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

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.