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Background: Health problems are prevalent in football, and in both elite youth and professional football, players are expected to sustain several health problems per season. Also, at any given time of the season, the prevalence of health problems (both injuries and illnesses) exceeds 40% among elite youth players. Training load has recently emerged as a potential risk factor for health problems in football; subsequently, many teams, particularly those at an elite level, attempt to manage players’ training loads as a preventative measure to mitigate health problem risk. However, the evidence supporting this practice is limited, and its effectiveness is never tested. Therefore, this dissertation aims to improve our understanding of the relationship between training load and health problems and to guide preventative efforts. Methods: All studies were performed on Norwegian football players and coahces, Papers I, II and III in elite youth (U19 age category) and Paper IV in professional football. In the first study (Paper I), we registered daily training load and health data to assess methodological issues in the relationship between the Acute:chronic workload ratio (ACWR) and health problems. The second study (Paper II) was a cluster-randomised trial that assessed the effectiveness of a load management programme on health problem prevention. We followed 482 players for a full season, registering their monthly health problem prevalence. In Paper III, we surveyed 250 of the players included in Paper II about their attitudes, beliefs and experiences of load management and health problems. In Paper IV, we assessed the injury characteristics of two different football seasons in the Norwegian premier league. This explorative descriptive study collected injury data from eight teams participating in the 2019 and 2020 seasons. Main results: In Paper I, we found 24 (22%) significant associations between ACWR and health problems among the 108 analyses performed. These were spread across various methodological combinations. We did not observe any patterns of combinations that substantially increased the chance of a significant association. There was a considerable variation in the size and precision of the estimated strength of the association. In Paper II, the average prevalence of health problems was 65.7% (61.1% to 70.2%) in the intervention group and 63.8% (60.0% to 67.7%) in the control group. The prevalence was 1.8%-points (-4.1 to 7.7%-points; P=0.55) higher in the intervention group, and there was no reduction in the likelihood of reporting a health problem in the intervention group (Relative Risk, RR 1.01 (95% CI 0.91 to 1.12 ); P=0.84). In Paper III, we found that most players (88%) think scientific evidence for improved performance is a key facilitator to implementation. Similarly, the coaches reported that the most crucial facilitator was scientific evidence that the preventive measures were effective (100%). Players reported that the coach’s attitude to preventive measures was important (86%), and similarly, 88% of coaches reported that the player’s attitude was important. In Paper IV, the match incidence was 7.23 per 1000h lower in 2020 (22.82 per 1000h; CI 18.07 to 28.44; Incidence Rate Ratio; IRR 0.76) than in 2019 (30.05 per 1000h; CI 24.55 to 36.41); however, this was not a significant difference. There were no differences in either availability, severity or injury burden across the two seasons. Conclusion: Based on our findings, we conclude that the relationship between ACWR and health problems is highly affected by the methodological approach, which can lead to p-hacking and cherry-picking of results. Future training load studies should pre-register their definitions, hypotheses, models and report all performed analyses' results. Furthermore, managing training loads using ACWR in a onesize-fits-all approach does not appear to prevent health problems in elite youth football. When implementing future health problem preventive measures, practitioners and researchers should focus on time-efficient interventions and create buy-in from club and federation stakeholders as well as coaches and players by focusing on both performance and prevention. Finally, we found no differences in injuries comparing a match-congested season with a regular one, suggesting a congested season can be a safe alternative.
