Running is a popular sport for children in the United States. In 2012, participation in running was the second most common physical activity among boys ages 12 to 15 years (33.5%) and the most common physical activity among girls ages 12 to 15 years (34.9%) (1). During the 2016 to 2017 academic year, 492,310 high school (HS) student athletes participated in cross-country (266,271 boys and 226,039 girls) and 1,094,613 participated in track and field (600,136 boys and 494,477 girls) (2). Clearly, running is a common activity in middle-school and HS, but little is known about participation rates in younger girls and boys (3).
It is evident that youth athletes are sustaining running-related injuries. A study of 225,344 children presenting to U.S. emergency departments for running-related injuries showed a 34% increase in incidence from 1994 to 2007, with the highest injury rate (45.8 per 100,000 US population) in runners ages 12 to 14 years (4). A 1997 to 2007 study of physical education-related injuries presenting to the emergency department noted 25.1% were attributed to running (5). While there is some injury data for HS grades 9 to 12 cross-country (4000-5000 m) and track (800, 1600, and 3200 m) distance running, there is sparse data for longer distances in children and HS runners (3). In fact, there is only one, a 26-year retrospective study of 310 runners ages 7 to 17 years that completed the Twin Cities Marathon. In this study, four youth runners required finish area medical assistance, and all were for minor medical issues; a rate about half that of adults (6). Unfortunately, the limited amount of published data to support the safety of endurance activity has led to speculation by some that long-distance running is not safe for youth runners (3,6,7). There also are emerging data suggesting that shorter intense bouts of activity found in vigorous free play or integrative neuromuscular training may be better for developing motor skills compared with endurance running (8).
This review summarizes current long-distance running injury risk in runners who are HS age or younger with a focus on developmental factors that may contribute to injury. Our secondary aim is to provide strategies to address health concerns associated with long distance running in this unique population to assist young athletes, parents, coaches, and health care providers with decision making regarding adaptation to growth and development, body energy requirements, potential training errors, and reasonable training schedules for young runners.
Intrinsic Risk Factors
A progressive and safe running program for the youth athlete should be based on understanding normal child growth and development. During the transformation from child to adult, there are several unique intrinsic factors that may impact the risk of injury for a distance runner (Table 1).
Reducing injury in a skeletally immature runner requires an understanding of the unique progression of biologic age compared with chronologic age (3,9,10). Peak height velocity, defined as the rate of growth in height, will vary in each child, but typically occurs around age 12 years for girls and age 14 years for boys (11). During these periods of rapid growth, the long bones lengthen more rapidly than the muscle-tendon complex, creating tension on the tendon apophysis and potential traction injuries on the growth plate (11). There also is potential for increased compression on the long bone growth plates (9,11,12). The size of the joint increases concurrently, resulting in changes to the articular cartilage that might impact injury, though this is not well understood (9,12). The variable rates of change in bone, growth plate, tendon, cartilage, and muscle growth may impact running biomechanics and load tolerance. In addition, hormonal stimuli during puberty may adversely influence growth plate tissues making the growth plate vulnerable to running load injury (9,13).
Youth sport athletes, including runners are accruing bone mass during growth. Bone mineral content (BMC) is at its lowest level just prior to peak height velocity (12). The increase in BMC generally follows the peak height velocity at age 12 to 14 years for girls and 16 to 18 years for boys (14), and adolescents recruit almost half of their adult BMC during peak bone mineral deposition (15). By age 20 years, young adults have more than 90% of peak bone mass (15). Though evidence is lacking, youth runners may be at increased risk for injury during these periods of biologic change (i.e., peak height velocity and BMC) that do not occur at the same time and do not correspond to chronologic age.
Hip, core, and lower-extremity muscle weakness may contribute to injury risk in the youth runner, though there is conflicting evidence in the literature. In a cohort of HS cross-country runners (mean age, 16.2 years), weak hip abductors were associated with anterior knee pain/patellofemoral pain (PFP) syndrome (16). However, a recent meta-analysis of runners showed a discrepancy between cross-sectional and prospective studies of hip weakness and PFP, suggesting that hip weakness may be secondary to PFP in some cases (17). Finally, a study of HS cross-country runners showed improved race times following a 6-wk pelvic and core strengthening program (18).
