Camic, C. L., Taddy, M. L., Zuniga, J. M., Housh, T. J., Traylor, D. A., Bergstrom, H. C., Schmidt, R. J., & Johnson, G. O. (2012). Electromyographic responses across repeated maximal isometric and concentric muscle actions. Presentation 2168 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.

This study examined the pattern of electromyographic (EMG) responses associated with fatigue across repeated maximal isometric and concentric muscle actions in females (N = 12). Ss performed isometric and concentric muscle actions of the leg extensors on a Cybex 6000 isokinetic dynamometer on three separate days. The first visit was structured as an orientation session to familiarize Ss with the testing protocols. During the second and third visits, Ss performed randomly ordered isometric and concentric fatigue protocols separated by 72 hours. The intermittent isometric fatigue protocol involved 30 maximal isometric muscle actions that were sustained for three seconds followed by three seconds of rest. The concentric fatigue protocol involved 30 consecutive maximal concentric muscle actions at 30°/second. In addition, EMG signals were recorded from the vastus lateralis during the isometric and concentric fatigue protocols. The relationships for torque, EMG amplitude, and EMG mean power frequency versus repetition number were examined using polynomial regression analysis.

There were significant decreases across the 30 repeated maximal isometric muscle actions for torque, EMG amplitude, and EMG mean power frequency. In addition, there were significant decreases across the 30 repeated maximal concentric muscle actions for torque, EMG amplitude, and EMG mean power frequency.

Implication. The pattern of responses for EMG amplitude indicated that both the isometric and concentric muscle actions led to a fatigue-induced de-recruitment of activated motor units and/or a reduction in the global firing rate that occurred after 10 to 15 maximal repetitions. These findings suggest that complete muscle activation during the maximal isometric and concentric muscle actions required multiple repetitions. Furthermore, the decreases in EMG mean power frequency during both the isometric and concentric muscle actions were parallel to the linear and quadratic decreases in torque, respectively. These findings suggest that the mechanisms of fatigue responsible for decreases in torque were consistent between the isometric and concentric muscle actions.

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