Reliability and validity of surface electromyography for identifying contractile muscle fatigue during cycling in people with COPD

Surface electromyography (sEMG) may provide reliable biomarkers of contractile muscle fatigue (CMF) during exercise in COPD. This study aimed to: (1) determine the test-retest reliability of amplitude- and frequency-based sEMG metrics recorded from the quadriceps muscle during a constant-work-rate cycling test (CWRT); (2) assess their criterion validity for detecting CMF against potentiated twitch force (TWp); and (3) explore associations between sEMG metrics and physiological outcomes. Adults with COPD referred for pulmonary rehabilitation performed two CWRTs to the limit of tolerance ( T _lim ) at 75-80% of the individual peak workload, with physiological responses recorded. CMF was assessed by: (1) amplitude- (root mean square, RMS) and frequency-based sEMG metrics recorded from the right quadriceps, analyzed per pedal-stroke and expressed as Z-score changes at T _lim relative to a fatigue-free baseline; and (2) the post-exercise change in TWp of the right quadriceps. Relative and absolute test-retest reliability, and criterion validity of sEMG against TWp were assessed. Twenty-five participants completed the assessments. Among the sEMG metrics, only vastus lateralis RMS showed adequate relative reliability [intraclass correlation coefficient (95% CI) = 0.79 (0.74–0.85)] but a non-negligible standard error of measurement of 0.58. Moreover, vastus lateralis RMS showed 86% sensitivity and specificity versus TWp in identifying CMF. An increase of ≥0.65 Z-scores identified the threshold for sEMG-based CMF. Changes in vastus lateralis RMS were moderately correlated with physiological responses at T _lim (p < 0.05). In conclusion, vastus lateralis RMS derived from sEMG can be used as a biomarker of CMF during cycling in individuals with COPD.