Objective Reliable quantification of deep-cervical muscle stiffness is clinically relevant, yet the reproducibility of shear-wave elastography (SWE) for the obliquus capitis inferior (OCI) has not been established. This study aimed to describe a standardized SWE procedure for measuring OCI stiffness and to determine its intra- and inter-examiner reliability in patients with chronic mechanical neck pain. Materials and methods We conducted a longitudinal reliability study in 17 adults with neck pain ≥6 months (mean age: 19.7 years; mean symptoms duration: 13.4 months). Seventeen participants underwent bilateral OCI imaging by two examiners with different ultrasound experience using fixed scanner presets. For each acquisition, a longitudinal region of interest encompassing the full muscle thickness was analyzed to obtain shear-wave velocity (SWV) and shear modulus (SM). Intraclass correlation coefficients (ICCs), standard error of measurement (SEM), and minimal detectable change (MDC) were calculated. Results Inter-examiner reliability was good-to-excellent (single measurements ICC3,2 = 0.897-0.938; mean of two measurements ICC3,2 = 0.951-0.968), with negligible between-examiner bias and lower SEM/MDC when two acquisitions were averaged. Test-retest reliability was excellent for both examiners (ICC3,1 = 0.925-0.947), with small SEM/MDC relative to the stiffness magnitude. Conclusions A rigorously standardized long-axis SWE protocol provides highly reproducible OCI stiffness measurements in young adults with mechanical neck pain. For longitudinal monitoring, assessments should preferentially be performed by the same examiner, and in multi-operator settings, decisions should be based on the mean of at least two acquisitions per side to improve sensitivity to true change.
