Background

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 (VO_2max) 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

Risk factors

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.

Injury prevention

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.