This study investigates the amplitude modulation of the soleus H-reflex during walking and standing in humans. The H-reflex, a monosynaptic reflex mediated by the soleus muscle, was found to be strongly modulated during the walking cycle, with the highest amplitude during the stance phase. In many subjects, the peak reflex amplitude coincided with the peak soleus electromyographic (EMG) activity, while in others, it occurred earlier. The variation in H-reflex amplitude during the step cycle differed from that of the EMG, indicating that the reflex is influenced by central mechanisms beyond the level of α-motoneuron excitation. During standing, the H-reflex amplitude was significantly larger than during walking, especially at low levels of EMG activity. This suggests that the reflex is more sensitive during standing, possibly due to changes in reflex sensitivity and threshold. The modulation of the H-reflex during walking is not simply a passive consequence of α-motoneuron excitation but is influenced by central mechanisms that also affect the reflex threshold and sensitivity. The study also compared H-reflexes during tonic contractions and walking, finding that the reflex amplitude was larger during standing than during walking. The relationship between H-reflex amplitude and EMG level was steeper during walking than during standing, indicating a greater sensitivity of the reflex during walking. The modulation of the H-reflex during walking and standing is likely due to central neural mechanisms that influence the synaptic transmission between Ia afferents and α-motoneurons. These mechanisms may include changes in reflex sensitivity, threshold, and presynaptic inhibition. The results suggest that the H-reflex is a useful measure of changes in synaptic efficacy between muscle afferents and α-motoneurons, as it is less influenced by peripheral factors such as fusimotor effects. The study highlights the functional importance of the H-reflex in maintaining body posture and balance during walking and standing. The reflex is most active during the stance phase, helping to maintain the upright position of the body against gravity. During the swing phase, the reflex is smaller, opposing ankle flexion. The findings suggest that the H-reflex is modulated by central mechanisms that are not solely dependent on the level of α-motoneuron excitation. The results also indicate that the H-reflex may be used to assess the functional state of the stretch reflex in humans, with implications for understanding motor control and posture regulation.This study investigates the amplitude modulation of the soleus H-reflex during walking and standing in humans. The H-reflex, a monosynaptic reflex mediated by the soleus muscle, was found to be strongly modulated during the walking cycle, with the highest amplitude during the stance phase. In many subjects, the peak reflex amplitude coincided with the peak soleus electromyographic (EMG) activity, while in others, it occurred earlier. The variation in H-reflex amplitude during the step cycle differed from that of the EMG, indicating that the reflex is influenced by central mechanisms beyond the level of α-motoneuron excitation. During standing, the H-reflex amplitude was significantly larger than during walking, especially at low levels of EMG activity. This suggests that the reflex is more sensitive during standing, possibly due to changes in reflex sensitivity and threshold. The modulation of the H-reflex during walking is not simply a passive consequence of α-motoneuron excitation but is influenced by central mechanisms that also affect the reflex threshold and sensitivity. The study also compared H-reflexes during tonic contractions and walking, finding that the reflex amplitude was larger during standing than during walking. The relationship between H-reflex amplitude and EMG level was steeper during walking than during standing, indicating a greater sensitivity of the reflex during walking. The modulation of the H-reflex during walking and standing is likely due to central neural mechanisms that influence the synaptic transmission between Ia afferents and α-motoneurons. These mechanisms may include changes in reflex sensitivity, threshold, and presynaptic inhibition. The results suggest that the H-reflex is a useful measure of changes in synaptic efficacy between muscle afferents and α-motoneurons, as it is less influenced by peripheral factors such as fusimotor effects. The study highlights the functional importance of the H-reflex in maintaining body posture and balance during walking and standing. The reflex is most active during the stance phase, helping to maintain the upright position of the body against gravity. During the swing phase, the reflex is smaller, opposing ankle flexion. The findings suggest that the H-reflex is modulated by central mechanisms that are not solely dependent on the level of α-motoneuron excitation. The results also indicate that the H-reflex may be used to assess the functional state of the stretch reflex in humans, with implications for understanding motor control and posture regulation.