Test–retest reliability of wavelet – and Fourier based EMG (instantaneous) median frequencies in the evaluation of back and hip muscle fatigue during isometric back extensions

The present study aimed at assessing the test–retest reliability of wavelet – and Fourier derived (instantaneous) median frequencies of surface electromyographic (EMG) measurements of back and hip muscles during isometric back extensions. Twenty healthy subjects (10 males and 10 females) performed a modified Biering-Sørensen test on two separate days, with a 1-week interval between the two tests. Surface EMG measurements were bilaterally performed from the latissimus dorsi, the thoracic and lumbar parts of the longissimus thoracis, the thoracic and lumbar parts of the iliocostalis lumborum, the multifidus, the gluteus maximus and the biceps femoris. In addition, three-dimensional kinematic data were recorded of the subjects’ lumbar vertebrae. The (instantaneous) median frequencies were calculated from the EMG signals using continuous wavelet (IMDF) – and short-time Fourier transforms (MDF). Linear regressions performed on the IMDF and MDF data as a function of time yielded slopes (IMDFslope and MDFslope) and intercepts (IMDFinit and MDFinit) of the regression lines. Test–retest reliability was assessed on the normalized slopes and intercept parameters by means of intraclass correlation coefficients (ICC) and standard errors of measurements expressed as percentages of the mean values (% SEM). The results of IMDFslope and MDFslope parameters indicated ICCs for back and hip muscles between .443 and .727 for IMDFslope, values between .273 and .734 for MDFslope, % SEM between 7.6% and 58.9% for IMDFslope and % SEM between 8.2% and 25.3% for MDFslope, respectively. The ICCs for IMDFinit and MDFinit parameters varied between .376 and .907 for IMDFinit and between .383 and .883 for MDFinit, and % SEM ranged from 2.7% to 6.3% for IMDFinit and from 2.6% to 4.7% for MDFinit, respectively. These results indicate that both wavelet – and Fourier based (instantaneous) median frequency parameters generally are reliable in the analysis of back and hip muscle fatigue during a modified Biering-Sørensen test.