Exercise-Related Injuries Among Women: Strategies for Prevention from Civilian and Military Studies

The material in this report was prepared for publication by:

National Center for Injury Prevention and Control 
Stephen B. Thacker, M.D., M.Sc.
Acting Director

Division of Unintentional Injuries Prevention 
Christine Branche, Ph.D.
Director

Exercise-Related Injuries Among Women: Strategies for Prevention from Civilian and Military Studies

Julie Gilchrist, M.D.
Bruce H. Jones, M.D., M.P.H.
David A. Sleet, Ph.D.
Division of Unintentional Injury Prevention
National Center for Injury Prevention and Control

C. Dexter Kimsey, Ph.D., M.S.E.H.
Division of Physical Activity and Nutrition
National Center for Chronic Disease Prevention
and Health Promotion

Abstract

Scope of the Problem: The numerous health benefits of physical activity have been well documented, resulting in public health support of regular physical activity and exercise. Although beneficial, exercise also has corresponding risks, including musculoskeletal injuries. The incidence and risk factors for exercise-related injury have been poorly assessed in women. Many civilian exercise activities (e.g., jogging, walking, and aerobics) have corollaries in military physical training; injury incidence and risk factors associated with military physical training have been more thoroughly studied.

Etiologic Factors: Injury risks increase as the amount of training increases (increased exposure). The same exercise parameters that can be modified to enhance physical fitness (i.e., frequency, duration, and intensity) also influence the risk for injury in a dose-response manner. Higher levels of current physical fitness (aerobic fitness) protect the participant against future injury. A history of previous injury is a risk factor for future injury. Smoking cigarettes has been associated with increased risk for exercise-related injury. Studies conducted in military populations suggest that the most important risk factor for injuries among persons engaged in vigorous weight-bearing aerobic physical activity might be low aerobic fitness rather than female sex.

Recommendations for Prevention: Because of the limited scientific research regarding women engaging in exercise, general recommendations are provided. Women starting exercise programs should be realistic about their goals and start slowly at frequency, duration, and intensity levels commensurate with their current physical fitness condition. Women should be informed about the early indicators of potential injury. Women who have sustained an injury should take precautions to prevent reinjury (e.g., ensuring appropriate recovery and rehabilitation).

Research Agenda: In general, a combination of factors affects the risk for exercise-related injury in women. How these factors act singly and in combination to influence injury risk is not well understood. Additional research regarding exercise-related injury in women is needed to answer many of the remaining epidemiologic questions and to help develop exercise programs that improve health while reducing the risk for injury.

Conclusion: Exercise is an important component in improving and maintaining health; however, injury is also an accompanying risk. A review of key military and civilian research studies regarding exercise-related injuries provides some clues to reducing these injuries in women. Greater adherence to exercise guidelines can help decrease these risks.

BACKGROUND

In 1996, the U.S. Surgeon General's report on physical activity brought together for the first time current knowledge regarding the health benefits of regular physical activity (1). The report concluded that persons who are inactive can improve their current and future health by becoming moderately active on a regular basis. In addition, the report indicated that activity does not need to be strenuous to achieve some health benefits, but that greater health benefits can be achieved by increasing the amount (frequency, duration, or intensity) of physical activity. Although many studies have documented the hazards of inactivity, few have assessed the adverse effects of increased physical activity (e.g., injury). Increased physical activity increases the risk for injury. Although opportunities for women to participate in sports and organized fitness activities have increased substantially during the preceding century, little is known about the risks for injuries associated with increased physical activity and exercise for women. This report reviews key military and civilian research studies regarding musculoskeletal injury associated with common weight-bearing exercise (e.g., running, walking, and aerobics) and provides general recommendations for preventing exercise-related injuries among women.

Recent public health reports have reviewed the scientific evidence supporting the association between physical activity and several health benefits (1,2). Documented health benefits of regular physical activity include reducing the risk for coronary heart disease, noninsulin-dependent diabetes, hypertension, colon cancer, osteoporosis, and other disorders (1). Physical activity decreases the symptoms and might reduce episodes of anxiety and depression (1). In addition, regular physical activity improves physical fitness (e.g., cardiorespiratory endurance and muscle strength); reduces body fat; and builds and maintains healthy bones, joints, and muscles (1). Physical activity enhances strength, balance, and coordination (1). These benefits might be particularly important in preventing falls and maintaining independence in older adults. As a consequence of these health benefits, regular physical activity is highly recommended for women and men of all ages (1).

