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Understanding the risks and benefits of physical activity important in public health

This is an excerpt from Foundations of Physical Activity and Public Health by Harold Kohl III & Tinker Murray.

In public health, it is important to understand not only the health benefits of a certain behavior or intervention you may be promoting, but also the risks. Knowledge of a downside to any program is critical to having a complete picture. Vaccination programs, environmental changes for cleaner air, educational programs for HIV prevention—all of these are examples of situations where programs with good intentions might have unintended risks to people and populations you are targeting. Understanding the risks as well as the benefits of particular programs is practicing “responsible” public health.

Two primary unintended consequences (risks) of physical activity that have been extensively studied are musculoskeletal injury and exertion-related sudden cardiac death. Here we review musculoskeletal injury and put it in perspective in terms of the costs and benefits of increasing physical activity.

A physical activity-related musculoskeletal injury involves some type of acute disorder in a bone, muscle, joint, or connective tissue that is attributable to physical activity or exercise. Such injuries can occur suddenly, such as an ankle sprain, or over a period of exposure, such as a gradual pain in the shoulder of an electrician who frequently works over his head installing circuits. Ligament tears, sprains, strains, bone fractures, bruises, and joint dislocations are common musculoskeletal injuries that can result from physical activity and exercise. Clearly, such injuries can occur without physical activity (e.g., in a motor vehicle accident), but this chapter addresses only those that result from some type of body movement.

A difficult problem when studying musculoskeletal injuries involves the definition of injury. What qualifies as an injury? You probably know what an injury means to you, but does your mother have the same definition? How about a world-class sprinter or a heavy machine operator? Chances are that each person has a unique idea of what an injury is. Many times it involves pain, loss of function, and an inability to work or socialize. Some people have a higher threshold for pain than others do, so the same incident in two people may be classified as an injury by one person and a “bump” by the other.

Severity is also an issue. How long does the pain or loss of physical function have to last before the incident is called an injury? Thirty minutes? Thirty days? Must a person see a physician or other health professional before an incident can be classified as an injury? What if that person does not have health insurance? Surely someone without health insurance would be less likely to see a doctor or health care professional for an injury than someone with insurance and the same injury. This could result in a study or survey counting one occurrence but not the other, simply because the latter could not be “found.” Does someone have to be injured during an exercise session for the injury to be considered exercise related, or does an injury caused by cumulative exposure to exercise (e.g., arthritis in the knee) also count as an exercise-related injury?

The point is that, although the scientific literature is replete with studies that have examined musculoskeletal injuries as related to physical activity and exercise, a consistent definition has rarely been used. This makes comparing studies nearly impossible. Studies that rely on participants' self-reports of “any injury” are not comparable to those that require a doctor's diagnosis prior to being classified as an injury. What we know about the rates and risks of musculoskeletal injuries due to physical activity and exercise is therefore limited compared to the wealth of information available on the health benefits.

What is the prevalence of exercise-related injury? This simple question, unfortunately, does not have a simple answer because of many complications. First, as discussed, the definition of injury is varied. Most people would count an event that was serious enough to require a trip to an emergency room, but what about something less serious? Is an event that requires you to take a few days off work, but not a trip to the emergency room, serious enough to be considered an injury? What about simply taking a few aspirin and self-treating for a few days? The main limitation to studies of physical activity and musculoskeletal injury is the use of inconsistent (and incomplete) definitions.

Another complicating factor in determining the prevalence of exercise-related injury is the fact that different types of physical activity have different participation rates and may result in different types of injury. Low-impact and noncontact exercise activities and sports are likely less risky for musculoskeletal injury than are high-impact and contact sports. These differences make it somewhat meaningless to discuss the prevalence of injuries in the same manner we talk about the prevalence of diabetes or myocardial infarction, both of which are single diagnoses. Similarly, we expect weight-bearing activities such as walking or running to be associated with injuries of the lower extremities more often than other parts of the body. Racket sports such as tennis and squash may be more likely to result in injuries of the upper extremities (shoulders and arms) or the head (from being struck by a racket or a ball).

Finally, in the United States there are no routine surveys or systems from which to generate a picture of exercise-related injuries. There have been periodic studies (Powell et al. 1998), studies of catastrophic injuries (Mueller and Cantu 1991), and studies that focus on numerators (Gotsch et al. 2002), but none of these have tracked these problems over time. This situation obviously makes it impossible to truly understand the risks of physical activity, which limits public health professionals' ability to give an accurate risk/benefit assessment.

Although population-based exercise-related prevalence data are limited, we do know some things about what might be expected to occur for several types of physical activity in a defined time period. The data in table 10.1 are from a national survey of injury (Powell et al. 1998). By asking respondents what they were doing when they became injured, the investigators were able to compare various types of common physical activities. Obviously, each of the reported activities is fairly safe; fewer than 3 people out of 100 were injured in any 30-day period. Outdoor bicycle riding appears to be the least risky activity in terms of musculoskeletal injury, and weightlifting was the riskiest.

