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Early Specialization in Youth Sport: A Biomechanical Perspective
Abstract
This article examines, from a biomechanical perspective, three issues related to early specialization: overuse injuries, the developmental aspects, and the performance aspects. It concludes that "there is no evidence that early specialization causes overuse injuries or hinders growth and maturation." At the same time, early specialization has "clear performance benefits" in some sports where a specific physique is favorable, such as figure-skating.
... However, it is important to investigate the specific types of practice behaviors that performers engage in to determine how this affects the level of expertise that an individual attains along with their rate of development. This can assist with the design of appropriate training environments for developing expertise in players (Baker, 2003;Baker, Cobley, & Fraser-Thomas, 2009;Côté, Baker, & Abernethy, 2003;Ericsson et al., 1993;Ericsson & Smith, 1991;Mattson & Richards, 2010). Some researchers argue that practice has to be deliberate and purposeful to attain expertise (Charness, Tuffiash, Krampe, Reingold, & Vasyukova, 2005;Ericsson et al., 1993), while others argue that engaging in a wide range of activities is beneficial for expertise development as an enduring characteristic (Bridge & Toms, 2013;Goodway & Robinson, 2015). ...
This study followed a longitudinal design to objectively monitor practice behaviors of professional and semi-professional esports players over a year. Publicly available data were collected from 30 male Counter-Strike: Global Offensive players (age: 23.76 ± 2.88y). Players were classified into two groups: professional (n = 18) or semi-professional (n = 12). The total hours of practice (all game-specific practice) and the competitive hours of practice (time spent in competitive modes only) were collected weekly. Generalised Estimating Equations were used to compare the practice behaviors of the two groups. Professional and semi-professional esports players completed an average of 30.9 ± 8.2 h and 24.7 ± 3.6 h per week of total game-specific practice, respectively, and 19.6 ± 6.9 and 15.0 ± 2.7 h of competitive practice, respectively. A significant week∗group interaction was observed for total practice time (Wald χ2 = 9.48, p = 0.002) and total competition practice time (Wald χ2 = 7.54, p = 0.006). Specifically, professional esports players completed 6.6 (SE = 2.2) hr per week more of total practice hours than semi-professional players, of which 4.8 (SE = 1.8) hr were competitive practice. This sample of expert esports performers complete high volumes of practice which can be monitored via publicly accessible repositories.
... Overall, extensive engagement in domain-specific activities (e.g., competition, organised training, and individual practice) is necessary to attain expert performance (Ericsson et al., 1993;Ward, Hodges, Starkes, & Williams, 2007). In many domains, the attainment of expertise can be influenced by the time engaged in practice (Baker, 2003;Baker, Cobley, & Fraser-Thomas, 2009;Mattson & Richards, 2010). As a result, researchers have focused on identifying which type of practice is most beneficial for developing expertise, as this information can assist with improving the effects of practice (Ericsson et al., 1993). ...
This study aimed to examine the influence of the quantity of practice and the in-game performance during practice of professional esports players over an eight-week period immediately prior to a major esports tournament. Data was collected from 43 male professional esports players (age: 23.52 ± 2.50 y). A range of measures were collected on a weekly basis to describe the quantity of practice and represent in-game performance during practice. The relationship between practice and tournament performance was examined using individual linear mixed-effects models for each week prior to competition. In a final linear mixed-effects model which incorporated the relevant variables identified within the weekly models a significant average kill/death ratio + average score main effect on tournament performance was identified (p < 0.001, R 2 = 0.30). With every standard deviation increase in average kill/death ratio, there was a 7.94% increase in tournament score (95% CI: 3.86-12.18%, t = 3.89, p ≤ 0.001). With every standard deviation increase in average score, there was a 6.40% increase in tournament score (95% CI: 2.40-10.56%, t = 3.17, p = 0.003). Overall, the quantity of practice and in-game performance during practice explained a small proportion of the variance in tournament performance. More specifically, the variables that were most associated with better tournament performance were kill/death ratio and the score obtained in practice during the lead up to competition. Interestingly, the quantity of accumulated and weekly practice had limited association with better tournament performance. Whether the association between practice and performance differs depending on players' expertise levels requires future research.
