Reliability of Sternocleidomastoid Muscle Stiffness Assessment Using Shear-Wave Elastography Under a Standardized Protocol with Novice and Experienced Examiners: An Intra- and Inter-Examiner Reliability Study

<i>Background and Objectives</i>: Sternocleidomastoid (SCM) dysfunction is commonly implicated in several musculoskeletal conditions. Accordingly, shear-wave elastography has been used to characterize SCM stiffness in asymptomatic and clinical cohorts. However, the only reproducibility study available reported limited reliability, so clinical interpretations should be made with caution. Therefore, this study revisits key methodological aspects of that protocol to assess intra-examiner reliability and includes two examiners with different levels of expertise to evaluate inter-examiner reliability. <i>Materials and Methods</i>: A longitudinal observational study was conducted, recruiting twenty-five asymptomatic participants. Two examiners with different experience levels participated in this study after following structured training. For each side, images were obtained in immediate succession in the sequence experienced-novice-experienced-novice (with side order randomized), using an ROI spanning full muscle thickness, stabilizing approximately 10 s before freezing to record Young's modulus and shear-wave speed. <i>Results</i>: Inter-examiner agreement was good-excellent: single-measurement ICCs were 0.77-0.86, improving to 0.79-0.87 when averaging two trials, which also reduced the standard error of measurement (SEM) and minimal detectable changes (MDCs). Between-examiner mean differences were small and nonsignificant (<i>p</i> ≥ 0.068). Intra-examiner reliability was excellent (ICC ≈ 0.93-0.94) with small absolute errors. Precision was high (SEM ~5-6 kPa; 0.22 m/s), and MDCs were ~15-16 kPa and ~0.60 m/s, with no trial-to-trial bias (all <i>p</i> ≥ 0.311). <i>Conclusions</i>: The revised protocol showed excellent intra-examiner repeatability and good-excellent inter-examiner reliability with minimal bias. Averaging two acquisitions improved precision, while a single operator optimized longitudinal stability.