The U.S. Surgeon General's report indicated that approximately 60% of adult women in the United States did not engage in the recommended amount of physical activity, and 25% did not participate in physical activity during their leisure time (1). Physical inactivity is more common in women than men (1,3). To help increase the proportion of persons engaged in regular physical activity, two of the Healthy People 2010 objectives are to a) reduce to <20% the proportion of persons aged >18 years who engage in no leisure-time physical activity (objective 22-1) and b) increase to >30% the proportion of persons aged >18 years who engage in regular, preferably daily, moderate physical activity for at least 30 minutes per day (objective 22-2). Because regular physical activity is considered essential to health, it has been included as one of the leading health indicators for health promotion and disease prevention in the United States (4).

Although physical activity has many health benefits, exercise has corresponding injury risks. Participants are at risk for exercise-related traumatic or overuse injuries. Some of the consequences of these injuries can be long-term (e.g., osteoarthritis and adverse health effects resulting from inactivity because of an injury). Injury causes many persons to stop participating in exercise (2,5 ). Efforts to increase physical activity and exercise in women must also be balanced with efforts to prevent injury.

Because lifestyles have become more sedentary and work has become less physically demanding, planned physical activity intended to improve physical fitness has become more important. Consequently, many adults choose to participate in exercise programs or sports. Health-related exercise programs and sports are excellent ways for women to increase their physical activity.

Opportunities for young women to participate in sports have substantially increased in recent decades. Since passage of the 1972 Title IX legislation that prevented sex discrimination in educational settings, the number of young women who participate in high school athletics has increased from approximately 300,000 during the early 1970s to nearly 2.7 million (one in three high school women) in the 1998-99 school year (6,7). This increased participation in high school athletics has fostered increased participation in college and elite athletics as well. Women now comprise approximately one third of all college athletes and 37% of U.S. Olympic athletes (7).

Many adult women participate in recreational aerobic activities. The National Sporting Goods Association reported that an estimated 37.4 million women participate more than twice a week in common aerobic activities (i.e., aerobic dance, cycling, exercise walking, exercising with equipment, calisthenics, swimming, and running) (8). Walking is the most prevalent physical activity among adults in the United States (1,9). If trends of increased participation in women's sports expand to include increased participation in recreational and other physical activities, the number of exercise-related injuries can also be expected to increase.

Injuries occur in association with physical activity, exercise, and sports (10-13), but the incidence and underlying causes of such injuries are not well understood. At the peak of the fitness boom in the 1980s, researchers knew little about exercise-related injuries and injury rates, even for common activities (e.g., walking and running) (12). During that period, researchers were only beginning to study the epidemiology of and risk factors for exercise-related injuries (12,14). Today, injury risk factors for physically active men remain poorly defined, and the specific risks for women who exercise are even less understood. Studies of runners have provided the most thorough examination of injury incidence and some related risk factors in civilian populations (5,12,14-17).

Studies of military populations provide sex-specific information on injury risks associated with physical training and exercise; activities are controlled, and complete and detailed health records, physical examinations, and physical fitness assessments are available (18,19). Studies of basic combat training, which occurs in all branches of the military and involves running, marching, and other weight-bearing aerobic activities, can often provide information relevant to civilian populations. Uniformity of training within military units provides unique control for the variability observed in exercise routines in the civilian population. Examination of military studies provides some data on exposure risks (18,20) and intrinsic risk factors (e.g., sex, previous injuries, health behaviors, sports participation, physical fitness, and anatomic factors) (19-24).

This report describes civilian and military research related to weight-bearing aerobic exercise and injuries. Aerobic exercises (e.g., running, walking, and aerobic dance) are highlighted in this report because they are popular and commonly prescribed activities. Military studies of training-related injuries are presented to identify shared and sex-specific intrinsic risk factors. Risks for men will be discussed briefly for comparative purposes. This report focuses on modifiable risk factors, which underlie the recommendations for prevention and future research.

Definitions

In this report, distinctions between the terms "physical activity," "exercise," and "physical training" are important. Physical activity has been defined as movement created by skeletal muscle contractions, resulting in energy expenditure. Exercise is a type of physical activity that is planned, repetitive, and designed to improve or maintain at least one of the health-related components of physical fitness (25). Physical training (as used in the military) is organized exercise intended to enhance fitness. The terms exercise and physical training are used interchangeably. Physical fitness can be categorized into five health-related components: a) cardiorespiratory endurance (aerobic fitness), b) muscle endurance, c) strength, d) flexibility, and e) body composition (1,25). The focus of this report is on exercise for women aimed at enhancing cardiorespiratory endurance (aerobic fitness). When discussing research results from cited literature, the terms "significant" and "not significant" refer to a documented p-value of <0.05 or >0.05, respectively, unless otherwise stated.

