Sex differences in myoelectric manifestations of shoulder fatigue in time-frequency and complexity domains during a...

Previous studies have shown sex differences in muscle activation amplitude and frequency spectrum during fatiguing shoulder-height tasks. However, time-frequency and complexity analyses may reveal new features of the shoulder's injury exposure with fatigue, but have never been compared between sexes. Fifty-five (29 females) asymptomatic young adults performed a repetitive, forward-backward shoulder height task until scoring 8/10 on a Borg CR10 scale while surface electromyography (EMG) of upper trapezius, anterior deltoid, biceps brachii, and triceps brachii was recorded. Findings show that 1) all muscles displayed decreased sample entropy and mean frequency with fatigue; 2) fatigue led to decreased major frequency (i.e., frequency with the highest power on the wavelet scalogram) of anterior deltoid, biceps brachii, and triceps but not upper trapezius; 3) females' biceps brachii mean frequency and complexity were significantly higher than males'; all other frequency components were similar between sexes; 4) females showed significantly lower complexity of anterior deltoid. Results suggest that entropy and continuous wavelet transform analyses are capable of revealing new effects of sex and fatigue in myoelectric manifestations of repetitive shoulder tasks. This information may be useful towards modeling shoulder joint loads and predicting sex-specific shoulder injury risks in future applications.