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

Background: 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.

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

Methods: 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.

Results: 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⁻). 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).

Conclusion: 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.