2003 Volume 52 Issue 5 Pages 599-608
This study hypothesized that gravity induced somatosensory input inhibits the spinal monosynaptic reflex circuit assessed by Hoffmann (H) reflex in man. To test this hypothesis, we investigated the effect of graded head-out water immersion (HOWI) on soleus H-reflex during upright standing. Eleven healthy male subjects aged from 21 to 28 (mean=23.8) without sensorymotor or orthopaedic history signed an informed consent. Subjects maintained an upright standing position without visual input and any background activities in soleus and tibialis anterior. H-reflex was elicited by constant current stimulation at popliteal fossa. The recruitment curve for H-reflex and motor (M) response was measured under the static tilt condition (from supine to standing) and the graded HOWI condition (the control condition, the navel condition, the neck condition, and post immersion condition) in that order. Under the HOWI condition, the water (34 degrees Celsius) was filled in a minute. No change in H- and M-wave form was observed. The amplitude of test reflex at 1.05 times motor threshold significantly decreased in the standing condition compared with the supine condition (P<0.01) . Furthermore, significant change was observed in the HOWI condition (P<0.01) . A contrast test reveled a significant increase in the navel condition (P=0.03) and the neck condition (P<0.01) compared with the control condition. The results showed that graded HOWI enhances soleus H-reflex during upright standing. It was suggested that gravity-induced somatosensory input from mechanoreceptors in the soles and joints inhibits the spinal H-reflex circuit and plays an important role for the static postural control mechanism in man.
