Introduction

Youth athletes are participating in sports training for longer and more intense hours than ever before. Travel and club sports teams have replaced recreational sports participation and have seemingly turned youth sports into a professional business that targets the dream of creating an elite athlete. The youth athlete who desires to compete at higher levels often train longer, harder, and with intense dedication which presents unique challenges to the developing athlete’s body. Individuals who specialize in a single sport are at increased risk of overtraining syndrome, stress fractures, and common overuse injuries, such as apophysitis (^26,27). Despite this, the volume of young athletes becoming single-sport specialists continues to grow.

In 2015 to 2016, the National Federation of State High Schools Associations (NFHS) participation survey demonstrated an increase in the total number of high school athletes for the 27th consecutive year with 7,868,900 participants. Cross country totaled 480,207 athletes and track totaled over 1 million participants nationwide, making it the second largest sport among both sexes. NFHS data revealed over 300,217 swimming and diving participants, and U.S. swimming membership exceeded 330,000 for year-round membership in 2016 (^1,38,50). These data support that both runners and swimmers are participating in endurance sports in high numbers. The training regimens these athletes follow extend beyond a traditional sport season and places them at high risk for overtraining, stress fractures, and common overuse injuries.

The sports medicine physician should advocate for individualized training efforts that recognize the unique developmental period when prepubescent (child) and postpubescent (adolescent) athletes are maturing physically and emotionally. Definitive statements regarding the recommended training regimens for youth endurance athletes are not uniformly accepted; however, conservative increases in frequency, duration, and intensity of sports participation may help decrease injury risk. The physician should consider age, physical and psychological development, nutritional needs, and the sport specific demands imposed on the pediatric athlete when evaluating for endurance injuries.

The Youth Runner

Running has become increasingly popular among adolescents, but participation in running often begins in early childhood. This is in part due to parents who are themselves runners and who strive to include their children in personal recreational activities. City marathons offer family runs and kid fun runs that range from one mile to 5K. Girls on the Run is a growing program offered to young girls in grades third through fifth that trains individuals to complete a 5K while offering discussions and lectures that focus on personal development, self-empowerment, and self-esteem (²). Such programs should focus on skill and fun rather than competition to support healthy and active lifestyles among the youth.

Competitive running programs emerge as early as middle school. A 2008 initiative by the United States Government has encouraged these programs at middle and high schools to increase physical activity and battle obesity (¹⁴). During summer months, high school runners are encouraged to self-train and increase training distances to 35 to 40 miles weekly before the start of official practice. High school cross-country athletes will then customarily run 45 to 55 miles·wk⁻¹ in season in preparation for a 5K race. Sudden changes in running mileage, noncompliance to summer training regimens, and nonsupervised running may increase an athlete’s risk for injury (⁴³).

There also is growing attention directed toward the pediatric marathon runner. Students Run Los Angeles marathon-training program produced greater than 16,000 marathon finishers from 1987–2005 (³⁶). The aspiring young endurance runner may cover 10 to 15 miles daily while training for a marathon, despite a high rate of overuse injuries (⁴⁴). In response to this, the International Marathon Medical Directors Association recommended an age minimum of 18 yr for marathon entry (⁴⁵). However, a study of 310 marathon runners aged 7 to 17 yr who completed the Twin Cities Marathon between 1982 and 2007, had a medical encounter incidence of 12.9/1000 finishers, lower (but not statistically significant) than the relative risk for an adult finisher (⁴⁶). Still, no widely accepted guidelines for running training in youth athletes are published. The American Academy of Pediatrics Council on Sports Medicine and Fitness (AAP COSMF) advises against high training volume and recommends that running participation should be driven by the child instead of by the coach or parent.

Consensus emphasizes enjoyment of running and physical fitness rather than competition to decrease risk for burnout, acute, and overuse injuries. The child who trains with unrealistic expectations may encounter psychological grief, especially when repetitive mechanical stress causes the young runner to suffer an overuse injury. A young runner should have a comprehensive medical team composed of a physician, physical therapist, nutrition specialist, as well as athletic trainers, and coaches, who may provide a reasonable training program that is specific to age, development, and skill level (¹⁰).

