Fatigue-induced changes in jump and hop performance and the limited explanatory value of Fatigue Index in professional Indian male soccer players- an analytical cross-sectional study

BACKGROUND: Fatigue can impair neuromuscular performance during sport-specific lower limb tasks. Although hop-based assessments are commonly used in sports rehabilitation and functional performance testing, rested assessments may not fully capture fatigue-related performance decline. Fatigue Index (FI) is commonly used to quantify decline in sprint performance, but its ability to explain fatigue-related changes in hop performance remains uncertain. METHODS: This cross-sectional study included 195 professional Indian male soccer players who completed a vertical jump assessment using the validated MyJump2 application and standardised hop-for-distance, and side-hop tests before and immediately after a repeated-sprint ability fatigue protocol. FI was calculated as a positive sprint decrement, defined as the ratio of total sprint time to ideal sprint time, where ideal sprint time was set to the fastest sprint time multiplied by 6. Paired non-parametric analyses were employed to examine pre- and post-fatigue changes. Robust regression models were used to evaluate the independent association of FI with performance change after adjusting for age and playing experience. Sensitivity analyses using winsorised and outlier-excluded datasets assessed the stability of the estimates. Correlation network analysis was performed to explore relationships among baseline hop-performance variables. RESULTS: The documented FI was 7.08 ± 0.90%, indicating a physiologically plausible positive sprint-decrement value. Fatigue significantly reduced all hop and jump performance outcomes. Median jump height decreased by 4.11 cm, contact time increased by 11.63 ms, mean power decreased by 39.33 W, and reactive strength index-modified decreased by 0.02 m/s. Hop-for-distance performance decreased by approximately 6.5-7.0 cm, while side-hop performance declined by 3-4 repetitions. After adjustment for age and playing experience, FI values showed minimal independent association with most fatigue-related change outcomes. Age showed stronger and more consistent associations, particularly with contact time and the modified reactive strength index. Sensitivity analyses indicated stable findings across primary, winsorised, and outlier-excluded models. Correlation network analysis identified an explosive-performance cluster in which jump height, reactive strength index-modified, and mean power were positively associated, whereas contact time was inversely associated with these variables. CONCLUSION: Hop-performance assessment under fatigued conditions revealed a consistent decline in explosive and reactive lower limb performance in this cohort of Indian male soccer players. FI values showed limited value as an isolated explanatory metric for fatigue-related performance decline. Outcome-specific changes, particularly contact time, reactive strength index-modified, jump height, and mean power, may provide more useful information than a single fatigue metric during fatigue-inclusive functional performance assessment. TRIAL REGISTRATION: The study was prospectively registered with the Clinical Trials Registry of India (CTRI/2024/08/072445) on August 12th, 2024.