Impact of High-Intensity Interval Training on Aerobic Capacity, Muscle Strength, and Agility Among Football Athletes: A Systematic Review and Meta-Analysis.

High-intensity interval training (HIIT) is increasingly incorporated into football conditioning programmes to enhance aerobic fitness, muscular strength, and agility. This systematic review and meta-analysis evaluated the effects of HIIT on these performance outcomes in football players and explored the availability of evidence relating to injury outcomes. Six electronic information sources were searched for randomised and non-randomised controlled trials published between January 2010 and December 2024. Methodological quality was assessed using the Modified Downs and Black checklist, and certainty of evidence was evaluated using GRADE. Ten studies including 329 football players were included. Aerobic-fitness outcomes were synthesised narratively because heterogeneity in outcome measurement and reporting precluded reliable quantitative pooling. Individual studies demonstrated favourable changes, although superiority over comparator training was not consistently established. Muscle-strength outcomes were also synthesised narratively because sufficiently comparable independent-study data were unavailable. Two RCTs contributed to the agility meta-analysis; following harmonisation of the direction of time-based outcomes, no significant advantage of HIIT over comparator training was observed (SMD = -0.29, 95% CI -0.83 to 0.25;