There are limited studies regarding how a child (prepubescent) or adolescent (postpubescent) should start a running program. Safe building base running mileage, varying running surfaces and alternating short and long mileage days are recommended for summer training for high school cross-country runners to help prevent injuries early in the season (^12,43). Supervised summer programs and coach-led education for the runners before summer training also may be beneficial to children and adolescents beginning a running program (⁴³). The weekly training schedule should be well designed with consideration given to age of the runner, safe running conditions, and appropriate education on endurance training. Limiting weekly mileage to 30 to 40 miles·wk⁻¹ in adolescents has been recommended (²⁸). It is advised that a child (prepubescent) not train more hours per week than the child’s age in years. Also, it is recommended that an adolescent (postpubescent) not train more than 16 h·wk⁻¹ because there is a correlation with injury risk above that level (⁹).

The Youth Swimmer

Swimmers follow rigorous training programs and oftentimes begin training at a young age. Competitive and elite swimmers may practice 6 to 7 d weekly equaling 20 to 30 h and cover between 10,000 and 14,000 m·d⁻¹ (^4,23). Many swimmers will maintain practice year-round resting only 3 wk out of the 12-month calendar year. Even holiday breaks from school allow time for coaches to work in double practice sessions. During a typical training week, each shoulder will perform an estimated 16,000 to 25,000 revolutions, and the majority of training is spent on the freestyle stroke (^4,47). The repetitive nature of swimming and volume of upper extremity motion predisposes the athlete to experience muscle soreness, muscle fatigue, and overuse injuries, such as impingement and stress fractures (²³). Shoulder laxity coupled with improper stroke mechanics can place the pediatric swimmer’s shoulder at further risk for these injuries (⁴).

A paucity of literature exists to offer clear guidelines regarding proper training volume and dryland conditioning programs with respect to the athlete’s age or pubertal development. Rehabilitation programs that focus on modification of training, flexibility, range of motion, and balanced shoulder strengthening may guide the swimmer with overuse injuries to train and compete in a safe manner, but future research should focus specifically on the pediatric swimming athletes (⁴⁹).

One common overuse injury in these endurance athletes is swimmer’s shoulder, which presents with diffuse shoulder pain during practice. Diffuse and constant shoulder pain first presents as the fatigued swimmer pushes through practice. Pain with overhead activities of daily living, such as combing hair, showering, or changing clothes then develops. Musculoskeletal examination often reveals slumped posture with anterior positioned shoulders due to weakened posterior scapular musculature, tight anterior pectoral muscles, and poor scapular control. Mild strength deficits may be present. Scapular winging is prevalent on examination and becomes more pronounced with shoulder abduction when performing simple maneuvers such as wall push-ups. Impingement signs may be positive as well. Management should include resting from swimming once pain is present (²³). Physical therapy should aim to address scapular stabilization and anterior chest wall stretching. Stroke assessment for arm cross-over, lack of lumbar roll when breathing, and leading with the elbow during stroke advancement should be performed (⁴).

Overtraining Syndrome

Individuals who participate in intense sports training, including running and swimming programs, may be at increased risk for overtraining syndrome due to the increased demands the rigorous training places on the athlete’s physical and psychological health. Oftentimes adolescents will forego participation in academic, social, and free play activities to prepare for competition. Overtraining syndrome may result as sport-specific isolation preventing equal attention to emotional, social, and psychological development during a critical period of adolescence.

Overtraining syndrome has been described as chronic fatigue and psychological burnout that impairs training, competition, emotional health and non–sport activities. It may present with vague complaints, training fatigue, poor sleep, and/or disinterest. Other common symptoms include chronic pain, elevated heart rate, lack of appetite, or loss of body weight. Worsening sports performance, deficient academic performance, difficulty concentrating, or failure to complete tasks throughout the school day may be signs of overtraining or burnout (³⁷).

The athlete who presents with fatigue should be screened for overtraining, burnout, and depression. Comprehensive training history, nutrition intake, sleep habits, sport and school performance, and changes in mood should be reviewed. Screening laboratory tests to rule out physiologic causes of fatigue include comprehensive metabolic panel, complete blood count, erythrocyte sedimentation rate, C-reactive protein, iron studies, creatine kinase, and thyroid-stimulating hormone (²⁹). Differential diagnosis should include hormone imbalance, nutritional deficiencies, acute illness, such as Epstein-Barr virus (EBV), and ergogenic supplement or illicit drug use. Consultation and frequent follow-up with a sports medicine provider and/or mental health professional may work to promote psychological and physiologic rest during recovery.