Musculoskeletal injuries related to exercise can be classified as either traumatic (acute) injuries (e.g., sprains and fractures) or overuse injuries (e.g., tendinitis, bursitis, and stress fractures). A distinction is also made between extrinsic and intrinsic risk factors for musculoskeletal injury. Extrinsic risk factors refer to the parameters of training (e.g., frequency, duration, and intensity) and the conditions associated with the environment in which the exercise takes place. Intrinsic risk factors refer to the personal and internal characteristics of the participant (Table 1).

SCOPE OF THE PROBLEM

Findings from Civilian Studies

The incidence of exercise-related injury among women in the civilian population is not well documented. Civilian studies of male and female exercise participants provide some indication of the frequency of such injuries. Surveys demonstrate that the incidence of self-reported running-related injury is high. Annually, approximately 25%-65% of male and female runners report being injured to the extent that they reduced or stopped training (5,13,15-18,26). In addition, 14%-50% of these injured runners seek medical care for their injuries (5,13,15-18), representing substantial health-care costs for treatment and rehabilitation. Prospective studies that incorporated follow-up of injury among runners and other persons involved in vigorous physical activities suggest that the incidence of injuries might be even higher (11,27-29).

In an 18-month study of runners training for a marathon, 85% experienced >1 injury, and 174 injuries were reported among the 73 participants (159 injuries per 100 runners per year) (27). In a 12-week study of aerobic dancers, 200 (49%) of 411 participants reported complaints associated with aerobics, and approximately 25% had to modify or stop participation because of an injury (28). In a study of participants engaged in several recreational sporting activities, 475 injuries occurred among 986 participants during a 12-week period (192 injuries per 100 participants per year) (11). In a 6-month study of walkers who averaged 14 miles per week, 21% stopped walking for >1 week because of injury (29). Although injuries during fitness activities are common, few studies of women or men who participate in recreational fitness activities are available to quantify risk or identify modifiable risk factors.

Findings from Military Studies

Many civilian fitness activities (e.g., walking and jogging) have corollaries in military physical training (e.g., marching and running). The incidence of injury and related intrinsic risk factors for these activities have been more thoroughly studied in military populations than in civilians. Because physical fitness is required for military readiness, recruits undergo a vigorous basic training (BT) course, and substantial research has been devoted to methods of enhancing fitness and understanding the causes of training-related injuries. Studies from the U.S. Army 8-week BT have documented cumulative injury rates from 42% to 67% among women during the course of training (19,20,30). Of women in the U.S. Air Force, 33% incurred an injury during the 6-week BT (20). Similarly, 22% of women in the U.S. Navy sustained an injury during the 9-week BT, and 49% of women in the U.S. Marine Corps were injured during the 11-week BT (20). The range of injury incidence (22%-67%) among women in the different services and over time might be explained by differences in the duration and intensity of BT.

Most of the injuries to both women and men engaged in military BT are overuse injuries (e.g., achilles tendinitis, patellar-femoral syndrome, plantar fasciitis, and stress fractures). Most injuries occur to the lower extremities. Studies during Army BT indicate that 60%-80% of BT injuries are related to overuse, and 80%-90% occur to the lower extremities (21,22,30).

Injuries in the military have substantial effects on training and combat readiness because they require greater rehabilitation and recovery time than illnesses. Approximately 50% of health-care visits in these young, vigorously active military populations are injury-related (20). The rate ratios of injury-to-illness sick call visits for women in the Army, Marine Corps, and Air Force are 1.0, 1.0, and 0.8, respectively. Furthermore, the rates of limited duty days (i.e., days when a trainee cannot fully function on duty) are often substantially higher from an injury than from illness (20,24). In one Army study, women were assigned 129 injury-related limited duty days per 100 female trainees per month compared with 6 illness-related limited duty days per 100 trainees per month. The rate ratio between injury and illness limited-duty days was 22, even though 50% of sick-call visits were for illnesses (20). Among men, the rates of limited duty for injury were five times higher than the rates of limited duty for illness. In the physically active and generally healthy military populations, injury can be expected to account for a substantial proportion of morbidity, health-care costs, and rehabilitation time in comparison with illnesses. The burden of injuries among physically active civilian populations might reflect a similar pattern.

