Reliability of electromyographic power spectral analysis of back muscle endurance in healthy subjects

Objective and Design: To investigate the reliability (within-day and between-days) of measurements of electromyographic (EMG) power spectral values in measurement of the fatigue rate of the back muscles. Methods: Twelve healthy male subjects were tested in the unsupported trunk holding position for 60 seconds. Two trials were performed on each of two separate sessions 3 days apart. Surface recording electrodes were placed over the iliocostalis lumborum and multifidus and a branched electrode technique was used to decrease cross-talk. Outcome Measures: Median frequency (MF) was extracted from the EMG signals by fast Fourier transform. Initial MF and the MF slope over time were computed from linear regression analysis. The reliability of the initial MF and the MF slope of the iliocostalis lumborum and multifidus was examined by Pearson's product moment correlation coefficients (Pearson's r), paired t tests, and a one-way analysis of variance (ANOVA) from which intrasubject coefficients of variation (CV(intra)) and intraclass correlation coefficients (ICC) were derived. Results: For the initial MF, within-day and between-days reliability of the iliocostalis lumborum and multifidus were good (Pearson's r = .79-.94 nonsignificant paired t test, CV(intra) = 6.5% to 8.5%, ICC = .79-.93). The MF slope showed moderate variability for the iliocostalis lumborum (Pearson's r = .39-.55, nonsignificant paired t test, CV(intra) = 33.0% to 48.7%, ICC = .37-.56) while better reliability was found for the multifidus (Pearson's r = .77-.87, nonsignificant paired t test, CV(intra) = 25.8% to 27.5%, ICC = .78- .82). Conclusion: The present study indicated that the trunk holding test with the use of EMG power spectral analysis can be a reliable method to measure the fatigue rate of the back muscles if adequate measures are employed to minimize cross-talk. The better reliability of monitoring the fatigue rate of the multifidus may lead to its future use as a clinical measure.