Relative skeletal maturity status affects injury burden in U14 elite academy football players

1400 | Scand J Med Sci Sports. 2022;32:1400–1409.wileyonlinelibrary.com/journal/sms Received: 31 July 2021 | Revised: 7 December 2021 | Accepted: 11 June 2022 DOI: 10.1111/sms.14204 ORIGINAL ARTICLE Relative skeletal maturity status affects injury burden in U14 elite academy football players XabierMonasterio1,2 | IraiaBidaurrazagaLetona1 | JonLarruskain2 | Jose A.Lekue1,2 | GontzalDiazBeitia1,2 | Juan M.Santisteban1,2 | ImanolMartinGaretxana1,2 | Susana M.Gil1 1Department of Physiology, Faculty of Medicine and Nursing, University of the Basque Country (UPV/EHU), Leioa, Spain 2Medical Services, Athletic Club, Lezama, Spain Correspondence Xabier Monasterio, Department of Physiology, Faculty of Medicine and Nursing, University of the Basque Country (UPV/EHU), Barrio Sarriena s/n, 48940 Leioa, Spain. Email: xabier[email protected] Funding information Euskal Herriko Unibertsitatea; Eusko Jaurlaritza Maturation progresses at different times and at different rates between individuals. Thus, differences in maturity status exist among players in the same chronological agebased category, especially in U14 players. The purpose of this prospective study was to describe injury burden according to the relative skeletal maturity status in U14 elite academy football players. From 2011 to 2020, injuries and individual exposure (training and match) were prospectively recorded in 183 male U14 players. Skeletal age (SA) was assessed using the TannerWhitehouse 2 method. Relative skeletal maturity status [SA minus chronological age (CA)] was classified as follows: early (SA– CA > 0.5), ontime (SA– CA ± 0.5), and late (SA– CA < −0.5). Overall and specific injury burden (days lost/1000 h) and rate ratios for comparisons between groups were calculated. Overall injury burden was 2.8 times higher (3.6 times in training) in early maturers compared with late maturers. Growthrelated injuries were the most burdensome injuries in all three groups, but significant differences were not found between groups. Muscle injuries were 4 times more burdensome in early maturers compared with ontime and late maturers. Besides, joint/ligament injuries were 7 and 12 times less burdensome in late maturers than in ontime and late maturers, respectively. Significant differences between groups in overall and specific injury burden were not found in matches. Our results showed different injury patterns in U14 early, ontime, and late maturers. Hence, monitoring maturity seems crucial to detect potential injuries that cause the greatest disruption, and facilitate design of targeted injury prevention programs. KEYWORDS adolescence, burden, epidemiology, football, growth and development, injury, maturation, youth This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2022 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd. 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | 1401 MONASTERIO et al. 1 | INTRODUCTION Professional football clubs invest many resources in their youth academies to develop young football players, with the aim of providing new players to the First team. Remaining free of injury is a key factor for progressing,1 not only that, injuries lead to increased susceptibility for future injuries and longterm health risks in adulthood.2 Thus, preventing injuries is a priority in football. Injury incidence (injury frequency) and burden (the product of injury frequency and severity) peaks since early- (Under U12– U14) and midadolescence (Under U15– 17),3 when marked somatic growth and significant musculoskeletal and physiological development occur. U14 age group generally coincides with the peak height velocity (PHV) in males, which is the period of maximal growth during adolescence.4 However, due to individual variance in pubertal timing, not all boys will experience the growth spurt at the same age.4 Consequently, interindividual differences in maturity status can be found among players of the same agebased category, especially in U14s.5,6 The large variations in maturity status within chronological age groups