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WARREN, G. L., K. M. HERMANN, C. P. INGALLS, M. R. MASSELLI, and R. B. ARMSTRONG. Decreased EMG median frequency during a second bout of eccentric contractions. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 820–829, 2000.Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is evidence that fast muscle fibers are more susceptible to eccentric contraction-induced 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 the EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to eccentric contraction-induced injury after a single bout of eccentric exercise.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 second bout could be a consequence of injury to fibers in fast motor units, the anterior crural muscles of mice were electrically stimulated to perform 50 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occasions 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.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 despite torque deficits similar to those observed in the humans.The data indicate that with repetition of maximal voluntary eccentric contractions, there is an increased activation of slow motor units and a concomitant decrease in activation of fast units.