The flywheel hip extension test demonstrated questionable reliability and convergent validity with isokinetic knee flexor strength, while the flywheel leg curl test exhibited greater between-session variability with a learning effect, suggesting both tests require sufficient familiarization before use in assessment.
Key Findings
Results
Isokinetic knee flexor torque demonstrated excellent between-session reliability in both concentric and eccentric modes.
ICC values exceeded 0.90 for isokinetic knee flexor torque in both contraction modes
Twenty-five physically active male and female participants with prior experience in strength training and flywheel exercise were tested across two sessions
Isokinetic dynamometry was used to assess concentric and eccentric knee flexor torque
Test-retest reliability was evaluated using intraclass correlation coefficients (ICC)
Results
The flywheel hip extension (FWHE) test showed questionable between-session reliability.
ICC values for FWHE were greater than 0.60, categorized as 'questionable' reliability
Both concentric and eccentric mean power were obtained during the FWHE test
The reliability of FWHE was notably lower than isokinetic dynamometry (ICC > 0.90)
Between-session variability was present but less pronounced than in the FWLC test
Results
The flywheel leg curl (FWLC) test exhibited greater between-session variability and a significant learning effect.
Significant improvements in both concentric and eccentric power were observed between sessions for FWLC
The learning effect indicates that performance on the FWLC test improved from session one to session two
The FWLC showed greater between-session variability compared to the FWHE test
Sufficient familiarization was identified as a prerequisite before FWLC can be used confidently for assessment
Results
Both flywheel tests demonstrated moderate to large associations with isokinetic knee flexor torque in both concentric and eccentric modes.
FWHE demonstrated 'moderate to large associations with isokinetic knee flexor torque in both contraction modes'
FWLC outcomes also demonstrated 'moderate to large correlations with isokinetic knee flexor torque' despite greater variability
Convergent validity was examined using Pearson correlation coefficients
The associations held across both concentric and eccentric contraction modes for both flywheel tests
Conclusions
The study concludes that flywheel hip extension and leg curl tests should be used with caution and require sufficient familiarization before assessment use.
The FWHE test was suggested as having 'potential use as a complementary assessment of hamstring strength'
Both tests showed 'good convergent validity' with isokinetic knee flexor strength
Greater between-session variability relative to isokinetic dynamometry limits their standalone use
The learning effect observed in the FWLC test specifically underscores the need for familiarization sessions prior to formal testing
What This Means
This research suggests that two flywheel-based hamstring strength tests — the flywheel hip extension (FWHE) and flywheel leg curl (FWLC) — are less consistent between testing sessions compared to the gold-standard isokinetic dynamometer. The study tested 25 physically active men and women twice to see how repeatable the results were. The isokinetic machine produced highly consistent results (excellent reliability), while the flywheel hip extension showed only questionable consistency, and the flywheel leg curl showed even more variation, with participants actually performing better in the second session — a sign that they were still learning the exercise.
Despite these reliability concerns, both flywheel tests produced results that had moderate to large correlations with the isokinetic strength measurements, meaning they do capture meaningful information about hamstring strength. This suggests that flywheel tests can provide useful complementary information about hamstring function, even if they are not as precise as isokinetic testing.
The practical takeaway from this research is that coaches and practitioners who use flywheel devices to assess hamstring strength should ensure athletes are thoroughly familiar with the exercises before collecting data intended for tracking or decision-making. Without adequate practice sessions beforehand, test results may reflect learning rather than true strength changes, which could lead to misleading conclusions about an athlete's hamstring strength or progress.
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Nutarelli S, Biancofiore F, Mattina A, Beato M, Perna P. (2026). Reliability of flywheel hip extension and leg curl tests and their associations with isokinetic hamstring strength.. Journal of biomechanics. https://doi.org/10.1016/j.jbiomech.2026.113357