1 Division of Adult and Community Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA.
2 Division of Nutrition and Physical Activity, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA.
3 Department of Epidemiology and Biostatistics, School of Public Health, University of South Carolina, Columbia, SC.
4 Department of Health, Physical Education, Recreation, and Safety, Middle Tennessee State University, Murfreesboro, TN.
5 Cooper Institute for Aerobics Research, Dallas, TX.
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ABSTRACT |
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exercise; logistic models; musculoskeletal system; physical fitness; wounds and injuries
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INTRODUCTION |
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The risk of injury is one of the few adverse effects of a physically active lifestyle (3, 5
8
). Among vigorously active populations, a clear dose-response relation exists between weight-bearing exercise volume and risk of lower extremity injury (7
, 9
19
). Studies of runners and military trainees have found that higher levels of cardiorespiratory fitness are associated with lower risk of exercise-related musculoskeletal injury (13
, 15
, 20
, 21
). However, the relation among physical activity, cardiorespiratory fitness, and musculoskeletal injury has not been defined for the civilian general population that participates in the moderate types of exercise recommended by national health organizations, such as walking, jogging, or sport activities. Identifying the relation between different levels of physical activity and musculoskeletal injury may help public health practitioners promote safe physical activities for adults. The purpose of this study was to identify the association among physical activity type and duration, cardiorespiratory fitness, and musculoskeletal injury in a group of adults with various physical activity and cardiorespiratory fitness levels.
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MATERIALS AND METHODS |
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Of the 18,806 baseline clinical examinations through 1981, only subjects (n = 11,972) that returned a follow-up survey in 1982 were mailed the 1986 survey. A total of 5,028 men and 1,283 women returned the 1986 survey. After deceased subjects and incorrect addresses were taken into account, the response rate of the 1986 survey was 72 percent. Subjects eligible for this study were adults aged 2085 years (at follow-up) who completed the baseline examination and returned a follow-up survey in 1986. Analyses were limited to the 4,034 men and 967 women who returned the survey and had complete data for all study variables. Additional details of the Aerobics Center Longitudinal Study methods have been described previously (1, 2
).
Injury definition
An injury was any self-reported muscle, tendon, bone, ligament, or joint injury occurring in the 12 months before the mailed follow-up survey. The cause of injury was defined as "activity related" if injury was due to participation in a run/walk/jog or strenuous sport exercise program (e.g., racquet sports, cycling, swimming, aerobic dance, basketball, soccer, and so on). It was defined as "other injury" if the injury was due to a cause other than run/walk/jog or strenuous sport participation. Subjects not reporting a musculoskeletal injury in the previous 12 months were classified as uninjured.
Physical activity
Physical activity status was categorized into four mutually exclusive groups according to the usual type of physical activity reported during the preceding 12 months. Sedentary subjects reported no participation in a run/walk/jog program, strenuous sports, racquet sports, bicycling, or swimming. Walkers reported participation in a run/walk/jog program with an average pace per mile of 15 minutes or slower (1 mile = 1.6 km). Runners participated in a run/walk/jog program and reported an average pace of less than 15 minutes/mile. For run/walk/jog participants who did not report information about their pace (45 percent of the sample), walking and running status were identified by a question that asked how much time subjects spent walking during their run/walk/jog program. Walkers were those who reported walking for at least 75 percent of their run/walk/jog program, and runners walked less than 25 percent of their run/walk/jog program. Run/walk/jog participants with missing information on pace and who reported walking between 25 and 75 percent of their run/walk/jog program (n = 165) could not be clearly classified as a runner or walker and were excluded from this analysis. Sport participants were those who exclusively engaged in one or more sporting activities (e.g., strenuous sports, racquet sports, bicycling, or swimming) but who did not participate in a run/walk/jog program.
The weekly duration of physical activity was calculated by summing the hours of all physical activity per week, including time spent participating in a run/walk/jog program, racquet sports, strenuous sports, swimming, and bicycling. Subjects were classified into physical activity categories based on their total reported hours per week. The low duration group included subjects with a total physical activity time of less than 1.25 hours/week. The moderate duration group was defined as those subjects who reported 1.253.75 hours/week, and the high duration group comprised those who reported more than 3.75 hours/week. These categories correspond with less than 15, 1545, and greater than 45 minutes per day of physical activity for 5 days per week.