have been proposed as factors that may influence injury risk in young footballers. On the one hand, it is assumed that early maturers, whose maturation occurs in advance of the mean and are likely to be bigger, heavier, and stronger,4 may be at higher risk due to greater engagement and involvement in competitive play. Besides, inappropriate training load in groups of players with mixed maturity and developmental status is another related worry.7 However, available data on growth, maturation, and injury risk in football provide conflicting evidence due to methodological differences and limitations. Skeletal maturation is considered the single best maturity indicator, as the start (skeleton of cartilage) and endpoints (skeleton of mature bone) are known and can be followed throughout the maturation process with precise and reliable estimates.4 Nevertheless, to date, most of the studies have used noninvasive methods with limited accuracy and only 3 studies have considered the effect of skeletal maturity on injuries in elite youth football players.8– 10 Available research assessing skeletal age suggests that early maturers, who have advanced skeletal age compared to their chronological age (relative maturity status), have a higher injury incidence compared with ontime and late maturers.8– 10 Regarding specific injury risk, early maturers seem to be more prone to muscle and tendon injuries, while ontime and late maturers have higher incidence for growthrelated injuries.8,9 Available research which measured skeletal age also has inherent limitations. Firstly, they measured injury incidence, which does not account for injury severity. Identifying injuries with the highest injury burden is vital to detect potential injuries that cause the greatest disruption, and facilitate design of targeted injury prevention programs.11 Secondly, they did not account for individual exposure data. Recent research suggests that advanced maturity status in U11, U13, and U14 groups is associated with higher grades in coaches' evaluations of match performance.12 This may translate to more playing opportunities, and therefore, research which includes individual training and match exposure is needed to allow for direct comparisons and more nuanced interpretation.13 This study aimed to build on these limitations by using 9season injury, exposure (training and match), and skeletal maturation data to describe overall and specific injury burden in U14 early, ontime, and late maturing elite academy football players. 2 | MATERIALS AND METHODS 2.1 | Study design and study population A prospective observational study of 183 male U14 players of an elite Spanish football academy was carried out between the 2011– 2012 and 2019– 2020 playing seasons. The male professional team plays in the Spanish LaLiga, and the development of academy players is key to the club's success because only players developed in the academy or born in the Basque Country can play in this club. Players were single sports athletes, trained 4 times per week, and played a game every weekend. This study was approved by the Ethics Committee of the University of the Basque Country (UPV/EHU). Written informed consent to use regularly collected data for research purposes was obtained from the players guardians. 2.2 | Recording of injuries and exposure Players' training/match exposure in minutes (daily) and injuries were recorded. The club's medical staff diagnosed, treated, and recorded all timeloss injuries following the consensus on definitions and data collection procedures outlined by the International Federation of Association Football (FIFA).14 Injuries were recorded when a player was unable to participate in a future training session or match due to a physical complaint. A player was considered injured until the medical staff cleared the player for full participation. Growthrelated injuries were not explicitly considered by the 2006 consensus statement on injury definitions.14 Thus, we included an additional category for “growthrelated injuries,” defined as unique injuries not seen in adults but common in skeletally immature athletes (e.g., 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 1402 | MONASTERIO et al. growth plate fractures, apophysitis, apophyseal avulsion fractures, and greenstick fractures).15 Injuries were classified into three groups based previous research3,5,16,17 and practitioner experience: (1) growthrelated injuries (Sever's disease, Osgood Schlatter disease, apophyseal injuries [osteochondrosis or avulsion] of the anterior inferior iliac spine, ischial tuberosity, or anterior superior iliac spine and spondylolysis), (2) muscle injuries (quadriceps, hamstring, and adductor), and (3) joint/ligament injuries (knee and ankle). The same doctor recorded injuries since the start of the study, thereby reducing the chance of bias, differences in injury interpretation, and changes in observation methods between doctors. 2.3 | Anthropometric measurements Standing height (±0.1cm; Añó Sayol, Barcelona, Spain) and weight (±0.1kg; (Seca) were measured by doctors before afternoon trainings. The same two doctors measured players during the entire study period, thereby reducing chance of bias. 2.4 | Skeletal maturity status Skeletal maturation was assessed at the beginning of the season, using Xray images of the athlete's left hand and wrist complex. Skeletal age was determined using the Tanner– Whitehouse radiusulnashort bone protocol (TW2 RUS) using sample data of the Basque Country.18 This allowed us to make better comparisons with reference to sample data as our academy only admits players born or developed in the Basque Country. TW2 RUS has been shown to be the method of choice for those using the Tanner– Whitehouse protocol with youth football players.19 The images were interpreted by two experienced doctors who were based at the academy for over 20 years and were in place throughout the 9season period of the present study. Doctors were trained in the use of this method and have a decade experience carrying out skeletal age assessments using TW2 RUS. To test the reproducibility of the assessments of the bone age, the investigators reevaluated randomly selected hand– wrist radiographs from 10 subjects in 2021. The coefficients of intraobserver and interobserver reliability were 0.99 (0.98– 1) and 0.95 (0.84– 0.99) with an error of 0.10 and 0.25 years, respectively. Players were assigned into three categories according to their relative skeletal maturity status: early, ontime, or late. Ontime maturers refer to players whose SA is within 0.5 years of the CA, early maturers refer to players whose SA is older than CA by more than 0.5 years and late maturers refer to players whose SA is younger than CA by more than 0.5 years. Mature players (SA > 18 years) with hand and wrist fully ossified were not found. Adult height was estimated using bone age rates according to TW2RUS method,18 and percentage of predicted adult height was calculated using players standing height. 2.5 | Data analysis Differences in exposure time were compared using Kruskal– Wallis test. Injury burden was calculated to account for both frequency (incidence) and severity (mean absence) of injuries11 and was presented as the number of days lost/ 1000 player hours.13 Injury burden of early versus ontime versus late maturers were compared by calculating rate ratios (RRs). Rate ratios of 1.2, 1.9, and 3.0 can be taken as small, medium, and large, TABLE 1 Number of players, chronological age, skeletal age, and average exposure (hours per season) according to relative skeletal maturity status Early Ontime Late All Number of players 74 (40.6%) 80 (44%) 28 (15.4%) 182 Height 169.79 ± 6.45 162.57 ± 6.42 158.92 ± 6.58 164.62 ± 7.68 Weight 55.84 ± 7.1 47.68 ± 6.45 43.36 ± 6.05 50.29 ± 8.18 Predicted adult height 181.2 ± 5.57 179.95 ± 4.92 181.46 ± 5.81 180.69 ± 5.33 Percentage of predicted adult height 93.72 ± 2.82 90.33 ± 1.98 87.64 ± 2.80 91.30 ± 3.30 Chronological age 13.5 ± 0.5 13.5 ± 0.4 13.7 ± 0.4 13.5 ± 0.5 Skeletal age 14.8 ± 0.8 13.7 ± 0.6 12.6 ± 0.9 14 ± 1 Average total exposure 204.7 ± 64.8 201.3 ± 59.4 226.5 ± 45 206.6 ± 59.9 Average training exposure 175.7 ± 53.9 175.3 ± 50 196.2 ± 36.3 178.7 ± 50 Average match exposure 29.4 ± 12.9 26.7 ± 11.4 30.3 ± 10.6 27.9 ± 12.4 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | 1403 MONASTERIO et al. respectively.20 Zeroinflated negative binomial models were fitted to account for the excess of zeros and overdispersion in the data21 using the glmmTMB package in R (version 3.6.2). Confidence intervals were calculated at the 95% confidence level for rates and RRs using a parametric bootstrap procedure. pvalues were adjusted for multiple comparison with the Benjamini and Hochberg method using R function p.adjust on the package stats. The significance level was set at <0.05. 