The number of children and adolescents participating in organised athletic activities worldwide is increasing. However, physical fitness levels among youth are lower today than in previous decades. The combination of increased exposure and decreased preparedness for sports participation has led to an epidemic of both acute and chronic sports-related injuries in this population. Poor physical fitness, in addition to having negative health consequences, seems to be a risk factor for sports-related injury. Accurate injury surveillance data are required to better define the magnitude of the problem of injury in youth sports, as well as to identify specific risk factors for injury. From these data, targeted intervention strategies incorporating fitness training may be developed with the goal of preventing sports-related injury. Preliminary experience with several specific injury patterns—anterior cruciate ligament injuries and ankle sprains—has demonstrated the efficacy of such targeted prevention strategies.
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The participation of children in organised sports continues to grow worldwide. While there are no accurate statistics available to describe this international trend, an idea of the scope of the development of youth sports participation can be gained from experience in the USA, where it is estimated that 44 million children participated in organised sports during the year 2008.1
Despite this noted increase in sports participation, there is accumulating evidence that physical fitness levels in children are declining. While these trends seem contradictory, the fact that children's fitness levels are decreasing makes good sense in light of our rapidly changing lifestyles and increasing reliance on technology. For example, today's youth are spending less time on obligate physical activity, such as bicycling or walking to and from school. Additionally, with increasing amounts of time spent performing sedentary activities, such as using the computer and watching television, less time is spent engaged in free play. As a result, organised sporting activities may represent the majority of physical activity that a growing child is likely to experience.
Data to support declining fitness levels in children come from a number of studies in countries throughout the world. Studies from Canada undertaken from 1981 to 2009, for example, have found a significant decrease in fitness levels in that nation's youth over time.2 Research performed in Australia from 1995 to 2000 evaluated fitness level in children 12–15 years old; these investigators, too, reported a significant decrease in fitness level over time.3 Similar findings have been reported in Lithuania, Korea and the USA; it seems as if no country is immune to the development of declining physical fitness in children and adolescents.4,–,6
Interestingly, participation in organised sporting activities does not seem to ensure an adequate level of physical activity. One recent study from the USA used accelerometers to monitor 200 children aged 7–14 years while they were participating in a variety of sports and sports training programmes. These authors found that, despite participation in organised sporting activities, only 24% of school-aged children spent 60 min daily engaged in moderate to vigorous daily physical activity. Additionally, fewer than 10% of 11–14- year olds and 2% of female softball players met the recommended guidelines for daily physical activity. Male subjects, children aged 7–10 years and soccer players spent more time being physically active than their counterparts. These authors also noted a wide range of physical activity level across different sports, with the highest levels occurring in soccer and the lowest occurring in baseball and softball.7
It is perhaps not surprising given the trends towards increasing sport participation and decreasing physical fitness that the numbers of sports-related injuries in young people are on the rise. A 2002 report by the Centers for Disease Control and Prevention estimated that 4.3 million school-aged children in the USA were treated in an emergency room for a sports-related injury during the 1-year period spanning July 2000–June 2001.8 Importantly, these data represent only acute injuries and, therefore, likely underestimate the total number of sports-related injuries occurring in this population. Indeed, injuries resulting from repetitive use or overuse—injuries usually evaluated in the ambulatory care setting—have been estimated to account for approximately half of all sports-related injuries in children and adolescents.9 When one considers the short-term disability (school absence, physical discomfort) as well as potential long-term consequences (osteoarthritis) of these injuries, it is easy to understand why youth sports injuries have received so much recent attention in both the scientific literature and the lay press.