Despite the need for more research, strength training has many benefits for children including an improved cardiovascular risk profile, better weight control, enhanced psychosocial well-being, improved motor performance skills, and decreased sports-related injuries (8,19). Targeted spine and hip strengthening exercises may be effective for rehabilitation and/or reduction of common running acute and overuse injuries including Achilles tendinosis, ankle sprain, hamstring strain, iliotibial band syndrome, PFP, plantar fasciitis, and tibial bone stress injury (BSI) (20) (Table 2). These exercises should incorporate running-specific functional movements progressing from double-leg to single-leg squats and hops to more demanding plyometrics.
The effects of maturation and training on the developing cardiovascular and respiratory systems are not well researched with respect to injury risk. It is difficult to accurately measure maximal oxygen uptake (V˙O2max) during exercise testing in children and the most reported metric is peak oxygen uptake (V˙O2peak) (28). V˙O2peak increases with age and maturation, largely as a function of increases in lean body mass, and is higher in boys than girls (28–30). Elite youth athletes have larger maximal stroke volumes from training that account for greater improvements in V˙Opeak (29). In addition, youth athletes have lower blood lactate accumulation during submaximal exercise than untrained youth likely due to enhanced oxidative function from training (29). Thus, age- and sex-specific maturational processes plus training may account for the observed differences in physical fitness, but it is unclear how this impacts the risk of injury or illness in the youth runner.
Cognitive, Behavioral, and Emotional Development
Both acute and chronic exercise bouts have a positive effect on cognitive development (31–33). The benefits of exercise include improved metacognition, behavior regulation (34), and the acceleration of both behavioral and emotional engagement (33). Unfortunately, young athletes, coaches, and parents often do not understand either cognitive or emotional development and frequently set unattainable expectations leading to athlete frustration, low self-esteem, burnout, and injury (35).
Although formal studies have not focused on behavioral issues in youth endurance training, a growing number of middle and HS students training for marathons report no negative psychological or social issues (36). Two mentored groups, Dreamfar and Students Run LA (SRLA), target at-risk students who have difficulty participating in other organized sports for socioeconomic reasons and may have poor social skills and/or lower self-esteem. SRLA, established in 1990, has more than 66,000 alumni and more than 2000 participants each year. Dreamfar High School Marathon is a smaller program of 200 runners each year. Dreamfar reviewed 5 years of injury data and found no psychological issues or significant injuries from running (36). The limited findings might suggest that a well-structured long-distance running program should not negatively impact psychologic or social development.
Bone density is altered by mechanical loading and energy availability. Most land-based sports that involve high impact mechanical loading and multidirectional movements improve bone density; however, the mechanical load of running alone does not predictably improve skeletal health (20). In addition to mechanical loading, low-energy availability has a negative impact on the biological factors that influence bone response to sports participation (37,38). Low bone mass for age is defined by bone mineral density (BMD) or BMC Z-scores < −1.058 and affects up to 39% of girl runners (39). Boys are not as well studied as girls, but boy runners also appear to have high rates of impaired bone health (40). The high stress fracture rates for both girls and boys running HS cross-country may be a clinical consequence of impaired bone health and not exclusively due to training load (41).
Risk factors for BSI in youth runners can be classified as biological or biomechanical. Static anatomical issues, including leg length discrepancy (true or functional), pes cavus or planus type foot (42–46) and dynamic biomechanical loading patterns (47,48) of running may contribute to injury. For example, greater average vertical (47,48) and higher peak acceleration (18,47) are both risk factors for tibial BSI in female runners. Although characterized in older athletes, higher peak hip adduction, tibial internal rotation, and rearfoot eversion represent a common motor pattern in youth runners that also may contribute to injury (17,47,48). Further research is needed to assess whether these findings are secondary to an immature neuromusculature, especially during periods of development (47,48).