3 | RESULTS A total number of 277 injuries (157 during training and 120 in matches) were recorded which caused 7301 days lost (4405 in training and 2896 in matches). Descriptive data are shown in Table1. Significant differences between groups were not found for average total, training and match exposure. Overall injury burden was 2.8 times higher in early maturers compared with late maturers (Table2). This difference was more pronounced during training sessions, as injury burden was 3.6 times higher in early maturers compared to late maturers (Table3). Significant differences between groups were not found in the matches (Table4). Growthrelated injuries were the most burdensome injuries in all maturity status groups. Despite not finding significant differences among maturity groups, Sever's disease and AIIS apophyseal injuries were more burdensome in late maturers while ASIS apophyseal injuries and spondylolysis were more burdensome in early maturers (Tables2, 3 and 4). Muscle injuries in early maturers were 4 times more burdensome compared with ontime and late maturers (Table2). Differences in injury burden were more pronounced during trainings [RR for early versus ontime: 7.30 (1.78– 89.98)] (Table3). This trend remained similar for hamstring injuries, in which early maturers had 6 times higher (12 times in trainings) burden than ontime maturers (Tables2 and 3). Significant differences between maturity groups were not found in matches (Table4). Joint/ligament injuries in late maturers were 7 and 12 times less burdensome than in ontime and early maturers, respectively (Table2). In training sessions, significant differences were only found in early versus late maturers; with early maturers showing 6 times higher burden (Table3). Concerning specific injuries, significant differences for ankle join/ligament injuries were also found, being injuries in early maturers 16 times more burdensome than in late maturers (Table2). 4 | DISCUSSION This study aimed to describe and compare overall and specific injury burden in early, ontime, and late maturing U14 academy football players. Injuries of early maturers were more burdensome than those of late developers. Concerning specific injuries, muscle injuries were more burdensome in early versus ontime/late maturers while joint/ligament injuries were more burdensome in early/ ontime versus late maturers. In line with previous data in elite football academies, our research showed a clear underrepresentation of U14 late maturers (15%). It must be highlighted that the less conservative band of ±0.5 years applied in our study and in previous research12,22– 24 identified a greater proportion of early maturers than those studies using a more conservative ±1 band.22,25 Significant differences between early, ontime, and late maturers were not found for total, training and match exposure. This is in accordance with previous research which only found longer pitch time in U9 and U10 early maturers.26 Nevertheless, recent studies suggested that advanced maturity status in U14s is associated with higher grades in matchrunning metrics6 and coaches' evaluations of match performance12; which might lead the coaches to rely more on early maturers and thus, to higher match exposure. However, apart from coaches' perceptions, other factors such as availability might also have an impact in player's average exposure. Although we did not study player's availability, our results showed 2.8 times higher injury burden (3.6 times in trainings) in early versus late