The purpose of this study was to investigate posture induced changes in the H-reflex and maximal M-wave (M_max) amplitude in soleus muscle (SOL). The hypothesis of the present study is that both H-reflex amplitude and M_max amplitude change with posture. Nine healthy males were tested under two randomly administered conditions; prone and standing position. The ankle joint angle was set at the same angle in each postural condition by monitoring the electrical goniometer. H-reflex was elicited in the SOL from the right leg every 5 seconds using electrical stimulation to the tibial nerve in the popliteal fossa. Surface electromyography (EMG) was recorded from the middle surface of the SOL and the tibialis anterior muscle (TA) using silver bipolar electrodes. Absolute values of H_max, M_max amplitude and H_max/M_max ratio in each postural condition obtained from the H-M recruitment curve were compared. All H_max/M_max ratios, H_max amplitudes and M_max amplitudes were significantly lower in the standing position. The hypothesis of the present study was verified, and M_max amplitude in the SOL was inhibited by the standing posture. There is a possibility that M_max amplitude is changed in conjunction with the posture induced physiological response
