Can You Trust Your Strength Assessment? Evaluating the Reliability and Validity of Handheld and Externally Fixated Dynamometers for Measuring Knee and Hip Strength.

BACKGROUND: The gold standard for measuring hip and knee strength is isokinetic dynamometry. This study evaluated the reliability and concurrent validity of a handheld dynamometer (HHD) and a externally fixated dynamometer (ForceFrame [FF]) against isokinetic dynamometry, considering their practicality, cost-effectiveness, and ease of use in clinical practice. HYPOTHESIS: The FF and HHD are reliable and valid for assessing hip and knee strength. STUDY DESIGN: Controlled laboratory study. LEVEL OF EVIDENCE: Level 4. METHODS: Maximal isometric hip strength (extension, flexion, abduction, adduction, internal and external rotation) and knee strength (extension and flexion) of 14 healthy persons (7 men; age, 24.4 ± 3.1 years; height, 176.3 ± 9.7 cm; weight, 68.5 ± 10.8 kg) was assessed with an isokinetic dynamometer, HHD, and FF. Measurements were repeated on 2 different days for test-retest reliability and by 2 different investigators on 1 day for interrater reliability. Intraclass correlations (ICC) were calculated, and Pearson correlation coefficients assessed concurrent validity. RESULTS: Moderate to good test-retest reliability was found for both the HHD (ICC, 0.62-0.88; MDC, 10-21%) and FF (ICC, 0.61-0.82; MDC, 15-38%). Interrater reliability was poor to good (HHD ICC, 0.15-0.80; FF ICC, 0.14-0.77). Concurrent validity was better for the HHD than FF. CONCLUSION: Both the HHD and FF provide stable measurements of hip and knee strength over a 1-week interval. The calculated MDCs (HHD, 10-21%; FF, 15-38%) suggest that only differences exceeding these thresholds can be interpreted as true changes, rather than measurement errors. Inter-rater reliability was rather low, indicating that repeated evaluations are best performed by the same tester. Poor concurrent validity cautions against substituting the HHD and FF for isokinetic dynamometry. CLINICAL RELEVANCE: HHD and FF offer practical and cost-effective alternatives for a single tester to evaluate strength changes over time.