To investigate the effects of gravity-related somatosensory information on spinal human reflexes, the soleus H-reflex was recorded in ten healthy subjects walking on a treadmill at 2.0 km/h on land and in water. The modulation pattern of the soleus H-reflex was determined in ten different phases of the step cycle. While the subjects were walking in water, the background electromyographic activity (BGA) of the soleus was lower than that on land; on the other hand, the soleus H-reflex amplitude while the subjects were walking in water showed no significant differences throughout the step cycle compared with that while the subjects were on land; the phase-dependent soleus H-reflex modulation pattern was well preserved while walking in water. There was a linear relationship between the BGA and the H-reflex amplitude in each condition; however, the soleus H-reflex gain while walking in water was significantly higher than that on land. These findings suggest that the somatosensory graviception can markedly reduce the spinal reflex excitability. Our findings are discussed in relation to human gait; therefore, further studies are needed to clarify the effect of somatosensory graviception on human neural mechanisms.
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