... From a social point of view, the hierarchy of the role and benefits of figure skating practice will hopefully lead to the formation of fair behaviors and collectivity behavior of fair play in skating rinks. Studying the impact of practicing recreational figure skating among students that would eventually become the teachers that will promote the sport with young children is of tremendous importance because studies have shown that if the teachers are familiar with the sport that they will be teaching they can increase the attractiveness for pupils [25][26][27]. The literature review revealed insufficient evidence related to the appreciation of the benefits of practicing figure skating by students from the physical education and sports specialization who will become future school promoters of this sport. ...
The study aims to highlight the impact of practicing recreational figure skating by evaluating the relationship between the perceptions of motor, mental and social benefits of practicing figure skating and the frequency and duration allocated for this recreational activity. The study included 143 students of physical education and sport specialization. The questionnaire was designed to evaluate the students’ perception about the benefits of practicing recreational figure skating; it included 30 items divided in three parts: motor, mental and social benefits. Each of them contained 10 items to be assessed by students using the Likert scale, with 2 items related to the duration and frequency of practicing figure skating during recreational time. The results were processed using SPPS 24. The results were statistically significant at p < 0.05. The Cronbach’s Alpha coefficient of the questionnaire was α = 0.965, suggesting that the items had high internal consistency. Using the Likert scale, we found the following: concerning the high score 5 points—the motor capacity 62.9%, the mental capacity 49.7%, the social capacity 49.7%; and for a low score of 1 point—mental and social capacity 1.4%, motor capacity 0.7%. The effect size was medium for almost all items. No statistically significant correlations were found between the result of the questionnaire and the frequency and duration of practicing figure skating during free time. Figure skating is considered by students to be one of the activities through which a series of parameters of physical development and level of physical fitness can be improved through the expansion of motor skills. Also, the practice of figure skating contributes to the formation of proactive behaviors by improving the mental and social abilities of practitioners.
... De lichaamsbouw van kinderen (klein en licht) vergemakkelijkt de acrobatische bewegingen. Momenten van inertie zijn beperkt en de rotaties worden bevorderd [13]. Vooral in de puberteit van meisjes is het een hele uitdaging om een biomechanisch gunstige lichaamsbouw te behouden. ...
Enkele wetenschappelijke studies uit de jaren 90 toonden aan dat wie in bepaalde domeinen zoals sport een zeer hoog niveau wil bereiken, al heel vroeg en intens moet beginnen te trainen. Wie te lang wacht, maakt maar weinig kans om ooit de top te bereiken. De media-aandacht voor jonge sporters, de organisatie van belangrijke jeugdcompetities en de prestatiedrang op erg jonge leeftijd, hebben bijgedragen tot vroege specialisaties. Echter toonden de onderzoekers aan dat die aanpak niet bijzonder efficiënt is en bovendien de ontwikkeling en de gezondheid van het kind in het gedrang kan brengen. De laatste jaren worden alternatieve en kindgerichte oplossingen voorgesteld als ontwikkelingsmodellen op lange termijn.
... • It is the quality of instruction through good coaching, not the quantity that is most important in youth sport (Brylinsky, 2010). • Although there are limited biomechanical benefits of SSYS, the hazards of overuse and reduced recovery time provided by some coaches are apparent (Mattson & Richards, 2010). • The physiological effects of SSYS have rarely been examined. ...
The purpose of this study was to determine the knowledge and opinions on concussions of high school coaches from a geographically large yet rural state in the northern Rocky Mountains of the United States. Few medical issues in sport are more important, or have had as much publicity recently, as concussions. The exposure gleaned from tragic health issues among professional athletes postconcussion has motivated college and professional sport agencies to modify and enact strict guidelines in the diagnosis and treatment of suspected concussions. Yet underdiagnosis and mistreatment by some (former University of Michigan coach in 2014) highlight the need for continued education and enforcement of the current guidelines. Obviously, when a head coach from a major NCAA Division I university still places an athlete's health in jeopardy by disregarding a head injury, it is imperative that coaches' knowledge of concussions in smaller, more rural and remote programs is considered as well. We used a commercial electronic product to survey coaches across the state. Coaches responded voluntarily with their opinions and their current knowledge of issues related to concussions. The purpose was to identify the current status of knowledge and attitudes in hopes that any weaknesses can be addressed in additional coach education opportunities. Keywords concussions; high school coaches; rural state; knowledge; attitudes Language: en
... 12 However, numerous potentially negative effects have also been associated with early specialization, including physical, psychological, and social aspects. 10,13,14 For instance, the risks of early specialization typically include an increased risk of injuries, 15,16 eating disorders, 17 psychological stress, and dropping out of sports. 18,19 Conversely, several studies 7,[20][21][22] have demonstrated that numerous elite athletes participated in a wide range of sports prior to specialization in one sport during mid-adolescence. ...
