Load-dependent dissociation between H-reflex amplitude and background EMG activity during oscillatory loading:...

Under static postural loading and graded contractions, H-reflex amplitude may be reduced despite increased background EMG activity (BGA), indicating a dissociation between H-reflex amplitude and motor output. Whether oscillatory mechanical loading, such as whole-body vibration (WBV), induces a similar dissociation remains unclear. The bone myoregulation reflex (BMR) is a load-dependent response to oscillatory stimuli. This study investigated whether WBV produces a dissociation between H-reflex excitability and motor output, evaluating the BMR's potential contribution to this dissociation. Fifteen healthy volunteers were assessed during quiet standing under varied postural (single/double-leg) and platform displacement (zero/1.1/2.2 mm) conditions. Soleus H-reflex amplitude (H/Mmax) and normalized BGA were derived from the recorded EMG signals. The reflex evoked during WBV was identified as the BMR. To quantify BMR activity, the rectified EMG signal was integrated over the 8-s vibration period, representing the cumulative motor output induced by the mechanical stimuli. BMR latency was sensitive to postural load distribution. Increasing oscillatory loading magnitude produced a marked reduction in the H/Mmax ratio (p < 0.001, η_p² = 0.71). In contrast, BGA increased in a platform displacement-dependent manner (p < 0.001, η_p² = 0.53). However, linear mixed-effects modelling revealed that when BMR activity was included in the model, it emerged as a significant predictor of BGA (p < 0.001, η_p² = 0.27), showing a positive linear relationship. In conclusion, the results demonstrate that oscillatory mechanical loading produces a load-dependent dissociation between H-reflex amplitude and motor output. The findings establish a consistent link between the load-sensitive BMR response and increased motor output, while suggesting a potential mechanistic basis for the accompanying load-dependent H-reflex dissociation.ClinicalTrials.gov registration: NCT07043595 (06/22/2025).