Health Consequences of Youth Sport Specialization.

Sport specialization is a training method now commonly used by young athletes who hope to achieve elite-level success. This may be defined as (1) choosing a main sport, (2) quitting all other sports to focus on 1 sport, and (3) year-round training (greater than 8 months per year). A number of sports medicine organizations have published recommendations based on the limited evidence available on this topic. The objective of this article was to perform a narrative review of the currently available evidence and sports medicine organizational recommendations regarding sport specialization and its effects on health to guide athletic trainers and sports medicine providers. To accomplish this goal, we conducted a narrative review of articles and position statements on sport specialization published from 1990 through 2018. Injury, overuse injury, serious overuse injury, and lower extremity injury were likely associated with higher degrees of sport specialization in various populations. Sports medicine organizations in general recommended against sport specialization in young athletes and instead promoted multisport participation for physical and psychological benefits. Few long-term data suggest that sport specialization has negative health-related quality-of-life consequences. Higher degrees of sport specialization likely pose risks of overuse injury; however, the age of specialization at which this risk occurs is not known. Because different populations and sports activities may put children at risk for certain injuries, future researchers should monitor large populations with sport-specific prospective active surveillance.

Sport specialization is a training method now commonly used by young athletes who hope to achieve elite-level success. This may be defined as (1) choosing a main sport, (2) quitting all other sports to focus on 1 sport, and (3) year-round training (greater than 8 months per year). A number of sports medicine organizations have published recommendations based on the limited evidence available on this topic. The objective of this article was to perform a narrative review of the currently available evidence and sports medicine organizational recommendations regarding sport specialization and its effects on health to guide athletic trainers and sports medicine providers. To accomplish this goal, we conducted a narrative review of articles and position statements on sport specialization published from 1990 through 2018. Injury, overuse injury, serious overuse injury, and lower extremity injury were likely associated with higher degrees of sport specialization in various populations. Sports medicine organizations in general recommended against sport specialization in young athletes and instead promoted multisport participation for physical and psychological benefits. Few long-term data suggest that sport specialization has negative health-related quality-of-life consequences. Higher degrees of sport specialization likely pose risks of overuse injury; however, the age of specialization at which this risk occurs is not known. Because different populations and sports activities may put children at risk for certain injuries, future researchers should monitor large populations with sport-specific prospective active surveillance.
Key Words: athletic injuries, stress, pain, athletes, children, adolescents I n the United States, nearly 60 million youths participate in organized athletics, and nearly 8 million adolescents participate in high school interscholastic sports each year. 1,2 However, recent estimates 3,4 indicated that approximately half of adolescent athletes participated on club teams in addition to their school-based teams in order to train year-round in a chosen sport. This increasing emphasis on specialization in youth sports (ie, athletes 18 years old or younger) is a growing concern among athletic trainers (ATs), physicians, and other health care providers. A number of position statements 1,5-9 from medical organizations have provided recommendations for youth athletes to reduce the risks of specialization.
Recent authors 3,10-13 have reported prevalence rates of sport specialization among youth athletes of 17% to 41%. Rates vary depending on factors such as athlete sex, age, sport, socioeconomic status (SES), school size, and geographic location. 3,[10][11][12][13] The prevalence rates indicate that specialization is a relatively common occurrence in modern youth sports. One proposed consequence of increasing rates of specialization is the potential for psychological burnout among youth athletes who specialize and train year-round starting at an early age. [14][15][16][17] The degree of specialization, as measured by the Jayanthi scale, is also thought to contribute to these adverse health effects. 18 Of even greater concern may be the effect of SES on sport specialization, which creates the potential for reduced access to sports among those youths who cannot afford (the time or money) to focus on a single sport yearround. 12,19 However, of more immediate concern to ATs and other clinicians working in the youth sports arena may be the potential physical risks of specialization and overtraining, such as overuse injury, acute injury, and illness. Therefore, the aim of our narrative review was to provide an overview of the current state of evidence related to the association of sport specialization with injury and illness in youth athletes. We also highlighted the recommendations made in an attempt to reduce the physical risks of specialization and the evidence supporting those recommendations (or lack thereof).

