The validity of a portable strain-gauge apparatus versus a commercial isokinetic dynamometer for evaluating knee extension kinetics

Purpose: Isokinetic dynamometers are the gold standard for assessing joint and muscle-specific kinetics. However, commercial dynamometers are often prohibitively expensive and lack portability. Therefore, we compared kinetic data captured via an affordable and portable strain-gauge with a treatment plinth against a isokinetic dynamometer during maximal isometric knee extensions. Methods Healthy participants (8 males and 6 females; age 30.2 ± 7.1 years) volunteered and performed knee extensions at a 90° knee angle on a dynamometer and a treatment plinth with a portable strain-gauge. Peak force (PF), peak rate of force development (PRFD), rate of force development (RFD 2080 ) and impulse (IMP 2080 ) from 20–80% of onset to peak force were assessed. Results No meaningful differences were identified between isokinetic and strain-gauge devices (all p  ≥ 0.268, ES ≤ 0.35). However, greater (2.5–9.5%) outputs were observed with the isokinetic dynamometer. Very large between-device correlations were found for PF ( r  = 0.77, p  = 0.001) and PRFD ( r  = 0.73, p  = 0.003), while small and moderate between-device correlations were found for RFD 2080 ( r  = 0.48, p  = 0.079) and IMP 2080 ( r  = 0.59, p  = 0.060). Bland-Altman plots did not reveal apparent biases from high to low performers. Conclusions The examined strain-gauge device produces valid maximal and rapid force expression measurements. Similar results can be obtained without extreme rigour and constraint, such as those quantified via an isokinetic device. This study's overall findings support the use of the practically relevant treatment plinth and strain-gauge combination as a suitable alternative to the isokinetic dynamometry for the measurement of PF and PRFD. Therefore, more rehabilitation and sports performance practitioners can confidently assess knee extension kinetics.