Reliability and validity of isokinetic and isometric strength testing: a systematic review

Objective Isometric muscle strength testing is easier to apply, but isokinetic test results are deemed to offer a better transfer for training purposes. This systematic review aims to elucidate differences in terms of validity and reliability, if any, between isometric and isokinetic approaches. Material and Methods In January 2024, the PubMed database was searched for studies reporting reliability or validity measures for healthy adults for both isokinetic and isometric strength testing devices. Investigations of certain handheld types of dynamometers or publications older than 30 years were excluded. Results Thirteen studies met the inclusion criteria. Isometric maximal strength testing tended to demonstrate higher short-term reliability coefficients (ICC: 0.87-0.96) compared to isokinetic approaches (ICC: 0.69-0.99), while absolute measures for reproducibility were somehow similar for isometric (CV: 6.9-9.4%, SEM: 4.5-11.0%) and isokinetic testing (CV: 2.3-12.4%,SEM: 4.8-14.0%). Validity was confirmed for both isometric and isokinetic testing, but correlation coefficients varied markedly depending on the external criteria (EMG r>0.9, 5-10-RM r>0.8, Cross Sectional Area r>0.7, specific isokinetic comparisons r>0.5). Conclusions Confirming general validity for both, existing evidence suggests negligible advantages for isometric compared to isokinetic devices in terms of reproducibility measures. Future research should address comparisons seeking for side-effect properties of novel isometric and isokinetic devices such as accessibility of normative data for certain populations or time economy.