Effect of weight bearing on the soleus H-reflex during upright standing under the head-out water immersion condition in humans

Abstract

To test the hypothesis that somatosensory inputs inhibit the soleus Hoffmann (H-reflex) circuit during upright standing, ten subjects were investigated under the head-out water immersion (HOWI) condition. Subjects maintained an upright standing without any activity in the soleus, tibialis anterior and vastus lateralis muscles. Testing was conducted 1) dry condition, 2) neck level water immersion to reduce the gravitational effect by 95 percent by buoyancy (HOWI condition) and 3) 95 percent of the subject's body weight bearing under HOWI condition (weight-load condition). The test soleus H-reflex and motor (M) response was elicited at a stimulation intensity of 1.05 times the motor threshold. No significant differences in the recruitment profile of the M response were observed. The amplitude of the H-reflex normalized by the maximum M response (M(sub max)) was significantly different, even while the stimulation intensity remained constant. The H-reflex in the HOWI was significantly stronger as compared with its strength in the dry condition, while the H-reflex in the weight-load condition was significantly weaker as compared with its strength in the HOWI condition. It was concluded that somatosensory inputs due to gravity exert an inhibitory effects on the soleus H-reflex circuit during upright standing in humans.