The purpose of this article is to review the current theories regarding prevalence, mechanism, and prevention strategies for overuse injuries in a young athletic population. This information provides valuable insight into the state of the current evidence regarding overuse injuries in young athletes as well as the potential future directions in the development of overuse injury prevention interventions.
Key points
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The prevalence of overuse injuries in young athletes in increasing.
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The mechanism of overuse injuries in young athletes is multi-factoral and can be classified as either intrinsic or extrinsically related.
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Identification of a mechanism unique to the individual athlete is important to apply targeted intervention strategies.
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Targeted neuromuscular interventions developed to prevent acute lower extremity injuries may also have a role in the reduction of certain overuse injuries, as well.
Introduction
Participation in organized sports is increasing in the United States. An estimated 30 to 45 million children participate in organized sports annually. Concurrent with this increase in participation is an upward trend in year-round participation in athletics in either one or multiple sports. The benefits of athletic participation in children as a means to stay active and physically fit are well documented ; however, an increased prevalence of athletic injury in young athletes has raised concern regarding the safety of intense athletic participation at a young age. Although many of these injuries may represent traumatic incidents, as many as one-third to more than 50% of these injuries are estimated to be a result of overuse.
Overuse injuries include a broad spectrum of injuries within sports medicine. Classically, they are defined as chronic injuries related to “constant levels of physiologic stress without sufficient recover time.” Globally, they can be perceived as the outcome of the difference between the volume of the stress or force applied to the body and the ability of the body to dissipate this stress or force. Injury may result from repetitive microtrauma imposed on otherwise healthy tissue or the repeated application of lesser magnitudes of force to pathologic tissue. Either scenario can lead to the sequelae of tissue breakdown. Unfortunately, the mechanism by which this stress ultimately leads to overuse injuries is not consistent among young athletes.
In the absence of a well-defined mechanism, the development of targeted intervention strategies is more difficult. Traumatic injuries, such as ligament tears, are typically the result of a single macrotrauma on otherwise healthy tissue, which results in tissue failure. Many injury-prevention programs attempt to develop the athlete’s neuromuscular control mechanisms to help dampen these external forces and reduce the likelihood of traumatic tissue failure. In the case of overuse injuries, there is significantly less evidence regarding the most efficacious program to reduce the incidence of these injuries. Therefore, current intervention programs that attempt to address potential underlying mechanisms or target specific risk factors that may contribute to abnormally high stress with repeated activities are still in development. The purpose of this article is to highlight the prevailing theories of overuse injury mechanisms as well as review the best available evidence for the implementation of prevention strategies designed to target overuse injuries in both endurance and pivoting/cutting sports.
Introduction
Participation in organized sports is increasing in the United States. An estimated 30 to 45 million children participate in organized sports annually. Concurrent with this increase in participation is an upward trend in year-round participation in athletics in either one or multiple sports. The benefits of athletic participation in children as a means to stay active and physically fit are well documented ; however, an increased prevalence of athletic injury in young athletes has raised concern regarding the safety of intense athletic participation at a young age. Although many of these injuries may represent traumatic incidents, as many as one-third to more than 50% of these injuries are estimated to be a result of overuse.
Overuse injuries include a broad spectrum of injuries within sports medicine. Classically, they are defined as chronic injuries related to “constant levels of physiologic stress without sufficient recover time.” Globally, they can be perceived as the outcome of the difference between the volume of the stress or force applied to the body and the ability of the body to dissipate this stress or force. Injury may result from repetitive microtrauma imposed on otherwise healthy tissue or the repeated application of lesser magnitudes of force to pathologic tissue. Either scenario can lead to the sequelae of tissue breakdown. Unfortunately, the mechanism by which this stress ultimately leads to overuse injuries is not consistent among young athletes.
