Intraobserver and interobserver reliability for the strength test in the Constant-Murley shoulder assessment

This study evaluates the standardized strength test in the Constant-Murley shoulder assessment of adults with healthy shoulders in a randomized, single-blind design. The following questions were to be answered: (1) Can the spring balance and a digital dynamometer both yield the same result? (2) What is the intraobserver and interobserver reliability of the strength test? (3) Is the strength test sensitive to change in technique or affected by calculation with mean or maximum values? Ten subjects were included in a comparison of the Handyscale (digital dynamometer) and the mechanical spring balance for concurrent validity, resulting in intraclass correlation coefficient values ranging from 0.96 to 0.99. For intraobserver and interobserver reliability, 2 observers tested 20 subjects with the Handyscale and retested them after 2 weeks. Regardless of technique during testing, this resulted in almost perfect agreement (intraclass correlation coefficient range, 0.89–0.98). The digital dynamometer can replace the conventional spring balance. The standardized strength test in the Constant-Murley shoulder assessment is reliable in young subjects with healthy shoulders, independent of technique or whether calculated with mean or maximum values.