Risk Factors for Exercise-Related Injuries

Risk factors for exercise-related injuries can be either extrinsic or intrinsic to the participant (Table 1). This report focuses on extrinsic training factors, perhaps the most important factors in determining injury risks, and addresses selected intrinsic factors. The association between training parameters and injury risks in civilian and military populations will be examined first because they are potentially the most important.

Extrinsic Training Factors

The same training parameters that are modified to achieve a training effect (i.e., frequency, duration, and intensity of exercise) are also the most important factors related to injury. Several surveys of distance runners indicate a relation between a higher number of miles run per week and a higher incidence of injury in both women and men (5,10,13,15,26). Several studies have demonstrated that the relative risk (RR) of injury among civilian women and men is a function of the miles run per week (Table 2) (5,15,26). One classic study indicated that as the average weekly training mileage increased in 10-mile increments from <10 miles per week to >50 miles per week, the incidence of injury for women increased from 29% to 57%. The incidence of injury for men increased in a similar manner (5). Two additional studies reported similar sex-specific trends (15,26). The annual incidence of injury among female and male runners was approximately the same, and the RRs of injury for both sexes increased with increasing miles run. These and other studies suggest that, for weight-bearing exercise (e.g., running), injury rates increase as the amount of training increases in a dose-response manner.

In a study that examined the benefits of aerobic fitness and injury risks associated with increased duration or frequency of training among men, injury rates increased with duration of exercise (when frequency and intensity remained constant). Participants received limited additional aerobic fitness benefits when they exercised 45 minutes compared with 30 minutes. As duration of running increased from 15 minutes to 30 minutes to 45 minutes per workout, injury rates increased from 22% to 24% to 54%, respectively, whereas aerobic fitness (measured by maximal oxygen uptake) improved only 9%, 16%, and 17%, respectively. Although a plateau in fitness occurred, more exercise increased the incidence of injury. This study also demonstrated that frequency of exercise (number of training sessions per week), although positively related to aerobic fitness, was also positively related to injury (31).

A similar study of male walkers and joggers demonstrated that injury rates were more related to total mileage walked and jogged than to the intensity of exercise. This study controlled the total amount of activity in two groups of participants during a 6-month period. Both groups exercised the same duration per day (30 minutes); however, the walkers exercised more frequently (more days per week) than the joggers to accumulate approximately the same mileage. The walkers averaged 120 minutes of exercise per week, and the joggers averaged 90 minutes per week; however, the total distance accumulated by both groups was approximately the same (13.7 km per week and 14.7 km per week, respectively). At the end of the 6-month study period, the two groups had similar injury rates: 21% of the walkers and 25% of the joggers had sustained injuries sufficiently severe to require terminating their activity for >1 week (29). Studies such as these indicate that the total amount of training is an important determinant of injury risk. These studies were conducted with men, and similar studies of women are needed.

Studies of military populations have also examined the relation between training frequency and duration, gains in cardiorespiratory fitness, and injury risk. As mileage during physical training increases, both aerobic fitness and the risk of injury increases. Similar to the findings in civilian populations (31), military studies have documented thresholds in physical training, above which increased training does not improve fitness levels but continues to increase the likelihood of injury (18-20). These studies of military populations examined the association between training parameters and injury risk among men only. Additional studies among women are needed.

Intrinsic Training Factors

Military BT provides a unique opportunity to study some intrinsic risk factors for exercise-related injuries. Unlike civilian fitness participants, regimentation in military training requires that trainees do the same type and amount of training. Researchers studying military populations have systematically examined several intrinsic factors and their relation to musculoskeletal injury risk. The most consistently identified intrinsic risk factors have been a) sex, b) age, c) history of previous injury, d) adverse health behaviors (e.g., smoking tobacco), e) previous physical activity (e.g., sedentary lifestyle), and f) current level of physical fitness.

Sex. Sex has consistently been identified as a risk factor for injury in military BT. In studies from the 1980s to 1997 that examined women and men at the same training site who performed essentially the same physical training, incidences of injuries for women were 1.7-2.2 times higher than those for men (19,20,21,30,32,33) (Table 3).

In addition, rates of some specific injuries during military training (e.g., stress fractures) are higher for women than men (20,24,30,33,34). In Army training, RR for stress fractures is 210 times higher for women than men engaged in the same training regimen (20,21,30,34-36). In the Marine Corps recruit training, the risk for stress fractures is 3.7 times higher for women than men (20).

