Using EMG Amplitude and Frequency to Calculate a Multimuscle Fatigue Score and Evaluate Global Shoulder Fatigue

The authors developed a function to quantify fatigue in multiple shoulder muscles by generating a single score using relative changes in EMG amplitude and frequency over time. Evaluating both frequency and amplitude components of the electromyographic signal provides a more complete evaluation of muscle fatigue than either variable alone; however, little effort has been made to combine time and frequency domains for the evaluation of myoelectric fatigue. Surface EMG was measured from 14 shoulder muscles while participants performed simulated, repetitive work tasks until exhaustion. Each 60-s work cycle consisted of four tasks (dynamic push, dynamic pull, static drill, static force target matching task) scaled to participants’ anthropometrics and strength. The function was generated to calculate a multimuscle fatigue score (MMFS) based on changes in EMG frequency, amplitude, and the number of muscles showing signs of myoelectric fatigue (increase in EMG amplitude; decrease in EMG frequency). The function was evaluated through changes in MMFS over time: first (31.8 ± 14.6), middle (47.6 ± 25.3), last (58.6 ± 35.5) reference exertions (p < .05). The evaluation of the relationships between MMFS and changes in strength (r = ？0.510) and MMFS and perceived fatigue (RPF) (r = 0.298) showed significant relationships over time (p < .05). MMFS scores increased over time (p < .05) with significant relationships between MMFS and strength changes and RPF (p < .05). The MMFS allows for comparisons between workplace tasks, which can aid in workplace design to mitigate the development of fatigue