Despite heightened awareness, many concepts pertaining to youth sports injuries remain unknown, including an understanding of all the factors that place the youth athlete at risk for injury and the best strategies for prevention.1,2 The most commonly researched risk factors include age, sex, sport selection, growth/maturation phase, pattern of previous injuries, biomechanics, degree of sports specialization, and sleep patterns. For risk factors that are modifiable, such as muscle strength and movement patterns, there has been research to investigate the effectiveness of targeted prevention programs (eg, neuromuscular training to improve muscle strength and teach movement patterns). By understanding the risk factors for injury and evidence-based prevention strategies, pediatricians can play a critical role in making youth sports safer by providing individualized advice regarding injury risk and prevention.
Patterns of sport participation and injury vary by sex. A recent epidemiologic investigation of sports injuries sustained by athletes age 5 to 17 years reported that boys, who tend to participate in more team and contact/collision sports than girls, sustained more injuries that were traumatic and bony in nature.3 In comparison, girls sustained more overuse injuries, and experienced patellofemoral pain syndrome (PFS) at a rate of approximately 3 times greater.3 Girls and boys also appear to possess different injury risk factors,4–8 perhaps due to the different movement patterns they have been documented to demonstrate.9,10 The differences in injury patterns are also, in part, related to sport selection. A recent study investigating overuse injuries in athletes age 5 to 17 years found that girls sustained more overuse injuries than boys (63% vs 40%, respectively), and 46% of this difference could be attributed to their chosen sports' characteristics (eg, contact/noncontact, team/individual).11
Injury Risk Factors
There is consistent evidence that the strongest predictor of future injuries is a previous injury.15 This finding highlights the important role of the preparticipation evaluation (PPE) in injury prevention. The PPE provides the pediatrician with a unique and valuable opportunity to identify previous injuries that may be incompletely healed or rehabilitated, and recommend appropriate treatment before clearing the athlete for full participation.16 For example, an athlete may report an ankle sprain that occurred 2 months prior, and is currently pain-free but demonstrates some weakness in the muscles around that ankle. In a typical case of recurrent ankle sprain, a rehabilitation program of strength and proprioception exercises should be recommended before clearing the athlete for full participation.
The concept of the growth process as a risk factor for sports injury in young athletes was highlighted by Micheli and Fehlandt,17 who observed that children may be more susceptible to repetitive stress injury than adults due to the unique vulnerability of growth cartilage and growing soft tissues as well as the growth process itself. For example, prepubertal children sustain significantly more injuries that were classified as bone-related and traumatic. The pubescent athletes, in comparison, were more likely to sustain soft tissue and overuse-type injuries.18 This is mainly due to physiologic and anatomic changes that occur during growth, which affect risk factors such as growth plate activity/vulnerability, muscle strength, and flexibility.
In young girl athletes, the period of rapid growth or peak height velocity (PHV) just prior to the onset of menarche may be the most vulnerable period for musculoskeletal injury. PHV has been associated with a transient decrease in bone density, which may lower the threshold for acute, traumatic fracture as well as repetitive stress injury to bone (ie, stress fractures).19 Further evidence that injury risk may be related to maturation phase is the higher injury rate observed in peripubertal girls who are gymnasts (Tanner stages 2 and 3) compared with gymnasts at either higher or lower stages of development, regardless of competitive level.20 Similarly, DiFiori et al.21 found that adolescent gymnasts between age 10 and 14 years are more likely than older gymnasts to have chronic wrist pain, even after adjusting for intensity of training, age of initiation of training, years of training, and sex. As such, a temporary reduction in training load or restructuring the training schedule to allow for more rest between training sessions may be effective strategies for reducing the risk for injury during these vulnerable periods of growth.
