Reliability and Validity of a Low-Cost and Portable Dynamometer: Toward a Simple Clinical Test for Assessing Plantar...

Plantar flexor weakness is common in orthopedic and neurological disorders and requires reliable strength assessment tools. This study evaluates the validity and reliability of a portable strain gauge dynamometer compared with a gold-standard instrumented dynamometer. Thirty-five healthy adults were divided into 2 groups: one for validation against the reference device and another for test-retest reliability assessment. Pearson correlations, Bland-Altman analyses, and intraclass correlation coefficients were performed on plantar flexor maximal force and torque data. A strong correlation (r = .84, 95% CI: .59-.94, P < .001) confirmed the portable strain gauge dynamometer's validity, with a slight torque underestimation (-4.6% [20.5%]). Bland-Altman analysis indicated a mean bias of -4.34 N·m with 95% limits of agreement ranging from -27.9 to 19.2 N·m. Reliability was excellent across sessions and raters (intraclass correlation coefficients = .889-.926; standard error of measurement = 43.2-55.6 N·m). Absolute reliability analyses suggested that only changes above ∼120 to 155 N exceeded measurement error. These results support that the strain gauge dynamometer, fixed to a stable surface, is a valid and reliable tool for assessing plantar flexor strength in clinical settings.