maturers, and as a consequence, early maturers might have been less available due to longer periods of absence caused by injuries.11 The higher injury burden (product of incidence and severity) found in early maturers is in line with results in previous research which found a higher incidence in this group.8– 10,27 Concerning the severity of injuries, contrary to our results, Le Gall et al.8 found longer average layoff time per injury and longer periods of absence in U14 ontime and late maturers. Nonetheless, comparisons with previous studies must be made cautiously as they applied different methods for skeletal age estimation, used different cutoff values for classifying players and were carried out in different populations. Most importantly, they did not account for individual exposure data. There are multiple reasons that might explain the higher overall injury burden in early maturers compared with U14 ontime and late maturers. In line with our results, previous research has already shown that players who are closer to their biological maturity have more burdensome injuries17,24,28 and early maturers might be more mature than their ontime and late peers. Furthermore, covering greater distances at higher speeds,6,29 a more 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 1404 | MONASTERIO et al. TABLE 2 Number of days lost/1000 h (95% CI) according to relative skeletal maturity status in U14 football players, and rate ratios (95% CI) for comparisons between groups Number of days lost/1000 h RateRatios Early Ontime Late Early versus Ontime Early versus Late Ontime versus Late Overall burden 392.1 (253.7– 559.4) L265.2 (172.8– 377.4) 140 (60.4– 256) E1.48 (0.88– 2.57) 2.80 (1.32– 6.81)* 1.89 (0.89– 4.62) Growthrelated injuries 205.5 (104.1– 327.6) 184.2 (106.6– 280.8) 100 (35.3– 190.9) 1.12 (0.51– 2.31) 2.06 (0.81– 6.32) 1.84 (0.76– 5.71) Sever's disease 2.2 (0.2– 5.2) 5.8 (0.6– 13.5) 9.1 (0.9– 20.2) 0.37 (0.03– 4.45) 0.24 (0.02– 2.68) 0.64 (0.05– 6.89) OsgoodSchlatter's disease 6.4 (0.6– 15.4) 12.5 (3.1– 26.5) 0.52 (0.04– 3.24) Ischial tuberosities injuries 11.5 (0.1– 30.5) 17.6 (5– 36.3) 18.9 (0.1– 51) 0.66 (0.01– 3.02) 0.61 (0.01– 9.16·107)0.93 (0.21– 1.25·108) AIIS injuries 19.9 (1.9– 47.6) 49.7 (19.6– 83.7) 64.1 (13.2– 132.8) 0.40 (0.03– 1.40) 0.31 (0.03– 1.80) 0.78 (0.25– 4.14) ASIS injuries 63.5 (0.1– 252.5) Spondylolysis 67.6 (14.1– 144.5) 73.3 (9.6– 168.7) 0.92 (0.16– 12.06) Muscle injuries 76.2 (3– 144.5) O,L 17.7 (5.3– 35) E18.8 (2.9– 44.9) E4.30 (1.34– 16.48)* 4.06 (1.23– 31.02)* 0.94 (0.21– 6.62) Hamstring 35 (7.4– 73.5) O5.6 (0.7– 14.1) E7.1 (0.1– 22.4) 6.23 (1.07– 53.03)* 4.95 (0.78– 2.71·108)0.79 (0.10– 5.49·107) Quadriceps 35.9 (3.1– 89.4) 4.5 (0.1– 14) 3.4 (0.1– 16.7) 7.99 (0.68– 1.67·109)10.59 (0.74– 4.20·109)1.32 (0.01– 2.77·108) Joint/Ligament injuries 35.5 (12.6– 62.9) L19.11 (6.6– 35.3) L2.84 (0.2– 7.5) E,O 1.86 (0.59– 6.06) 12.51 (3.35– 175.09)** 6.73 (1.69– 93)* Knee 0.4 (0.11– 1.5) 8.2 (0.6– 19.6) 0.9 (0.1– 3.9) 0.05 (0.01– 830) 0.43 (0.01– 1.58·109)9.55 (0.66– 3.14·1014) Ankle 31.3 (10.3– 58.4) L8.9 (1.2– 20.1) 1.98 (0.1– 5.7) E3.52 (0.87– 25.14) 15.76 (3.37– 1.85·108)** 4.47 (0.52– 6.17·107) Note: Significant differences (*p < 0.05, **p < 0.01) between maturity status groups: E(early), O(ontime), or L(late). Abbreviations: AIIS, anterior inferior iliac spine; ASIS: anterior inferior iliac spine. 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | 1405 MONASTERIO et al. TABLE 3 Number of days lost/1000 h (95% CI) in trainings according to relative skeletal maturity status in U14 football players, and rate ratios (95% CI) for comparisons between groups Number of days lost/1000 h RateRatios Early Ontime Late Early versus Ontime Early versus Late Ontime versus Late Overall burden 292.4 (170.6– 434)L188.9 (101– 315) 81.6 (23.9– 174.3) E1.55 (0.75– 3.28) 3.58 (1.42– 12.75)* 2.31 (0.88– 8.92) Growthrelated injuries 165 (77.7– 283) 127.5 (53.3– 227) 59.5 (7.2– 153.7) 1.29 (0.52– 3.76) 2.77 (0.85– 23.21) 2.14 (0.63– 18.25) Sever's disease 2.54 (0.15– 6.07) 4.6 (0.1– 13.6) 10.2 (1– 22.6) 0.56 (0.03– 6.28·109)0.25 (0.02– 2.60) 0.45 (0.01– 