Reliability and Validity of Portable Muscle Measurement Device for the Assessment of Isometric Muscle Strength in the Knee of Healthy Adults

Objective: This study aimed to verify the reliability and validity of the portable knee strength measurement device IB-LS in assessing isometric knee strength in healthy adults by comparing its performance with a criterion standard dynamometer (CSMI) and exploring its potential applicability in clinical and community settings. Design: A cross-sectional methodological study employing repeated measures for test-retest reliability and criterion-referenced validation against a standard isokinetic dynamometer was conducted. Methods: Twenty-nine healthy adults in their 20s and 30s participated. Isometric knee flexion and extension strength and isokinetic strength at 60°/s were measured using both the IB-LS and CSMI devices. Test-retest reliability was evaluated using intraclass correlation coefficients (ICC), and the validity between devices was assessed using Pearson correlation analysis. Results: The IB-LS demonstrated excellent test-retest reliability (ICC＝0.962－0.986). The concurrent validity between the IB-LS and CSMI for isometric strength was high (ICC＝0.799－0.895), and the correlation between the IB-LS isometric strength and CSMI isokinetic strength at 60°/s was also strong (ICC＝0.809－0.876). Conclusions: The IB-LS is a practical, reliable, and valid tool for evaluating isometric knee strength in healthy adults. Further large-scale studies, including diverse populations and long-term follow-ups, are warranted to confirm its broader clinical applicability. These findings highlight the potential of IB-LS as a convenient alternative to laboratory-based dynamometry, which may facilitate broader applications in clinical rehabilitation and field-based assessments.