Data Sources
A search of PubMed using the terms (youth) AND (sport specialization) AND (injury OR illness) for peer-reviewed articles was performed in July 2018. Additionally, the authors contributed relevant articles and used select reference lists to identify relevant papers that were not found in the initial search. Position and consensus statements that provided recommendations regarding sport specialization were gathered from medical and athletic organizations. Because we chose to conduct a narrative review rather than a systematic review, we did not strictly follow the guidelines for ''Preferred Reporting Items for Systematic Reviews and Meta-Analyses'' (PRISMA; http:// www.prisma-statement.org/).

Study Selection
The criteria for article inclusion were (1) original research or consensus or position statement, (2) written in English, (3) published in 1990 or later, and (4) an aspect of sport specialization (such as number of sports played, months or hours per week participating in a single sport, playing on multiple teams of the same sport at the same time) was examined as a risk factor for injury or illness in youth athletes. Articles were excluded if they were (1) narrative or systematic reviews or meta-analyses, (2) editorials, (3) critically appraised topics, (4) abstracts, or (5) based on animal research.

Data Extraction
After the initial search, we extracted and read the relevant articles. Final decisions for article inclusion were made by consensus of the study team, which consisted of 2 sports medicine physicians (N.A.J., T.C.L.), 1 AT with a doctoral degree (E.G.P.), and 1 orthopaedic surgeon with sports medicine training (P.D.F.), all of whom were established researchers in the area of sport specialization. Any articles that initially lacked consensus for inclusion were discussed by the team before a decision was reached. Relevant consensus and position statements that provided recommendations regarding sport specialization are listed in Table 1. Relevant original research articles examining aspects of sport specialization as a risk factor for injury or illness are presented in Table 2.

FINDINGS AND DISCUSSION
What Do Organizations Recommend Regarding Sport Specialization?
Organizations including the American Academy of Pediatrics (AAP), American Medical Society for Sports Medicine (AMSSM), American Orthopaedic Society for Sports Medicine (AOSSM), Fédération Internationale de Médecine du Sport (FIMS), International Olympic Committee (IOC), National Athletic Trainers' Association (NATA), and National Strength and Conditioning Association (NSCA) have released position statements regarding early sport specialization and youth athlete development. The recommendations can be subdivided into several categories-the official position on the topic, risks associated with early specialization, benefits of sports sampling, appropriate age of specialization, and miscellaneous suggestions-to be considered when analyzing a young athlete's decision to train or specialize in a single sport ( Table 1).

Definition of Sport Specialization
No consistent definition of sport specialization exists. One of the original definitions suggested that sport specialization is ''intense, year-round training in a single sport at the exclusion of other sports.'' 1,6,28 The 3 key components of this definition (year-round training, choos-ing a main sport, and quitting other sports) became the basis for the most commonly used method of classifying specialization, the 3-point Jayanthi scale. 3,4,[10][11][12][13]18,19,29,30,31 This scale classifies an athlete as having a low, moderate, or high level of specialization based on his or her responses to 3 yes/no questions. To determine if a young athlete is specialized, one should ask (1) whether the athlete has quit all other sports to focus on the main sport (or only ever played 1 sport), (2) if the primary sport was viewed as more important than other sports, and (3) if more than 8 months in the year was spent training or participating in the primary sport. A yes answer to a question receives 1 point: a total score of 0 to 1 points is classified as low specialization, 2 points as moderate specialization, and 3 points as high specialization.

Sports Sampling
All 7 of the organizations listed in Table 1 support early sports sampling due to the numerous health benefits provided. 1,[5][6][7][8][9] Early exposure to a variety of sports is thought to promote longer-term success in and enjoyment of sports. 5,7 It also allows athletes who do eventually specialize at an older age to identify which sport best fits their interests, physique, and capabilities. 7 Moreover, movement diversity allows young athletes to acquire a broader range of neuromuscular patterns that may be protective against overuse injury. [6][7][8] The AAP, AMSSM, and AOSSM recommended participation in multiple sports at least until physical maturation. 1,5,6 Early Sport Specialization The AAP and AMSSM did not specify an age threshold for ''early'' specialization; however, the consensus was that specialization before adolescence may increase the risks of overuse injuries and burnout. 1,5,6 The AOSSM position statement first introduced the concept of early sport specialization, which involved choosing a single sport earlier than age 12. 5 Currently, no strong evidence supports this age recommendation, but further studies may help to identify an age (or stage of development) when these risks of chronic injury and burnout increase. Although these organizations consistently discouraged early specialization and promoted exposure to a wide array of sports, sports that require skill development and peak performance before puberty ends may be the few exceptions. Examples provided by the AAP, AMSSM, AOSSM, and Council on Sports Medicine and Fitness were dancing, diving, figure skating, and gymnastics. 1,5,6 Early specialization may be inevitable at the elite level in particular sports, as these athletes tend to perform optimally in their teens and early 20s. 6 Not all organizations view sport-specialized training as universally detrimental. The IOC suggested in 2015 that with proper precautions and a solid support system, it is indeed possible for athletes who specialize at an early age to have a positive experience. 20