AAP COSMF has published guidelines that address the development of overuse injuries, overtraining and burnout in youth sports (^10,11). The recommendations include limiting participation in one sport to a maximum of 5 d·wk⁻¹ with at least 1 d off per week from any organized physical activity, having at least 2 to 3 months off per year from a particular sport and increasing total training intensity or mileage by no more than 10% per week (¹⁰).

Nutrition

Youth athletes must follow a high-calorie, well-balanced diet to support the energy demands of sports participation (⁵). The American College of Sports Medicine (ACSM) and the American Dietetic Association (ADA) recommend a diet balanced in macronutrients (carbohydrates, protein and fats) to support sport performance and decrease the risk of energy imbalance (³²). Parents and athletes may require education in selecting appropriate type, quantity, and quality of food as well as timing of food consumption. Proper nutritional intake can optimize energy availability, support physical development and meet the athlete’s specific training needs (²⁴). Table 1 may serve as a reference for ideal food choices and timing of consumption in relation to training. Consultation with a registered dietician who specializes in sports nutrition may be helpful for the young athlete.

Athletes with fatigue or poor performance, those who limit caloric intake, limit food groups, train intensely, or suffer from overuse injuries may benefit from laboratory assessment to evaluate for vitamin and nutritional deficiencies. Laboratory workup should include complete blood count (CBC) with differential, reticulocyte count, iron studies, ferritin, electrolytes, thyroid screening laboratories, and 25-OH vitamin D. (²⁹). Dietary sources of calcium, vitamin D, and iron should be encouraged due to improved bioavailability over nutritional supplements. Nutritional and vitamin supplementation may be considered to correct deficiencies after diet has been optimized. The AAP recommends a daily intake of 1300 mg calcium for children and adolescents ages 9 to 18 yr (²²). Daily intake of at least 600 IU vitamin D also is advised for children and adolescents ages 1 to 18 yr, and higher doses may be advocated for those who live in areas of limited sun exposure (³). The Endocrine Society has defined vitamin D deficiency as 25-OH Vitamin D concentration <20 ng/mL and insufficiency as 25-OH vitamin D concentration between 21 and 29 ng/mL (²⁵). For those who are vitamin D deficient, a supplementation of vitamin D2 or D3 50,000 IU weekly for 6 to 8 wk followed by maintenance dose of 600 to 1000 IU daily is advised (²²). The daily recommended intake of iron for children is 6 to 8 mg until age 11 yr and increases to 10 to 13 mg daily for adolescents ages 12 to 18 yr to support expansion of blood volume, increased lean body mass, and menstruation in girls (⁴²).

Female Athlete Triad

Female athletes may experience loss of menses during periods of intense training. Although this is often perceived by teenage female athletes, coaches, and parents to be a normal occurrence, menstrual irregularity may be a sign of inadequate nutrition that, when prolonged, has been shown to correlate with loss of bone strength. Primary amenorrhea is defined as the absence of menarche by age 15 yr or within the 3 yr of thelarche (breast bud development). Secondary amenorrhea is defined by the lack of menses for 90 consecutive days after menarche. All active females should be thoroughly evaluated for all three components of the female athlete triad: low energy availability with or without disordered eating, menstrual dysfunction, and low bone density. Further evaluation should be pursued if one or more of the components is identified. Females whose energy intake does not meet or exceed energy output are at risk for decreased estrogen secretion and stress fractures. If low energy availability is prolonged, female athletes are at risk for osteopenia and osteoporosis later in life. Early diagnosis and comprehensive management should focus on proper nutrition to prevent further risk (³⁰).

AAP COSMF recommends complete nutritional, menstrual, fracture, and exercise history be obtained (⁵¹). The Female Athlete Triad Coalition has developed a physician screening tool that focuses on the athlete’s goals regarding body weight, food consumption and attitude towards food as well as menstrual patterns and history of stress fractures (¹⁶). A history of primary or secondary amenorrhea should be assessed. A complete physical examination and pelvic examination, as indicated, should be performed. In addition to evaluating for nutritional and vitamin deficiencies as previously discussed, laboratory evaluation for causes of menstrual dysfunction may include pregnancy test, thyroid stimulating hormone, parathyroid hormone, bone specific alkaline phosphatase, and reproductive hormone levels (¹⁶). Electrocardiography is used to evaluate for arrhythmia if bradycardia is present. Bone mineral density testing by dual-energy x-ray absorptiometry (DXA) is indicated if history identifies eating disorder, body mass index (BMI) less than 18.5 kg·m², menstrual dysfunction, history of recurrent or high-risk stress fractures or if the athlete is on medication or has a chronic illness that adversely affects bone health (⁵¹). A multidisciplinary team of nutritional, psychological, and adolescent or sports medicine providers may then provide education and intervention techniques to normalize eating habits, improve menstrual regularity, and promote balanced training.