Cardiorespiratory fitness
Cardiorespiratory fitness was assessed at the baseline clinic visit by walking on a motorized treadmill during a physician-supervised graded exercise examination using the modified Balke testing protocol (22). Subjects were monitored for blood pressure, heart rate, and rhythm by a trained technician before and during the test (23
). The treadmill test began at a speed of 3.3 miles per hour and 0 percent grade for the first minute. The grade was raised to 2 percent during the second minute, and thereafter the grade increased 1 percent every minute for 25 minutes. At 25 minutes, the grade remained constant and the speed increased 0.2 miles per hour each minute until the subject reached volitional fatigue or was stopped by the physician for medical reasons (23
). All subjects reached at least 85 percent of their age-predicted (220 age) maximum heart rate. The maximal time on the treadmill in seconds was the measure used to classify cardiorespiratory fitness. The total treadmill time is highly correlated with maximal oxygen uptake in men (r = 0.92) and women (r = 0.94) (24
, 25
). Cardiorespiratory fitness levels were categorized by age and gender-specific quintiles of total treadmill time. The lowest quintile (bottom 20 percent) was classified as low cardiorespiratory fitness. The middle two quintiles (middle 40 percent) and the top two quintiles (top 40 percent) were classified as moderate and high cardiorespiratory fitness, respectively. The low cardiorespiratory fitness group in this cohort has been previously established as a high-risk category for several health outcomes, including mortality and cardiovascular disease, among this same cohort (2
, 26
, 27
).
Covariates
Age (years) and body weight (pounds; 1 pound = 0.45 kg) were self-reported on the follow-up survey. Height (inches; 1 inch = 0.025 m) was measured at the baseline or during a subsequent clinic visit. Body mass index was calculated as weight (kg)/height (m)2. Body mass index was dichotomized as less than 25 (normal) and 25 or greater (overweight/obese). Other covariates included a history of a previous musculoskeletal injury (prior to 1985) and participation (yes/no) in weight training, stretching, or calisthenic exercises at least twice per week during the preceding 12 months.
Statistical analyses
Frequency tables were developed for all study variables and are presented by injury type for men and women separately. Polytomous logistic regression was used to assess the relation between injury status (activity related, "other," and none) and each of the three exposures: the type and duration of physical activity and cardiorespiratory fitness level. The potential modifying effects of body mass index on the relation among physical activity, cardiorespiratory fitness, and injury status were assessed by the inclusion of interaction terms in the regression models using the dichotomous body mass index variable. The following variables were included in the final model as categorical variables: history of previous injury and participation in weight training, stretching, and calisthenic exercises. Age (years), cardiorespiratory fitness level (seconds), and body mass index were included in the final model as continuous variables. Statistical significance was set at the = 0.05 level for all analyses. All analyses were conducted using SAS statistical software (SAS Institute, Inc., Cary, North Carolina).
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RESULTS |
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The multivariate relation between activity-related injury and activity type, duration, and cardiorespiratory fitness in nonsedentary subjects (n = 2,931) is shown in table 3. Compared with that in walkers, the risk of activity-related injuries was not different for runners and sport participants for any duration of activity per week. Among walkers and sport participants, an increasing duration of activity per week did not significantly increase the risk of injury. In contrast, among runners the risk of injury increased with increasing duration of activity per week.
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DISCUSSION |
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The foot and ankle were the most frequent sites of activity-related injuries while the back was the most common site of "other" injury. Musculoskeletal injuries among physically active populations tend to occur in the lower extremity (7, 9
19
). Activity-related back injuries, however, tend to be sport specific, because sports such as gymnastics, golf, and weight lifting have a higher incidence of back injuries than other sports (28
, 29
). Population-based and health club studies describing the distribution of general musculoskeletal injuries among adults also report the back as one of the top three sites of injury (30
, 31
).
