Somatosensory graviception inhibits the soleus H-reflex in standing man - a parabolic flight experiment-

The purpose of this study was to investigate how gravity level affects the excitability of the soleus muscle (SOL) motoneuron pool to la afferent input while erect posture is maintained in humans. Three healthy male subjects participated in an experiment whereby three different gravity conditions (micro gravity (MG), normal gravity (NG), and hyper gravity) were imposed using a parabolic flight procedure. The SOL H-reflex was evoked every 2 seconds while the subjects kept an erect posture. The background electromyographic activity (BGA) of the SOL was almost absent during MG. The SOL H-reflex amplitude was significantly larger during MG than during NG. These results suggest that the somatosensory systems detecting a load at the lower limbs and/or vertebral column play a role in reducing the excitability of the SOL motoneuron pool to la afferent inputs by presynaptic inhibition.