6.43) OsgoodSchlatter's disease 6.4 (0.3– 14.7) 7.8 (1.4– 16.2) 0.83 (0.07– 4.98) Ischial apophyseal injuries 9.9 (0.1– 24.3) 10.4 (0.1– 28.9) 0.95 (0.01– 4.73*108) AIIS apophyseal injuries 10 (0.1– 33.6) 25.3 (0.1– 58.5) 49 (0.1– 159.6) 0.40 (0.01– 3.21) 0.20 (0.01– 8.65·107)0.52 (0.07– 2.49·108) ASIS apophyseal injuries 38.3 (0.1– 117.2) Spondylolysis 64.9 (9.9– 145.5) 56.8 (0.1– 160) 1.14 (0.09– 1.29·109) Muscle injuries 79 (26.3– 150.3) O,L 10.8 (0.9– 28.1) E19 (2.6– 47) E7.30 (1.78– 89.98)* 4.16 (1.04– 29.77)* 0.57 (0.04– 5.62) Hamstring 37.2 (6.7– 88.9) O0.3 (0.1– 1.5) E7.9 (0.1– 23.6) 12.17 (1.24– 6.15·1010)* 4.73 (0.70– 1.44·109)0.04 (0.01– 1.41·107) Quadriceps 37.2 (5.2– 80.9) 4.9 (0.1– 17.1) 3.8 (0.1– 18.9) 7.60 (0.94– 1.01·1010)9.71 (0.96– 2.69·109)1.28 (0.01– 1.99·108) Joint/Ligament injuries 19.1 (3.9– 40) L17.4 (3.3– 36.7) 3.3 (0.2– 8.3) E1.09 (0.22– 6.45) 5.84 (1.11– 116.49)* 5.34 (0.89– 77.94) Knee 0.4 (0.1– 1.9) 9.4 (0.2– 24.9) 1 (0.1– 4.5) 0.05 (0.01– 1.87) 0.43 (0.01– 7.74·108)9.40 (0.26– 2.57·1010) Ankle 13.9 (1.1– 35) 7.1 (0.1– 20.1) 2.3 (0.1– 6.6) 1.97 (0.2– 6.20·109)6.13 (0.60– 1.57·108)3.12 (0.01– 8.92·107) Note: Significant differences (*p < 0.05) between maturity status groups: E(early), O(ontime), or L(late). Abbreviations: AIIS, anterior inferior iliac spine; ASIS, anterior inferior iliac spine. 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 1406 | MONASTERIO et al. TABLE 4 Number of days lost/1000 h (95% CI) in matches according to relative skeletal maturity status in U14 football players, and rate ratios (95% CI) for comparisons between groups Number of days lost/1000 h RateRatios Early Ontime Late Early versus Ontime Early versus Late Ontime versus Late Overall burden 773.7 (417.8– 1192) 450.5 (232.1– 723) 736.8 (213.9– 1491.1) 1.72 (0.77– 3.66) 1.05 (0.43– 3.85) 0.61 (0.24– 2.33) Growthrelated injuries 496 (207.8– 886.8) 265 (110.3– 453.1) 463.4 (0.1– 1304.6) 1.87 (0.66– 5.47) 1.07 (0.26– 5.60·108)0.57 (0.14– 2.96·108) Sever's disease 5.4 (0.1– 14.5) 4.3 (0.1– 14.9) 1.23 (0.01– 8.84·1037) OsgoodSchlatter's disease 44.6 (0.1– 167.3) 3.9 (0.1– 18.3) 11.39 (0.01– 1.31·1012) Ischial apophyseal injuries 104.5 (10.6– 238.4) 20 (0.1– 50) 64.7 (0.1– 228.5) 5.23 (0.41– 2.37·108)1.62 (0.09– 8.77·108)0.31 (0.26– 1.85·108) AIIS apophyseal injuries 46.9 (5.6– 103.6) 122.2 (45.3– 212.9) 320.3 (0.1– 703.4) 0.38 (0.05– 1.25) 0.15 (0.02– 1.28·108)0.38 (0.11– 3.61·108) ASIS apophyseal injuries 196.3 (0.1– 603.9) Spondylolysis 119.1 (0.1– 341.1) Muscle injuries 19.7 (0.5– 51.6) 39.5 (9.2– 83.9) 52.3 (2.3– 143.9) 0.44 (0.01– 1.17·109)0.25 (0.01– 9.64·107)0.56 (0.01– 1.17·108) Hamstring 10.3 (0.1– 32.4) 23.1 (0.1– 63.1) 41.2 (0.1– 114.5) 0.44 (0.01– 1.17·109)0.25 (0.01– 9.64·107)0.56 (0.01– 2.38·108) Quadriceps 1 (0.1– 4) Joint/Ligament injuries 35.2 (0.1– 99.5) 92.2 (8– 213.4) 48.3 (4.6– 118.8) 0.38 (0.01– 4.22) 0.73 (0.01– 8.46) 1.91 (0.17– 27.30) Knee Ankle 35.2 (0.1– 89.4) 84.2 (5.5– 212.5) 48.3 (2.4– 115.5) 0.42 (0.01– 6.22) 0.73 (0.01– 12.14) 1.74 (0.12– 38.08) Abbreviations: AIIS: anterior inferior iliac spine; ASIS: anterior inferior iliac spine. 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | 1407 MONASTERIO et al. aggressive way of playing football and assuming more leadership roles30 might lead to more burdensome injuries in early maturers. Besides, different injury patterns in early, ontime, and late maturers might also have contributed to differences in total injury burden. Growthrelated injuries were the most burdensome injuries in all maturity status groups, which is in line with previous research which found that growthrelated injuries were the most common timeloss injuries in U14 players.3 Significant differences between early, ontime, and late maturers were not found for absolute growthrelated injury burden. However, different specific growthrelated injury burdens were found in early, ontime, and late maturers, which might be explained by the distal to proximal sequence of the physeal endochondral ossification from cartilage to complete bony fusion.31 Our results showed that distal growthrelated injuries (e.g., Sever's disease) were