Concurrent Validity and Within-Session Reliability of a Wireless Surface Electromyography Device (MR EMG) Compared to a Criterion Measure During the Leg Extension Exercise

Purpose: The aim of this study was to assess within-session reliability and concurrent validity of a wireless surface electromyography device (MR EMG) relative to a wireless criterion system (Delsys Trigno) during dynamic leg extension at varying loading intensities. Methods: Thirty-one resistance-trained adults performed four repetitions at 30%, 60%, and 90% of their predicted one-repetition maximum (1RM) on the plate-loaded leg extension machine, with EMG amplitude recorded simultaneously by both systems for the vastus lateralis (VL) and vastus medialis obliquus (VMO). Root mean square (RMS) amplitudes were normalized to maximal voluntary isometric contraction and compared across devices using intraclass correlation coefficients (ICC), coefficients of variation (CV%), standard error of measurement, Pearson correlations, and Bland–Altman analyses. Results: Both devices demonstrated excellent within-session reliability (ICC = 0.92–0.97; CV = 7.8–11.3%). Validity differed by muscle. Across all loads, VMO agreement was good (ICC = 0.82–0.85; r = 0.83–0.86), while VL agreement was moderate (ICC = 0.53–0.57; r = 0.54–0.64). Bland–Altman analyses revealed systematic positive bias across all conditions, with proportional bias present for the VL but absent for the VMO. Conclusions: MR EMG demonstrated excellent within-session reliability but muscle-dependent concurrent validity, with good VMO agreement and moderate VL agreement with the criterion system.