Some specific injuries (e.g., anterior cruciate ligament tears in the knee) occur more frequently in female athletes (37). However, in studies comparing civilian runners (the most extensively studied civilian recreational fitness activity), the overall rates of exercise-related injury are similar among women and men. Researchers suggest that female civilian runners have the same injury rates as men because they can modulate their training frequency, duration, and intensity (unlike military trainees) to accommodate their fitness levels and the minor overuse injuries that might occur (10). Injury studies among military populations suggest that without controlling for physical fitness, at any fixed level of activity, women will be at greater risk for injury than men (Table 3).

Age. Results of military studies regarding the effects of age on training and injuries have been inconsistent. Some studies have revealed that during BT, female (38) and male trainees aged >23 years are at greater risk for injury (22,38,39). Other military studies have indicated no statistically significant difference in injury risk by age (20,36,40). Studies of civilian runners have also had inconsistent results. Some studies have demonstrated that age is not an important risk factor, whereas others have demonstrated that rates of injury decrease with age (10,13,15,16,26). Among civilian women, older age was not associated with elevated risk (10). Unlike military trainees, older participants in civilian studies might have been able to decrease their risk by modulating the frequency, duration, or intensity of their personal training regimens (10). Alternatively, a "survivor effect" might exist, whereby persons who have sus tained injury change activities or cease participation and thus are unavailable for inclusion in studies (10). Data from military and civilian studies suggest that among adults aged <45 years, age alone is not a strong predictor of injury in exercise.

History of Previous Injury. A history of previous musculoskeletal injury has also been reported as a risk factor for injury in both civilian and military studies. In a systematic review of the literature regarding the prevention of ankle sprains in sports, the most commonly identified risk factor for an ankle sprain was a previous ankle sprain (41). Overuse injuries occurred twice as frequently in trainees with a previous history of ankle sprain (19). A previous ankle sprain is also a risk factor for injuries among male trainees in Army BT (22). In addition, data from the Marine Corps suggest that previous injuries pose a risk for future injury (20,40). These findings are consistent with civilian studies of female and male distance runners, in which RR for an injury in a person who has had an injury during the preceding year was 1.8-2.4 for women and 1.7-2.7 for men (15,26).

Health-Related Behaviors. Health behaviors engaged in before entry into military service (e.g., smoking tobacco and participating in regular physical activity) can influence a woman's injury risk during BT.

Smoking. Both female and male smokers who participate in Army or Marine Corps BT are at a significantly higher risk for injury than nonsmokers (20). Women who were smokers on entry into the Army were 25% more likely to be injured in BT; injury rates were 77% for smokers and 62% for nonsmokers (20). Similarly, the risk for injury among women in the Marine Corps who smoked before beginning BT was 1.7 times higher than for those who were nonsmokers (20). Male smokers in Army and Marine Corps BT were 1.9 times and 2.3 times more likely to have an injury, respectively, than their nonsmoking male counterparts (20,22). Studies have not indicated whether civilian athletes or exercise participants who smoke tobacco are at greater risk for injury. However, in a literature review of the potential association between smoking and injuries, researchers estimated that smokers were two times more likely than nonsmokers to sustain unintentional injuries in the workplace, although some of these injuries might not be related to physical activity (42). Data from these studies suggest that women who smoke are at a higher risk for training-related injuries than women who do not smoke.

Previous Physical Activity. Although some health behaviors (e.g., smoking) might increase injury risk, previous regular physical activity might be protective against injury. This protective effect has been documented in men in the Army and Marine Corps (20-22,32,39). Among male trainees in the Army, running before entry into the service might be protective. For military women, the association between previous regular physical activity and injury risk has not been documented (20,32,36). Researchers documented that, for men, more years of participating in running was protective against injury; however, for women, more years of participating in running might be associated with higher risk for injury (15). These results are difficult to interpret because of possible survivor effects (e.g., injured runners cease to run). Because no comparable data in civilian populations of women exist, no conclusions can be drawn regarding the influence of previous regular physical activity as a protective factor against injury among women. Further research is needed regarding the influence of previous physical activities and exercise-related injury risk among women and men in both military and civilian populations.

Current Level of Physical Fitness. A person's current level of physical fitness has been one of the most important predictors of injury in military studies (19-21,24,32,33,40). Of the five health-related components, low levels of aerobic fitness and, to a lesser extent, low muscular endurance have consistently been associated with injury risk during BT. Other factors (e.g., body composition and strength) demonstrated weaker and less consistent associations with injury risk.

