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4 Physical Activity and Riskâ Maximizing Benefits Physical activity can pose risks in two main areas: musculoskeletal injury and fatal and nonfatal cardiac events. This chapter first addresses injury risk and then cardiovascular risk. Both sections of the chapter in- clude evidence regarding reduction of the risk. The discussion section provides an overview of riskâbenefit considerations and of osteoarthritis and joint injuries in relation to physical activity, and it offers a brief summary of comments raised by the workshop participants. RISKS OF MUSCULOSKELETAL INJURY Presenter: Bruce H. Jones1 This presentation included background information about risks and the incidence of musculoskeletal injury, a summary of factors affecting risks of injuries, and a summary of key conclusions about the causes, risk factors, and prevention of physical activity-related injury. Background In 1984, the Centers for Disease Control2 held a workshop on the public health aspects of physical activity. Injuries were identified as a risk about which limited information was available (Koplan et al., 1985). 1 Dr. Jones acknowledged assistance from Dr. Joseph Knapik. 2 Renamed the Centers for Disease Control and Prevention in 1992. 73
74 PHYSICAL ACTIVITY WORKSHOP To help prevent injuries related to physical activity, answers to the fol- lowing questions are needed: â¢ How big is the problem? â¢ What causes the problem? â¢ Are there modifiable risk factors? â¢ What works to prevent the problem? To answer these questions, it is useful to focus on the evidence for weight-bearing physical activityâthe injury problem that has been most studied. Data are available from studies of runners, walkers, and military trainees. Self-reported injury rates (counting both overuse and trauma) have been estimated to be between 25 to 65 injuries per 100 person-years for runners (Jones et al., 1994), 42 per 100 person-years for walkers (Suter et al., 1994), and 110 per 100 person-years for cyclists (Wilbur et al., 1995). Injury rates from studies based on routine clinical follow-up or medical records are much higher for activities such as aerobic dance (Garrick et al., 1986), exercise and recreational sports (Requa et al., 1993), and army basic training (Jones et al, 1994). Thus injury can be a substantial problem for a variety of exercise groups. Factors Affecting the Risk of Injury The factors that affect the risk of physical activity-related injury in- clude the amount and type of current physical activity, the amount of past physical activity, other health behaviors, physical fitness level, his- tory of previous injury, selected demographic factors, and anatomical factors. Current Physical Activity Amount of physical activity The risk of injury increases as the distance run per week increases (Koplan et al., 1982). However, the injury risk per 100 kilometers run decreases with an increase in the number of kilo- meters run per week (Jones et al., 1994). A number of studies confirm
75 RISKâMAXIMIZING BENEFITS that greater amounts3 of running result in higher rates of injury among civilian runners (Brunet et al., 1990; Colbert et al., 2000; Macera et al., 1989; Marti et al., 1988; Walter et al., 1989) and among military recruits (Almeida et al., 1999). For walking, the risks of injury are similar over the range of less than 15 minutes of walking per day to more than 30 minutes per day (Colbert et al., 2000). For previously sedentary individuals exercising three days per week, a threshold for elevated injury risk appeared between about 30 and 45 minutes per day of running (Pollock et al., 1977). That is, injury risk more than doubled, whereas endurance improved little or not at all. A study of army trainees showed a similar threshold of elevated injury risk with no gain in aerobic fitness at between 5 and 10 miles of running per week (Jones et al., 1994). Type of physical activity Few studies allow the comparison of different types of activities using the same definitions of injury and the same measures of exposure. Hootman and colleagues (2001), however, pro- vide useful data (Table 4-1). Notably, sedentary individuals have about a 15 percent risk of injury. By comparison, walkers are at only slightly higher risk than sedentary people, while runners and those involved in sports (especially men) are at substantially higher risk. TABLE 4-1 Injurya Risks Among Men and Women by Type of Physical Activity Risk by Sex, percent per year Type of Activity (relative risk) (Number of Subjects) Men Women 14.6b (1.0) 16.8b (1.0) Sedentary (1,608 M, 501 W) Walking (508 M, 206 W) 16.5 (1.14) 19.9 (1.18) Running (2,445 M, 405 W) 24.7 (1.78) 23.2 (1.38) Sports (467 M, 101 W) 27.6 (1.89) 26.7 (1.59) NOTE: M = men, W = women. a Self-reported injury in the previous year. b Significantly lower than all activities, Chi squared p < .05. SOURCE: Adapted from Hootman et al. (2001). 3 Amount refers to duration x frequency x intensity.
