Reliability, Validity, and Sensitivity of a Novel Smartphone-Based Eccentric Hamstring Strength Test in Professional Football Players

Purpose: To evaluate the test-retest reliability, sensitivity, and concurrent validity of a smartphone-based method for assessing eccentric hamstring strength among male professional football players. Methods: A total of 25 healthy male professional football players performed the Chinese University of Hong Kong (CUHK) Nordic break-point test, hamstring fatigue protocol, and isokinetic hamstring strength test. The CUHK Nordic break-point test is based on a Nordic hamstring exercise. The Nordic break-point angle was defined as the maximum point where the participant could no longer support the weight of his body against gravity. The criterion for the sensitivity test was the presprinting and postsprinting difference of the Nordic break-point angle with a hamstring fatigue protocol. The hamstring fatigue protocol consists of 12 repetitions of the 30-m sprint with 30-s recoveries between sprints. Hamstring peak torque of the isokinetic hamstring strength test was used as the criterion for validity. Results: A high test-retest reliability (intraclass correlation coefficient = .94; 95% confidence interval, .82-.98) was found in the Nordic break-point angle measurements. The Nordic break-point angle significantly correlated with isokinetic hamstring peak torques at eccentric action of 30 degrees/s (r=.88, r(2)=.77, P <.001). The minimal detectable difference was 8.03 degrees. The sensitivity of the measure was good enough that a significance difference (effect size = 0.70, P < .001) was found between presprinting and postsprinting values. Conclusion: The CUHK Nordic break-point test is a simple, portable, quick smartphone-based method to provide reliable and accurate eccentric hamstring strength measures among male professional football players.