Gravity-induced inhibition of the H-reflex and the modulatory potential of the bone myoregulation reflex

The osteocytic network is capable of responding to mechanical stimuli such as gravitational loading or whole-body vibration (WBV), which may potentially initiate the bone myoregulation reflex (BMR). This study explored whether increasing gravitational load is associated with progressive suppression of the soleus H-reflex, whether cyclic (vibratory) versus continuous loading might differentially influence this suppression, and whether the BMR could contribute to this modulation. Twelve healthy young adults participated in two experiments. In Experiment I, H-reflex amplitude was assessed under both static and dynamic loading conditions: in the static condition, 0, 50, and 100% body weight (BW) were applied to the right heel; in the dynamic condition, 50% BW was combined with WBV (30 Hz, 2 mm peak-to-peak displacement, 60 s duration). In Experiment II, the latencies of the BMR and the tendon reflex (T-reflex) were recorded. Mean H/Mmax ratios were 0.30 ± 0.15 (0% BW), 0.24 ± 0.14 (50% BW), and 0.15 ± 0.09 (100% BW), showing a significant progressive decrease with increasing load (p < 0.001). During WBV, despite a similar average load, the H/Mmax ratio markedly declined to 0.01 ± 0.02, indicating significantly greater suppression than under static loading of equivalent magnitude. In Experiment II, the latency of the BMR was approximately 10 ms longer than that of the soleus T-reflex. These findings indicate that suppression of the H-reflex tends to increase progressively with greater gravitational loading under static conditions but appears to become substantially more pronounced during dynamic loading (vibration). BMR-mediated afferent input may partly contribute to the greater inhibition observed, especially under dynamic conditions. From a translational perspective, understanding these mechanisms may inform countermeasure strategies for astronauts, where targeted exercise and vibration protocols could help mitigate neuromuscular and skeletal challenges associated with microgravity.