76 PHYSICAL ACTIVITY WORKSHOP Past Physical Activity Five well-designed studies report that greater frequency or duration of past physical activity is associated with a lower risk of current injury (Jones et al., 1993b; Knapik et al., 2001; Rauh et al., 2006b; Shaffer et al., 1999, 2006). Among male U.S. Army trainees, for example, those who ran at least 4 days per week the month before initial entry training had a 20 percent risk of injury, whereas those who ran 3 or fewer days per week had about twice the risk. Similar results are reported for other types of exercise. Physical Fitness Levels Health-related components of fitness include endurance, muscle en- durance, muscle strength, flexibility, and body composition (Caspersen et al., 1985). A very consistent finding in the military studies is that the more aerobically fit a person is, the lower the incidence of injury (Jones et al., 1993a; Knapik et al., 1993, 1998, 2001; Rauh et al., 2006b; Rey- nolds et al., 1994; Shaffer et al., 2006). Data on improvement in 2-mile run times among army recruits shows that the trainees in the slowest quartile of trainees reduce their time by an average of 5.5 minutes while the trainees in the fastest quar- tile reduce their time by an average of 1.1 minutes (Knapik et al., 2006). Data such as these suggest that the least aerobically fit trainees on entry to the service may move themselves into a lower risk category and re- duce their risk of injury by 30 to 50 percent over the current 9-week pe- riod of basic training. Data on civilian populations suggests the opposite effect from mili- tary trainees, that the most fit civilians may be at highest risk (Hootman et al., 2001). It has been suggested that the most fit runners and exercise participants may place themselves at higher risk because they do more exercise at higher intensities than their less fit peers (Hootman et al., 2001). Associations of other health-related components of fitness with in- jury vary: â¢ Compared with aerobic fitness, the muscle endurance of military personnel shows similar association to injury risk, but the re-
77 RISKâMAXIMIZING BENEFITS ported associations are weaker (Bell et al., 2000; Jones et al., 1993b, 1994; Knapik et al., 1998, 2001). â¢ Muscle strength shows inconsistent association with injury risk (Jones et al., 1993b; Knapik et al., 2001; Westphal et al., 1995). â¢ Flexibility has a bimodal association with injury risk (Jones et al., 1993b; Knapik et al., 1992, 2001). Those who are loose (most flexible) and those who are tight (least flexible) have the highest incidence of physical activity injury, which is about twice as high as those of average flexibility. â¢ Body composition has inconsistent associations with injury risk (Hootman et al., 2001; Jones et al., 1992, 1993a; Macera et al., 1989; Reynolds et al., 1994; Taunton et al., 2003). When fitness data are stratified by body mass index, it appears that the leanest, slowest individuals are at greatest risk for injury (NRC, 2006). Health and Health Behaviors Among the common health and health behavior risk factors for physical injury are: â¢ past injury (Hootman et al., 2001; Jones et al., 1993b; Macera et al., 1989; Walter et al., 1989), â¢ amenorrhea (Barrow and Saha, 1988; Lloyd et al., 1986; Rauh et al., 2006b; Shaffer et al., 2006), â¢ sedentary lifestyle (Gardner et al., 1988; Jones et al., 1992, 1993b; Knapik et al., 2001), and â¢ cigarette smoking (Altarac et al., 2000; Jones et al., 1993b; Knapik et al., 2001; Reynolds et al., 1994). Selected Demographic Factors Age Studies of associations of older age with the risk of injury have pro- duced inconsistent results. Eight studies indicate that older age is associ- ated with higher risk of injury (Brudvig et al., 1983; Gardner et al., 1988; Jones et al., 1993b; Knapik et al., 2001, 2006; McKean et al., 2006; Shaffer et al., 1999; Taunton et al., 2003). Four studies show that older age is associated with lower risk (Carlson et al., 2006; Colbert et al., 2000; Hootman et al., 2001; Knapik et al., 1993). Most of the studies that