Sex-specific factors for injury
The Female Athlete Triad (Triad) is defined by the interrelationship of energy availability, menstrual function, and BMD (13,37). In the International Olympic Committee consensus statement, Relative Energy Deficiency in Sports (RED-S) expanded the concept of low-energy availability to impaired physiological function involving multiple processes that have health consequences in both male and female athletes (38). Adolescence is a time of rapid growth and unmet energy demands associated with inadequate calorie intake that may have detrimental effects on musculoskeletal health (49). Runners are at risk for low-energy availability, highlighting the need for strategies to ensure appropriate nutrition (including supplementation) for normal menstrual function and bone mass deposition during training. HS athletes with components of either the Triad or RED-S also are at increased risk for musculoskeletal injury (49,50).
A risk factor unique to girl runners is delayed or loss of menstrual function (49,50). Late menarche is defined as lack of menses after age 15 years and girl runners with late menarche have more stress fractures (50). Following menarche, girls normally establish 10 to 13 regular annual menstrual cycles within a year of menarche (37). Reduction in the number of periods or cessation of menses may be a presenting symptom of low-energy availability. Therefore, any girl athlete with primary amenorrhea (no menarche by age 15 years) or secondary amenorrhea (missing three or more consecutive periods or less than six periods in the last 12 months) requires further evaluation to exclude other medical conditions associated with menstrual dysfunction and ensure appropriate treatment of nutritional deficits associated with the Triad and RED-S (37,38).
Boys also can develop RED-S and impaired skeletal health, but there are very few studies evaluating impaired bone health in boys (40,51,52). A study of 70 adolescent male athletes (52 runners and 18 nonrunners) identified several risk factors for low BMD Z-score (<−1.0) including running > 30 miles per week, a history of stress fracture, <85% expected body weight for height, and consuming <1 serving of calcium rich food per day (40). Separately, 9 (21%) of 42 male adolescent runners with BMD Z-scores ≤ −1.0 had two risk factors for impaired bone mass, a BMI ≤ 17.5 kg·m2 and a “yes” response to the question “do you believe that being thinner leads to faster running performances?” (52). Collectively, these studies suggest that young male runners with lower BMI, prior stress fracture, or poor dietary intake are at risk for impaired bone health and may require interventions to improve energy availability and reduce BSI.
Burnout is a part of a spectrum of conditions that includes overreaching and overtraining (7,35). Burnout is withdrawal from a previously enjoyable activity by a young athlete in response to chronic stress that was not mitigated by inherent coping mechanisms. Failure of the complex interactions between motivation, perfectionism, and coping strategies may lead to burnout (53,54). Some have suggested a relationship between early sports specialization and burnout, though further research is needed to assess the direct link in the youth runner (53,54). While problem-focused coping takes an active approach to psychological stress reduction, youth athletes who have tendencies to adopt avoidant coping strategies involving denial and behavioral disengagement can develop higher anxiety preceding burnout (55). Either controlled (i.e., parental and coach-created) motivation or autonomous motivation, which is related to perfectionism may result in training distress and greater burnout (55,56). Perfectionism-related thought impacts both girls and boys (56). Perfectionism is associated with higher psychopathology including depression, anxiety disorders, obsessive-compulsive disorder, and eating disorders in athletes and nonathletes (45). More than 50% of all pediatric sports injuries are due to overuse (7,31), so it is reasonable to monitor youth runners for the psychological factors that are associated with burnout.
Understanding the history of previous running injuries may help reduce future injury. In a retrospective study of 748 HS runners (442 girls and 306 boys), 68% of girls and 59% of boys reported a previous running injury (57). A prior history of a stress fracture is a risk factor for future stress fracture as shown in a prospective study of 748 competitive HS runners (442 girls and 326 boys) with nearly 1 year average follow-up (50). A single-season prospective study of 421 runners found 38.5% had at least one injury and injury rates of 19.6/1000 athletic exposures (AE) for girls and 15.0/1000 AE for boys (58). In this study, injury during summer training prior to the season for girls and a history of previous running injuries in boys were important predictors of in-season injury (58). The majority of injuries (77.6%) were minor, leading to less than 1 wk away from running. Further research is needed to assess the relationship between interventions based on screening and reduction in injury risk in the youth runner.
