Gender and muscle differences in EMG amplitude and median frequency, and variability during maximal voluntary contractions of the quadriceps femoris

The purpose of this study was to evaluate gender and muscle differences in electromyographic (EMG) amplitude and median frequency mean and standard de viation during maximal voluntary contractions of the quadriceps femoris. Th irty recreationally active volunteers were assessed for isometric EMG activ ity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during three 5-s maximal isometric voluntary contractions (MVC s). Median frequency of the three muscles was assessed through a power spec tral analysis (fast Fourier transformation, Hanning window processing, 512 points). The power spectral analysis was performed during the middle 3 s of each contraction over 11 consecutive, 512 ms epochs overlapping each other by half their length (256 ms). The median frequency (F-med) for each of th e 11 windows was determined for each muscle. The mean and standard deviatio n of the F-med across the 11 overlapping windows were then calculated for e ach contraction and muscle. EMG amplitude was determined by calculating the root mean square (RMS-50 ms time constant) over the same contraction perio d for each muscle. The mean amplitude and standard deviation about the mean value were then determined. A three-factor ANOVA with repeated measures wa s performed on the calculated F-med mean and standard deviation values, and RMS standard deviations, to assess any gender, muscle, or trial difference s, or interactions. A two-factor (gender by muscle) ANOVA with repeated mea sures was performed on the RMS mean amplitude for each muscle. Intraclass c orrelation coefficients (ICCs-2,1), standard errors of measurement (SEMs), and associated 95% confidence intervals were then calculated for maximal qu adriceps torque and F-med for each muscle. The results from this study demo nstrated that the VL muscle displayed significantly higher F-med values tha n the RF and VM muscles. The RF muscle showed significantly higher F-med va lues (mean of 11 overlapping windows) than the VM muscle. Intrasession reli ability was found to be high for the calculated mean values (ICC=0.85-0.96) , but was shown to be low for variability (ICC=0.13-0.45). The major findin gs of this study support: the notion that the EMG signal is "quasi-random" in nature, as demonstrated by the reproducible F-med means and unreliable v ariability. (C) 2000 Elsevier Science Ltd. All rights reserved.