78 PHYSICAL ACTIVITY WORKSHOP show higher risk with older age are military studies. It may be that older civilian exercise participants modulate their risk of injury by performing less exercise at lower intensities. Sex During army basic training where male and female trainees do the same physical training, females are at 1.6 to 2.1 times greater risk for injury than males (Bell et al., 2000; Bensel et al., 1983; Canham et al., 1998; Jones et al., 1992; Knapik et al., 2003; Kowal, 1980). After adjust- ing for physical fitness (Canham et al., 1998)âor for physical fitness, age, and race (Bell et al., 2000)âthe risk is essentially the same. Anatomical Factors Some anatomical factors (e.g., foot morphology, knee Q-angle) ap- pear to be associated with the risk of injury (Brunet et al., 1990; Cowan et al., 1994, 1996; Giladi et al., 1985; Rauh et al., 2006a), but such fac- tors may not be amenable to correction. Evidence indicates that bone, muscle, tendons, and ligaments adapt to increased physical activity and exercise (Maffulli and King, 1992; Maganaris et al., 2004; Sharma and Maffulli, 2006). Structured regimens to improve the resistance of these tissues to injury have not yet been tested. Evidence for the Prevention of Weight-Bearing Physical Activity-Related Injuries In 2003, a new standardized program to prevent over-training during army basic training found that the traditional physical training program posed a significantly higher risk than did the injury prevention program (Knapik et al., 2004b, 2005). Features of the standardized injury preven- tion program include reduced miles run, performance of distance runs by ability groups, the addition of speed drills, and a more balanced program of activities. Other trials of similar methods have been successful (Knapik et al., 2003, 2004a; Rudzki and Cunningham, 1999; Shaffer et al., 1996). Other prevention strategies appear to have a limited effect or no effect on reducing risk. These other strategies include stretching (Herbert and Gabriel, 2002; Rome et al., 2005; Shrier,1999; Thacker et al., 2004), warm-ups (Fradkin et al., 2006), and shock absorbent devices
79 RISKâMAXIMIZING BENEFITS for footwear (Dâhondt et al., 2002/2005; Jones et al., 2002; Rome et al., 2005; Withnall et al., 2006; Yeung and Yeung, 2001). Concluding Remarks Greater amounts of physical activity increase the risk of injury, and thresholds of physical activity exist above which more activity increases injury risk but not fitness. More past physical activity and higher fitness levels protect against injuries. In addition to lack of fitness, modifiable risks include amenorrhea, inactivity, and smoking. The prevention of overtraining can reduce injury risk and improve fitness. Dr. Jones ex- pressed the view that (1) sufficient evidence is available to establish gen- eral principles of physical activity-related injury causation and prevention, and (2) the application of those principles is expected to maximize the benefits and minimize the risks of physical activity. PHYSICAL ACTIVITY AND CARDIOVASCULAR DISEASE RISKS Presenter: David S. Siscovick The primary focus of this presentation was on sudden cardiac arrest among individuals older than about 30 years, with some attention to non- fatal myocardial infarction. Because the overall effect of exercise on car- diovascular health reflects a potential balance of benefits and risks, the presentation included a review of evidence regarding the magnitude of risk, factors that modify risk, data on the balance of the risks and bene- fits, and implications for public health and for clinical care. Magnitude of the Problem About 10 to 15 percent of all deaths from coronary heart disease are due to sudden cardiac arrest. Approximately 5 percent of cardiac arrests occur during vigorous exercise, and about 10 percent of cardiac arrests occur during moderate-intensity exercise. The magnitude of the risk var- ies with the nature of the physical activity, the environment, characteris- tics of the population, and other factors. Reported values for absolute risk
80 PHYSICAL ACTIVITY WORKSHOP per exerciser range from 1 cardiac arrest in 5,000 individuals per year at an aerobics center to 1 in 20,000 individuals per year in the general population during vigorous activity. Examined in a different way, the ratio of the risk during the activity compared to the risk at other times ranged from 4.5 for cross-country skiing in Finland to 7 for joggers in Rhode Island. Factors that Modify Risk Factors that modify the risk of cardiac arrest during exercise can be organized by characteristics of the exercise, characteristics of the exer- ciser, environmental factors, and personal habits. Little evidence is avail- able about the impact of environmental factors (temperature, humidity, altitude, air pollution) or about the impact of personal habits (such as the use of alcohol or tobacco) on the risk of cardiac arrest during exercise. Thus Dr. Siscovick focused on characteristics of the exercise and of the exerciser. Characteristics of the Exercise Both the intensity of the exercise and the habitual level of physical activity can affect the risk of cardiac arrest. In a large study in Finland, Vuori (1986) showed that there was a 9-fold increase in the risk of hav- ing a sudden cardiac arrest during vigorous exercise compared to other times and a 3.2-fold increase in risk during nonvigorous exercise. To ex- amine the risks of the exercise in relation to the potential benefits of physical activity, Siscovick and colleagues (1984) collected data on the type, intensity, frequency, and duration of activity; and then they esti- mated the time spent on activity to examine the risks, both during and not during activity, in terms of incidence density. Characteristics that Modify Absolute Risk Clinical heart disease In the presence of clinical heart disease, both the risk during activity and the risk not during activity are increased. How- ever, the magnitude of the increase in risk during activity compared to
