Convergent validity, reliability and sensitivity of a running test to monitor neuromuscular fatigue

Purpose: The aim of this research was to investigate the convergent validity, reliability and sensitivity over a week of training of a standardized running test to measure neuromuscular fatigue. Methods: Twenty male rugby union players were recruited for the study, which took place during preseason. The standardized running test consisted of four 60 m runs paced at ≈5 m•s-1 with 33 seconds of recovery between trials. Data from micromechanical electrical systems (MEMS) were used to calculate a running load index (RLI) which was a ratio between the mechanical load and the speed performed during runs. RLI was calculated by using either the entire duration of the run or a constant velocity period. For each type of calculation, either an individual directional or the sum of the three components of the accelerometer were used. A measure of leg stiffness was used to assess the convergent validity of the RLI. Results: Unclear to large relationships between leg stiffness and RLI were found (r ranged from -0.20 to 0.62). Regarding the reliability, small to moderate (0.47 to 0.86) standardized typical errors were found. The sensitivity analysis showed the leg stiffness presented a very likely trivial change over the course of one week of training, while RLI showed very likely small to a most likely large change. Conclusion: This study showed that RLI is a practical method to measure neuromuscular fatigue. Additionally, such a methodology aligns with the constraint of elite team sport set up due to its ease of implementation in practice.