Amplitude of the maximum motor response (Mmax) in human muscles typically decreases during the course of an experiment

It was shown that the amplitude of the soleus M-max and H-max responses decreases in the course of long-lasting H-reflex studies. The peak-to-peak amplitudes of the M-max and H-max responses in the soleus muscle (and the M-max in the tibialis anterior muscle and small hand muscles) were measured repeatedly for 1-3 h in 20 subjects. 3-5 M-max responses and 5-10 H-max responses were elicited about every 3 min while the subject was at rest. Decreases in the soleus M-max response of up to 50.5% (mean 20.5% SEM 2.2) and of the soleus H-max of up to 49.7% (mean 19.1% SEM 3.7) in relation to the amplitudes measured at the beginning of the experiment were seen in 17 subjects. In 3 subjects no M-max amplitude decrease was seen. The maximum decrease was reached between 10 and 100 min (mean 44.2 min SEM 4.3). An M-max amplitude decrease was also seen in the tibialis anterior muscle and in two small hand muscles. In some subjects the decrease of the M-max response seemed to be initiated by the infrequent supramaximal stimulations. The possible causes for this amplitude reduction, as well as the methodological consequences of these findings for H-reflex studies and fatigue studies, are briefly discussed.