As a result of the large number of sports-related injuries occurring in youth, injury prevention has become a worldwide healthcare priority. Perhaps the best-known example of a major governing body taking an active role in sports-related injury prevention is the Fédération Internationale de Football Association (FIFA).10 FIFA has recently established a FIFA Medical Assessment and Research Centre (F-MARC), whose primary goal is to undertake sports medicine research worldwide through a network of Medical Centres of Excellence. One major focus of F-MARC is sports-related injury prevention. Through F-MARC, the programme ‘11+’ has been designed and implemented among young soccer players, with preliminary data demonstrating efficacy of this neuromuscular training programme in preventing soccer-related injuries.11
Unfortunately, the increase in worldwide awareness of the problem of injury in youth sports has not yet been translated into salient information. As early as 1998, the International Federation of Sports Medicine and the World Health Organisation recognised the dearth of existing data on paediatric sports-related injuries and recommended that systematic injury surveillance be a primary component of all organised sports programmes for children worldwide.12 Despite this, such injury surveillance programmes are a rarity. We currently have little reliable data regarding the nature and extent of injuries in youth sports, as well as the potential risk factors associated with these injuries and the possible strategies that might be implemented to prevent them.
We do know this: physically active children reap myriad health benefits, including improved cardiovascular health, bone health and mental health. Strength, endurance and proprioceptive skills serve as protective factors against sports-related injury. Additionally, there is good evidence to support the efficacy of multifaceted training programmes targeted specifically to injury prevention. The most notable of these are prevention programmes for anterior cruciate ligament (ACL) injuries in young female athletes. Finally, it is important to note that inasmuch as physical fitness and training are crucial for optimising and maintaining lifelong health, young athletes are not immune from the hazards of overtraining, burnout and overuse injuries.
Guidelines for physical activity in children were first established by the American College of Sports Medicine in 1988 with the goals of optimising bone health, muscular strength, flexibility and general health. The initial recommendation of the convened expert panel was for 20–30 min of vigorous exercise daily.13 Subsequent recommendations established in 1994 by the International Consensus Conference on Physical Activity Guidelines for Adolescents were also based largely on expert opinion. With their dual goals of promoting physical and psychological well-being during adolescence as well as enhancing future health, this panel concluded that (1) ‘all adolescents should be physically active daily or nearly every day’ and (2) ‘adolescents should engage in three or more sessions per week of activities that last 20 min or more at a time and require moderate to vigorous levels of exertion.’14 Subsequent proposals increased the recommended amount of daily activity for children to 60 min of at least moderate-intensity activity, and this recommendation has had some support in the international scientific literature.15 16
As researchers attempt to establish the optimal frequency, duration and intensity of physical activity for children and adolescents, training recommendations for this population are regularly adjusted in accordance with the available data. For now, however, determination of the ‘ideal’ amount of training for young athletes has remained an elusive goal.
Benefits of fitness
There are abundant data to support the idea that physical fitness is associated with health benefits, including improvements in cardiovascular health, bone health and mental health. Numerous studies have investigated the relationship between physical fitness and clustered risk factors (eg, systolic blood pressure, triglyceride level, total cholesterol/high-density lipoprotein ratio and insulin resistance) for cardiovascular disease and diabetes mellitus in children and adolescents.15,–,19 There is now a well-established inverse relationship between aerobic fitness and these clustered cardiovascular risk factors. In fact, a recent Cochrane review of school-based physical activity programmes found a consistent association between physical activity in the school setting and improved peak oxygen consumption (VO2max) and serum cholesterol levels.20 Importantly, this relationship reliably ‘tracks’ into late adolescence and adulthood, meaning that good physical activity habits acquired at a young age may yield long-term cardiovascular benefits. Building upon this foundation of evidence, some researchers have recently worked to uncover ethnic differences in cardiovascular risk profiles, with the aim of identifying the most at-risk youth for targeted prevention programmes.21
