The objective of the study was to investigate the amplitude and modulation of the human soleus Hoffmann (H) reflex during walking and during running at different speeds. 2. EMGs were recorded with surface electrodes from the soleus, the medial and lateral head of the gastrocnemius, the vastus lateralis and the anterior tibial muscles. The EMGs and the soleus H reflex were recorded while walking on a treadmill at 4.5 km h-1 and during running at 8, 12 and 15 km h-1. 3. The amplitudes of the M wave and the H reflex were normalized to the amplitude of a maximal M wave elicited by a supramaximal stimulus just after the H reflex to compensate for movements of the recording and stimulus electrodes relative to the nerve and muscle fibres. The stimulus intensity was set to produce M waves that had an amplitude near to 25 % of the maximal M wave measured during the movements. As an alternative, the method of averaging of sweeps in sixteen intervals of the gait cycle was applied to the data. In this case the amplitude of the H reflex was expressed relative to the maximal M wave measured whilst in the standing position. 4. The amplitude of the H reflex was modulated during the gait cycle at all speeds. During the stance phase the reflex was facilitated and during the swing and flight phases it was suppressed. The size of the maximal M wave varied during the gait cycle and this variation was consistent for each subject although different among subjects. 5. The peak amplitude of the H reflex increased significantly (P = 0.04) from walking at 4.5 km h-1 to running at 12 and 15 km h-1 when using the method of correcting for variations of the maximal M wave during the gait cycle. The sweep averaging method showed a small but non-significant decrease (P = 0. 3) from walking to running at 8 km h-1 and a small decrease with running speed (P = 0.3). The amplitude of the EMG increased from walking to running and with running speed. 6. The relatively large H reflex recorded during the stance phase in running indicates that the stretch reflex may influence the muscle mechanics during the stance phase by contributing to the motor output and enhancing muscle stiffness.
