Decreased EMG median frequency during a second bout of eccentric contractions

Purpose: Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is eviden ce that fast muscle fibers are more susceptible to eccentric contraction-in duced injury. We tested the hypothesis that during a second bout of maximal eccentric contractions 1 wk after the first, there would be a reduction in the electromyographic (EMG) median frequency (MF) with minimal change in t he EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to ecc entric contraction-induced injury after a single bout of eccentric exercise . Methods: Human subjects performed 50 maximal voluntary eccentric (N = 10) or concentric (N = 10) contractions of the anterior crural muscles on two occasions separated by 1 wk. To determine whether MF changes during the sec ond bout could be a consequence of injury to fibers in fast motor units, th e anterior crural muscles of mice were electrically stimulated to perform 5 0 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occa sions separated by 1 wk. In both the humans and mice, torque production and tibialis anterior muscle RMS and MF were measured during the two exercise bouts. Results: In human tibialis anterior muscle, MF was 30% lower (P < 0. 01) during the second eccentric bout although RMS was the same. In the mice , RMS and MF were unchanged at any time after the first eccentric bout desp ite torque deficits similar to those observed in the humans. Conclusions: T he data indicate that with repetition of maximal voluntary eccentric contra ctions, there is an increased activation of slow motor units and a concomit ant decrease in activation of fast units.