Long-lasting supernormal conduction velocity after sustained maximal isometric contraction in human muscle

Local muscle fatigue (1 min maximal voluntary contraction) and recovery were studied by means of surface and invasive EMG on elbow flexors to record the changes in muscle fiber conduction velocity (MFCV), median power frequency (MPF), integrated EMG (IEMG), and force. The main finding was a long-lasting "supernormal" MFCV during recovery, for at least 1 hour. After a normalization phase, the MFCV and MPF continued to increase reaching a steady state at supernormal values after 10-12 min. Mean MFCV increase at 20% MVC after 15-min recovery was 0.58 m.s-1 (12%). Postfatigue IEMG values were increased at all contraction levels. In combination with near normal force levels, this resulted in a decrease in "neuromuscular efficiency" (force/IEMG). We suggest that this IEMG increase is mainly a result of the MFCV increase. The MFCV changes in fastest and slowest fibers found with the invasive method indicate a relatively equal effect on type I and II fiber types. A possible explanation of the supernormal MFCV is muscle fiber swelling, in combination with altered membrane properties.