The development of expert-level performance in sports is discussed against the background of two different pathways. The early specialization approach emphasizes both early onset and high volumes of sport-specific practice in a desired main sport. On the other hand, the diversification approach promotes diversified involvement in a range of other sports with later specialization. This study examines the way in which youth coaches translate the antagonistic concepts of specialization and diversification into their coaching strategies during the early stages of talent development. Using qualitative research methodology, 44 expert German youth coaches (M age45.1 years, SD 7.5; 39 male and 5 female) in 24 different sports, with an average of 21.7 years accumulated coaching experience (SD 7.0) were included in the inductive thematic analysis. Analysis showed a differentiated understanding of the process of specialization that considered multiple ways to apply diversification both within sports and across several sports. Although all of the coaches appeared to acknowledge the importance of within-sports diversification, there was considerable variation in the reported purposeful implementation and significance of sporting activities other than the main sport leading to a nuanced typology of strategies. The typology could be divided into the following four categories based on the preferred strategies that the coaches described: (I) changers and late entrants; (II) early engagement in DS + secondary sports; (III) early engagement in DS + supplementary sports; and (IV) early engagement in DS + specialization. The findings strengthened existing suggestions of a gradual and multidimensional understanding of diversification prior to necessary special- ization. Furthermore, it can be assumed that the preferred strategies are fundamentally influenced by individual cognitions and contextual aspects that acknowledge the complex and ideographical nature of coaching.
... • It is the quality of instruction through good coaching, not the quantity that is most important in youth sport (Brylinsky, 2010). • Although there are limited biomechanical benefits of SSYS, the hazards of overuse and reduced recovery time provided by some coaches are apparent (Mattson & Richards, 2010). • The physiological effects of SSYS have rarely been examined. ...
The efficacy of sport specialization in youth sport is a source of parental confusion and potential conflicts of interest with coaches. Sport specialization is the exclusive participation in one sport in the belief that it will increase the chances of receiving an athletic college scholarship and/or being able to pursue a career as a professional athlete. While there is continued debate over sport specialization, it is imperative that coaches convey the most current and valid information to parents in a non-biased manner. Failure to present all aspects of sport specialization erodes the professional and ethical responsibilities of coaches and places many young athletes' health at risk.
... Third, one needs to be careful to draw generalisable recommendations from such an exceptional minority of elite youth players to the overwhelming number of football players world-wide who may neither have the ability nor desire to reach such a level (FIFA, 2007; Malina, 2010). Many authors have underlined the lack of consensus about the potential benefits or negative consequences of early specialisation (Gould, 2010; Kaleth & Mikesky, 2010; Malina, 2010; Mattson & Richards, 2010 ). Haugaasen and Jordet (2012) highlighted the need for implementing more sport-specific nuances to such a debate, where both sportspecific studies, such as the present study, but also comparative studies across sports would represent important contributions (e.g., Baker et al., 2003). ...
Abstract Despite the large amount of research available on how engagement in football practice relates to future performance level among football players, similar information about the contribution of non-football activities is scarce. Based on data from 745 elite youth players this study aimed to identify the characteristics and contribution of diverse participation towards elite youth and senior professional status. The data were collected using a retrospective questionnaire where the players reported the amount of time spent in other sports than football, in addition to their perceived contribution of different non-football activities for developing football skills. The accumulated hours of time spent in other sports of players who had obtained a senior professional contract were compared to non-professional players, using multilevel modelling (n = 558), while a t-test compared the activity ratings to each other. No significant differences were identified between professional and non-professional players' engagement history, but overall, the players rated sports similar to football to be significantly more relevant for developing football skills than other sports. The results suggests that spending time in non-football activities did not contribute to present differences in performance attainment in football, but also that potential advantages of such activities may be related to their characteristics.