How athletes use their muscles during athletic maneuvers and the resulting joint movement patterns can affect the risk for injury, especially to the anterior cruciate ligament (ACL). Throughout puberty, girl athletes have higher rates of ACL injuries compared to boys in similar sports.22,23 Although sex-based anatomic and physiologic changes may partly explain this difference in injury rates, including greater ligamentous laxity in girls,24 neuromuscular differences between girls and boys that develop during puberty appear to be the most important factor that contributes to girls being at an increased risk for ACL injury.25 During puberty, girls and boys experience relatively rapid increases in body mass and height, and elevation of their centers of mass; boys have a surge of testosterone during this time. This testosterone surge during puberty for boys who are athletes results in an increase in muscle mass and strength, both serving to improve muscular control about the joints and increase core stability. Girls, however, do not have this same burst of testosterone, and thus do not experience as much muscle growth as boys. Therefore, pubertal girls tend to have less effective muscular control of their joint movements and decreased core stability during athletic maneuvers, which is thought be the main reason girls have higher ACL injury rates than boys.25 A key tenet of this decreased core stability for girl athletes is the absence of increases in strength and muscle recruitment at the hip and trunk.26 These muscles appear to play an important role in supporting and stabilizing the knee.
Biomechanical investigations have identified “dynamic knee valgus” (inward collapse of the knee in the frontal plane due to a combination of hip internal rotation and hip adduction) during athletic movements as a contributing risk factor for both ACL injury and PFS.27 In fact, dynamic knee valgus has been identified as the most sensitive (73%) and specific (78%) risk factor for future ACL injury in adolescent girl athletes.25 As girls go through pubertal maturation, they tend to perform athletic maneuvers (eg, cutting and jump-landing) with increasing amounts of dynamic knee valgus, presumably due to relative weakness in the hip and trunk muscles.28–30 Given that hip strength plays a key role in modifying dynamic knee valgus, neuromuscular training programs aimed at building hip and core strength have been investigated for their preventive effect on ACL injuries.31 A recent meta-analysis of 14 ACL injury prevention studies found significantly lower rates of ACL injuries in those who performed preventive neuromuscular training as compared to those who maintained their usual training routine.32,33 Some training programs also reduced rates of PFS and ankle sprains.34,35 The most effective programs included three main components: (1) strengthening exercises for the hip and core (Figure 1 and Figure 2),36 (2) plyometrics (repetitive jumping exercises),36,37 and (3) feedback to athletes about proper form (ie, avoiding dynamic knee valgus).38 The largest effect on ACL injury reduction was seen for girls age 4 to 18 years.32,34,39 Pediatricians can play a key role in educating girls and their parents about these evidence-based prevention programs, especially those in their early teens who participate in sports with the highest rates of ACL injury (ie, soccer, basketball, gymnastics).40–42 More information about ACL injury prevention including links to specific training programs for athletes and coaches can be found on the American Academy of Pediatrics Council on Sports Medicine and Fitness website.43
Training targeting improving hip muscle strength by using a slide board. From The Micheli Center for Sports Injury Prevention (Waltham, MA) with consent of both participants.
Training for peri scapular and upper trunk muscles, and core muscle strength and stabilization. From The Micheli Center for Sports Injury Prevention (Waltham, MA) with consent of both participants.
Early Sports Specialization
Early sports specialization is a new trend in the culture of youth sports that may add to injury risk. Sports specialization has been defined as “year-long, intensive training in a single sport at the exclusion of other sports.”44 Young children are playing sports with increased intensity and more of them are specializing in a single sport, starting as young as age 7 years.45 Hall et al.46 have shown that adolescent girls who specialized in one sport had increased risk of anterior knee pain disorders, when compared to multisport athletes. An even larger study of youth athletes demonstrated that sport specialization is a risk factor for overuse injury, independent of training volume.47 No evidence currently exists to support the notion that sports specialization before puberty is necessary to achieve elite status for the majority of sports.48 In fact, research has shown that participation in multiple sports may facilitate transfer of skills from one sport to another, and in doing so, may better promote youth athletic development than specialization.49 Additionally, participation in a variety of sports exposes the growing body to a wider range of physical challenges, which may result in more balanced neuromuscular development and less repetitive stress, theoretically reducing the risk for injury. Both the American Academy of Pediatrics45 and the American Medical Society for Sports Medicine13 recommend that children diversify sports training during early and middle adolescence, and delay specialization until late adolescence.