Intensive Training and Age of Specialization
A major concern is that early specialization has been associated with increased training and competition loads. 1,8 From a biomechanical standpoint, these may result in excessive exposure to a narrow spectrum of repetitive body   ,.0001 movements without an adequate interval for recovery. 8 The AAP 6 and NATA 7 recommended counseling athletes who do specialize to take at least 3 nonconsecutive months off in 1-month intervals each year; furthermore, the AAP advised young athletes to take 1 to 2 days off per week to ensure adequate recovery. These athletes should also discuss their sport aspirations with appropriate personnel and assess the practicality of these goals. 5 The AAP, 6 AMSSM, 1 and AOSSM 5 endorsed delaying specialization until late adolescence. By this age, athletes have already honed the physical, psychological, and social skills necessary to effectively specialize in a single sport. 6 Although the recommended level of intensity was not specified, most organizations advised youth athletes to take more than 1 month off from their sport in a year, potentially pursue fewer weekly training hours than their age, and have a good support system both in sport and at home to reduce the psychological effects of intense training.
What Are the Risks Associated With Sport Specialization?
An expanding body of primary scientific literature has explored the associations between youth sport specialization and an increased risk of overuse injury ( Table 2). Before 2015, primary research consisted largely of smaller cross-sectional or survey studies, which may have been susceptible to recall and observer bias. These investigations typically focused on a single sport, age demographic, or injury type; a few focused on several categories. However, the results began to guide clinical recommendations and inform more recent research involving larger prospective surveys and active surveillance epidemiologic studies.