In this study we measured four of five components of physical activity that can contribute to the risk of musculoskeletal injury: cardiorespiratory fitness and type, frequency, and duration of activity. We do not have a measure of the intensity of exercise for all subjects, primarily those reporting participation in sports activities such as swimming, cycling, and other strenuous sports. Our findings regarding high cardiorespiratory fitness level as a risk factor for injury contrast with results from military studies reporting low cardiorespiratory fitness levels being associated with increased training injuries (13, 20
, 21
). One explanation may be that the cardiorespiratory fitness level in our study is acting as a surrogate for activity intensity or some other unmeasured component of activity. However, among nonsedentary subjects, a higher cardiorespiratory fitness level was still associated with an increased risk of activity-related injury, even after adjusting for the type and duration of physical activity. Furthermore, among just the runners and walkers, cardiorespiratory fitness was still associated with a nonsignificant 5074 percent increased risk of injury after controlling for type (running vs. walking), duration (hours per week of activity), and intensity (average pace). Even with these findings, these results suggest that other factors, such as biomechanical or structural abnormalities or personality traits, may also play a role in the relation between physical activity and the risk of musculoskeletal injuries.
Differences in the study design and sample between our study and military studies may also explain our contrasting findings regarding cardiorespiratory fitness as a risk factor for injury. The military population is young (1823 years old), while our subjects range from 20 to 85 years. Being older on average, our subjects may have sustained more injuries over their lifetime than younger persons, thereby increasing their risk of subsequent injuries. We controlled for previous injuries. However, some subjects may not have reported injuries that occurred a long time in the past. In military studies, cardiorespiratory fitness was measured immediately preceding entrance into a vigorous basic training program, and subjects most likely did not have sufficient exercise experience prior to testing to have high cardiorespiratory fitness levels. Because their lifetime exercise experience is higher, our subjects' baseline cardiorespiratory fitness levels may have been higher than those of military trainees. Cardiorespiratory fitness is usually measured by 2-mile-run times in military studies, and therefore we cannot directly compare our baseline cardiorespiratory fitness measure (seconds on treadmill) with theirs. It would be interesting to know how cardiorespiratory fitness measured after a 6-week basic military training program, when subjects have accommodated to the increased activity levels, correlates with injuries reported over the next 12 months. When measured over a year as in our study, it may be that even military trainees with high cardiorespiratory fitness levels will be at high risk of injury.
Military trainees engage in a well-controlled physical training program, and training volume is essentially equal among subjects. Our subjects are free to vary their activity level and therefore have a wide range of activity time per week. One military study that did report some variation in hours per week of training found, similar to ours, that training time per week was correlated with injuries (32). In addition, military studies use a stricter definition of injury requiring that an injury must have required medical attention and be recorded in the medical record. We included any injury the subject attributed to physical activity. In summary, the association of a higher level of cardiorespiratory fitness with higher injury rates in this cohort most likely reflects our subjects' high lifetime activity levels. A higher level of cardiorespiratory fitness may also be acting as a surrogate for some unmeasured risk factor for injury such as biomechanical or structural abnormalities, personality traits, or exercise behavior patterns.
Identifying activities that provide an adequate amount of physical activity to improve health but have a lower risk of injury is an important public health goal. Walking is a popular form of moderate physical activity; an estimated 138 million US adults report walking for exercise (33). According to recent national estimates, 1.4 percent of US adults are injured while walking for exercise (33
). Several exercise training studies report a lower frequency of injury during walking activities than during jogging (34
36
). In an epidemiologic study comparing runners and walkers, walkers had a lower risk of activity-related injury (37
). In addition, the injury risk among runners significantly increased with increasing duration of running per day, while increased duration of walking per day did not significantly increase the injury risk among walkers. We report similar findings that, among walkers and sport participants, the risk of injury did not increase with increased duration of activity per week. In contrast, among runners the risk of activity-related injury increases with longer duration of physical activity per week in a classic dose-response pattern.
None of the relations among physical activity, cardiorespiratory fitness level, and the risk of "other" injuries was statistically significant, which is likely due to the small number of "other" injuries reported (approximately 8 percent). However, a consistent trend is evident, suggesting that the type of physical activity and cardiorespiratory fitness level may provide some protection against an "other" injury. Running decreased the risk of an "other" injury 13 percent among men, and walking decreased the risk of "other" injury 35 percent among women. Moderate cardiorespiratory fitness also decreased the risk of an "other" injury 30 percent among men and 21 percent among women. A strong and efficient neuromuscular system is better able to adapt to acute stresses and subsequently may be more resistant to physical injury (3841
). Thus, moderate physical activity in amounts to achieve a moderate level of cardiorespiratory fitness may provide an adequate stimulus to strengthen the neuromuscular and skeletal systems and decrease the risk of injury, regardless of the cause of injury.