Stress Fractures

A stress fracture is a common sports related injury that has been reported as high as 20% of all overuse injuries with the highest incidence occurring in running sports (⁷). Stress fractures occur as a result of repetitive microtrauma often associated with sudden increases in training or limited rest intervals (^7,35,41). The short rest interval, such as double practice sessions, does not allow for adequate healing of the bone via osteoblastic activity. Continual training on relatively weakened areas of the bone can ultimately lead to a stress fracture (⁴⁰). Onset of pain is typically insidious, worsens with impact or repetition of sports movement, and eventually remains after training. The pain will localize to the site of the stress fracture. The location is tender to palpation, and pain can be reproduced by impact activity, such as a hop test, if the stress fracture occurs in a weight-bearing bone.

A comprehensive history should focus not only on onset, timing, and location of pain but also on volume and frequency of sports training as well as any acute changes in schedule. Plain radiographic films can often miss stress fractures, especially if obtained within the initial 3 to 4 wk of pain. Magnetic resonance imaging (MRI) is considered as the gold standard to diagnose stress fractures if the clinical suspicion is high despite negative radiographs (^40,41).

Between 2005 and 2013, High School Reporting Information Online (RIO) recorded the highest rates of stress fractures(in descending order) among athletes were in the lower leg, foot, low back, and pelvis (¹⁵). The following are considered to be low-risk stress fractures: posteromedial tibia, tarsal bones (except for navicular and talus), distal metatarsals 2 to 4, fibula, and femoral shaft (⁴¹) (see Fig. 1). These low-risk areas typically heal with conservative treatment of relative rest, immobilization, and occasionally non–weight-bearing status (⁴⁰). Acetaminophen or nonsteroidal anti-inflammatories (NSAIDs) could be added for pain control; however, there is controversy with NSAIDs because of a study showing delayed bone healing in acute fractures in animals (^40,52).

High-risk stress fractures characteristically occur in areas of high tensile load and poor blood supply. Sites of high-risk stress fractures include tension-sided femoral neck (superiolateral femoral neck), patella, anterior tibia, medial malleolus, talus, tarsal navicular, proximal fifth metatarsal, and great toe sesamoids (^35,41). These locations are associated with delayed or nonunion, progression to complete fractures, and often require surgical fixation (³⁵).

After recovery of low- or high-risk stress fractures, a gradual return to activity with recommendations to cross-train is strongly emphasized (^35,40,41). There also is strong evidence to support physical therapy to address underlying strength and flexibility deficits as well as biomechanical issues, such as pes planus correction with orthotic (⁴⁰).

A stress fracture of the pars interarticularis of the lumbar spine, called spondylolysis, is a unique stress fracture that should be considered in the endurance athlete with low back pain. Individuals who consistently perform back extension or rotation are at risk for spondylolysis. The microtrauma experienced from repetitive motion and the vulnerability of the skeletally immature spine contribute to the endurance athlete's risk (¹⁸). One prospective study of pediatric athletes found that 40% of high school athletes with low back pain persistent for greater than 2 wk may have lumbar spondylolysis (³⁹). Pain localized to the lower-lumbar region and exacerbated with lumbar extension while swimming butterfly or breast stroke, or during lumbar rotation, such as with arm crossing over midline while running or swimming, is concerning for spondylolysis. Spondylolysis is diagnosed by history, focal lumbar spine tenderness to palpation, pain with range of motion (extension and rotation) and radiographic imaging. Plain radiographs of the lumbosacral spine may reveal a stress fracture of the pars interarticularis, but most often confirmation by MRI or computerized tomography (CT) scan is necessary (see Fig. 2). To relieve the athlete's pain, avoidance of painful activity is warranted. A thoracolumbar orthotic brace may be used for comfort, but it may not affect healing rates (²⁰). A comprehensive physical therapy protocol that focuses on a neutral to flexion-based core strengthening program and lower-extremity flexibility should be prescribed. Pain-free progression to extension-based core exercises should be encouraged before clearance. Return to sports should be postponed until an athlete is pain free with sports specific exercises, however low impact cross training can often resume earlier. Cessation of sports for 3 months is recommended to allow for healing and safe return to sport (¹⁹).

