Effect of maximal voluntary contraction on the amplitude of the compound muscle action potential: Implications for the interpolated twitch technique

Abstract

The compound muscle action potential (M(MAX)) during a maximal voluntary contraction (MVC) may be measured to determine if the motor nerve has been supramaximally stimulated during the interpolated twitch technique (ITT). Ten males performed isometric knee extension MVCs. M(MAX) for the vastus medialis was recorded during MVC and rest. To examine the effect of stimulating electrode movement, the M(MAX) of the thenar group and antidromic sensory nerve action potentials (SNAPs) to the third digit were recorded in a separate experiment. M(MAX) during MVC was reduced by 18% (P < 0.0001) and 43% (p < 0.0001) for the quadriceps and thenar group, respectively. The SNAP amplitude was not different between rest and MVC (P = 0.18). Reduction of M(MAX) during MVC suggests that some motor axons are refractory and unable to respond to a superimposed maximal stimulus. These results have implications for the sensitivity of the interpolated twitch technique.