Reliability of isokinetic decay slope is superior to using fatigue indices for shoulder horizontal abduction

Introduction: Reliable and valid measurements for shoulder muscular endurance should be available for clinical use. The posterior shoulder endurance test offers a potential clinical assessment, but its construct validity isn't available. Since a criterion measure of muscular endurance is not available, this study's purpose was to determine a reliable method for testing shoulder muscular endurance using an isokinetic dynamometer. Methods: The test-retest reliability, standard error measurement, and minimal detectable change were calculated on four different paradigms to quantify muscular fatigue using two isokinetic speeds (60°sec⁻¹,180°sec⁻¹). Calculation paradigms included peak torque fatigue index (FI), average torque FI, area-under-the-curve FI, and peak torque decay slope. Testing occurred on two days. Repeated measures analysis of variance compared the two peak torque decay slopes across both testing days. Results: Superior reliability was found within the decay slope measurements at both 60°sec⁻¹ (ICC = 0.941) and 180°sec⁻¹ (ICC = 0.764) speeds, with the 60°sec⁻¹ decay slope being the highest reliability between the two angular velocities. There was a greater amount of fatigue in the 60°sec⁻¹ decay slope compared to the 180°sec⁻¹ decay slope. Conclusion: Using the decay slope of isokinetic shoulder horizontal abduction at 60°sec⁻¹ is a reliable method to validate other muscular endurance clinical measures. Rehabilitation specialists should utilize the decay slope of the isokinetic dynamometry to monitor responsiveness.