In the absence of a well-defined mechanism, the development of targeted intervention strategies is more difficult. Traumatic injuries, such as ligament tears, are typically the result of a single macrotrauma on otherwise healthy tissue, which results in tissue failure. Many injury-prevention programs attempt to develop the athlete’s neuromuscular control mechanisms to help dampen these external forces and reduce the likelihood of traumatic tissue failure. In the case of overuse injuries, there is significantly less evidence regarding the most efficacious program to reduce the incidence of these injuries. Therefore, current intervention programs that attempt to address potential underlying mechanisms or target specific risk factors that may contribute to abnormally high stress with repeated activities are still in development. The purpose of this article is to highlight the prevailing theories of overuse injury mechanisms as well as review the best available evidence for the implementation of prevention strategies designed to target overuse injuries in both endurance and pivoting/cutting sports.
Mechanism of overuse injury
Factors that increase the likelihood of overuse injuries can be classified as either intrinsic or extrinsic risk factors. Intrinsic factors are categorized as implicit or unique to the individual that may increase the likelihood of sustaining an injury. Maturational status, body mass index (BMI), gender, anatomic variations, and biomechanical movement patterns are all examples of intrinsic risk factors. Theoretically, these factors can affect the ability of the athlete’s tissue to dampen or respond to stress. For example, if an athlete possess a varus knee alignment, he or she is more likely to experience an increase load on the medial compartment of the knee. Over time, this may lead to more articular cartilage breakdown in the medial compartment. Anatomic variants, such as knee alignment, in the absence of a surgical intervention to realign the knee, are unmodifiable risk factors. Conversely, intrinsic risk factors, such as BMI, strength deficits, or altered movement patterns, would generally be considered modifiable risk factors, which have the potential to improve with an injury-prevention intervention.
Extrinsic risk factors are those factors that, when applied to the athlete, may increase the risk of injury. These factors may include training methods, equipment, and environment and may have an effect on the magnitude, stress, or force applied to the body. Training regimes are often implicated as a potential mechanism of overuse injury. Hogan and Gross identified 3 scenarios that may increase an individual’s likelihood of developing an overuse injury. The first scenario involves the athlete who attempts to rapidly increase his or her training load after a period of inactivity or decreased activity. In this situation, the body has an insufficient adaptation period to respond to a higher level of stress and, therefore, is not adequately prepared to dissipate repetitive forces. Investigations of the high incidence of stress fractures during the initial stages of training in the military support this theory of overuse injury caused by rapid increases in activity.
A second category of extrinsic risk factors includes athletes who attempt to participate at a level that exceeds their individual skill level. In theory, this mismatch of individual skill or fitness level to imposed stress and physical demands can lead to tissue breakdown. Finally, consistent participation at an exceptionally high level is theorized to lead to overuse injury. This group may suffer from excessive microtrauma over time with insufficient rest, ultimately leading to tissue breakdown. Athletes who continuously participate in sports without rest or who specialize in one sport throughout the year are anecdotally thought to be included in this high-risk category; however, the current evidence is sparse.
In summary, overuse injuries are generally a product of the application of an applied load to the body and the body’s ultimate inability to dampen the applied load. This inability may be caused by intrinsic factors that limit the body’s ability to dampen the load or extrinsic factors that increase the load that is applied. Using this theory, programs designed to prevent overuse injuries should target impairments that decrease the individual’s ability to dampen forces applied to the body and encourage participation in appropriate progressions of training to increase the individual’s ability to dampen the applied load.
Preventing overuse injuries in young endurance athletes
Running is an endurance sport that continues to grow in popularity among middle school and high school athletes; more than 450,000 young athletes participated in cross country during 2010 to 2011. Concomitant with an increase in running participation comes an associated increase in injuries. The annual incidence rate among high school cross-country runners is reported to be as high as 17.0 per 1000 athletic exposures (AEs). Unfortunately, the literature on running-related injury prevention is sparse and often contradictory. In addition, the potential confounding effects of growth and maturation on running biomechanics and injury risk in children may also limit the generalizability of adult literature to the pediatric running population. Although these challenges exist, focusing on known factors that contribute to pediatric running-related injuries, recognizing the hallmark signs and symptoms of these injuries, as well as having a strong understanding of the underlying biomechanics of distance running may serve to guide clinicians interested in the prevention of these injuries in a young population.
