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

OBJECTIVE: 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. BACKGROUND: 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. METHOD: 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). RESULTS: 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). CONCLUSION AND APPLICATION: The MMFS allows for comparisons between workplace tasks, which can aid in workplace design to mitigate the development of fatigue.