81 RISKâMAXIMIZING BENEFITS not during activity is about seven-fold (Siscovick, 1997). This is similar to that for the general population. Sex The absolute risk of sudden cardiac arrest overall and during bouts of moderate to vigorous exertion are much lower in women than in men, as is the relative risk. During moderate or vigorous exertion by women, the absolute risk of sudden cardiac arrest increased about two-fold, com- pared to other times, but the magnitude of the risk was very much lower than that in men (Whang et al., 2006). As with men, the training or ha- bitual activity level was associated with a lower risk during a bout of moderate to intense exertion. Age Data from Vuori (1986) indicate that the absolute risk of sudden death during exercise increases with age, but sudden death is very un- common among persons younger than 45 years. The relative risk of sud- den cardiac arrest during vigorous exercise compared to other times is much higher in men younger than 45 years than in older men because the absolute risk of sudden cardiac arrest that occurs during inactivity is very low in younger men. Habitual physical activity The data in Table 4-2 indicate that the mag- nitude of the transient increase in risk during exercise was influenced by the level of habitual vigorous exercise. The increase in risk during exer- cise was lowest among the men who were engaged in more than 124 kilocalories (kcal) (20 minutes) of vigorous exercise per day. The impact of habitual activity on the transient increase in risk during a bout of ac- tivity is consistent with a training effect, possibly due to reduced physio- logical and metabolic stress among men who engage in habitual vigorous exercise. Some years later, Albert et al. (2000) reported similar findings using data from the Physiciansâ Health Study. The overall occurrence of sud- den death during vigorous exercise was very small. However, the evi- dence suggested a much greater risk of having a sudden cardiac arrest during vigorous exertion among the physicians who exercised less than once per week than among those who exercised at least five times per week. These investigators also examined the data to look for a circadian variation in the risk of exertion, but they found no difference in the tran- sient increase in risk during vigorous exertion occurring in the morning, afternoon, or at night.
82 PHYSICAL ACTIVITY WORKSHOP TABLE 4-2 Sudden Cardiac Arrest During Vigorous Exercise: Balance of Cardiac Risk and Benefits Cases/108 Person Hours (N) Not During Overalla Habitual Vigorous Exercise, kcal/day During Activity Activity 0 NA 18 (30) 18 (34) 1â16 732 (2) 13 (44) 14 (46) 17â123 66 (3) 5 (32) 6 (35) >124 21 (4) 4 (10 5 (22) NOTE: 108 = 100 million. a The weighted average of the risk during and not during exercise in which the weights are the amount of time spent in exercise. SOURCE: Siscovick et al. (1984). Looking at vigorous exertion as a potential trigger of acute nonfatal myocardial infarction, Mittleman et al. (1993) demonstrated that a tran- sient increase in the risk of a nonfatal myocardial infarction occurs within the hour after a bout of vigorous exertion. As with the investiga- tors above who examined increases in the risk of sudden cardiac arrest, Mittleman and colleagues (1993) found that the relative risk of nonfatal myocardial infarction for heavy exertion decreased dramatically with increased frequency of regular physical exertion, reported as numbers of bouts per week. Balance of Cardiac Risks and Benefits The data in column 4 of Table 4-2 also indicate that the overall inci- dence of sudden cardiac arrest (the weighted average of the risk during and not during exercise where the weights are the amount of time spent in exercise) was highest among those who did not engage in habitual vigorous exercise. With increasing levels of habitual activity, the overall risk of sudden cardiac arrest decreases substantially, despite the transient increase in risk during activity compared to the risk at other times. Physical activity can be compared to surgeryâsimilar to surgery, physi- cal activity poses acute risks, but it also has the potential for supporting a longer, healthier life.