Bone health, too, is positively affected by physical activity. As reviewed by Boreham and McKay elsewhere in this issue, the growing skeleton is supremely responsive to mechanical strain placed upon it by weight-bearing physical activity, adapting to this stress by increasing overall bone mass.22 In fact, the adolescent skeleton is capable of vast quantities of bone mineral accrual, with an estimated 26% of the total adult bone mass gained in merely 2 years during the adolescent peak of bone mineral accumulation.22 As a result, some authors have viewed this age as a ‘window of opportunity’ during which physical activity might be used to build bony stores and theoretically protect against future fracture.22
Finally, there is a widely held belief that physical activity is associated with positive changes in mental and emotional well-being, and there is some scientific evidence in support of this assertion, as detailed by Biddle and Asare in this issue.23 In general, there seems to be a consistent positive association between physical activity and lower scores on scales of anxiety and depression symptoms, higher levels of self-esteem and improved academic performance.16 23 24
In order for prevention programmes to successfully reduce the incidence of sports-related injuries in children and adolescents, a good understanding of the factors placing young athletes at risk is necessary. Traditionally, these risk factors have been broken down into extrinsic factors (those that are external to the athlete, such as weather) and intrinsic ones (those that are inherent to the athlete, such as gender). Extrinsic risk factors typically include sport played, position within that sport, sport-specific rules, level and duration of play, playing surface, type and quality of protective equipment, coaching quality and experience and environmental factors such as weather and season.25 Frequently cited intrinsic risk factors include gender, age, physiological maturation level, anatomical alignment, physical fitness level, flexibility, strength, muscle–tendon imbalances, joint stability, coordination, proprioceptive skill level, nutrition status, history of previous injury and psychological and social factors.25
Looking more closely at these lists of risk factors for injury, it is clear that some are potentially modifiable. Of the extrinsic factors, for example, sports-related factors such as rules, playing surface and equipment may all be modified to better ensure the safety of the child athlete. However, some extrinsic factors that place young athletes at risk for injury are non-modifiable. Indeed, the choice of sport itself may pose a risk. Interestingly, the three sports most frequently associated with injury in boys are hockey, basketball and football. Girls are most at risk participating in gymnastics, basketball and soccer.25
Intrinsic risk factors, too, may be either modifiable or not. Non-modifiable risk factors include gender (boys are more likely to be injured participating in sport), left-handedness, age and history of previous injury. Importantly, many risk factors inherent to the young athlete are potentially modifiable. Fatigue (a surrogate for inadequate physical fitness) increases the risk of injuries in hockey players and baseball pitchers.25 Early evidence in support of the relationship between poor physical fitness and activity-related injury came from investigation of army trainees; history of inactivity, higher body mass index (BMI) and low aerobic fitness were all believed to contribute to physical-training-related injuries in this population26 In fact, there are good data demonstrating that youth with increased BMIs have a significantly higher risk of sustaining a sports-related injury than their normal-weight peers. In a review of the available literature on obesity and injury, McHugh reported that in 11 of the 13 studies included in his analysis, a higher BMI and/or a high percentage of body fat was associated with an increased risk of sports-related injury (specifically ankle sprains, medial collateral ligament tears and dental injuries).27 The reported increases in injury risk ranged from 1.4 to 3.9 times the risk identified for the normal-weight control groups. Proposed mechanisms for this finding in overweight and obese children include poor postural control (leading to problems with balance and coordination), poor physical fitness (associated with muscle fatigue and subsequent injury) and low preparticipation physical activity levels (associated with impaired neuromuscular and motor learning).27
Additionally modifiable intrinsic risk factors include strength, muscle–tendon imbalances, joint stability, coordination and proprioceptive skill level. It is these factors that have been targeted with initial success in the prevention of non-contact ACL injuries in young female athletes.
