Injury in Youth Elite Footballers: From Surveillance to Prevention

Introduction: Elite youth players experience increasingly rigorous training and match demands from youth to senior football whilst being exposed to psychological (e.g., academic demands) and physiological stressors (e.g., physical maturation), potentially affecting injury risk negatively. To prevent injuries, frameworks suggest a stepwise approach from injury surveillance to testing an intervention in a controlled setting, before evaluating its effectiveness within ‘real-world’ context. Despite a growing number of injury surveillance studies in youth (elite) footballers, no research with third party data collection methods has yet investigated the impact of time-loss injury on different football participation levels, i.e. no participation as ‘full’ time-loss and reduced / modified participation as ‘partial’ time-loss), and none is available from Australia. Additionally, considering Australian youth elite players are engaged in part-time football academies plus full-time schooling, it is unclear whether certain yearlong patterns such as season phases, school holidays or periods of returning to school affect incidence rates or burden.

Multimodal injury prevention systems in youth football commonly contain exercise-based injury prevention programs, such as the 11+, which is known to reduce injuries and improve physical performance. The duration to complete the full 11+ program at training presents a notable barrier to implementation, with re-scheduling parts of the 11+ outside of training as a possible solution. Football Australia’s “Perform+” adapted the 11+ to provide flexible scheduling of strength, balance and power exercises (11+ Part 2), with the running exercises (11+ Parts 1 and 3) maintained as warm-up, but adherence and compliance to the Perform+ in youth elite footballers has not been investigated to date.

Hence, the aims of this thesis were to describe injuries in Australian elite youth footballers with focus on recording methods to distinguish between full and partial time-loss injury burden in Chapter 2; investigating patterns of injury incidence rates and burden within combined Pre-, Start-of-, Mid-, and End-of-Season and school-holiday phases in Chapter 3; in Chapter 4, to assess injury and performance outcomes when Part 2 of the 11+ is either performed at training or at home 3 × / week; and finally, to describe adherence to individual exercises within Football Australia’s Perform+ (performance and running parts), the preference or timing of exercises and the remaining barriers in Chapter 5.

Methods: Medical attention injury (including time-loss and non-time-loss), individual football exposure, body mass and growth data were collected from 118 male elite footballers (U13–U18) over three consecutive seasons (two full seasons from 2017 – 2019, season 2019/2020 was interrupted by 16 weeks of COVID-19 lockdown or training restrictions) according to the Football Consensus Statement. This injury surveillance forms part of Chapters 2 to 4. In Chapter 2, the number of total, full and partial days lost per 1000 hours of exposure were used to calculate the respective injury burden with the proportion of partial time-loss described for different injury characteristics such as injury severity. In Chapter 3, pooled injury incidence rate and burden per season or school / holiday phases from 2017 – 2020 (until the COVID-19 lockdown) were calculated for all injuries. To compare between season or school / holiday phases, injury rate ratios (IRR) for injury incidences were assessed using Generalised Linear Mixed Models whilst including growth rates as a covariate and offsetting for football exposure. 99% confidence intervals were used to determine differences in injury burden between phases. In Chapter 4, the rescheduled 11+ program efficacy was established by assessing differences between home (HG) and training groups (TG) regarding time to stabilisation, eccentric hamstring strength and countermovement jump height four times during the 2019 football season, in addition to the established injury surveillance. 65 male elite footballers (U13 – U16) were stratified according to age, known previous injury history and baseline hamstring strength before matched pairs were randomly assigned to the home or training groups. Linear mixed-models were used to evaluate main and interaction effects of group and time with either rate of growth or rate of body mass included as covariate. Bonferroni post-hoc tests were used to account for multiple comparisons. Differences in time-loss and non-time-loss injuries were determined using a 2-tailed Z test for a comparison of rates. Weekly adherence of the home group was established via an online survey. Finally, in Chapter 5, a cross-sectional online survey was disseminated to top-tier female and male U13-U18 Australian and New Zealand footballers between 2021 and 2022. Statistical comparisons were made between the adherence to Perform+ parts (performance / running) and playing groups (goalkeeper / field players and females / males). Barriers were identified via thematic analysis.