... In the last significant review of this area, it was concluded that the literature is inadequate to resolve this issue (Baker, Cobley, & FraserThomas, 2009). Neither in psychology (see Gould, 2010), physiology (see Kaleth & Mikesky, 2010) nor biomechanics (see Mattson & Richards, 2010) have researchers been able to draw clear conclusions. Malina (2010) points out the athletes' activity environment as one of the key issues, where coaches and parents are important factors in both reducing and enhancing potential negative effects of sport participation. ...
About 265 million people regularly play football, of which only 0.04% play in a professional league. This suggests that reaching expertise specifically in football is difficult and highly competitive. In recent decades, research on the development of football expertise has increased substantially. However, most of these studies have focused on isolated aspects of the developmental process, and consequently there has been a lack of overview of how these different aspects interact to produce football experts. The aim of this study is to review available research on the development of expertise specific for football. The review is limited to papers found in the electronic SportDiscus database. The 2007 Developmental Model of Sport Participation (DMSP) has been used as the conceptual framework. The results highlight the importance of football-specific practice in early developmental phases in becoming an expert in football. It is also proposed that football-specific nuances may need to be applied to the DMSP in order to make the debate of early specialization versus diversification sufficiently valid for this particular context. Consequences for practice engagement and future research are discussed.
Introduction: While practitioners and organizations advise against early specialization, the lack of a consistent and clear definition of early specialization reduces the impact of recommendations and policies in youth sport. An important first step in understanding the consequences of early specialization is establishing what early specialization is. Objectives: This PRISMA-guided systematic review aimed to determine the types, characteristics, and general content of early specialization papers within the literature, and examine how early specialization has been defined and measured in order to advance knowledge toward a clear and consistent definition of early specialization. Data sources: Four different electronic databases were searched (SPORTDiscus, Web of Science, Sports Medicine and Education Index, and Scopus). Both non data-driven and data-driven studies were included to ensure a comprehensive understanding of the literature. Eligibility Criteria: In order to be included in the review, the paper must: (a) Focus on specialization and explicitly use the term “specialization” (b) Focus on sport and athletes (c) Be papers from a peer-reviewed (d) Be in English. And finally, (e) be available in full text. Results: One thousand three hundred and seventy one articles were screened resulting in 129 articles included in the review after applying inclusion/exclusion criteria. Results indicated a clear discrepancy between key components of early specialization and the approaches used to classify early specializers. Conclusion: Future research should work toward developing a valid and reliable approach to classifying early specializers and establishing a consistent definition across studies.
This paper presents an excerpt from a comprehensive study on the concepts of long-term athlete development. Following a qualitative research methodology and using a hermeneutic-reconstructive procedure the purpose of this paper was to gain insight into expert youth sports coaches’ understanding of the two main trajectories leading to elite performance: early specialization and early sampling. Analysis of the transcribed interviews suggests that the training strategy is primarily the result of a complex decision making process. Additionally, coaches perceived a trend towards early specialization which conflicts with existing considerations of long-term athlete development. The results of this study provide an opportunity to understand the subject of diversification in youth sports and to raise awareness for the problems of practitioners.
The role of early specialization in the development of sport expertise is a point of contention among researchers. While there is consistent evidence linking quantity of training with level of proficiency attained, a focus on specialized training during early stages of development has been linked with several negative consequences. Diversified involvement in a number of sports during early stages of development has been presented as a possible alternative to early specialization. Considering the consequences of advocating the early specialization ap- proach and research suggesting the effectiveness of early diversification, coaches and sport scientists should consider the early diversification approach as an alternative. Further research is required to expand our understanding of the relative contributions of diversified versus specialized training. The acquisition of expertise in sport is the result of complex interactions among biological, psychological, and sociological constraints (Singer & Janelle, 1999). Successful negotiation of these constraints can lead to the highest levels of perform-
In November 2005, the International Olympic Committee's Medical Commission issued a statement on Training the elite child athlete recommending that “more scientific research be done to better identify the parameters of training the elite child athlete”. This paper focuses on a specific issue related to training the child athlete: early specialization. While associations between early specialization and expertise development are highlighted, much research also suggests early specialization is associated with a range of negative consequences affecting physical, psychological, and social development. Alternately, some researchers have proposed that an early diversification approach does not disadvantage athletes in acquiring expertise, and is important for the development of intrinsic motivation and skill transferability. However, this review suggests that inadequate evidence exists to resolve the issue in favor of either approach.