Risk of Overuse Injury
Since the introduction of the Jayanthi scale 18 in 2015, the authors of 8 of 10 identified studies (Table 2) used some or all components-year-round training (more than 8 months), Abbreviations: HR, hazard ratio; OR, odds ratio; OSD, Osgood-Schlatter disease; RR, risk ratio; SLJ, Sinding-Larsen-Johansson syndrome. a Physical activities included aerobics, bicycling, golf, gymnastics, Pilates, running, skiing, swimming, waterskiing, weight lifting, and other activities. b Injuries to the groin, hip, thigh, knee, lower leg, ankle, or foot were included in lower extremity injuries. c Defined as .60 primary sport competitions within the past year. d Defined as an injury that occurred during sport and that caused the athlete to seek medical care. e Sport-specialization criteria: year-round training (.8 mo), choosing a main sport, and quitting all other sports to focus on 1 main sport (low ¼ meets 0 or 1 criterion, moderate ¼ 2, high ¼ 3) from Jayanthi et al. 18 f Defined as those for which the physician-recommended treatment included at least 1 mo of rest from sports. g Unspecified injury, ranging from ankle sprains to operative conditions such as anterior cruciate ligament ruptures and ulnar collateral ligament ruptures. h Defined as participation in only 1 sport. i Serious overuse injuries included spondylolysis, pars stress injuries to the spine, osteochondritis dissecans, overuse elbow/ligament injuries, and all other ankle, foot, leg, knee stress fractures. j Self-reported, in the past 12 mo. Response choices were often, sometimes, and never. k Adjusted for age, body mass index, and region (geographic). l Defined as a pitching-related injury (such as shoulder or elbow pain) that caused the athlete to miss pitching in at least 1 practice or game in the past 12 mo. m Defined as any acute-, gradual-, or recurrent-onset injury to the lower extremity musculoskeletal system. In addition, each lower extremity injury must have (1) occurred as a result of participation in an interscholastic sport practice or competition and (2) required medical attention by an athletic trainer or physician. All lacerations, abrasions, and contusions to the lower extremity were excluded. n Defined as the percentage of yes responses to ''Did you ever sustain an injury that you attributed to specializing in 1 sport?'' o A higher percentage of current collegiate athletes specialized to play a single sport during their childhood/adolescence (high school ¼ 45.2%, collegiate ¼ 67.7%, professional ¼ 46.0%; P , .001). This may suggest that early sport specialization can be a factor in the occurrence of early sport-related injury, but of course, multiple factors affect the occurrence of injury in sport.
choosing a main sport, and quitting all other sports to focus on 1 main sport-to describe the degree of sport specialization. Sport specialization is then categorized as low (0 or 1 criterion), moderate (2 criteria), or high (3 criteria). Despite the methodologic limitations of some of the existing research, consistent findings have linked high degrees of sport specialization with higher rates of injury, particularly overuse injuries. These self-reported injuries included strains, Osgood-Schlatter disease, patellofemoral pain, and Sinding-Larsen-Johansson syndrome. Additionally, serious overuse injuries are typically defined as those that result in the loss of 1 month or more of playing time or participation. These include spondylolysis, osteochondritis dissecans, elbow ligament injuries, and stress fractures. Serious overuse injuries have been noted more frequently in specialized youth athletes in individual sports, 22 with a high degree of sport specialization adjusted for hours of training and age, 13,18 and participating in more hours of sports per week than their age in years. 13,18 Acute Injury Risk Paradoxically, researchers have shown no link between a high degree of sport specialization and acute injury risk. Furthermore, a ratio .2 : 1 hours per week (adjusted for age and hours training in sports) of organized sports to free playtime may be associated with a reduced risk of acute sports injuries. 18 This may reflect the sport selection: specialized athletes may be more likely to participate in individual sports, which carry a greater risk of overuse injury. On the other hand, team-sport athletes may be more likely to have acute, traumatic injuries and be less specialized. 22 Improved sport-specific skills and movement patterns in specialized young athletes may protect them from acute injuries despite perhaps placing them at higher risk for overuse injury due to increased training volumes.

Sport Type and Regions of Injury
Lower extremity-dominant sports such as basketball and soccer have been studied most frequently, but mixed and upper extremity-dominant sports (eg, baseball, tennis, volleyball) have also been addressed. In summary, risk factors such as playing on more than 1 organized team, engaging in competition (not just training) year-round, participating in more hours of sports per week than one's age in years, and a high degree of sport specialization have consistently been found to increase young athletes' risks for lower extremity pain and overuse injuries. 3,13,21 Additionally, overuse injuries to the lumbar spine may include more serious injuries that are typically seen in specialized athletes. 18 In studies focusing on baseball, pitching for multiple teams, pitching year-round, playing catcher in addition to pitching, and pitching on consecutive days were all associated with arm pain. 24,26,32 Pitching with arm pain was more closely associated with pitching-related injuries. 26 Logically, lower extremity overuse injuries are more common in running and repetitive lower extremity impact sports than in overhead sports, where upper extremity overuse injuries predominate.

Sex Differences
Few investigators have thoroughly explored sex differences with regard to sport specialization and overuse injuries in young athletes. The largest study to date, by Post et al, 3 indicated that, in a cohort of 1544 high school athletes, girls were more likely to have higher participation volumes, be active on club sports teams, and be highly specialized. Considered with other sport-specialization literature, these factors may place them at increased risk for overuse injury. In another study, 33 young female athletes were more likely than young male athletes to demonstrate increased risk for overuse injuries. This may reflect the choices of young female athletes to be involved in more individual, technical sports (eg, dance, gymnastics, tennis).

