Reliability of flywheel hip extension and leg curl tests and their associations with isokinetic hamstring strength

Flywheel (FW) resistance training is used to develop and monitor hamstring strength in team-sport athletes. The purpose of this study was to assess the between-session reliability of flywheel hip extension (FWHE) and flywheel leg curl (FWLC) tests and to examine their associations with isokinetic knee flexor strength. Twenty-five physically active male and female participants with prior experience in strength training and flywheel exercise completed testing across two sessions. Concentric and eccentric mean power were obtained during FWHE and FWLC tests, while concentric and eccentric knee flexor torque were assessed using isokinetic dynamometry. Test-retest reliability was evaluated using intraclass correlation coefficients (ICC), and convergent validity was examined using Pearson correlation coefficients. Isokinetic knee flexor torque demonstrated excellent reliability in both concentric and eccentric modes (ICC > 0.90). The FWHE test showed questionable between-session reliability (ICC > 0.60) and demonstrated moderate to large associations with isokinetic knee flexor torque in both contraction modes. The FWLC test exhibited greater between-session variability, with significant improvements in concentric and eccentric power. Despite this variability, FWLC outcomes demonstrated moderate to large correlations with isokinetic knee flexor torque. In conclusion, the FWHE test demonstrated questionable reliability and convergent validity with isokinetic knee flexor strength, suggesting its potential use as a complementary assessment of hamstring strength. Although the FWHE and FWLC tests showed good convergent validity, their greater between-session variability and, in the case of the FWLC, a learning effect, suggest that these tests should be used with caution and sufficient familiarization is required before they can be used confidently for assessment.