The relative contribution to the plantar flexor torque of the soleus motor units activated by the H reflex and M response in humans.

This study proposes a method of quantifying the relative contribution to the plantar-¯exor torque of soleus H and M responses evoked by tibial nerve stimulation. For ten subjects, the amplitude of the twitch produced by the H wave was plotted against the corresponding potential, for stimuli producing H without M (i.e. in the ascending portion of the Hrecruitment curve). It was then assumed that the contribution of H to twitches produced by M plus H was similar for similar H waves on the curve-descending portion. Hence, the contribution of Mwas estimated, for the range of Mwaves including those accompanying Hmax. The estimated mechanical contributions of H and M wave increase linearly with their potentials, the slope of the relationship being greater for H than M. The interpretation of this ®nding is discussed. A simple formula for assessing the contribution of the H wave to the twitch torque is given.