H-reflex changes with loading and unloading

Soleus H-reflex has been tested clinically in lying position. Stresses on the spine vary during lying, standing, weight lifting and unloading. This may influence the H-reflex. The purpose of this study was to measure the changes that might occur in the soleus H-reflex during such loading and unloading conditions. Twenty healthy volunteers (20-50y) with no history of significant low back pain or radiculopathy participated in the study. Cadwell Excel EMG unit was used to elicit and record the soleus H-reflex. The tibial nerve was stimulated at the popliteal fossa using 1 ms pulses and 0.2pps of H-max. Each subject was tested under four different conditions; prone, free standing, standing while lifting 20% of the body weight and standing unloaded by 25% of the body weight using the ZUNI II. H-reflex maximum peak-to-peak amplitudes and onset latencies from eight trials were averaged for each lower extremity. Two factor ANOVAs with repeated measures over each factor were used to test the effect of the position and the side on the H-reflex amplitude and latency with alpha = 0.025. Results showed no significant effect of the side on the H-reflex amplitude and latency. Compared to prone lying, there was a significant reflex inhibition during free standing, loading and unloading conditions. No significant difference was recorded in the reflex latency among different conditions. These results imply a significant interplay between peripheral and central mechanisms on the spinal motoneurons. It suggests testing of the H-reflex amplitude and latency in functional postures to detect subtle changes in root impingement.