Development and performance verification of an isometric dynamometer for lower extremity

Lower limb isometric strength is crucial for predicting diseases, monitoring rehabilitation, and assessing activity levels. Manual testing lacks quantitative evaluation, while handheld dynamometers (HHDs) require skilled raters and isokinetic dynamometers are expensive and complex. Existing devices often focus on single-joint measurements for specific populations. To address the need for multi-joint quantitative muscle strength assessment, along with portability, affordability, and ease of use, this study developed the isometric dynamometer for the lower extremity (IDLE) to measure hip flexion, knee extension, knee flexion, and ankle dorsiflexion strength. Its validity and reliability were evaluated in 20 healthy adults (50% female). The IDLE demonstrated excellent validity compared to a strap-fixed HHD (Pearson's r ≥ 0.907, ICC ≥ 0.908, P < 0.01). Intra-rater reliability was excellent (ICC ≥ 0.926) for male knee extension (bilateral), left knee flexion, and right ankle dorsiflexion; as well as for female right hip flexion, knee extension (bilateral), and right knee flexion, and good (ICC ≥ 0.808) for other measurements. Inter-rater reliability was excellent (ICC ≥ 0.901) for all measurements except male right ankle dorsiflexion. The IDLE is a valid and reliable device for measuring lower extremity isometric strength in healthy adults, with further validation in clinical populations recommended.