83 RISKâMAXIMIZING BENEFITS Limitations of the Available Evidence Several limitations of the available evidence need to be considered. There is a lack of consistency in the definition of cardiac events during exertion. Specifically, some investigators define events within 30 to 60 minutes of a bout of exertion as âduring exertion.â In addition, most studies have only a small number of events during exertion. In general, there is a lack of detail about prior habitual activity and the presence of prior heart disease and coronary risk factors, and current data are not yet sufficient to predict cardiac events during exertion. Concluding Remarks Dr. Siscovick presented his assessment of public health implications and clinical implications as follows: Public Health Implications Exercise has acute cardiac risks, but the absolute risk of a cardiac event during exercise is low, and the transient increase in risk is out- weighed by the long-term cardiac benefits of exercise. A number of fac- tors modify risk. In advising the public, it is important to consider both risks and benefits. Clinical Implications When providing clinical care, an important measure is targeting ex- ercise, and considering environmental factors and habits. This requires asking patients about the circumstances under which they exercise. In part because the sensitivity of pre-exercise evaluation is low in adults, medical supervision of exercise is indicated for selected individuals (such as those with clinical coronary heart disease). Automated external defibrillators in settings of supervised exercise may be very helpful in resuscitating individuals who have a cardiac arrest.
84 PHYSICAL ACTIVITY WORKSHOP DISCUSSION Physical Activity-Related Injuries: Is the Benefit Worth the Risk? Discussant: Jennifer Hootman Risk Associated with Physical Activity Among civilian, community-dwelling adults in the United States, the incidence of physical activity-related injury is very low overall, but it ranges widely for specific groups (for example, the range is 8 to 80 per- cent for recreational and competitive runners). The more active a person is, the higher the probability of injury (Hootman et al., 2002; Pollock et al., 1977). Nonetheless, about 15 percent of sedentary people report an activity-related injury. As indicated earlier by Dr. Jones, with increases in activity, the risk per unit of exercise exposure may be lower. Using unadjusted data, Hootman and colleagues found that people who meet the Surgeon Generalâs physical activity recommendation (de- fined as 1,000 kcal or more of physical activity-related energy expendi- ture per week) have a risk of injury that is about 77 percent higher than that of inactive people (zero energy expenditure from physical activity per week). After adjusting for a variety of risk factors (mainly the modi- fiable risk factors identified by Dr. Jones), however, the increase in risk is not statistically significant for both men and women. The results of a multivariate analysis suggest that (1) even insufficient exercise (1 to 999 kcal of exercise per week) may confer a small amount of protection against activity-related injury, and (2) twice weekly weight training and stretching exercise each may also confer a small amount of protection against activity-related injury. A few studies report injury rates per 1,000 hours of physical activity. Reported rates include 0.19 to 1.5 for commuting and lifestyle activities (Parkkari et al., 2004), 2.2 for strength and aerobic exercise in older adults (Colbert et al., 2000), 5.9 to 7.8 for adult fitness activities (Requa et al., 1993), and 6.6 to 18.3 for recreational sports (Parkkari et al., 2004). According to Dr. Hootman, the injury rate can be estimated to be about one injury every 4 years for the average person who meets the minimal public health recommendation for physical activity. The basis for the estimate is in Box 4-1.