Common Pediatric Running-Related Injuries
The most common location of pediatric running-related injuries is shin injuries for girls and knee injuries for boys. The 2 most common shin injuries are medial tibial stress syndrome (MTSS) and tibial stress fractures. In adolescent runners, the most common knee injuries are patellofemoral syndrome, iliotibial band syndrome, and injuries to the apophysis, such as Osgood-Schlatter disease (OSD).
MTSS is characterized as “an exercise-induced, localized pain along the distal two thirds of the posterior-medial tibia” and affects more female than male long-distance runners. The risk factors for sustained MTSS include reduced running experience, a previous history of MTSS, and a higher BMI. Although runners with a history of MTSS are more likely to report orthoses use than those runners without a history of MTSS, the evidence supporting an association between a pronatory foot type and MTSS is mixed. Adult athletes diagnosed with MTSS have significantly reduced plantar flexor muscle endurance compared with uninjured athletes. Although retrospective in nature, this adds support to the theory that a lack of endurance of the plantar flexor muscle group may lead to a higher force transfer to the tibia.
Stress fractures occur along a continuum of repetitive loading, bone remodeling, and microdamage accumulation and can be classified as either fatigue or insufficiency fractures. A fatigue stress fracture occurs in healthy bone as the result of repetitive loading and mechanical stress, whereas an insufficiency fracture is the result of normal loading on pathologic bone. Most stress fractures that occur in pediatric long-distance runners can be classified as fatigue stress fractures, with tibial stress fractures as the most common type of stress fracture.
Of all running injuries, patellofemoral syndrome (PFPS) is the most common and the cause is still unclear. Also known as runner’s knee or anterior knee pain, PFPS is typically described as pain to the peripatellar region that increases with activities, such as running, stair ambulation, squatting, jumping, and/or prolonged sitting with the knees flexed. Although many etiologic theories exist, 2 biomechanical pathways have received the most attention. First, a lack of proximal stability caused by impaired hip strength and/or hip muscular activation leads to excessive patellofemoral joint stress by increasing the dynamic quadriceps angle acting on the patellofemoral joint. Second, excessive and/or mistimed pronation may lead to alterations in frontal and transverse plane mechanics at the patellofemoral joint resulting in patellofemoral compression, overuse, and pain.
Proximally, the gluteus medius and gluteus maximus function to eccentrically stabilize the femur in the frontal and transverse planes, respectively, while running. Recent systematic reviews suggest adolescent and young adult women with PFPS demonstrate deficits in hip strength and adult runners with PFPS exhibit delayed and shorter muscle activation of the gluteus medius. Together, these studies indicate an association between the lack of proximal pelvic girdle stability and PFPS. Furthermore, interventions targeted at improving the strength and neuromuscular control of the hip abductor, hip extensor, and hip external rotator musculature may be efficacious as part of an injury-prevention program. Distally, as the foot and ankle complex pronates the tibia internally rotates because of the anatomic wedging of the talus in the distal tibia-fibular mortise. Some investigators theorize that excessive or mistimed pronation would lead to excessive patellofemoral stress because of the disruptions in transverse and frontal plane timing of the transverse plane tibiofemoral joint. However, there is mixed evidence regarding the association between excessive foot pronation and PFPS.
Iliotibial band syndrome (ITBS) is the most common cause of lateral knee pain in runners, with an annual incidence rate of up to 12%. Although originally thought of as a sagittal plane disorder secondary to a tight ITB fractioning over the lateral femoral epicondyle, more recent evidence supports the theory that ITB occurs as a result of a lack of frontal and transverse plane control of the femur and tibia. Antiinflammatory treatment is thought to be effective in the acute treatment of patients with ITBS, whereas hip abductor strengthening has been shown to be effective at improving hip strength and returning injured runners back to function.