A group of 453 elite young athletes (231 boys, 222 girls) in five two year age groups from 8-16 years of age was followed up for two years in order to identify self reported injuries over that period. Four sports were studied, namely football (soccer), gymnastics, tennis, and swimming. The injury rate was low with just over half the children suffering one or more injuries per year, with the majority of those injured sustaining one injury only. Over the two year period of intensive sporting activity this amounted to less than one injury per 1000 hours of training. The highest risk of injuries was in football (67%) and the lowest in swimming (37%). Most injuries (70%) were acute and of a minor nature, although overuse injuries did require longer periods off training and competition than acute injuries (20 v 13 days). Footballers appeared to sustain more significant injuries than other sports as judged by the time required to resume training and/or competition (16 days after acute and 57 after overuse). No significant associations were found between injury rate, injury severity, sex, and pubertal status with the single exception of female gymnasts in whom more injuries occurred in the latter stages of puberty. Only four of the 453 athletes reported injury as a reason for retiring from their chosen sports. Most injuries in elite young athletes are minor, their prevalence is low and, at least in the short to medium term, do not constitute a significant health problem.
The theoretical framework presented in this article explains expert performance as the end result of individuals' prolonged efforts to improve performance while negotiating motivational and external constraints. In most domains of expertise, individuals begin in their childhood a regimen of effortful activities (deliberate practice) designed to optimize improvement. Individual differences, even among elite performers, are closely related to assessed amounts of deliberate practice. Many characteristics once believed to reflect innate talent are actually the result of intense practice extended for a minimum of 10 years. Analysis of expert performance provides unique evidence on the potential and limits of extreme environmental adaptation and learning.
Children involved in sports should be encouraged to participate in a variety of different activities and develop a wide range of skills. Young athletes who specialize in just one sport may be denied the benefits of varied activity while facing additional physical, physiologic, and psychologic demands from intense training and competition. This statement reviews the potential risks of high-intensity training and sports specialization in young athletes. Pediatricians who recognize these risks can have a key role in monitoring the health of these young athletes and helping reduce risks associated with high-level sports participation.
In 1987, during two great skating contests--the Universiade in the Tatra Mountains and the Gold Pirouette in Zagreb--a total of 42 world class skaters were asked through a questionnaire if they had ever in their career suffered from a stress fracture. Of the 42 skaters, 9 had stress fractures. Four stress fractures occurred during preseason training (two fibular, one second metatarsal, and two fourth metatarsal stress fractures). Increased mileage was reported by three skaters and the fourth had done too much speed training on hills. Five stress fractures occurred during the season (one tibial and two tarsal navicular stress fractures and two stress fractures of the base of the fifth metatarsal). In all cases, the fracture occurred in the take-off leg. All of the subjects were competitive figure skaters with a daily training period of 3 to 8 hours, six times a week. The time from the onset of symptoms to definite diagnosis ranged from 2 to 10 weeks. Of the nine injured skaters, eight were treated conservatively and one skater with Jones' fracture was treated surgically. All of the skaters were able to resume a preinjury level of activity 3 to 7 months after treatment began. In conclusion, it may be emphasized that stress fractures in figure skaters are not rare and should, therefore, always be considered as a possibility.
Reports dating back to the 1920s have unequivocally established that efficiency of movement improves during infancy and childhood, and generally through adolescence. The inclination from 1940 to 1960 to record movement in quantitative values has given way during the last two decades to a reemphasis of qualitative assessment. Investigators commonly agree that movement skills change in an orderly manner, but controversy continues over the degree of accuracy obtainable with the various descriptive forms, and over the utility of products resulting from such assessments. Comparison of the quantitative changes in movement skills of children is difficult because standardized procedures of test administration have not been applied. When comparisons are possible because of similar or identical testing protocols, improvement in selected motor tests is evident in both boys and girls until adolescence. At approximately 13 years of age the performance of girls in some tests reaches a plateau, and may even decline thereafter, while boys continue to improve in skills requiring strength, power, and muscular endurance. Exceptions to these generalizations occurred in arm and shoulder girdle muscular endurance, as measured by the flexed-arm hang, where boys had superior performances beginning at age 7, and in flexibility, as measured by the sit and reach test, where girls excelled at age 5 and thereafter. Stability of motor performance was greater for tasks that required all-out effort than for those emphasizing accuracy or total body coordination. Relationships between successive measures taken during early and middle childhood are likely to decline more rapidly than those taken after adolescence. Girls generally were more stable in motor performance than boys, except in the Motor Performance Study, where the values for boys across a range of 5 to 6 years were clearly more stable. This review underscores the need for careful documentation of the conditions under which data on motor performance are obtained. Numerous reports were examined and excluded by the authors because information that would have qualified the data for comparative analysis was not available. Essential ingredients in such reports are descriptions of the sampling techniques and the manner of calculating chronological ages, socioeconomic status, ethnic and racial characteristics, evidence of secular changes, geographic and environmental characteristics, and a detailed account of the testing procedures. Differences in maturational age for a given chronological age, and accompanying assumptions about body size, may account for differences in motor skills that might otherwise be attributed to changes in p
To compare the bone mineral density (BMD) of competitive female teenage figure skaters with a history of fracture with the BMD of skaters without fracture and to compare each group to age-matched, nonathletic controls.