Extrinsic Risk Factors
Extrinsic risk factors can impact running-related injuries (Table 1). These factors may be influenced by peers, coaches, parents, and medical providers, so coordination of care and communication is important.
There is no consensus and sparse research regarding the appropriate age to start running, training volume or intensity, race distances, or progression through longer distances for young runner safety and health. An HS cross-country coaches survey (26.4% response rate) elicited the collective opinion regarding appropriate race distances (≤½ mile to 6.2 miles [10K]) for children (59). For kindergarten through second grade, 43% of coaches suggested one mile and 34% preferred ≤½ mile; for grades 3 to 5, longer distances were considered appropriate with 37% choosing 5K, and 29% selecting either the 1- or 2-mile distances (59).
Most of the research on training workload and schedule has focused on the adolescent runner. A cross-sectional study of 13- to 18-year-old runners identified an association between higher weekly mileage (average of 17 miles) and overuse injuries in boys, which was not observed in girls (57). HS runners were more likely to be injured during the fall cross-country season if they did not alternate daily high and low running volume, ran <8 wk, ran a higher percentage of total volume on hills, or ran on irregular terrain during summer training (60). Based on a prospective evaluation of competitive HS runners, boys with prior participation in basketball had an 81% decreased risk for stress fractures and girls who participated in dance or gymnastics had a three-fold increased risk of stress fractures, suggesting prior sports participation may influence injury risk (50). Cumulative participation in “lean sports,” including running, may increase risk for BSI in girls (39,50,57).
Reducing sport-related injury also involves developing a training plan that limits weekly and yearly participation time to reduce repetitive movements and increase scheduled rest periods within each week, season, and annual training cycle (7,35). This can be difficult for some adolescent distance runners who compete in cross-country in the autumn, indoor track in the winter, outdoor track in the spring, and are expected to train during the summer “off-season.” This schedule can get even more complicated when runners participate in other endurance sports throughout the year or during the “off-season” (e.g., Nordic skiing or triathlon). In addition, some school-based programs allow 6th, 7th, and 8th grade athletes to participate at the varsity level. Thus, the potential for injury is increased if coaches do not carefully and individually increase the training loads for the younger runners. Despite the lack of research regarding optimal training loads and injury rates, it seems logical that periodization of training throughout the year is important to enhancing performance, peaking at the right time, and avoiding injury (35,53). Clearly education for young runners, parents, and coaches is critical to reduce overuse injuries, burnout, and overtraining with a long-term goal to increase lifelong participation in the sport (3).
Sports specialization is defined as training > 8 months per year, focusing on a single sport, and/or quitting all other sports to participate in one sport (53). Highly specialized youth athletes report more injuries than peers independent of age, sex, and weekly organized sport volume (54). Sport specialization is associated with a unidimensional identity with the sport and an athlete’s perception of lack of control with sports participation. Both may be factors that lead to burnout (35).
Despite the trend for early specialization in organized sports, specialization in running may be far less common. In a NCAA study of 21,000 collegiate athletes, sports specialization before the age of 12 years was highest in gymnastics (87%), tennis (72%), and soccer (62%) compared with track (13%) (61). Of all sports surveyed, track participants experienced the lowest attrition rates in men (9%) and women (13%). Similar trends are seen in HS sports. In the Project Play Survey (Aspen Institute) of 47,000 young athletes from grade 8 to 12, cross-country had the lowest attrition rate at 2% (62). Both studies found that athletes who had early specialization withdrew from their sport either due to injury or burnout (61,62).