Aerobic fitness, as measured by timed performance of 1- to 2-mile runs during Army or Marine Corps physical fitness entry tests, has been the single most consistently and strongly associated intrinsic risk factor for subsequent training-related injury. During Army BT, women who scored in the slowest quartile on the initial entry physical fitness test experienced 1.5-1.7 times greater injury risk than women in the fastest quartile (21,36) (Table 4). Findings were similar for women in Marine Corps BT: women in the slowest quartile experienced 2.4 times greater risk for injury than women in the fastest quartile. Women and men with the slowest run times (i.e., least aerobically fit) were consistently at greater risk for injury than those with the fastest run times (i.e., most aerobically fit). Comparable trends were documented among female Army cadets at West Point Academy, New York (24). Among men, the inverse relation of aerobic fitness and injury risk is similar to that of women. Male trainees with slower run times were at greater risk for injury than those who ran the fastest (20,36).

In addition to being at greater risk for injury, women who had the slowest run times experienced 2.5 times the risk of stress fractures and stress reactions compared with women who had faster run times (20,32). Similar findings were documented among women in Marine Corps BT (20,40). Researchers demonstrated that the least aerobically fit and least physically active trainees were 3.5 times more likely than persons who were the most fit and most active to sustain a stress fracture (23).

A prospective study of Army trainees in BT demonstrated an association between maximal oxygen consumption (ml O₂ per kg body weight per minute), which is a measure of aerobic fitness, and subsequent risk for injury. Maximal oxygen consumption (VO₂ max) was measured in trainees running on a treadmill before the start of BT. For women in successive tertiles of VO₂ max, risk for injury increased from 39% in the highest tertile to 50% in the middle tertile, to 55% in the lowest. Similarly, men with the lowest VO₂ max were at greatest risk for injury (36). Prospective studies among civilians examining the association between aerobic fitness and injury are not available. Military research suggests that higher levels of baseline physical fitness is protective, at least at the start of a training program. Further research is needed to determine the degree and duration of this protection.

Higher levels of muscular endurance and strength can also be protective against injury in military BT. For both women and men, greater muscular endurance (measured by the number of push-ups completed in 2 minutes) was associated with fewer training-related injuries (20). When categorized into quartiles, risk for injury decreased for women who could do more push-ups. The cumulative incidence of injury was 57% for women who completed the least number of push-ups in 2 minutes and 38% for women who did the most push-ups. Similarly, Army women who could not lift >34 kg had RR for injury of 1.4 compared with women who could lift >46 kg (20).

The relation of body composition to exercise-related injury risk is complex. Some studies indicate no association between body composition and exercise-related injury risk (20,36). When an association between measures of body fat and injury incidence for women in Army BT has been identified, the relation has been bimodal (U-shaped). Women with the least and the most body fat were at greater risk for injury (21,32,40). Among women in Army BT, the risk for injury varies by body mass index (BMI). To obtain BMI, weight in kilograms is divided by height in meters squared (weight [kg]/[height squared [m²]). The cumulative incidence of injuries in successive quartiles of increasing BMI were 56% (lowest quartile), 46%, 38%, and 63% (highest quartile). The corresponding RRs were 1.5, 1.2, 1.0, and 1.6, respectively. BMI for women ranged from 18 kg/m² to 27 kg/m² (32). A study of civilian male distance runners demonstrated a statistically significant bimodal relation between BMI and injury (16). A study of civilian female runners indicated a statistically not significant but also bimodal relation between BMI and injury (15). Additional research is needed to better determine the relation between body fat, BMI, and incidence of injury; these studies should control for physical fitness and previous physical activity.

The Relation Between Sex and Level of Physical Fitness

The observation that low levels of physical fitness on entry into BT is related to injuries during BT is particularly relevant to the issue of injuries among women. The incidence of injuries among women in Army BT is consistently 1.6-2.1 times higher than the incidence for men in Army BT. However, several studies also document that on entry into the Army, women are less physically fit than men (20,21,32,35,43). On average, women have slower run times, perform fewer push-ups, and have a higher percentage of body fat than men.

