Fatigue associated EMG behavior of the first dorsal interosseous and adductor pollicis muscles in different groups of subjects

We have studied the fatigue-associated behavior of surface EMG in two histochemically different muscles of the hand: first dorsal interosseous (FDI) and adductor pollicis (AP; relatively more type I fibers in AP than in FDI). During a fatigue test evoked by electrical stimulation of the ulnar nerve, the mean amplitudes of compound muscle action potentials (M-waves) exhibited the same overall pattern for both muscles: a rapid phase of potentiation followed by a gradual decline. However, if the group of subjects was subdivided on the basis of hand length, significant differences emerged in the reactions of AP: in large hands, no fatigue-associated M-wave decline was seen, whereas in small hands a distinct decline was observed. A possible explanation for this phenomenon might be the presence of a greater amount of EMG contamination from other muscles in smaller hands. In the supposedly "cleaner" recordings from larger hands, significant differences between FDI and AP were observed with regard to their fatigue-associated EMG reactions (M-wave depression in FDI but not in AP). The direction of these differences was in accordance with expectations on the basis of known differences in histochemical fiber type composition.