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

Abstract

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 (IMDF slope and MDF slope) and intercepts (IMDF init and MDF init) 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 IMDF slope and MDF slope parameters indicated ICCs for back and hip muscles between .443 and .727 for IMDF slope, values between .273 and .734 for MDF slope, % SEM between 7.6% and 58.9% for IMDF slope and % SEM between 8.2% and 25.3% for MDF slope, respectively. The ICCs for IMDF init and MDF init parameters varied between .376 and .907 for IMDF init and between .383 and .883 for MDF init, and % SEM ranged from 2.7% to 6.3% for IMDF init and from 2.6% to 4.7% for MDF init, 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.