OSD is the most common apophyseal disorder affecting adolescents and presents as anterior knee pain, swelling, and tenderness to palpation over the tibial tubercle. OSD typically occurs during a growth spurt and is seen most often in adolescents’ participation in sports involving repetitive running and/or jumping. Tightness to the rectus femoris is associated with OSD. Interventions typically include relative rest, gentle quadriceps stretching, and quadriceps strengthening. Because OSD is associated with recent growth, these athletes may benefit in the reduction of total running volume as part of an injury-prevention effort.
Risk Factors for Sustaining a Pediatric Running-Related Injury
Intrinsic risk factors
The intrinsic risk factors most often associated with running-related injuries in a pediatric population include sex, anatomic morphology, running mechanics, hip strength, and nutrition. Epidemiologic evidence of sex differences is sparse. Rauh and colleagues prospectively tracked high school cross-country runners and noted that female runners had a higher injury rate as compared with boys (19.6 per 1000 AEs vs 15.0 per 1000 AEs) and sustained more injuries causing more than 15 days lost from running than boys. Compared with boys, the total injury rate for girls was significantly higher than boys for all injuries except those resulting in 5 to 14 days lost. Retrospectively, Tenforde and colleagues assessed a large cohort of long-distance runners and noted that girls had a higher overall injury rate than boys. The investigators noted the following most common injuries: tibial stress injuries, ankle sprain, patellofemoral pain, ITBS, and plantar fasciitis.
The risk factors related to anatomic morphology in runners include a quadriceps angle (Q angle) at the knee and foot morphology. Rauh and colleagues report that high school cross country athletes with a standing Q angle of more than 20° were 1.7 times more likely to sustain a running-related injury compared with a standing Q angle of 10° to less than 15°. Runners with more than a 4° absolute right-left Q angle difference were at a 1.8 times greater risk compared with runners with a smaller difference. Runners with a Q angle of more than 20° were more likely to injure their knee, whereas runners with more than a 4° Q angle difference were more likely to injure their shin.
Anatomic variations related to foot morphology are often theorized to contribute to pediatric running-related injuries. Many investigators postulate that a pes planus foot type results is a more mobile foot leading to an increase in pronation excursion and excessive strain to medial soft tissue structures, whereas a pes cavus foot type results in a stiffer foot that is less well equipped to dampen ground reaction force (GRF) at the foot and ankle resulting in excessive bone stresses and lateral column injuries. Although multiple investigators cite structural deviations of the arch as either indirectly or directly contributing to the running injury incidence in both adult civilian and military runners, others have found no association. Further, few studies have prospectively assessed the effect of foot structure on the incidence of pediatric running-related injuries (PRRI) with contradictory conclusions. This contrary finding has lead one investigator to state that efforts to optimally “aligning the skeleton” with shoes and orthoses designed to mitigate anatomic variants should be reconsidered. Thus, further high-quality prospective studies are warranted to help delineate the effect of arch structure on the risk of sustaining a running-related injury.
Altered running mechanics are theorized to lead to injury. Recently, primary biomechanical faults cited for an increased risk of sustaining a running-related injury are excessive rearfoot eversion and altered stance phase impact forces. Although limited work has been undertaken in pediatric running athletes, some prospective evidence exists correlating dynamic pronation excursion with the occurrence of exercise-related lower limb pain in a heterogeneous cohort of college-aged physical education students.
Two studies have demonstrated altered running mechanics in adult women who have sustained tibial stress fractures. In separate cross-sectional, retrospective studies comparing 3-dimensional mechanics of female runners with a history of tibial stress fracture to an uninjured cohort, it was noted that the tibial-stress-fracture groups demonstrated increased peak rearfoot eversion, a component of pronation, compared with controls. Further, Milner and colleagues reported that the tibial-stress-fracture group also demonstrated significantly higher peak hip adduction, whereas Pohl and colleagues noted the variables of peak rearfoot eversion, peak hip adduction, and free moment (a measure of impact force) correctly classified 83% (50 out of 60) of runners into the tibial-stress-fracture or control group. Taken together, this suggests efforts aimed at reducing pronation and/or hip adduction may reduce the likelihood of developing a tibial stress fracture; however, caution must be noted because a cause-and-effect relationship cannot be ascertained from retrospective studies.