There are some recent data documenting an apparent association between sleep duration and sport-related injury risk.50 Among adolescent elite athletes, those who reported more than 8 hours of sleep per night on typical weekdays had significantly reduced odds of sustaining an injury.51 In a survey of young athletes age 6 to 18 years, Luke et al.,52 found that fatigue-related injuries were related to sleeping <6 hours the night before the injury. Furthermore, additional research supports a relationship between good sleep hygiene and improved sports performance.53 Thus, counseling young athletes about proper sleep hygiene may be a simple and effective way to reduce youth sports injuries. More information about the relationships between sleep and athlete health and performance can be found in an article by Copenhaver and Diamond (this issue).
Concussion Risk Factors and Prevention Strategies
Among high school athletes, concussion comprises up to 10% of all sport-related injuries.54 Despite concussion being a fairly prevalent injury, there are relatively few studies on prevention, and the strategies investigated to date, which include both primary and secondary prevention, have shown limited effectiveness. Primary prevention aims to prevent the injury before it ever occurs. Secondary prevention aims to reduce the adverse effects of a concussion that have already occurred. This is done by detecting and treating the injury as soon as possible, and making recommendations to prevent re-injury or prolonged recovery.
Protective equipment is commonly cited as a means of primary concussion prevention. However, evidence to support the notion that any protective equipment reduces the likelihood that an athlete will sustain a concussion is scant to date. Helmets, for instance, are often discussed as a means of concussion prevention in sports such as ice hockey or American football, as athletes are required to wear them as a part of their routine equipment. Modern helmets were designed to prevent skull fractures. However, there is no good evidence to show that any type of helmet prevents a concussion.55
One primary prevention method that may reduce concussion risk is neck strengthening. This is based on a study suggesting that greater neck strength was associated with reduced odds of sustaining a concussion in contact sport athletes.56 The theory is that stronger neck muscles may reduce head acceleration after a blow to the head.
Thus far, research shows the most effective strategies for primary concussion prevention are rule changes and rule enforcement. In a review of the available evidence regarding the effect of rule changes on concussion risk, Benson et al.57 concluded that introducing fair play rules and eliminating body checking in youth ice hockey are effective strategies for preventing concussions. Pediatricians can support policies that promote fair play and rule enforcement, and can advocate for rule changes that decrease concussion risk.
The most commonly researched method of secondary prevention of concussion is athlete education, as studies have shown that many athletes do not report their symptoms after a hit to the head and continue to play, putting themselves at risk for additional injury or prolonged recovery.58,59 The goal of education, therefore, is not to prevent the initial injury from occurring, but rather to improve clinical outcomes after injury. In a survey of approximately 500 college athletes, the two most common reasons given for not reporting symptoms were (1) feeling the concussion was not serious and that they could continue to play without any danger to themselves, and (2) the fear of being removed from play.60 Athlete education programs focus on how to recognize concussion symptoms and the risks of continuing to play with symptoms. Studies investigating the effect of such education programs have shown variable results. For example, studies of high school football players showed that despite demonstrating an increased knowledge about concussion symptoms and the danger of continuing to play, many did not change their attitude toward reporting symptoms or abstaining from play after a concussion.61,62 A similar study of nearly 500 high school athletes from a variety of sports showed that younger age, female gender, and soccer participation were more likely to be associated with better self-reported behaviors and attitudes toward reporting concussion symptoms.63 Thus, like many of the other sports injury prevention strategies discussed in this review, the effectiveness of concussion education may vary with age, sex, and sport.