Under the Americans with Disabilities Act (ADA), a disability is defined as "a physical or mental impairment that substantially limits one or more of the major life activities". One difficulty with estimating the number of people in the United States who have a disability, however, is in the way the term disability is defined. It is often defined from both a broad and a narrow perspective. If one chooses a narrow estimate of the number of Americans with disabilities, a key government report entitled Americans with Disabilities: 1991-92 noted that there are 48.9 million (19.4% of the population) persons with disabilities. The report also broke out the number of people classified as having a severe disability. Of the 48.9 million Americans with a disability, 24.1 million (9.6% of the population) were classified as having a severe disability, which was defined as "unable to perform one or more activities, or as having one or more specific impairments, or as a person who used a wheelchair or who was a long term user of crutches, a cane, or a walker" (p. 5). However, these data did not include an additional two million individuals living in nursing homes or in other residential institutions.

From the broader perspective, a disability may be classified as any type of disorder that limits a person's ability to perform a normal daily routine. For example, a person with arthritis may have difficulty performing activities that require adequate muscle strength and endurance, such as lifting a vacuum cleaner, carrying groceries, climbing stairs, or getting in and out of a bathtub. A child with asthma may find it difficult participating in a physical education class that requires a great deal of running, and may therefore have to be excused from class. An individual who is severely obese may be unable to sustain an 8-hour workday that involves standing or walking. All of these conditions impose limitations on a person's ability to interact successfully with the environment and could therefore be broadly construed as a disability.

Under this broader definition, arthritis is the most prevalent chronic disease in the United States, with an estimated 38 million Americans having the condition, most of them elderly. It is classified as the leading cause of activity limitation among Americans. The second most common disability is diabetes. Fourteen million people have either insulin-dependent (Type I) or noninsulin-dependent (Type II) diabetes, and a significant number have an accompanying physical disability such an amputation, severe obesity, or renal failure.

The United States Department of Health and Human Services reports that there are an estimated 10 million people in the United States who have asthma, resulting in millions of lost days from school or work annually. In a report published by the Department of Health, Education, and Welfare, 10 million Americans were classified as severely obese (95th percentile or higher on body mass index) and this figure does not include children. Approximately 7 million Americans over the age of 15 yr have been diagnosed as having a mental disability, which includes a learning or intellectual disability, Alzheimer's disease, senility or dementia, or any other mental or emotional condition. And there are an estimated one million people who have a spinal cord injury or neurological impairment, which includes such conditions as spina bifida, postpolio, cerebral palsy, and multiple sclerosis.

Using these figures, there are an estimated 80 million cases of disability, which does not include persons with blindness, hemophilia. AIDS, epilepsy, cancer, emphysema, stroke, and osteoporosis. Since many individuals, particularly older adults, have more than one disability (i.e., diabetes and osteoarthritis), a conservative estimate disability would be 30% of the U.S. population of 252 million, or roughly 75 million Americans.

From the standpoint of health promotion and disease prevention, one could argue that the broader definition of disability provides a more accurate picture of the number of people with disabling conditions in the United States who could benefit from physical activity. Although a large percentage of individuals with diabetes, for exam-pit, are not classified as having a disability according to the Bureau of the Census report Americans with Disabilities 1991-92, because the condition does not "substantially limit one or more of the major life activities," from a clinical standpoint a significant percentage of these individuals could potentially achieve important health benefits from physical activity by reducing their incidence of secondary complications such as hypertension, obesity, and coronary heart disease.

Regardless of which figure is used for estimating the number of people with disabilities in the United States, one thing is clear: Americans with disabilities represent a very large group of people in our society. Albrecht notes that part of the reason for this is that, "Advances in medical technology and practice have saved the lives of young people who earlier would have died. Now they survive but with impairments that cause activity limitations" (p.15). And as the population grows older, the number of persons with disabilities will grow exponentially. It is important for exercise physiologists, clinicians, scientists, and other health care providers to begin to address the physical activity needs of persons with disabilities in order to improve their quality of life and maximize potential for independence.