JMIR Publications

ABSTRACT

Background:

Degenerative loss of muscular strength, especially in the knee extensors and flexors, is a hallmark of aging that severely compromises functional independence and elevates fall risk among older adults. Precise quantification of muscular strength is therefore essential for effective diagnosis and intervention in geriatric rehabilitation. Although isokinetic dynamometry remains the reference standard for objective strength measurement, its cost, bulk, and operational complexity hinder its applicability in real-world clinical and community settings. Portable isometric testing devices offer a practical alternative, but rigorous validation studies are needed.

Objective:

This study aimed to establish the test-retest reliability and concurrent validity of the Leg Strength Analyzer (IB-LS), a compact isometric dynamometry device developed by InBody Inc., against a gold-standard isokinetic dynamometer (CSMI Humac Norm) in a sample of community-dwelling older adults.

Methods:

Twenty-one older adults (mean age: 65.10 ± 4.56 years; 11 men, 10 women) were recruited from senior community centers. Participants underwent standardized testing of bilateral knee extension and flexion strength using both the IB-LS and CSMI systems. The IB-LS protocol involved three maximal voluntary contractions per movement per limb, performed on the same day for test-retest assessment. A follow-up session was conducted 24 hours later for 17 participants, during which isokinetic strength testing at 60°/s was performed. Intraclass correlation coefficients (ICCs) were calculated to evaluate intra-device reliability and inter-device validity. Bland-Altman plots were used to assess agreement and bias across devices.

Results:

The IB-LS demonstrated excellent test-retest reliability, with ICCs ranging from 0.856 to 0.987 across all testing conditions. Concurrent validity analyses revealed strong correlations between IB-LS and CSMI-derived measurements: ICCs ranged from 0.826 to 0.946 for isometric comparisons and from 0.775 to 0.881 for isokinetic comparisons. Bland-Altman analyses indicated acceptable limits of agreement, with no evidence of systematic measurement bias. The mean difference in torque between devices ranged from 13.2 to 93.5 Nm for isometric data and 31.0 to 56.0 Nm for isokinetic comparisons.

Conclusions:

The findings substantiate the IB-LS as a reliable and valid instrument for assessing isometric knee strength in older adults. Its high agreement with standard dynamometry and user-friendly design position it as a feasible solution for strength assessment in clinical and community-based geriatric care. Clinical Trial: Not applicable.