more burdensome in U14 late maturers while injuries in the hip/trunk (ASIS apophyseal injuries and spondylolysis) peaked in early maturers. Nevertheless, we cannot forget that ossification centers in the same bone can follow different ossification patterns. For instance, the appearance and closure of the AIIS ossification center occur earlier than other centers in the pelvis (e.g., AIIS and ischial tuberosity)32 and may explain why AIIS apophyseal injuries peaked in late maturers despite being a proximal segment injury. On the contrary, the burden of muscle injuries was higher in early maturers compared with U14 ontime and late maturers and there are multiple factors which might have contributed to it. Firstly, in players with advanced skeletal maturity, which are more likely to have ossification centers closed in different regions of the body, muscle might be the structural point of for injuries.17 Besides, early maturers, who are physically superior to their ontime and late peers, might develop a more physical way of playing football33 that may predispose them to further muscle injuries. Moreover, recent evidence has suggested that in U14 category early maturers were quicker, made more accelerations and cover greater distances at high speed.29 Taking into account that highintensity running has been associated with hamstring injuries,34 the more physical way of playing of early maturers might have contributed to the higher burden of hamstring injuries observed in the results. Besides, the burden of joint/ligament injuries was lower in late maturers compared to ontime and late mature. Furthermore, 16 times lower burden for ankle join/ ligament injuries was found in late versus early maturers. U14 late maturers might be preor circaPHV, periods at which join/ligament injury burden has been shown to be lower.17 Moreover, ankle sprains in players with less delayed maturation and decreased bone density might lead to ankle epiphyseal injuries35; which were classified as growthrelated injuries in our study. Regarding late maturers way of playing, it is possible that smaller and lighter late maturers avoid contact and risktaking situations that could lead them to acute join/ligament injuries.36 4.1 | Methodological considerations The present study provides a robust injury recording methodology by having medical staff from the same club diagnose, treat, and record injuries, avoiding potentially unreliable data that might come from medical staffs at different clubs.37 Besides, some of the weaknesses identified for earlier research on growth, maturation, and injuries have been addressed; using goldstandard skeletal age instead of less accurate noninvasive methods, accounting for individual exposure, recording specific injury data and calculating injury burden to detect injuries that cause the greatest disruption. Yet, some important methodological limitations must be acknowledged. Concerning the skeletal age assessment, TW2RUS method overlooks the time lag between onset and completion of union in the criterion for the final stage of the distal radial epiphysis. Nevertheless, U14 players are rarely at this final stage (3% of our players), and this limitation might not have impacted our results. Furthermore, the less conservative ±0.5year band for classifying players according to their relative skeletal maturity status differs with previous research studying skeletal maturity status and injury risk.8– 10 However, this band has already been applied in other studies for SA– CA23 and relative somatic maturation.12,22,24 Criterion that is too conservative (SA– CA ± 1.0 years) may have failed to differentiate between ontime maturers that are markedly different in terms of maturity status, increasing the likelihood for type two errors. However, it should be noted that the less conservative criterion for determining maturity status (SA– CA ± 0.5 years) also increases the likelihood of the researcher making a type one error (i.e., detecting a bias when no such bias exists). The ±0.5year range used for the current study is well outside the range of the error of bone age assessment (~0.18 years) so