Validity and Reliability of a Low-Cost and Portable Option for Measuring Peak and Explosive Isometric Quadriceps and Hamstrings Strength

Abstract Purpose Dynamometers are commonly used to assess muscle force properties, with isokinetic dynamometers (IKD) considered the gold standard. However, their high cost and limited portability have led to the development of alternative devices. In this study, we investigated validity and reliability of a tension dynamometer (Tindeq) compared to an IKD and how different methods of determining peak torque and rate of torque development (RTD) influence measurement properties. Methods Twenty-eight healthy university students (23.14 ± 4.06 years) completed isometric knee extension (KE) and flexion (KF) testing. Validity and reliability of peak torque and RTD were assessed using ICC , minimal detectable change (MDC), correlation (R), and Bland Altman plots across three methods: average of three trials, highest of three trials, and a single trial. Results Peak torque demonstrated good validity ( ICC = 0.80–0.88) across all methods, only KF demonstrating minimal bias (< 2% of mean). Respectively, excellent and good reliability for peak torque was observed for KE ( ICC = 0.94–0.95, MDC = 24.90%–26.81%) and KF ( ICC = 0.80–0.82, MDC = 35.08%–35.79%). Most Tindeq RTD measures demonstrated poor-moderate validity and systematically underestimated RTD (bias: 9.6%–67.5% of mean), only KF average and highest methods RTD20%–80% performed good ( ICC = 0.80–0.84). RTD20%–80% methods performed moderately ( ICC = 0.70–0.75, MDC = 69.23%–91.60%), except for the first trial method which performed poorly ( ICC = 0.25–0.48, MDC = 112.44%–179.17%). Peak RTD methods demonstrated moderate-good reliability but with large MDC ( ICC = 0.73–0.88, MDC = 49.06%–75.63%). Conclusion Tindeq demonstrates strong validity and reliability for peak torque assessment, supporting it as a portable, low-cost alternative for knee strength testing. Tindeq RTD measurement properties were less consistent, likely stemming from differences in sampling rate and processing differences.