Although these findings may appear to be intuitive, they emphasize the need to quantify the risks of physical activity for application in the real world. For example, such information can be very useful to a program manager who is beginning a community-based walking program for sedentary adults. After reviewing the data in table 10.1, the manager now knows that she might expect one or two people in her program to be injured during the walking program in a given month (30-day period). This is useful information for program planning and evaluation. If she puts appropriate preventive strategies in place and none of her participants become injured, she can report that the participants in her program are injured less frequently than what one might expect given the literature.

Despite the problems in the scientific literature, we do know some things about the causes and risk factors for physical activity-related musculoskeletal injuries. These factors have been identified in the scientific literature from studies in epidemiology, biomechanics, physiology, and medicine. As with other health-related outcomes for physical activity and exercise, risk factors conveniently can be classified as modifiable (i.e., things that can change or be changed) and nonmodifiable (i.e., things that typically can't be changed or are difficult to change).

Modifiable Risk Factors for Musculoskeletal Injuries

  • Amount and type of current physical activity
  • Cigarette smoking
  • Low physical fitness level
  • Improper use of protective equipment
  • Adverse environmental conditions

Nonmodifiable Risk Factors for Musculo­skeletal Injuries

  • Age
  • Sex (for some types of injury)
  • History of injury
  • Amount of physical activity in the past (history)
  • Anatomical factors
  • Environmental, or external, conditions

Following are descriptions of the modifiable and nonmodifiable risk factors for musculoskeletal injury:

  • Amount and type of current physical activity. The more physical activity a person performs, the higher the risk of musculoskeletal injury associated with the activity. This finding has been demonstrated repeatedly in the literature. Moreover, different types of physical activity and exercise convey different risks. For example, contact sports are more likely to be related to injury than noncontact sports.
  • Cigarette smoking. Although exercise and cigarette smoking would appear contradictory behaviors, people whose occupations demand physical activity, such as construction workers and landscapers, may also smoke. Cigarette smoking seems to increase the risk of physical activity-related musculoskeletal injuries, possibly as a result of vasoconstriction, which restricts the amount of oxygen being delivered to the muscles or connective tissues. The structure of the site and the availability of metabolic nutrients are then altered, and the hypothesis is that this alteration makes the muscle or connective tissue more susceptible to injury.
  • Low physical fitness level. People who have higher physical fitness levels (measured as .VO2max) have been consistently shown to be at lower risk of musculoskeletal injury related to physical activity.
  • Improper use of protective equipment. Bicycle helmets, protective padding for skateboarders, breakaway bases for baseball players, mouthguards for certain sports, shoes—all of these are examples of protective equipment that, when properly used, can prevent musculoskeletal injuries associated with physical activity.
  • Adverse environmental conditions. Environmental conditions can be considered either nonmodifiable or modifiable. If conditions are not conducive to physical activity or could increase the risk of injury during physical activity, venues can be changed, activities can be rescheduled, or the type of physical activity can be modified (e.g., going to the gym rather than playing basketball in the rain).
  • Age. Changes in the musculoskeletal system that occur with aging result in older people being more likely to be injured than younger people doing the same activity.
  • Sex. Women's skeletal structure and sex hormones have been hypothesized to increase their risk of lower extremity injury (specifically, to the anterior cruciate ligament in the knee) compared to men doing the same activities.
  • History of injury. A history of injury is one of the most consistent risk factors for injury during physical activity reported in the literature. People who have been injured previously are more likely to be injured in the future than those who have not. This is a strong rationale for efforts to prevent injuries from occurring in the first place.
  • Amount of physical activity in the past (history). Much of what we know in the area of physical activity and musculoskeletal injury comes from studies of military recruits who participate in basic training involving substantial physical activity. Recruits who were physically active prior to the training were less likely to be injured during the training. Again, this finding makes a powerful case for injury prevention.
  • Anatomical factors. Each human body is unique, and a person's biomechanical and anatomical characteristics may increase the risk of an (or exacerbate an existing) exercise-related musculoskeletal injury. Among the many factors that have been hypothesized are varus, or bowlegs (an abnormal inward angle of a bone); valgus, or knock-knees (an abnormal outward angle of a bone); pes cavus (an abnormally high foot arch); and pes planovalgus (flat feet). Many anatomical problems can be reversed through medical intervention.
  • Environmental, or external, conditions. A frequently overlooked risk factor for exercise-related musculoskeletal injuries is environmental conditions. Traffic, damaged or wet playing fields or courts, and broken sidewalks are all examples of environmental, or external, conditions that could increase the risk of a musculoskeletal injury associated with physical activity.


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