Footwear and Technique
Research on running injury in children related to mechanics and footwear is very limited. In adults, habitual running in minimal footwear with no cushioning or bare foot promotes a forefoot strike (FFS) running pattern (63), as landing on the heel is painful (64). An FFS has many benefits including improved strength and stiffness of the Achilles tendon (65–67), reduced load to the anterior shin (68), and reduced contact stress in the patellofemoral articulation (69). In addition, an FFS pattern typically increases step rate. Increasing the step rate by 7.5% to 10% can substantially reduce the load rates to the hip and knee during running (70,71) and is associated with a reduction in shin injuries in youth runners (72).
Reduced shoe support may promote increased intrinsic foot muscle strength (73,74). A study of children from India reported that those who were habitually barefoot had a significantly lower incidence of pes planus than those who were shod (75). Adults who run in minimal shoes with no support significantly increase muscle size (and presumably strength) of the intrinsic and extrinsic foot muscles (76). An increase in foot strength might be protective against future foot and ankle injuries (74,76). The studies outlined above suggest that adopting an FFS and increasing the step rate may improve strength and reduce many of the common injuries in both youth and adult runners (68,74,77,78). Perhaps, starting toddlers, children, and young runners in footwear that promotes an FFS pattern will establish greater foot and ankle strength during development, along with a reduction of impact loading that may reduce the risk of future running injuries.
Readiness for running and reducing running-related injury in growing children should be based on a combination of physical, emotional, psychological, social, and cognitive factors. Aligning individual growth and development with the physical demands of running and meeting the body energy needs of an individual athlete may reduce injury, overtraining, and burnout. Running initiated by children, as opposed to parents, combined with an individualized training program that allows adequate rest and energy replacement may promote injury-free running, reduce burnout, and successfully transition young runners to a lifetime of running.
Strategies for Preventing Injury and Future Directions for Research
Based on this clinical review, these are the potential strategies for preventing injury in young runners.
- Readiness for running, especially longer distances, should be determined by growth and development rather than chronological age (3,9,10,12).
- Athletes should be screened for previous injuries to correct biomechanics and training errors (20).
- Improving neuromuscular control of the lumbopelvic region and lower extremity should be promoted to reduce biomechanical risk factors for injury (16,19,20).
- Single sport specialization in running should be discouraged until boys and girls reach puberty to reduce risk for burnout (35,53,54).
- Youth runners should have at least 1 rest day per week, 1 to 2 wk every 3 months, and limit participation to 9 to 10 months per year (3,35,53).
- Youth runners should participate in high impact and multidirectional activities at least through puberty to promote bone health (18,30,47).
- Flexible, light shoes without additional heel cushioning and support may help develop foot musculature and promote a forefoot or midfoot heel strike running pattern (72,74,78).
- Youth runners should vary training surfaces and training schedules to “mix” mechanical loading (72,74,78).
- Adequate calorie intake to ensure energy balance is essential for all runners (13,37).
- Supplements with 1300 mg calcium daily and 600 IU of vitamin D daily are recommended for the runner ages 9 to 18 years (13,37,38).
- Risk factors that require evaluation (37,38) include:
- ○ untreated disordered eating/eating disorder or related complications (i.e., arrhythmia, renal failure)
- ○ BMI ≤ 17.5 kg·m2 OR measured body weight below 85% of normal for age.
- ○ one high risk (i.e., femoral neck, proximal tibia, navicular) OR ≥ two BSI
- ○ female runners without menarche by age 16 years OR,<6 menstrual cycles in the past 12 months
- ○ BMC or BMD Z-score ≤ −2.0
Provisional return to running should be based on resolution of the risk factors under the guidance of a health care provider:
- Self-motivated children and adolescents should be allowed to participate in long distance running events with a supervising adult (6,7,28,35) if:
- ○ They follow an acceptable supervised training program.
- ○ They maintain normal growth in height and weight as well as normal menstrual function for girls during training.
- ○ They remain healthy with good nutritional intake, sleep patterns.
The authors declare no conflict of interest and do not have any financial disclosures.
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