H-reflex modulation preceding changes in soleus EMG activity during balance perturbation

When balance is compromised, postural strategies are induced to quickly recover from the perturbation and to regain balance. However, neuronal mechanisms underlying these strategies are not fully understood. In this article, we assessed the amplitude of the Hoffmann’s reflex (H-reflex) in the soleus  muscle (SOL, muscle of the calf) during forward and backward tilts of the support surface during standing. We found that: 1) during backward tilt, a significant decrease in H-reflex amplitude was observed from 75ms after tilt onset and significantly preceded the onset of the SOL EMG (electromyography) decrease (latency: 144 ± 16 ms); 2) during forward tilt, the amplitude of the H-reflex increased from 100 ms after tilt onset, which coincided simultaneously with the onset of the SOL EMG increase (127 ± 5 ms). These results establish the time course of change in SOL H-reflex excitability, its relation to the SOL EMG activity modulation and suggest that balance mechanisms may differ between forward and backward tilts.