Between-session reliability and measurement error of a commercial surface electromyography system across isometric and dynamic exercise conditions in resistance-trained adults

Introduction: Commercial surface electromyography (sEMG) systems are increasingly used to assess muscle activity, but their between-session reliability and measurement error require evaluation. Aim To evaluate the within-examiner, between-session reliability and measurement error of the MyoAct sEMG system during selected isometric and dynamic exercise conditions in resistance-trained adults. Methods Fourteen healthy adults (10 males) completed two test sessions seven days apart. sEMG was recorded from the vastus medialis obliquus (VMO) during knee extension and infraspinatus (IS) during shoulder external rotation. Participants performed maximal voluntary isometric contractions and three dynamic sets at 50%, 70% and 85% of estimated one-repetition maximum. Reliability was assessed using intraclass correlation coefficients (ICC), standard errors of measurement (SEM) and Bland–Altman analysis. Results Reliability varied by muscle and exercise condition. VMO reliability was good during maximal isometric contractions and dynamic contractions at 50% and 70% of one-repetition maximum, and moderate at 85%; all IS measurements showed poor reliability. SEM ranged from 14.2% to 16.2% of the grand mean for VMO and from 18.3% to 24.0% for IS. Limits of agreement were wide for both muscle–exercise conditions. VMO retest values were significantly higher at 50% and 70%; no other between-session differences were detected. Conclusion MyoAct reliability was muscle- and exercise-specific. Small between-session changes should be interpreted cautiously, particularly for IS and conditions with substantial measurement error. Findings are limited to within-examiner repeated assessments in resistance-trained adults and should not be generalised to patients, other examiners or other sEMG systems.