This study investigates the amplitude modulation of the soleus H-reflex during walking and standing in humans. The H-reflex, a monosynaptically mediated response, was measured using a computer-based analysis procedure. The results show that the H-reflex amplitude is strongly modulated during the walking cycle, peaking during the stance phase. In many subjects, the peak reflex amplitude coincided with the peak soleus electromyographic (EMG) activity, but in others, it occurred earlier. The reflex variation during the step cycle can differ significantly from the EMG produced during walking. At equal stimulus strength and EMG levels, the H-reflex was significantly larger during steady contractions while standing compared to walking. The large reflexes during standing are consistent with the need to maintain stable posture, while the reflexes during walking assist in maintaining upright posture against gravity. The reflexes are smallest during the swing phase, opposing ankle flexion. The modulation of the H-reflex during walking is not solely a passive consequence of α-motoneuron excitation but depends on central mechanisms. The study also compares the H-reflex during tonic contractions and walking, finding that the reflex sensitivity and threshold are lower during standing than during walking. The functional implications of these findings suggest that the H-reflex plays a crucial role in maintaining balance and posture during standing and locomotion.This study investigates the amplitude modulation of the soleus H-reflex during walking and standing in humans. The H-reflex, a monosynaptically mediated response, was measured using a computer-based analysis procedure. The results show that the H-reflex amplitude is strongly modulated during the walking cycle, peaking during the stance phase. In many subjects, the peak reflex amplitude coincided with the peak soleus electromyographic (EMG) activity, but in others, it occurred earlier. The reflex variation during the step cycle can differ significantly from the EMG produced during walking. At equal stimulus strength and EMG levels, the H-reflex was significantly larger during steady contractions while standing compared to walking. The large reflexes during standing are consistent with the need to maintain stable posture, while the reflexes during walking assist in maintaining upright posture against gravity. The reflexes are smallest during the swing phase, opposing ankle flexion. The modulation of the H-reflex during walking is not solely a passive consequence of α-motoneuron excitation but depends on central mechanisms. The study also compares the H-reflex during tonic contractions and walking, finding that the reflex sensitivity and threshold are lower during standing than during walking. The functional implications of these findings suggest that the H-reflex plays a crucial role in maintaining balance and posture during standing and locomotion.