More recently, emphasis has shifted toward assessing hip strength, hip muscular activation, and running gait mechanics to assess the relative impact these variables may have on the development of a running-related injury. A lack of hip strength has been associated with multiple injuries, including PFPS, ITBS, and tibial stress fracture. A lack of strength to the hip abductor, hip extensor, and hip external rotator musculature is theorized to place the femur into excessive amounts of adduction and internal rotation leading to alterations in joint coupling and mechanics to the knee, shank, and foot-ankle complex distally. Indeed, there is limited evidence to suggest that altered hip strength affects running mechanics in a healthy population and in a population of adult female runners with PFPS. However, although improving hip strength has led to reductions in pain and improvement in function, particularly in patients with PFPS and ITBS, they have not directly led to changes in running gait mechanics. This seeming contradiction has, in part, caused some investigators to assess the effect of alterations in hip muscular activation and running gait mechanics on the relative risk of sustaining a running-related injury.
Proper nutrition is important for maintaining health and reducing the risk of sustaining a pediatric running-related injury. High School cross-country runners with a higher BMI have a higher risk of sustaining MTSS than those runners with a lower BMI. Conversely, in a cohort of female collegiate track-and-field athletes, reduced nutritional fat intake was associated with an increased risk of sustaining a stress fracture. This finding speaks to the intrinsic risk factors unique to female athletes, such as delayed age of menarche and irregular menstruation. Both menstrual irregularity, such as oligomenorrhea and amenorrhea, as well as a delayed age of attaining the first menstrual cycle are associated with increased risk of sustaining a bony stress-related injury. Further, female long-distance runners who demonstrate signs or symptoms of disordered eating are at an increased risk of reduced bone mineral density than those runners with more typical eating habits.
Extrinsic risk factors
Extrinsically, improper training is identified as a contributor to pediatric running-related injuries. Progressive training regimes with a focus on a gradual increase in running to help acclimate the body to the rigors of running, thereby reducing the likelihood of sustaining a running-related injury, are common. Unfortunately, little evidence exists to support this widely held belief. A randomized controlled trial did not find a protective effect for a preconditioning program at reducing running-related injury rates (RRIR) in novice adult runners. This finding is in agreement with Buist and colleagues who found no difference on the RRIR of adult novice runners using a 13-week graded training program that followed “the 10% rule” for mileage increase when compared with an 8-week control group. Because many pediatric runners can be considered novice runners or runners who have been running for 2 years or less, these results may provide some insight into the effect of training on the PRRI rate.
The type and duration of previous sports participation may alter the risk of sustaining a running-related injury. In novice adult men training for a 4-mile race, those runners who had participated in sports without axial loading before training, such as swimming and cycling, were more than twice as likely to sustain an running-related injury than those men who had participated in sports such as basketball or soccer. This finding is in agreement with Fredericson and colleagues who noted that youth participation in basketball or soccer was protective against the future risk of sustaining a stress fracture in collegiate track athletes. Taken together, these studies impart 2 valuable clinical pearls. First, participation in sports that induce axial loading in a 3-dimensional fashion may enhance bone mass and, thereby reduce the likelihood of sustaining a stress-related running-related injury. Second, participation in sports whereby athletes are required to perform sprinting, cutting, and jumping maneuvers may enhance an athlete’s neuromuscular control capabilities, which might impart a reduced likelihood of sustaining a running-related injury.
