Relative and Absolute Reliability of Muscle Thickness Measurements Using a Smart Device-Connected Ultrasound Device

Objective: This study aimed to determine the relative and absolute reliability of muscle thickness measurements obtained using a smart device-integrated portable ultrasound system. Method: Thirty healthy adults participated in this study. Muscle thickness of the rectus abdominis and rectus femoris was measured. Two independent examiners performed repeated measurements on Day 1 to assess intra-rater and inter-rater reliability. One examiner repeated the measurements after 7 days to evaluate test-retest reliability. Relative reliability was assessed using intraclass correlation coefficients (ICC), while absolute reliability was evaluated using the standard error of measurement(SEM), minimal detectable change (MDC95), coefficient of variation (CV), and Bland-Altman analysis. Results: Intra-rater, inter-rater, and test-retest reliability demonstrated good to excellent agreement (ICC>.80). SEM and MDC95 values were within acceptable ranges, and no significant systematic bias was observed. Conclusion: The smart device-integrated portable ultrasound system demonstrated good relative and absolute reliability for muscle thickness assessment in healthy adults, supporting its potential utility in clinical and remote rehabilitation settings.