85 RISKâMAXIMIZING BENEFITS BOX 4-1 Estimation of Injury Rate for Individuals Meeting the Surgeon Generalâs Physical Activity Recommendation Physical activity recommendation = 30 minutes of exercise 5 days/week. â¢ 150 minutes/week = â¢ 7,800 minutes or 130 hours/year = â¢ 7.7 years to accumulate 1,000 hours of activity If the injury rate is approximately 2.0/1,000 hours, then the injury rate is about one injury every 4 years. SOURCE: Colbert et al. (2000). Possible Prevention Messages Dr. Hootman listed a number of possible prevention messages and the type of evidence that supports each, as follows: â¢ Choose activities wisely. Walking poses 50 percent less risk than does running. â¢ Moderate the total dose of physical activity. Exercising more than 20 miles per week increases the risk 1.7 to 2.1 times. â¢ Combine strengthening with aerobic exercise. Strength training reduces the risk about 40 percent in women. â¢ Maintain a normal body weight. Although it is not known if weight loss reduces the risk of injury, it improves biomechanics. â¢ Improve fitness level slowly before entering a rigorous training program. Military data show the importance of this approach. â¢ Obtain appropriate care and rehabilitation before returning to participation post injury. Previous injury is a risk factor for a fu- ture injury. Concluding Remarks Dr. Hootman expressed the view that the benefits do outweigh the risks and that there are ways to modify the risks.
86 PHYSICAL ACTIVITY WORKSHOP Physical Activity, Osteoarthritis, and Joint Injuries Discussant: William E. Garrett, Jr. In his discussion, Dr. Garrett focused on the potential association be- tween exercise and osteoarthritis (OA) rather than on a single injury. Among the many reasons people provide for not exercising are the asso- ciation between exercise and pain and the understandable concern that exercise-related pain could lead to OA. Although OA has been linked to âwear and tear,â some evidence suggests that exercise that promotes muscle strength and endurance may allow the muscles rather than the bones and joints to absorb the energy preferentially. By this concept, ex- ercise may even protect from OA (Hurley, 1999; Shrier et al., 2004). Diseases such as diabetes lead to muscle weakness and sensory neuropa- thy that can cause extreme arthritis in the form of Charcot jointsâeven with very little exercise. The clear associations between exercise and OA involve sports (e.g., soccer, football, and basketball) that may lead to joint injury such as ligament or meniscal damage in the knee. Data are unclear regarding whether there is an association between running and OA, but moderate levels of running or of other aerobic exercise do not appear to cause OA (Konradsen et al., 1990; White et al., 1993). Professional groups such as the American College of Rheumatology include aerobic and strengthen- ing exercises in the treatment of people with OA. Joint Injuries Relatively high rates of joint injuries occur in sports. These injuries can lead to changes of OA and are a better predictor of OA than sports participation itself (Thelin et al., 2006). The development and implemen- tation of injury prevention programs could help achieve the benefits of exercise without the risk of OA. Concluding Remarks Given all the health benefits of exercise and the lack of an associa- tion between exercise and OA, health care professionals can provide a
87 RISKâMAXIMIZING BENEFITS useful service by prescribing exercise regularly and alleviating patient concerns regarding arthritis. Group Discussion Moderator: Caroline Macera Points raised during the group discussion included the following: â¢ Few data are available regarding traumatic injury risk associated with exercise such as running, racing, and bicycling. â¢ Data are needed regarding physical activity-related injury in the context of all injury. For example, can physical activity provide protection from other types of injuries? â¢ Definitions related to injury need to be standardized. In general, the injuries that are counted are those that require some medical treatment and result in a change in activity level. â¢ The primary cause of sudden cardiac death among U.S. adults is coronary heart disease. Risk factors for coronary heart disease are likely to increase the risk of sudden cardiac death both during activity and during inactivity. For persons with previously ex- pressed cardiovascular disease, a supervised setting for exercise is beneficial. â¢ Screening guidelines for the general population and those with cardiovascular risk factors need clarification. Results of sub- maximal tests on middle-aged men with elevated low-density lipoprotein cholesterol concentration have low sensitivity. â¢ Findings from the Wisconsin Epidemiological Study on Diabetic Retinopathy related to weight lifting may provide useful infor- mation on adverse effects of physical activity on microvascular disease. â¢ An article by Carato and a review by Thompson (2005) provide potentially useful information about cardiovascular events during exercise among athletes and the potential impact of pre-exercise evaluations among young people.
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