–5 Hz) and broad band (0–10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from ~130° at the lowest contraction levels to ~50° as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31–69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47–95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.