Test–Retest Reliability of a Portable Surface Electromyography Device During the 30-s Sit-to-Stand Test in Patients with Chronic Kidney Disease

Chronic kidney disease (CKD) is a progressive systemic condition frequently associated with sarcopenia, leading to reduced muscle mass, strength, and functional capacity. Reliable and objective assessment tools are needed to monitor neuromuscular status and support rehabilitation strategies. This study aimed to evaluate the test–retest reliability of a portable surface electromyography (sEMG) device (mDurance) during the 30-s Sit-to-Stand (30-STS) test in patients with CKD. An observational cross-sectional study was conducted in 26 participants (80.8% male; mean age 50.35 ± 12.40 years). Muscle activation was quantified using the Root Mean Square (RMS) amplitude of the rectus femoris during a single-session test–retest protocol with a 20-min recovery interval. Reliability was assessed using the Intraclass Correlation Coefficient (ICC), Standard Error of Measurement (SEM), Minimal Detectable Change (MDC95), and Bland–Altman agreement analysis. The ICC was 0.828 (95% CI: 0.592–0.926) for single measures and 0.906 (95% CI: 0.744–0.961) for average measures, indicating good-to-excellent reliability. The SEM, calculated from the standard deviation of the paired differences on the original RMS scale, was 35.82 μV, and the corresponding MDC95 was 99.30 μV. Bland–Altman analysis showed a mean bias of −24.73 μV, indicating higher RMS amplitudes during Test 2, potentially reflecting familiarization or motor adaptation, although these mechanisms were not directly investigated. Internal consistency was high (Cronbach’s alpha = 0.924). Portable sEMG demonstrated good-to-excellent reliability for assessing quadriceps activation during the 30-STS test in CKD patients and may serve as a complementary tool for within-session neuromuscular assessment in renal rehabilitation settings.