Relationships Among Muscle Fiber Type, Electromyography, and Mechanomyography During Fatigue In Resistance- vs. Aerobically-Trained Subjects

Abstract

PURPOSE: Previous studies have suggested that the mechanomyogram (MMG) may be used as a noninvasive technique to globally estimate skeletal muscle fiber type composition. The purpose of this study was to examine the relationships among muscle fiber type and the time and frequency domains of the MMG and surface electromyogram (EMG). METHODS: Five resistance-trained (RT; age ± SD = 23 ± 4 years; stature =176 ± 8 cm; body mass = 102 ± 38 kg) and five aerobically-trained (AT; 33 ± 5 years; 176 ± 3 cm; 68 ± 4 kg) men 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 (mVrms and uVrms, respectively) and median frequency (Hz, discrete Fourier transform) 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. RESULTS: Differences (p <0.05) were observed for percent MHC IIa (RT=59.0 ± 4.2%, AT=27.4 ± 7.8%) and percent MHC I (RT=40.9 ± 4.3; AT=72.6 ± 7.8%), but not (p>0.05) for percent MHC IIb (RT=0.1±0.1 %, AT=0.0 ± 0.0%). MMG amplitude and median frequency were greater (p<0.05) for the RT group than the AT group, however, there were no differences (p>0.05) between groups for EMG amplitude or median frequency. MMG and EMG amplitude increased (p <0.05), EMG median frequency decreased (p <0.05), and MMG median frequency did not change (p>0.05) across time. The patterns of response across time for MMG and EMG were the same for the RT and AT groups. CONCLUSIONS: Surface EMG was unable to distinguish between RT and AT groups. Greater MMG amplitudes for the RT may have been due to greater muscle mass for the RT than the AT group, whereas higher MMG median frequencies may have been due to greater type IIa fiber type and consequent higher motor unit firing rates for the RT group. These findings suggested that the MMG signal may be useful for identifying global differences in fiber type composition.