Recurrent stress fractures raise the concern for low bone mineral density. DXA should be ordered when there is a history of multiple stress fractures, stress fractures unexplained by an increase in training, single stress fracture in a high-risk location, family history of osteoporosis, prolonged corticosteroid use, and for athletes with amenorrhea (³⁴). Normative data on DXA are based on postmenopausal women; consequently, caution is advised when interpreting results in children and adolescents who have not yet achieved peak bone mass. For analyses, Z scores should be used instead of T scores, and corrections should be made for size (²²). Bone mineral content or density that falls >2 standard deviations below expected should be considered “low for age.” DXA scans and dietary counseling should occur annually (⁶).

Apophysitis

Participation in endurance sports places youth athletes at risk of overuse injuries. One of the most common overuse injuries in the growing athlete is apophysitis. The apophysis is the location where a tendon attaches to a bone (see Table 2). The cartilaginous attachment is a weak link and can develop pain when the muscle is used repetitively and the tendon applies traction (^13,21). Athletes in all sports can develop apophysitis, but endurance athletes may be at increased risk because of the frequency and duration of their training (^8,33). Also, highly specialized athletes may be at increased risk for apophysitis due to repetitive movement patterns (¹⁷). There are many apophyses in the growing body, but for endurance athletes, the lower-extremity apophyses are at the greatest risk for developing apophysitis (³³). For the purposes of this article, apophyses of the lower-extremity sites are discussed.

Apophysitis is a clinical diagnosis. The most common symptom reported is pain at the apophysis which increases with physical activity. The pain may extend after sports participation and affect activities of daily living. Examination findings may include swelling and/or tenderness of the apophysis, pain at the apophysis with activation or flexibility testing of the corresponding muscles as well as decreased flexibility of the involved muscles. Range of motion is typically normal, although a stretch on the muscle may increase the pain. Functional examination may reveal limping and/or asymmetric hop test due to pain or weakness. Radiographs will display an open apophysis at the location of pain. Widening of the apophysis and/or ossicle formation also may be seen (²¹).

The main treatment goal for apophysitis is to limit pain so an athlete can participate comfortably. Athletes should be restricted from activity if pain is severe or they cannot participate without limping. General treatment consists of NSAIDs as needed, icing directly over apophysis, and limiting painful activity. Physical therapy can be prescribed to help treat pain, address mechanical issues, and improve flexibility and core strength. Good shoe wear should be encouraged to help with shock absorption. Immobilization is rarely used (²¹).

Location specific treatment also can be beneficial. Individuals suffering from apophysitis in the pelvis may benefit from wearing compression shorts that fully cover the painful area. Athletes with Osgood-Schlatter and Sinding-Larsen-Johansson may find relief from wearing a patellar tendon strap or Osgood-Schlatter knee brace while physically active. Quality viscoelastic (gel) heel cups can relieve symptoms associated with Sever, also known as calcaneal apophysitis (⁴⁸). Athletes and parents should be warned that signs/symptoms can reoccur during times of increased training intensity/frequency or during growth spurts as long as the apophysis is open (¹⁷). Parents also should be reassured that apophysitis is rarely linked with growth disturbance after the injury (³¹).

Summary/Conclusions

Youth sport participation should support lifelong physical activity, enjoyment, and healthy competition. Parents, coaches, and youth athletes should be educated regarding the signs and symptoms of common problems that youth endurance athletes may face, such as overtraining, nutritional deficits, female athlete triad, stress fractures, and overuse injuries. Prevention should focus on avoiding training errors, improper technique, excessive sport participation, inadequate rest, poor nutritional intake, and early specialization. Rest should be encouraged because it improves sports performance by allowing the athlete to recover from the physical demands of practice and competition, rehabilitate injury, and attend to school or social opportunities. By scheduling weekly nonpractice days and sport-free months, the developing athlete’s body may prevent burnout, overtraining, and overuse injuries. The athlete, rather than the coach or parent, also should drive training to focus on fun, skill development, and personal fitness goals. A comprehensive team of health care providers may serve as a resource to promote early recognition, proper evaluation, and thorough rehabilitation of overuse injuries to promote recovery and ensure lifelong enjoyment of active lifestyle.