The idea of implementing training programmes as a means of reducing sports-related injuries in young athletes is not new: as early as 1978, Cahill and Griffith examined the effect of preseason conditioning on the incidence and severity of American football injuries in high school players.28 These authors reported a significant decrease in the occurrence of sports-related injuries in the trained group when compared with the untrained controls. They postulated that increasing the strength of the bone, muscle and supporting connective tissue by implementing a focused preseason training programme served to increase the relative resistance of these tissues to mechanical stresses experienced during practice and competition, thereby serving as a protective factor against in-season injury. Subsequent researchers have replicated this finding, and today, there are numerous studies investigating the effects of various training programmes on sports-related injury prevention.11 27 29,–,44 The area that has perhaps received the most recent attention in the scientific literature has been the prevention of ACL injuries in adolescent female athletes through the implementation of various neuromuscular, plyometric and proprioceptive training programmes.29,–,33
Researchers have identified potentially modifiable factors placing young women at risk for ACL injury, including extended hip and knee-joint postures upon landing from a jump; decreased hamstring-to-quadriceps strength ratios; overall poor physical conditioning; low core, trunk and hip strength; and increased valgus knee moments with landing and squatting.31 The majority of interventional studies have used a combination of plyometrics, proprioceptive training, strengthening, stretching, aerobic training and risk awareness training to effect changes in these modifiable risk factors and ultimately decrease the rate of non-contact ACL injuries in the target population. Most of these studies have reported a significant association between the implementation of interventional training programmes and a decrease in the incidence of ACL injuries.29,–,33
One of the earlier investigations into the effects of proprioceptive training on the incidence of ACL injuries was performed in semiprofessional soccer players in Italy.29 Over a period of three soccer seasons, Caraffa et al followed up 600 young athletes, half of whom had undergone proprioceptive training. When comparing the trained group with the untrained control athletes, these authors noted a significantly lower incidence of ACL injuries in the trained group (10 ACL tears among the 300 athletes in the trained group vs 70 ACL tears among the 300 athletes in the untrained group).29
A protective effect of neuromuscular training programmes in preventing ACL injuries was subsequently demonstrated by multiple researchers using various types of training interventions. Hewitt et al, for example, used a 6-week neuromuscular training programme composed of stretching, plyometrics and weight training to investigate its effect on the rate of serious knee injury including non-contact ACL injury.32 These authors reported a rate of serious knee injuries as much as 3.6 times higher in the untrained group. They additionally noted that while five untrained female athletes sustained injuries to the ACL, not a single female athlete in the trained group sustained a similar injury.32
In a more recent study, Mandelbaum et al investigated the efficacy of their Prevent Injury and Enhance Performance Program for preventing non-contact ACL injuries in adolescent female soccer players.33 The experimental group received videotaped instruction on the performance of three basic warm-up activities, five stretching techniques, three strengthening exercises, five plyometric exercises and three soccer-specific agility drills; emphasis was placed throughout on using proper biomechanical technique. Athletes in both the experimental and the matched control groups were then followed for 2 years, and data concerning athletic exposure and ACL injuries were collected. Significantly, these authors reported six ACL injuries in the trained group, compared with 67 in the untrained group, corresponding to an average decrease in ACL injury of more than 70%.33
Other authors have examined the association between the implementation of neuromuscular training programmes and the rates of overall injuries in youth sports.34,–,40 Olsen et al investigated the effects of a neuromuscular training programme on 1837 Norwegian handball players aged 15–17 years and found a significant protective effect of training for all acute knee or ankle injuries (48 injuries occurred in the intervention group of 958 athletes vs 81 in the control group composed of 879 athletes).35 They additionally found a significantly higher number of ligamentous knee injuries in the untrained group. Recently, Soligard et al examined the effects of implementing a comprehensive warm-up programme geared towards improving the strength, awareness and neuromuscular control of female soccer players aged 13–17 years. Over an 8-month span, these authors noted that 121 players in the intervention group (1055 athletes) and 143 players in the control group (837 athletes) had sustained sports-related injuries to the lower extremity. They reported a significant reduction in all injuries, overuse injuries and severe injuries in the trained athletes.43 Finally, Emery and Meeuwisse recently published the results of their randomised controlled trial investigating the effect of training on injury rates in youth. These authors, too, reported a significant difference in injury rates between the trained and untrained cohorts; they additionally noted trends towards decreased ankle and knee sprains specifically in the trained group, although these did not achieve statistical significance.36
