Test-Retest and Inter-Rater Reliability of a Rotary Axis Encoder-Flywheel System for the Assessment of Hip Rotation Exercises

Background: Flywheel devices have found extensive use as a resistance training method. Performance monitoring during functional exercises can be achieved through a coupled rotary axis encoder. However, the reliability of a rotary axis encoder-flywheel system remains underexplored for isolated movements. This study aims to assess test-retest and inter-rater reliability of a rotary axis encoder-flywheel system for assessing hip rotation movements. Methods: Twenty-nine physically active participants were included. The Conic Power Move^® flywheel was used to perform hip internal and external rotation exercises. Mean and peak values for velocity, force, and power were collected using a Chronojump rotary axis encoder and the Chronojump software v.2.2.1. The intraclass correlation coefficient (ICC) and the coefficient of variation (CV) were calculated to assess relative and absolute reliability, respectively. Standard error of measurement and minimum detectable changes were also calculated. Results: Good to excellent ICCs (0.85–0.98) were achieved for test-retest and inter-rater reliability in all outcomes for both hip internal and external rotation exercises. There was acceptable test-retest absolute reliability (CV < 10%) for mean and peak velocity, and mean force of hip internal and external rotation (CV = 4.7–7%). Inter-rater absolute reliability was acceptable for mean and peak velocity, mean power, and mean force (CV = 4.7–9.8%). Conclusion: The rotational encoder-flywheel system demonstrated good to excellent relative reliability for assessing hip rotation exercises. Peak force and power values exhibit absolute reliability >10%, so the use of mean and peak velocity, mean force, and mean power seems more adequate for measurements with the rotary axis encoder-flywheel system.