Sport Specificity
Because different sports have different risk profiles, the activity in which a young athlete specializes and the level of specialization are important when predicting injury risk. In a recent case-control study, 22 young athletes who engaged in individual, skill-specific sports (such as dance, gymnastics, or tennis) were more likely to be highly specialized at younger ages than those who participated in team sports. They had high training volumes on average and 1.67 times greater odds of developing an overuse injury than nonspecialized athletes, indicating that these individual sports may portend a worse overuse injury risk than team sports. Furthermore, individual sports have been examined to determine sport-specific injury risks. Among 540 United States Tennis Association junior tennis players, injury in the previous year and practice volumes were associated with the risk of withdrawing from a match for medical reasons. 34 This association disappeared after the authors adjusted for the age of specialization, indicating that it was closely linked to training volumes and confounded the relationship between training volumes and the risk of match withdrawal due to injury. Other sport-specific injury rates have also been investigated, with associations noted between knee overuse injury and playing on multiple soccer teams 21 as well as training more than 8 months per year in basketball, soccer, tennis, or volleyball. 10,25 Baseball pitching more than 8 months per year conferred 5 times the odds of sustaining an elbow or shoulder injury that required surgery, 24 and pitching on multiple teams or consecutive days increased the odds of arm pain or tiredness, which in turn increased the odds of injury by up to 7.9 times. 27

Psychological Effects
In addition to physical stress, the intense training inherent in youth sport specialization is also thought to affect an athlete's psychosocial wellbeing, although evidence-based research on this relationship is lacking. 6,9 Numerous factors have been postulated to account for this potential association. Intense training can lead to social isolation, which can impede the normal formation of an athlete's identity. 5 Intrinsic characteristics, such as perfectionism, and external factors, such as unrealistic expectations set by parents and coaches, can result in excessive psychological stress. 20 This can, in turn, lead to maladaptive coping strategies, loss of motivation, mood disturbances, and ultimately burnout. 5 The AOSSM, FIMS, IOC, and NATA encourage life balance and a strong social support system involving family, coaches, and medical staff to minimize these adverse effects. 5,7,9,20 Furthermore, the IOC advocated for educational programs to help coaches strengthen their relationships and interpersonal skills when dealing with athletes. 20

Socioeconomic Factors
Two groups 12,19 have evaluated parental SES and sports participation. This aspect is important because high levels of sport specialization may be cost prohibitive to families of lower SES. Parents reported median spending of $1500 per year on club sports, which may only be feasible for families with higher household incomes. 12 In fact, this finding was upheld in the later study, 19 which indicated that youth athletes from families with high SES described more serious overuse injuries than athletes from families with lower SES. This may be due to higher rates of sport specialization by children from families of higher SES, resulting in more hours per week playing organized sports, a higher ratio of weekly hours in organized sports to free play, and greater participation in individual sports. In fact, the athletes of lower SES participated in more weekly hours of physical activity than those of higher SES but had a lower rate of serious overuse injuries, possibly from more weekly hours of free play and lower rates of sport specialization.

Geographic Factors
The influence of geographic location (rural or suburban) on sport-specialization rates and sport-participation characteristics has been evaluated. 11 Bell et al 11 observed that suburban high school athletes were more likely than rural high school athletes to be highly specialized, participate in a larger number of competitions per year, play in a league outside of school, train in their primary sport more than 8 months per year, and play their primary sport for more than 16 hours per week.
Rates of sport specialization likely also differ based on broader geographic location, with warm-weather climates being more conducive to year-round sport participation than colder climates, but the evidence in this area is limited. High school pitchers in warm-weather climates pitched for an average of 9 months per year compared with 6 months per year for pitchers in cold-weather climates. 35 Other Health-Related Effects of Sport Specialization Although position statements 6,9 have theorized negative effects of sport specialization on various noninjury aspects of wellbeing and health, few data currently exist to support this claim. Among parent-child pairs of specialized tennis athletes, health-related quality of life (HRQoL) measures were obtained and accelerometers were used to determine levels of physical activity in both the parents and children. 36 Both the parents and children had relatively high rates of HRQoL and moderate to vigorous physical activity. Interestingly, the parents of multisport athletes were 4 times more likely to pursue physical activity with their children. In a separate qualitative study, 37 researchers found that parent-child dyads of specialized versus multisport young athletes all had relatively high HRQoL. Parents expressed concerns about sport specialization but generally did not influence their child's decision to specialize. 37 Additionally, those athletes with high HRQoL had supportive environments with less emphasis on winning and outcomes. 37 The overall benefits of sports sampling by high school seniors included academic success, better nutritional habits, and less likelihood of engaging in addictive behavior. 38 In a recent study, 39 investigators evaluated the potential associations of sport specialization with daytime sleepiness in 647 youth athletes from a variety of sports. High levels of specialization were associated with higher levels of daytime sleepiness. Additionally, athletes who had sustained an overuse injury in the previous year or who regularly traveled out of state for their sport had greater daytime sleepiness than uninjured athletes or athletes who did not travel regularly. 39 Overall, the profile of an athlete at high risk for serious overuse injuries is a young female athlete in an individual sport who focused solely on that sport at 10 years old, who was highly specialized with yearround training (more than 8 months per year), and participated in more weekly hours than her age with limited free play. This young athlete would possibly miss some of the other health benefits of a diversified experience but might still have the potential for a high HRQoL if she had a supportive family and coaching environment.

