Acute effects of repeated sprints on performance measures during the countermovement rebound jump

The countermovement rebound jump (CMRJ) test is an alternative to traditional jump assessments, providing unique insights into slow and fast stretch-shortening cycle mechanics. This study investigated the effects of repeated sprint-induced fatigue (4 sets of 6 × 40-m sprints with 20-s inter-repetition rest) on CMRJ performance metrics, including jump heights, time to take-off, ground contact time, reactive strength index modified (RSImod), reactive strength index (RSI), countermovement depth, and leg stiffness, measured at baseline, during sprints, and over a 24-hour recovery period. Twenty-four elite male team-sport athletes completed CMRJ tests before and after each sprint set, with additional assessments at 0.5-hour, 1-hour, and 24-hour post-sprints. Results showed significant reductions in CMRJ heights, RSImod, and RSI immediately following sprints (effect size = 0.27 to 1.78, p ≤ 0.040), indicating acute decrements in jump performance. Strategy-based metrics, such as leg stiffness and contact times, remained stable (effect size = 0.09 to 0.57, p ≥ 0.079), suggesting limited sensitivity to fatigue. These findings support the use of CMRJ outcome metrics to evaluate neuromuscular fatigue effectively in an athlete population with a non-invasive solution suitable for field settings.