Reliability and Validity of a Crane Scale for Isometric Knee and Shoulder Strength Assessment.

Background: Due to cost barriers and/or space constraints, electromechanical and hand-held dynamometry are underutilized in rehabilitation settings. As a result, clinicians often rely on semi-quantitative methods that may fail to detect deficits in upper or lower extremity strength. The purpose of this study was to examine the between-trials test-retest reliability and concurrent criterion validity of an accessible crane scale for measuring isometric knee and shoulder strength.

Study design: Observational cohort, Cross-sectional study.

Methods: Twenty healthy, recreationally active adults underwent isometric knee and shoulder strength testing using a crane scale, electromechanical dynamometer, and hand-held dynamometer during a single session. Knee extension and flexion, and shoulder internal rotation, external rotation, and abduction were tested. Three maximal effort trials were performed per device and motion, with the order of device testing randomized. Between-trials test-retest reliability of the crane scale was assessed using intraclass correlation coefficients (ICCs). Concurrent criterion validity was examined using ICCs, absolute and relative error, Bland-Altman plots, and simple linear regression.

Results: Test-retest reliability of the crane scale was excellent for all motions (ICCs ≥ 0.90). Concurrent validity was poor to moderate with the electromechanical dynamometer (ICCs = 0.36-0.72) but excellent with the hand-held dynamometer (ICCs ≥ 0.90). There was a negative bias for the crane scale compared to alternative devices and evidence of a proportional bias for knee extension and shoulder abduction.

Conclusions: A crane scale provides values similar to hand-held dynamometry but is not comparable to results from electromechanical dynamometry. The lack of concurrent validity between the crane scale and electromechanical dynamometry may be partially attributable to differences in stabilization and participant positioning.

Level of evidence: 3b, reliablity and validity study.