Potential Interventions to Target Altered Mechanics and Reduce Injury Risk
Although attempts to reduce the incidence of running-related injuries in adults through graded training programs or by matching shoe wear to foot type have proven to be ineffective, a careful review of the literature on risk factors for sustaining a PRRI as well as treatments for the most common PRRI suggests a pathway toward prevention. The key variables to this pathway may include the following:
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Identifying at-risk populations
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The application of a preseason and in-season hip strengthening program
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Assessment of running biomechanics
Based on the epidemiologic reports of Rauh and colleagues and Tenforde and colleagues as well as the application of the reports on risk factors for novice runners, the following characteristics of pediatric and adolescent long-distance runners should be taken into consideration and can be broken down between intrinsic and extrinsic measures. The assessments of 4 intrinsic measures are recommended. First, measuring the standing Q angle is recommended based on the aforementioned studies by Rauh and colleagues. Specifically, runners with a Q angle of more than 20° and/or runners with a right-left difference of more than 4° should be noted and the application of modified training programs and/or targeted hip and quadriceps strengthening programs is recommended. Second, measuring the BMI should be considered because an increased BMI is associated with MTSS, whereas a reduced BMI may serve as a warning to coaches, parents, and health care professionals that the runner may be at risk for bony stress-related injuries, such as stress reaction and stress fractures. Third, the navicular drop test should be considered, but caution should be noted in interpreting the results because of the conflicting reports noting its association with MTSS, exertion-related lower limb pain (a condition that encompasses pain between the knee and ankle that occurs with exercise ), and stress fractures. Fourth, preseason measures of hip abductor, hip extensor, and hip external rotator strength should be considered because of the fair to strong evidence noting the relationship between hip muscle weakness and conditions such as PFPS and ITBS. Additionally, indirect evidence suggests a possible biomechanical link between hip muscle weakness and tibial stress fractures ; however, further prospective studies are warranted. Finally, with regard to female athletes, it is strongly recommended to regularly measure menstrual cycle status and regularity to reduce the likelihood of incurring bony stress-related injuries.
Dynamics evaluation of altered gait mechanics, with the potential for targeted gait retraining, is viewed as a potential new avenue in the evaluation, treatment, and perhaps prevention of pediatric running-related injuries. With regard to stress fractures, multiple investigators have noted that adult female runners who have sustained a tibial stress fracture demonstrate altered running kinetics, specifically increases in loading rate and ground reaction forces. This finding had led some investigators to attempt to retrain running gait mechanics of at-risk and/or injured runners to reduce these variables of impact shock that may be associated with injury. Techniques such as real-time visual feedback, mirror gait retraining, and increasing step rate have been used to target altered running mechanics. Taken together, both visual and auditory gait retraining provide clinicians with emerging interventions to address the high incidence of pediatric running-related injuries. However, future prospective research in a pediatric population is warranted to determine the effect of gait retraining on the reduction of injuries in at-risk populations.
With regard to modifiable extrinsic factors, 3 steps are recommended. First, cataloging an athlete’s preseason activity level by using such valid and reliable tools as the Tegner Activity Scale may be warranted to both determine the athlete’s readiness to run as well as his or her relative risk for sustaining a running-related injury. Based on weak to fair evidence in the adult population, determining the fitness level before running, the length of time running, the type and severity of prior running-related injuries, as well as the types of previous sporting activities are recommended. Second, modifying training volumes and intensity for runners who have recently undergone a growth spurt are recommended to reduce stressors to the apophysis. Finally, early and continued participation in ball sports and/or sports involving a 360° playing field are recommended based on weak to fair evidence. Providing runners individualized training programs that take into account their fitness level, growth and maturation, and prior running-related injuries and that afford runners an opportunity to perform cross-training activities that challenges their coordination and neuromuscular control are recommended to help modify the extrinsic risk factors associated with pediatric running-related injuries.
In conclusion, more high-quality prospective research is warranted to better illuminate the intrinsic and extrinsic risk factors associated with sustaining a running-related injury. Further, high-quality randomized controlled trials assessing the effects of preseason interventions on the incidence and severity of these injuries are necessary to help guide coaches, parents, student-athletes, and health care professionals in making quality decisions regarding evidence-based steps to reduce the likelihood of sustaining a running-related injury. Although these challenges remain, the continued participation in endurance sports, such as long-distance running, by pediatric and adolescent youths is recommended as a means of promoting a healthy and well-rounded lifestyle.
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