Relationships Among Muscle Fiber Type, Mechanomyographic, And Electromyographic Amplitude Response Patterns During Ramped Isometric Muscle Actions

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The purpose of this study was to examine the mechanomyographic (MMGRMS) and electromyographic (EMGRMS) amplitude vs. force relationships of the vastus lateralis (VL) for aerobically-trained (AT), resistance-trained (RT), and sedentary individuals (SED). Five RT (mean ± SD age = 23 ± 3 yrs; body mass = 101 ± 37 kg; height = 176 ± 8 cm), 5 AT (32 ± 5 yrs; 67 ± 4 kg; 176 ± 2 cm), and 5 SED (23 ± 4 yrs; 93 ± 32 kg; 180 ± 5 cm) men volunteered to perform two 6-s isometric ramp muscle actions from 5% to 100% of their maximal voluntary contraction (MVC) while MMG (m·s−2) and EMG (μV) signals were recorded from the VL muscle. Thigh skinfold measurements and Bergstrom muscle biopsies were taken from the VL. The muscle samples were analyzed for myosin heavy chain (MHC) isoform content. Simple linear regression models were fit to the natural log-transformed EMGRMS and MMGRMS vs. force relationships. The slope (b term) and the antilog of the Y-intercept (a term) were calculated for each relationship. For %MHC, the AT group had a greater percentage of type I fibers than the RT and SED groups (p ≤0.05). The RT group had a greater percentage of type IIa fibers than the AT group (p ≤0.05), and the SED group had a greater percentage of type IIx fibers than the AT and RT groups (p ≤0.05). For both the log-transformed EMGRMS and MMGRMS vs. force relationships, the mean a term for the AT group was higher than the RT and SED groups (p ≤0.05). For the log-transformed MMGRMS vs. force relationships, however, the b term for the AT group was lower than the RT and SED groups (p ≤0.05). There were no differences (p >0.05) among the b terms of the AT, RT, and SED groups for the log-transformed EMGRMS vs. force relationships. In addition, the mean skinfold for the SED group was higher than the AT group (p ≤0.05) (AT = 8.7 ± 2.2mm, RT = 15.4 ± 7.6mm, and SED = 25.4 ± 9.2mm). The group-related differences among the a terms for both the log-transformed EMGRMS and MMGRMS patterns might be best explained by the group differences in skinfold thickness. However, the fact that the AT group had a lower mean b term for the log-transformed MMGRMS vs. force relationships may suggest that the differences in motor control strategies between individuals with predominantly type I vs. type II fibers in the vastus lateralis could be detectible with the MMG signal, but not EMG. The lower b term for the AT group suggested that the MMG-force relationship may have been sensitive to the earlier achievement of rate coding, whereas the RT and SED groups relied more on motor unit recruitment to reach higher force levels. The b terms from the EMGRMS vs. force relationships were unable to differentiate among the groups. The log-transformed MMGRMS, but not EMGRMS, vs. force relationships may be useful as non-invasive measurements to determine the onset of rate coding, which may differ among muscles that are predominantly type I vs. type II fiber types. Therefore, the MMG signal may offer an attractive, simple technique for examining changes in the motor control strategies that govern incremental isometric force production.