Results: In Chapter 2, 243 time-loss injuries with an average of 21 days lost (median 9 days) per injury were observed in 94 players. Injury burden (137.2 days lost/1000h, 95% CI 133.4 – 141.0) was comprised of 23% (31.9 days lost/1000h, 95% CI 30.1 – 33.8) partial time-loss and 77% (105.3 days lost/1000h, 95% CI 102.0 - 108.6) full time-loss burden. Injuries of moderate severity (8-28 days lost) showed 40% of partial time-loss. Time-loss injury incidence rate (6.6 injuries/1000h, 95% CI 5.8 – 7.5), the number of severe injuries (16%) and the distribution of time-loss and non-time-loss injuries (56% and 44%) were comparable to other reports in elite youth footballers. In Chapter 3, considering the reduced number of weeks from the 2019/2020 season, 202 time-loss and 169 non-time-loss injuries were observed in 94 players, with growth rate significantly affecting time-loss injury incidence rate. Medical attention and non-time-loss injury incidence rates were higher during Pre-season (IRR 1.65, 95% CI 1.12 – 2.18, p =0.01; IRR 2.08, 95% CI 1.02 – 3.14, p =0.02, respectively), and Mid-season showed a higher non-TL incidence rate (IRR 2.15, 95% CI 0.97 – 3.33, p =0.02) and burden (69 days with injury/1000hrs, 99% CI 47 – 103) compared to End-of-Season (25 days with injury/1000hrs, 99% CI 15 – 45). Medical attention injury rates and partial time-loss injury burden were higher during school compared to holiday periods (IRR 0.6, 95% CI 0.37 – 0.91, p =0.04; 61 partial days lost/1000hrs, 99% CI 35 – 104; 13 partial days lost/1000hrs, 99% CI 8 – 23). Chapter 4 demonstrated that relative to baseline, eccentric hamstring strength (HG 4.3 kg, 95% CI to 5.7, p <0.001; TG 5.5 3 kg, 95% CI 4.3 to 6.6, p <0.001) and countermovement jump height (HG 3.5 cm, 95% CI 2.2 to 4.7, p <0.001; TG 3.2 cm, 95% CI 2.2 to 4.3, p <0.001) increased, with no difference between groups observed at the end of the season. The home group performed the exercises 2.8 × / week (95% CI 2.7 to 2.9). Injury outcomes were similar with 63 injuries observed in the home group and 76 observed in the training group. Chapter 4 showed that 374 players (49% female, 51% male) reported to perform running and performance exercises respectively at an average of 2.1 × and 1.8 × per week (p =0.01). Running exercises were mostly completed before training and was confirmed as the preferred timing for players. Performance exercises were mostly performed outside of the training environment or before sessions with a preference for scheduling to occur any time other than during training. No difference in adherence, timing or preferred timing of Perform+ parts existed between playing groups, but individual exercise-specific differences in adherence, current and preferred timing were evident between female and male footballers. Barriers for adherence were allocated to themes: Lack of information (27%); Attitudes and beliefs (25%); Program content (21%); Time constraints (17%); Unknown reasons (4%).

Conclusion: Australian youth elite footballers experience similar injuries to youth footballers from other countries and have shown for the first time, that whilst being injured, almost one quarter of the time-loss injury burden was still spent being part of some team football activities. Furthermore, considering the complex nature of injury occurrence, season phase and return-to-school may be considered as external risk factors for elite academy footballers. Within injury prevention programs, a flexible delivery of exercises can be successful and is preferred by the majority of youth players. To continue to improve injury prevention program adherence, educating youth players directly on injury and injury prevention may provide a practical solution in elite male and female youth football. Key stakeholders in football can use the findings of this thesis to support the optimisation of multimodal injury prevention systems.