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Objectives To investigate the association of maturity status with injury incidence in Middle-Eastern youth athletes. Design Prospective cohort study. Setting Four consecutive seasons (2010–2014), Aspire Academy, Qatar. Participants Male athletes (age range: 11–18 years) representing four disciplines enrolled and grouped into two categories: individual sports and racquet sports. Outcome measures Injury data collected over four seasons. Athletes’ anthropometric characteristics assessed to calculate age at peak height velocity. Predicted mature heights (PMHs) collected and categorised into four quartiles. Athletes had wrist and hand radiographs for assessment of skeletal age (SA). Early and late maturers with an SA of >1 year older or younger than their chronological age (CA). Results For the sample (n=67) across all groups, 43 (64%) athletes had one or more injuries: total of 212 injuries, 4.9 injuries per athlete across study. Survival analysis of maturity status using SA found early maturing athletes had two-fold greater injury risk compared with late maturers (HR 2.04, 95% CI 1.15 to 3.61, p=0.015). PMH associated with injury risk (HR 1.05, 95% CI 1.01 to 1.08, p=0.006). Athletes in fourth quartile (≥184 cm) had up to two-fold injury risk (HR 2.41, 95% CI 1.42 to 4.08, p=0.001). Racquet and individual sports involved similar injury risk (HR 1.14, 95% CI 0.86 to 1.52, p=0.37). Conclusion SA early maturity and PMH gradient were significant predictors of injury in youths. This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. Other information Published in: BMJ Open Sport & Exercise Medicine License: http://creativecommons.org/licenses/by-nc/4.0 See article on publisher's website: http://dx.doi.org/10.1136/bmjopen-2018-023284

Motor coordination and physical fitness are multidimensional concepts which cannot be reduced to a single variable. This study evaluated multivariate relationships among morphology, physical fitness and motor coordination in 74 pre-pubertal girls 8.0-8.9 years of age. Data included body dimensions, eight fitness items and four motor coordination tasks (KTK battery). Maturity status was estimated as percentage of predicted mature stature attained at the time of observation. Canonical correlation analysis was used to examine the relationships between multivariate domains. Significant pairs of linear functions between indicators of morphology and fitness (r(c) = 0.778, Wilks' Lambda = 0.175), and between fitness and motor coordination (r(c) = 0.765, Wilks' Lambda = 0.289) were identified. Girls who were lighter and had a lower waist-to-stature ratio and % fat mass attained better scores in the endurance run, sit-ups and standing long jump tests, but poorer performances in hand grip strength and 2-kg ball throw. Better fitness test scores were also associated with better motor coordination scores. Relationships between body size and estimated fatness with motor fitness suggested an inverse relationship that was particularly evident in performance items that required the displacement of the body through space, while motor coordination was more closely related with fitness than with somatic variables.

From the literature, it has been massively recommended that talent identification, development and selection processes in youth soccer should provide a longitudinal, holistic approach accounting for maturation and relative age. The sport of soccer systematically excludes those players who are later to mature and/or who are later born in the in the selection year, whilst these players might be as gifted as their earlier maturing and/or earlier born peers. There are often no or insufficient objective criteria that could support the evaluation process. The present thesis aimed to gain insight in young soccer players’ development of anthropometrical characteristics, physical fitness and motor coordination parameters with respect to maturation and relative age. Therefore, the conducted research was divided into four different chapters. The first chapter investigated (1) test-retest reliability and validity of the intermittent endurance performance, assessed by the Yo-Yo Intermittent Recovery test level 1 (YYIR1) in elite, suband non-elite players (study 1, n=228, 10-17 y; study 2, n=36, 13-18 y ), (2) the short- and long-term stability of anthropometrical characteristics and YYIR1 of 42 pubertal soccer players (study 3), and (3) the relationship between invasive (skeletal age) and non-invasive (estimation of age at peak height velocity) protocols to estimate adult stature on the one hand, and the agreement between methods assigning players to somatic maturity categories on the other in a mixed-sample of 160 Belgian and Brazilian elite players (study 4). Combining the results of the first two studies, the YYIR1 seems more reliable at elite level and at older ages (U17-U19) compared with sub-/non-elite level and at younger ages (U13-U15). Also, players with a relatively low YYIR1 performance at the age of 12 years are able to (however partially) catch-up the better performers over a four-year period, suggesting the need for individualization within the training process (study 3). Furthermore, the fourth study demonstrated that invasive and non-invasive protocols correspond well in estimating mature stature, although transforming estimated APHV into somatic maturity categories has proven to be problematic in elite youth soccer players. The second chapter focused on the influence of relative age on both aerobic (YYIR1) (study 5, n=606, U10-U19) and anaerobic performance measures (speed and explosive leg power) (study 6, n=374, U13-U17). A clear overrepresentation of players born in the first part of the selection year was found in both studies, although relative age did not confound aerobic as well as anaerobic performance measures. This might be explained by the fact that (1) the formation of homogenous players in terms of aerobic and anaerobic performances was already manifest before the age of 10 years, and (2) this reflects the variation in maturity status among players within the same age-cohort. The third chapter investigated the longitudinal development of the YYIR1 performance (study 7, n=162, 11-14 y) and explosive leg power (study 8, n=356, 11-14 y; study 9, n=555, 7-20 y) via multilevel analyses. Also, retrospective data were used to predict drop out, contract status and first-team playing time using anthropometrical, maturational, physical fitness and motor coordination characteristics (study 10, n=388, 8-16 y). Generally, the results highlighted that non-specific motor coordination contributed significantly to the development of aerobic and anaerobic performances, and that this parameter could distinguish between future successful and less successful young soccer players. Further, young soccer players possessing higher levels of explosive leg power from the age of 16 years are more likely to sign a professional contract and are receiving more playing minutes at the professional adult level. The final chapter described differences in 744 youth soccer players’ (9 to 18 y) anthropometrical characteristics and general fitness level through aerobic and anaerobic tests according to the playing position on the field (study 11). The results revealed that inherent anthropometrical and physical capacities (i.e., speed, power, agility) might select players in or reject players from certain positions, and the time around peak height velocity seems to be crucial in this selection process. However, the typical characteristics for the different playing positions at senior level are yet not fully developed among young soccer players between 8 and 14 years, although the typical anthropometrical characteristics of goalkeepers (i.e., taller and heavier) were already manifest at young age. In conclusion, the abovementioned studies in this thesis (1) emphasize the use of the YYIR1 as a valid, reliable and maturity-independent tool to assess a players’ intermittent endurance capacity, (2) highlight that the selection process is focused on the formation of homogenous groups of players in terms of anthropometrical, maturational and physical fitness parameters, independent of playing position, and (3) that non-specific motor coordination is essential in the development of physical fitness measures and should be included in the training process.