Retrospective age-matched cohort.
Tertiary care medical center and 3 local skating clubs.
Thirty-six adolescent female competitive skaters (10 with fracture, 26 without fracture) to 22 age-matched controls.
Not applicable.
BMD was estimated by quantitative ultrasound.
Skaters who had suffered stress fractures had BMD values comparable with those of healthy nonathletic controls. However, skaters who had not suffered stress fractures had calcaneal BMD values 15% to 24% greater than either the controls or skaters with fractures. Among the skaters without fracture, there was a 14% to 19% higher calcaneal BMD in skaters who executed triple jumps relative to skaters who performed only double jumps. Furthermore, there was 7% to 11% greater BMD in the landing foot of the skaters relative to the takeoff foot.
Stress fractures in adolescent skaters are not caused by low bone mass but may result from excessive forces placed on a normal skeleton. Our findings also support the hypothesis that higher peak forces are applied to the landing foot relative to the takeoff foot.
Forces that are repeatedly applied to the body could lead to positive remodeling of a structure if the forces fall below the tensile limit of the structure and if sufficient time is provided between force applications. On the other hand, an overuse injury could result if there is inadequate rest time between applied forces. Running is one of the most widespread activities during which overuse injuries of the lower extremity occur. The purpose of this article is to review the current state of knowledge related to overuse running injuries, with a particular emphasis on the effect of impact forces. Recent research has suggested that runners who exhibit relatively large and rapid impact forces while running are at an increased risk of developing an overuse injury of the lower extremity. Modifications in training programs could help an injured runner return to running with decreased rehabilitation time, but it would be preferable to be able to advise a runner regarding injury potential before undertaking a running program. One of the goals of future research should be to focus on the prevention or early intervention of running injuries. This goal could be accomplished if some easily administered tests could be found which would predict the level of risk that a runner may encounter at various levels of training intensity, duration, and frequency. The development of such a screening process may assist medical practitioners in identifying runners who are at a high risk of overuse injury.
The purpose of this paper is to review the biomechanics of triple and quadruple figure skating jumps, focusing on information that has implications for strength and conditioning programs. At a minimum, to complete the required revolutions in a jump, a skater must balance the average angular velocity with the time in the air. Vertical velocity at takeoff is similar in high revolution jumps to that in low revolution jumps; however, when comparing skaters of different abilities, those with higher abilities generate greater vertical velocities at takeoff for the same type of jump. Powerful extension of the legs is the primary factor in generating vertical velocity. Some jumps use asymmetrical extension of both legs, while other jumps involve extension of only one leg. Angular velocity is controlled primarily by the skater's moment of inertia, which means skaters must forcefully arrest the motion of the arms and legs after the propulsion phase and then quickly position the arms and legs close to the axis of rotation during flight. Training exercises that emphasize eccentric and concentric muscle actions and which are adaptable to asymmetrical or unilateral motions, such as box jumps and medicine ball throws, are a crucial component to off-ice training programs for figure skaters.
What do we know about early specialization? Not much! High Ability Studies Low injury in rates in elite athletes
- J Baker
- Cobley
- J Fraser-Thomas
Baker, J., Cobley, s., & Fraser-thomas, J. (2009). What do we know about early specialization? Not much! High Ability Studies, 20(1), 77-89. Baxter-Jones, a., maffuli, N., & Helms, P. (1993). Low injury in rates in elite athletes. Archives of Disease in Childhood, 68, 130-132.
- Mair S. D.
- American Academy of Pediatrics