In addition to the small number of "other" injuries reported, several other limitations of these data should be mentioned. First, inadequate recall of information used to classify each subject into exposure and outcome groups may introduce measurement error (42). Measurement error in this sample is expected to be low because the accuracy of recall of self-reported health information among subjects in the Aerobics Center Longitudinal Study has been previously reported as excellent (98 percent sensitivity, 99 percent specificity) (1
). Second, subjects reported if an injury was related to participation in a run/walk/jog program or a strenuous sport program or due to some "other" cause. The cause of injuries reported in the "other" category was not specified. Therefore, some injuries due to sports activities, such as golf, may not have been perceived by subjects to fit into the run/walk/jog or strenuous sports categories. These other types of sports injuries may have gone unreported or may have been included in the "other" category and, therefore, it is possible that some activity-related injuries may have been classified as other injuries. This type of misclassification would most likely be nondifferential and would bias the risk estimate toward the null. Third, 8 percent of sedentary subjects reported an activity-related injury. The sedentary category included subjects who did not report any participation in the specific activities listed on the survey. Therefore, sedentary subjects did not consider themselves to be regularly physically active but may have been injured during occasional participation in sport activities on weekends or at social gatherings. Lastly, cardiorespiratory fitness levels may change significantly from baseline to follow-up, an average of 8.2 years in this sample. However, a unique feature of this cohort is that cardiorespiratory fitness levels have been reported to be relatively constant over time. Using subjects from the same cohort, Blair et al. (43
) report that approximately 82 percent of subjects remained fit from the baseline treadmill examination to a second treadmill examination (the mean time between examinations was 5 years). In our sample, using body mass index as a proxy for cardiorespiratory fitness, over 90 percent of subjects maintained stable weight from baseline to follow-up.
An additional limitation is that, although the response rate was adequate, differences may exist between the subjects who responded to the follow-up survey and those who did not respond. Macera et al. (44) conducted a comparison of subjects who did and did not return the 1982 follow-up survey in this same cohort. Using baseline laboratory measurements and a personal health history, nonrespondents and respondents were equally healthy. However, participants reporting more positive health behaviors and more family history of chronic disease were more likely to respond to the mail survey. Assuming that the response patterns reported by Macera et al. are similar to those of the current study using the 1986 follow-up survey, subjects included in this sample most likely represent health conscious adults who demonstrate behaviors consistent with good health, such as engaging in physical activity and not smoking. Thus, the results of this study may not be applicable to all segments of the population.
Several aspects of these data and the statistical procedures contribute to the strengths of this study. Detailed data on physical activity, cardiorespiratory fitness, and injury were obtained from a large sample of adult men and women. Subjects reported a wide range of activity levels, including no regular physical activity, which allowed comparison of injury experiences across different levels and duration of activity. Cardiorespiratory fitness was directly measured by a treadmill graded exercise test and serves as an objective measurement of cardiorespiratory fitness level. Simultaneously estimating the association between each exposure and activity-related and "other" injuries using polytomous logistic regression allowed for direct comparison of the risk estimates for each injury cause (45).
It is well known that subgroups of vigorously active populations such as runners, scholastic athletes, and military trainees experience high rates of musculoskeletal injury (7, 9
19
, 21
). The results of this study indicate that the moderate types and duration of physical activity promoted by private and federal health organizations have lower injury risk than more vigorous types and longer durations of physical activity. Walking for exercise was not associated with a significant increased risk of activity-related injury, even among those walkers with the highest duration of activity per week. This suggests that for most adults, walking is a safe form of physical activity. However, to minimize the risk of injury, public health practitioners need to target injury prevention messages to adults engaged in vigorous types of physical activity.
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ACKNOWLEDGMENTS |
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The authors thank Dorothy Davis of the University of South Carolina Prevention Research Center and the staff of the Cooper Institute for Aerobics Research for collecting and managing the data used in this study.
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NOTES |
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REFERENCES |
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