Wavelet analysis of electromyography for back muscle fatigue detection during isokinetic constant-torque exertions

Study Design. An investigation of the effects of human trunk extensor muscl e fatigue on the temporal change in frequency content of the electromyogram as quantified using the Fourier and wavelet transforms during the performa nce of repetitive dynamic trunk extension. Objectives. To evaluate whether alterations in the Fourier and wavelet tran sform measures were consistent with a shift of the signal power to lower fr equencies, and to determine which measures were more highly correlated with the decline in maximal trunk extension torque. Summary of Background Data. Objective assessment of trunk muscle fatigue is likely to play a more important role in the rehabilitation and prevention of low back injuries, given the association between lack of trunk muscle en durance and acquisition of low back pain. Validation of new methods designe d to quantify the level of fatigue using the surface electromyogram is nece ssary before these techniques can be used in industrial rehabilitation sett ings. The wavelet transform is a recent development in the signal processin g of electromyograms that shows promise as a method for assessment of fatig ue. Methods. Trunk muscle electromyograms obtained from study participants perf orming repetitive isokinetic trunk extension endurance tests were analyzed using the wavelet and the traditional Fourier methods. Trunk extension torq ue was controlled at 35% and 70% of the participants' maximal voluntary con traction while they exerted at 5 and 10 repetitions per minute. The decline in maximal trunk extension torque was measured once per minute. Linear reg ression quantified the rate of change in Fourier and wavelet measures cause d by fatigue, whereas Pearson's correlation coefficient determined their as sociation with the decline in maximum torque. Results. Changes in the characteristics of the electromyogram were consiste nt with a shift to lower frequencies: The signal power at higher frequencie s was reduced, whereas the power at lower frequencies was elevated. The amo unt of change was dependent on the task conditions (exertion level and repe tition rate). The wavelet-based measures demonstrated as strong an associat ion with the decline in maximal torque output as the Fourier-based measures . Conclusions. This study demonstrates:that:, assessment of trunk muscle fati gue during isokinetic movementis possible using both Fourier and wavelet me asures. However, the methods were not as likely to change significantly dur ing lower rates of exertion. These methods, when implemented in a controlle d setting, may be used to document the rehabilitation process and guide pre ventive exercise training.