Intrarater Reliability and Criterion Validity of Ultrasound Measurements of Supraspinatus and Infraspinatus Cross-Sectional Area Compared With MRI

CONTEXT: Diagnostic ultrasound (US) provides a cost-effective, accessible alternative to the gold standard magnetic resonance imaging (MRI) for assessing muscle cross-sectional area (CSA). Since muscle CSA correlates directly with force production, accurate evaluation of rotator cuff CSA is essential for assessing shoulder strength and function. The present study investigated the same-day and between-day intrarater reliability of US measurements of supraspinatus and infraspinatus CSA, as well as the criterion validity of these measures compared with MRI. DESIGN: Cross-sectional study evaluating the intrarater reliability and criterion validity of US measures of supraspinatus and infraspinatus CSA. METHODS: Thirty-two right-hand dominant adults with unilateral shoulder pain (mean age: 43 [13] y; 22 males) were enrolled including 12 participants who underwent both US and MRI. CSA of the supraspinatus and infraspinatus was measured using a standardized US protocol. Same-day intrarater reliability was assessed in all participants, and between-day reliability was evaluated in 14 participants who returned for a second US session. Criterion validity was tested by comparing US measurements to corollary MRI CSA values in the MRI subgroup. RESULTS: US demonstrated excellent same-day intrarater reliability (intraclass correlation coefficients [ICCs] .97-.99, standard error of measurements 0.36-0.52 cm2) and excellent between-day (supraspinatus ICC: .87-.95, infraspinatus ICC: .92-.94; standard error of measurements 0.52-1.13 cm2). Criterion validity compared with MRI was also excellent (supraspinatus ICC: .97, infraspinatus ICC: .99) with small measurement differences (mean difference -0.29 to 0.41 cm2). CONCLUSION: US CSA measures of the supraspinatus and infraspinatus muscles demonstrated excellent criterion validity and same-day and between-day intrarater reliability. These findings support the use of US as a practical, accessible tool for monitoring rotator cuff muscle size and detecting clinically meaningful changes over time.