Complementing these studies are those that have focused specifically on the prevention of sports-related ankle sprains.37,–,40 Just as with prevention programmes for non-contact ACL injuries, the majority of interventions for ankle sprains in youth and young adults have incorporated some component of neuromuscular and/or proprioceptive training, often in combination with bracing or taping. One representative study of the effect of balance board training on the incidence of acute sports-related ankle sprains was performed by Verhagen et al in the Netherlands. These authors implemented a season-long balance board training programme in 641 volleyball players and compared the rate of ankle sprains in these young athletes (average age of 24 years) with that of the control group, who had received no intervention. At the conclusion of the season, they noted significantly fewer ankle sprains in the group that had undergone proprioceptive training.37 In a similar study, Emery and colleagues investigated the effects of balance board training on sports-related injury rates in 920 high school basketball players aged 12–18 years. These authors found that the implementation of this type of balance training programme was effective in reducing acute sports-related injuries (134 injuries sustained in 426 control athletes vs 109 injuries in 494 intervention athletes) over a 1-year period. There was additionally noted a ‘clinically significant trend’ towards a reduction in lower extremity injures and ankle sprains in the intervention group.44 Finally, in 2006, McGuine and Keene evaluated the efficacy of a balance board training programme on the rate of ankle sprains in 765 high school soccer and basketball players. These authors reported a significant decrease in the rate of ankle sprains in the intervention group (1.13 sprains per 1000 exposures in the trained group vs 1.87 sprains per 1000 exposures in the untrained group, p=0.04).38 The effectiveness of proprioceptive training in decreasing the rate of ankle sprains in young athletes has been replicated elsewhere in the literature.38,–,40
One novel approach to preventing sports-related injuries in the school-age population is the iPlay (Injury Prevention Lessons Affecting Youth) study from the Netherlands.41 42 These authors designed a programme combining education about sports injuries with a classroom training programme geared towards improving strength, speed, flexibility and coordination. The intervention was carried out over an 8-month period in 40 Dutch primary schools. At the end of this period, the authors noted a decrease in injuries in children who had received the intervention and had been previously classified as ‘low-active.’ They conclude that, especially for children who are less physically active, the implementation of this type of training programme significantly decreases the incidence of sports-related injury.41
Overtraining and overuse
Participation in organised sports clearly has the potential to promote health as well as to boost self-esteem and allow children to learn about teamwork. Ideally, sports participation is also fun. That said, the pressure to be successful on an individual, a national and an international level is felt at an early age. It is contended that regions or nations will not remain competitive without a systematic programme of youth development, and experience in a number of national development programmes for youth sports seems to bear this out. This pressure to be successful at the highest levels of competition has led to the phenomena of early sport specialisation, year-round training and simultaneous participation on multiple teams. Fundamental training concepts including periodisation, the ‘10% rule’ and mandatory breaks between seasons are often dismissed, with resultant declines in the athletes' physical and mental function. Just as adult athletes may suffer from overtraining syndrome, burnout and overuse injuries, children who perform excessive amounts of high-intensity, repetitive physical activity without adequate rest may be susceptible to the perils of overtraining.45
Overtraining syndrome and burnout are routinely associated with changes in a young athlete's cognitive and mood profile, with symptoms including fatigue, sleep disturbance, chronic muscle and/or joint pain, elevated resting heart rate, performance decline, mood disturbances and impaired academic performance.45 46 Specifically, ‘burnt-out’ athletes demonstrate decreased vigor while the less adaptive mood states of fatigue, confusion, depression, anger and tension are all increased—the precise opposite mood profile of a healthy, high-functioning athlete.46 The athlete's physiological response to the perceived stress of continued athletic participation may then result in increased muscle tension, narrowing of the visual field and distractibility, which predispose ‘burnt-out’ athletes to sports-related injury at significantly higher rates than their appropriately trained peers.24 45,–,48
In addition to the acute sports-related injuries that overtrained athletes may sustain, overuse injuries are frequently associated with overtraining and burnout. The major concern with overtraining in the skeletally immature athlete is damage to the growth cartilage.49 Injuries to the cartilage may be divided into articular injuries, apophyseal injuries and physeal injuries. Osteochondritis dissecans lesions, which are frequently found in the knee, elbow and ankle of physically active children and affect primarily the articular cartilage and subchondral bone, are common sequelae of overtraining. The apophyseal cartilage of the knee (Osgood-Schlatter's disease) and elbow (‘little leaguer's elbow’) are locations commonly affected by repetitive use, resulting in a painful traction apophysitis.49 The physis itself may be injured as a result of overuse: both ‘little leaguer's shoulder’ and ‘gymnast's wrist’ are examples of this phenomenon. Finally, frank failure of bone as a result of repetitive loading may also be seen in the overtrained athlete. Lumbar spondylosis, often seen in gymnasts, ballerinas, or wrestlers, is an example of this type of overuse injury.50
There are two additional peculiarities of the child athlete that put him or her at risk for injury: specifically, children have an increased susceptibility to the elements (especially heat exhaustion) when compared to adult athletes.46 Ensuring the safety of the child athlete therefore requires an awareness of weather conditions and modifying training sessions to accommodate them appropriately (ie, increasing the frequency of water breaks, shortening practice time and training during cooler hours). Also noteworthy is the fact that children may be susceptible to sustaining a sports-related injury during the time of their peak growth velocity.49 This is likely related to a combination of muscle imbalances resulting from asymmetric growth, relatively tightened muscles as the soft tissues lag behind the osseous structures in longitudinal growth and decrements in proprioception and balance resulting from adjustment to rapid bony growth.