What would be the effect of controlling for level of fitness when making comparisons between men and women? In several studies, injury risks were stratified by quartiles or quintiles of run times to enable comparison of groups of women and men who performed similarly on the initial-entry physical training test (20,32,35,43). In these studies, initial RRs of injury for women were higher than for men, with RRs ranging from 1.6 to 2.1. However, when stratified by aerobic fitness (run times), the stratum-specific risk ratios all approached 1.0, and the summary risk ratios declined (range = 0.9-1.2). In a logistic regression model that controlled for physical fitness (i.e., run times, numbers of push-ups and sit-ups, and strength), age, and race, the odds ratio for women versus men was 1.1 (20,43). Slower run times were the only component of fitness associated with increased odds of injury. Odds of injury progressively increased for successive quintiles of run time from fastest to slowest: 1.0, 1.4, 1.5, 2.5, and 3.2, respectively. In another logistic regression model, female sex was initially a risk factor, with an odds ratio of 2.5 for women compared with men, until run time was entered into the model. When corrected for run times, the odds ratio for females declined to 1.0; however, run time remained a significant predictor (32). These findings suggest that the most important underlying risk factor for injuries among military trainees engaged in vigorous aerobic weight-bearing activities (e.g., running and marching) is aerobic fitness level and not female sex (33,43). Studies that compare injury risks between men and women with similar fitness levels have not been conducted in civilian populations.

Because the findings in this report are derived from studies of special populations (e.g., runners and military trainees), they might not be generalizable to other U.S. populations. A review of the studies in these special populations provides guidance toward establishing general principles that will be valuable in preventing injuries and guiding research in the general population.

RECOMMENDATIONS FOR PREVENTION

Scientific research regarding injuries related to physical training and exercise has focused on men rather than women, on military trainees rather than physically active civilians, and on competitive rather than recreational athletes. In addition, the studies of military populations generally involve a young, healthy population. Studies of recreational athletes in the civilian population are difficult to conduct and might not be able to completely control for the frequency, duration, and intensity of activity, as is possible in studies of military populations. In addition, measures of current physical fitness might be difficult to obtain.

Based on the limited scientific research regarding physical activity, exercise, and injuries among women and generally agreed on "best practices," the following recommendations are made to reduce the risk of exercise-related injury among women:

• Although most healthy women do not need to visit their physician before starting a moderate-intensity exercise program, women aged >50 years or women who have either a chronic disease or risk factors for a chronic disease should consult their physician to ensure that their exercise program is safe and appropriate.
• The choice of an exercise program should be tailored to a woman's current physical fitness level. Resources that include examples of activities categorized by exercise intensity levels are available and can aid women in choosing activities based on their respective physical fitness levels.
• Decisions regarding the frequency, duration, and intensity of exercise should be individualized, based on the woman's current level of physical fitness, history of physical activity, and history of injury.
• Women, particularly those with lower fitness levels, should begin participating in exercise at a lower level of training (frequency, duration, and intensity) and progress slowly. Women who are sedentary and start a new exercise program or activity might need to begin with intervals of activity as short as 5-10 minutes of light-intensity activity and gradually increase to the desired intensity and/or duration of participation.
• Participants should be aware of early signs of potential injury (i.e., increasing muscle soreness, bone and joint pain, excessive fatigue, and performance decrements). Coaches, personal trainers, and instructors should be alert to these signs among the women they are supervising.
• When a participant senses any of the warning signs (i.e., increasing muscle soreness, bone and joint pain, excessive fatigue, performance decrements, or current injury), she should incrementally decrease training (i.e., reduce frequency, duration, or intensity) until symptoms diminish or cease participation temporarily, depending on the severity of injury.
• Women who sustain a musculoskeletal injury should allow sufficient recovery and rehabilitation time and take precautions to prevent reinjury.
• Women who smoke should be informed that smoking might increase their risk for exercise-related injury. They should make every effort to stop smoking, not only to reduce their risk for injury, but also to enhance their long-term overall health.
• Women should be realistic in setting their exercise goals by balancing the desire for measurable weight reduction, increases in endurance or strength, or other health-related fitness benefits with the risk for injury.

RESEARCH AGENDA

Research Needs

This report provides an overview of the relation between extrinsic training factors, selected intrinsic factors, and musculoskeletal injury risks during exercise. Some research exists regarding exercise-related injury risk factors among military or elite athletic populations; however, little research has been conducted among other physically active populations. Even less research specifically addresses the particular risks to women who exercise. These gaps in current knowledge limit the specificity with which recommendations can be made. Future research is needed to identify methods of promoting physical activity while preventing or reducing the risk for injury. As researchers continue to define the benefits of regular exercise, the following suggestions might help develop complementary research regarding injury risks:

Surveillance systems need to be developed to monitor physical fitness, health, injury, and other medical outcomes of physical activity and exercise. Questions regarding exercise-related injuries need to be incorporated into existing surveillance instruments that monitor physical activity participation levels. Measures of exposure in physical activity (i.e., frequency, duration, and intensity) by sex, age, and activity should be incorporated into data-gathering systems to better characterize population-based injury risks.