it might be relevant for the sample in this study. Besides, the skeletal age determination was based on the maturity of the hand and wrist, which does not necessarily reflect the maturation of other bones, tissues, and organ systems. Not only that, the wrist Xrays were only available in preseason. As maturation progresses at different rates (tempo),4 players' maturity status might change during the season. Besides, only U14 players were included in the study, and future research should consider studying other age groups. Finally, total match exposure was smaller than exposure 16000838, 2022, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/sms.14204 by Xabier Monasterio - Spanish Cochrane National Provision (Ministerio de Sanidad) , Wiley Online Library on [13/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 1408 | MONASTERIO et al. in training, and it might have contributed to not finding significant differences between groups in matches. 5 | PERSPECTIVE Our results showed higher overall injury burden in U14 early maturing academy football players and different injury patterns in early, ontime, and late maturers. Hence, monitoring maturity status seems crucial to detect players at higher risk of being disrupted by injuries and potential injuries that cause the greatest disruption. This would facilitate design of targeted injury prevention programs to reduce the impact of burdensome injuries which might negatively affect individual development and longterm performance.38 Growth and maturation are nonmodifiable risk factors, and there is little that can be done to influence these processes. However, educating key stakeholders (players, coaches, directors, parents) about the effect of maturation seems vital. Firstly, coaches should be aware that the tendency to rely more on early maturers in U14 category12 can lead them to higher exposure, more leadership roles and higher injury burden. In this context, load management strategies such as modifying training sessions (e.g., selecting players as “floaters” during possession drills) or utilizing maturitycategorized strategies (biobanding)7,30,33 would enable better management of the frequency and volume of exposure. Besides, prescribing activities to facilitate development (e.g., working on technical proficiency of movement, core strength or mobility) while preventing unwanted mechanical stress would enable better management of the frequency and volume of exposure to stressful activities.7 Considering injuries that cause the greatest disruption according to each players maturation profile would be crucial when prescribing those activities. Nonetheless, injuries are multifactorial, and we cannot forget other risk factors (training load, neuromuscular and biomechanical factors, physiotherapy, coaching, communication, psychosocial factors, etc.) when designing injury prevention programs.39 ACKNOWLEDGEMENTS This work was partially supported by the University of the Basque Country (UPV/EHU) (GIU20/006). The corresponding author was supported by a PhD studentship from the Basque Government (PRE_2020_2_0145). These organizations have no roles in the collection of data, analysis, interpretation, and publication of the paper. DATA AVAILABILITY STATEMENT The data that support the findings of this study are available from the corresponding author, [XM], upon reasonable request. ORCID Xabier Monasterio https://orcid. org/0000-0001-9409-0059 Susana M. Gil https://orcid.org/0000-0003-0596-7627 REFERENCES 1. Larruskain J, Lekue JA, MartinGaretxana I, Barrio I, Mccall A, Gil SM. Injuries are negatively associated with player progression in an elite football academy. Sci Med Footb. 2021;110. doi: 10.1080/24733 938.2021.1943756. Online ahead of print. 2. Maffulli N, Longo UG, Gougoulias N, Loppini M, Denaro V. Longterm health outcomes of youth sports injuries. Br J Sports Med. 2010;44(1):2125. 3. Materne O, Chamari K, Farooq A, et al. Injury incidence and burden in a youth elite football academy: a fourseason prospective study of 551 players aged from under 9 to under 19 years. 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