The role of the bone myoregulation reflex in impact-loading activities: a new view of feedback control mechanisms

The short-latency reflex (SLR), which occurs immediately after ground contact during jumping, is traditionally attributed to a muscle spindle-mediated stretch reflex, with a longer latency explained by slow muscle stretching. However, emerging evidence suggests that the bone myoregulation reflex (BMR) may provide a more physiologically parsimonious and biomechanically integrated explanation for this response.

This study compared the latencies of these reflexes and assessed the mechanical stimulus transmission delay to the muscle during impact.

Two experiments were performed in healthy adults. Experiment 1 measured the soleus tendon reflex (T-reflex), SLR, and BMR latencies via surface electromyography (EMG). Experiment 2 recorded delays from the mechanical stimulus to the muscle belly using intramuscular EMG.

The median latencies in Experiment 1 were 35.0 ms (T-reflex), 45.8 ms (SLR), and 43.0 ms (BMR). The SLR and BMR latencies were significantly longer than the T-reflex latencies (p = 3.6 × 10⁻¹¹). There was no difference between the SLR and BMR. Experiment 2 showed mechanical transmission delays of 4.31 ms (tendon stretch), 3.31 ms (tap), and 2.83 ms (whole-body vibration), without significant differences. The ~ 11 ms longer SLR latency than the T-reflex cannot be explained by slow muscle stretching. Normalized soleus EMG signals during landing (feedforward) were positively correlated with the SLR amplitude (feedback) (r = 0.554, p = 0.0003).

The latency characteristics of the SLR suggest that it more closely resembles the BMR than the classical stretch reflex does. It is speculated that as a bone-protective mechanism, BMR may underlie reflexive muscle contractions that deliver load-induced protective feedback during impact, potentially preserving both bone and muscle-tendon integrity.