It is important to recognise that because of the extreme variability in physical and psychological maturation among children of the same chronologic age, the frequency, duration or intensity of physical activity that is appropriate for one child may be frankly overwhelming, both physically and emotionally, for a same-age peer. This makes rigid prescriptive guidelines for physical activity in the child and adolescent athlete untenable; rather, each athlete must be evaluated individually and the onus lies on the athlete's coaches and parents to be aware of the signs of overtraining and burnout and to intervene in a timely fashion.
There is mounting evidence that fitness levels in children and adolescents worldwide are declining and that the associated health problems of obesity, diabetes mellitus and cardiovascular disease are rising to epidemic proportions. At the same time that physical fitness levels are falling, the number of young people participating in organised sports has risen dramatically. Unfortunately, simply participating in organised sports does not guarantee that a young person will meet the recommended daily requirements for physical activity and reap the health benefits associated with physical fitness. The imbalance between physical fitness levels and physical demand during athletic participation has likely played a role in the increasing numbers of children and adolescents worldwide who sustain sports-related injuries. Indeed, while the data concerning children's participation in sport and their risk factors for sports-related injury are meager, there is some evidence to demonstrate that poor physical fitness—most notably in obese children and adolescents—is significantly associated with an increased risk of sports-related injury. This risk is likely due to a combination of factors including fatigue, weakness and poor overall coordination.
Importantly, there is growing evidence that the risk of sports-related injuries in ‘unfit’ and ‘untrained’ youth may be mitigated by the implementation of targeted training programmes. While these programmes vary widely, most of them incorporate a combination of aerobic fitness, strength and flexibility training, neuromuscular training and injury prevention education. Success with this type of intervention programme has been reported both in reducing overall injury rates in young athletes and in reducing specific risks, such as non-contact ACL injuries and ankle sprains. Furthermore, there is some evidence that incorporating this type of injury prevention programme into the regular school day may help protect children and adolescents against sports-related injuries and their long-term sequelae.
Finally, no discussion of injury in youth sports is complete without recognising that, at the opposite end of the spectrum, overuse and burnout may occur in the ‘overtrained’ athlete and place him or her at risk for both acute and chronic injuries as well as decrements in overall health and function.
While it is ultimately difficult to formulate universally applicable recommendations for training the child athlete, there are several common-sense guidelines that should be followed. In general, fitness training programmes for children must be: (1) individual-specific, taking into account factors such as a child's gender, age, BMI, injury history, developmental level and skill set; (2) sport-specific and (3) context-specific, with awareness of the level of play, relevant weather conditions and season length informing all training recommendations. Modifiable risk factors, including poor physical fitness, should be identified and addressed to ensure that children may participate in sporting activities as safely as possible. Perhaps most important is to remember that it is the ultimate responsibility of involved adults—coaches, parents, trainers and teachers—to ensure the health and safety of each child.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.