CONCLUSIONS
Many pathways are available for young athletes to develop elite-level success, including specializing in a single sport versus engaging in a variety of sports. The authors 28 of a review of sport specialization summarized that most studies suggested that intense or specialized training should occur after age 12. A number of these studies were limited by smaller sample sizes, recall bias, lack of primary scientific evidence, and inconsistent definitions. Despite the potential negative injury and psychological effects of sport specialization, the perception may still be that it is worth the risk to attain the perceived benefits.

Injury Risk
Position statements from sports medicine organizations have generally been consistent in recommending against early sport specialization. Although many of these recommendations do not state the age when it can occur, the effects on performance, or the specificity of the sport involved, they generally recommend a multisport-sampling developmental pathway for young athletes. The potential risks of injury, burnout, and attrition are cited. These risks may be supported by a few investigators, but a less measurable outcome is the potential negative effect on sport participation. In addition to the perceived injury risk, overtraining, and less appealing nature of an intensive training environment, it is possible that youth participation has declined in various team and individual sports. 40 Injury risk in young athletes may be attenuated by an intervention of serial online counseling using evidencebased recommendations. 39 The source of the reduction in injury risk was unclear, but many of the recommendations originated from reputable sources, so adherence to evidence-based guidelines may help reduce injury risk. 1,7,18 Perceptions of Sport Specialization Despite growing evidence linking specialization and overuse injury, the trend toward early specialization seems to be driven in part by the perceptions of youth athletes, parents, and coaches. Surveys 23,29,41 of parents and youth athletes indicated that a desire to play at the collegiate or professional level was a major factor in the decision to specialize. This belief did not appear to be shared by youth sport coaches, as the vast majority reported that playing multiple sports during childhood was most beneficial for developing athletic ability. 31 Additionally, coaches appeared to be more concerned about specialization and more aware of the link between specialization and overuse injury than parents. 30,31 However, most coaches and parents were unaware of sport-participation recommendations for reducing the risk of overuse injury, highlighting the need for outreach to and education of those involved in youth sports. 30,31 In general, most recommendations by sports medicine organizations and based on the current available evidence in clinical and community-based cohorts of young athletes discourage early sport specialization. It is not clear what the appropriate age or stage of development is for specialization, and these factors may need to be sport specific. The current evidence and recommendations do suggest an independent association of sport specialization with overuse and serious overuse injury risks (but not acute injury risk). Other factors play a role in potentially modifying this injury risk, including sex, geography, SES, and type of sport. Although sport specialization is associated with potentially negative psychological health consequences, long-term effects have not been adequately demonstrated, and some specialized athletes with supportive family and coaching systems may maintain a high HRQoL.

Future Research
Most of the research into sport specialization has been cross-sectional and performed in different populations. The association with injury risk has been demonstrated in clinical, 8 community, 13 and high school 3,4,10,25 populations. Additionally, the organizational guidelines and position statements on this topic tend to offer general youth sports recommendations. These may be beneficial for broad recommendations and may establish some training guidelines regarding sport specialization and preventing trainingrelated overuse injury. However, in certain populations, particularly elite young athletes, sport-specific recommendations regarding the potential age of specialization, associated risks, and age for optimal performance must be developed. National sport-governing bodies, in addition to coaches in each sport, are more likely to adopt such measures. This method will allow investigators to meticulously gather data on specific sport activities and demographic groups, include performance-based criteria, and establish specific health-related consequences of sport specialization.