Portable Fixed Frame Dynamometry Shows Poor Agreement with Isokinetic Dynamometry During Isometric Knee Extension and Flexion Strength Tests

Abstract Purpose This study aimed to evaluate the level of agreement between the KangaTech (KT360) portable fixed-frame dynamometer and the Cybex isokinetic dynamometer (IKD) in assessing maximal voluntary isometric contraction (MVIC) of the knee flexor and extensor muscles. Specifically, the research investigated whether the KT360 provides comparable measurements to the IKD when quantifying isometric knee muscle strength. Methods Twenty-three physically active participants (16 males, 7 females; age: 27 ± 7 years) completed MVIC testing of the knee flexors and extensors using both KT360 and IKD. Testing order was randomised, and standardised protocols were followed. Peak force (KT360) and peak joint moment (IKD, converted to force) were recorded. Agreement was assessed using paired t -tests, intraclass correlation coefficients ( ICC 3,1 ), standard error of measurement (SEM), minimum detectable change (MDC), and Bland–Altman analysis. Results KT360 recorded 24% lower peak force for extensors and 17%–21% higher for flexors compared to IKD ( P < 0.001). ICC values ranged from poor to excellent (− 0.093 to 0.964). Bland–Altman analysis revealed systematic bias: KT360 underestimated extensor strength (~ 27%) and overestimated flexor strength (~ 17%–19%), with wide limits of agreement and significant heteroscedasticity. Conclusion KT360 and IKD are not interchangeable for assessing isometric knee strength. While KT360 offers portability and accessibility, its results should be interpreted with caution, especially in contexts requiring precise strength quantification such as injury risk assessment and rehabilitation.