Research regarding the etiology of exercise-related injuries is needed to determine the incidence of and risk factors for injuries in common exercise activities (e.g., walking, hiking, bicycling, and aerobic dance) and active sports (e.g., tennis, racquetball, basketball, and soccer) for women. Identification of the amount (i.e., frequency, duration, and intensity), type (e.g., jogging, walking, biking, and dancing), and progression of exercise that is appropriate for women of differing physical fitness levels and body composition is needed to maximize fitness while minimizing injury risk. To prevent overuse injuries among women, the appropriate amounts and balance of training and recovery for different types of exercise and activity need to be determined. In addition, sex-specific exercise-related injury risks need to be determined to guide the choices women make regarding exercise. These risks include but are not limited to a) anterior cruciate ligament rupture, b) stress fractures, c) pregnancy and postpartum injury risks, and d) risks (and benefits) of exercise in older women and women with osteoporosis.

Conducting longitudinal intervention trials to monitor injury occurrences while measuring changes in fitness is essential for developing and evaluating injury reduction measures. Determination of the long-term effects of exercise-related injuries on health outcomes (e.g., osteoarthritis, late or chronic sequelae, and disability) and future exercise participation is needed because a decrease in physical activity might increase the risk for chronic diseases. Finally, to document the impact of exercise-related injury and the importance of further research, the economic and social costs of exercise-related injury and any resultant nonadherence to exercise regimens need to be determined.

CONCLUSION

Persons who participate in vigorous exercise might incur a higher number of musculoskeletal injuries than more sedentary persons. However, several intrinsic and extrinsic factors interact to modify the risk for incurring an exercise-related injury. For activities other than running and military training, little data are available regarding the incidence or risk factors for such injuries. The data suggest that a combination of factors (e.g., sex, current level of fitness, previous exercise experience, smoking, previous injury, and body composition) might affect the risk for exercise-related injury in women. However, how these factors act singly and in combination to influence injury risk is not well understood. The following conclusions might help in the development of further research regarding the relation between exercise and the risk for injury:

• The most important risk factors for exercise- or training-related injuries are the frequency, duration, and intensity of the physical training activity. The total amount of exercise (e.g., the frequency, duration, and intensity) is the most consistently identified predictor for injury risk.
• Physical fitness is inversely related to injury risk; as physical fitness level increases, risk for injury decreases. Men and women who participate in the same activities and have the same physical fitness levels generally have similar incidences of injury. Thus, physical fitness rather than female sex is the underlying risk factor.
• A dose-response relation exists between the amount of weight-bearing exercise performed and the risk of injury for both women and men.
• A training threshold exists, above which increased training does not appreciably increase fitness but will substantially increase risk for injury. This threshold might be different for each person.
• Although higher current amounts of exercise or physical activity are risk factors for injury because of increased exposure, at any fixed amount of activity, men with a history of higher amounts of physical activity are at lower risk for injury. For women, the relation is unclear.
• At any given amount of aerobic weight-bearing activity, women and men who have the highest aerobic fitness levels can be expected to have lower subsequent injury rates.
• The combined findings of research regarding the association of training, previous physical activity, and current physical fitness levels suggest that tailoring exercise to accommodate a person's current level of fitness and previous physical activity reduces injury rates. Changes in frequency, duration, or intensity of exercise can have cumulative effects on injury risk. These findings are particularly important for persons who are the least fit or most sedentary because they are at the greatest risk for injury when initiating physical activity.
• The protective effect against injury of higher levels of aerobic fitness provides an incentive to become more physically active. It suggests that incremental increases in fitness are beneficial in terms of increasing health benefits and decreasing injury risks.
• The relation between previous injuries and higher risk for subsequent exercise-related injuries provides some indication of the importance of a) recovery and rehabilitation and b) consideration of the history of previous injuries when planning exercise programs.
• The association between smoking tobacco and higher exercise-related injury risks suggests another possible reason to discourage smoking, both for injury reduction in the short-term and increased overall health benefits in the long-term.
• Although the association of body composition with exercise-related injury risks is not completely clear, the bimodal relation that exists suggests that proper maintenance of body weight in the normal range (i.e., BMI 18.5 kg/m²-24.9 kg/m²) is important not only for health and appearance but also to reduce risks for injury.

Further research is needed to answer many of the remaining epidemiologic questions and to help develop exercise programs for women that improve health while reducing the risk for injury.

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