ALTERNATIVE ASSESSMENT OF MUSCLE FIBER TYPES
Weir, J. P., Fry, A. C., Cramer, J. T. Schilling, B. K., Beck, T. W., Falvo, M. J., Moore, C. A., & Housh, T. J. (2006). Relationships among muscle fiber type, electromyography, and mechanomyography during fatigue in resistance vs. aerobically-trained subjects. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1353.
Studies have suggested that the mechanomyogram (MMG) may be used as a noninvasive technique to estimate GLOBAL skeletal muscle fiber type composition. This study examined the relationships among muscle fiber type and the time and frequency domains of the MMG and surface electromyogram (EMG). Resistance-trained (N = 5) and aerobically-trained (N = 5) males volunteered to perform a 30-second isometric leg extension at 50% of the maximal voluntary contraction (MVC) force. MMG and EMG signals were recorded from the vastus lateralis muscle with an accelerometer and bipolar surface electrodes, respectively. MMG and EMG amplitude and median frequency were determined for each consecutive 1-sec epoch. Muscle samples from the vastus lateralis were extracted via needle biopsy, and analyzed for myosin heavy chain (MHC) isoform content.
Analysis differences were observed for percent MHC IIa and percent MHC I, but not for percent MHC IIb. MMG amplitude and median frequency were greater for the resistance-trained group than the aerobically trained group. There were no differences between groups for EMG amplitude or median frequency. MMG and EMG amplitude increased, EMG median frequency decreased, and MMG median frequency did not change across time. The patterns of response across time for MMG and EMG were the same for both groups.
Implication. Surface EMG was unable to distinguish between resistance- and aerobically-trained groups. Greater MMG amplitudes for the resistance-trained group may have been due to greater muscle mass than that of the aerobically-trained group, whereas higher MMG median frequencies may have been due to greater Type IIa fibers and consequent higher motor unit firing rates for the resistance-trained group. These findings suggested that the MMG signal may be useful for identifying global differences in fiber type composition.
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