cades, longer than they did in the past, although often with some degree of physical or mental impairment. In addition, advances in early diagnosis and treatment for heart disease, stroke, and other serious chronic diseases associated with aging have meant longer survival—also with various degrees of impairment—for many people in middle and late life.

Disability and health are dynamic in other respects. Over time, some health conditions, such as certain forms of multiple sclerosis and arthritis, exhibit a significant course of waxing and waning that affects the ease or difficulty that people with these conditions have performing daily activities and underscores the significance of environmental characteristics, such as the accessibility of transportation systems. People with other conditions, such as Down syndrome, tend to exhibit periods of long-term stability, although changes in their environment may enlarge or reduce the scope of their activities and participation. Yet other health conditions are characterized by a fairly steady course of increasing impairment (which happens with amyotrophic lateral sclerosis) or improvement (which is typical for certain types of serious injuries). The promotion of health and well-being and the creation of supportive environments play a part in the dynamics of disability in all these pathways.

Research on risk factors for disability has highlighted the interaction between individual characteristics and physical and social environments as a critical dynamic in the development of disability. This interaction was a theme in the Institute of Medicine (IOM) reports Disability in America (1991) and Enabling America (1997). In addition, the latter report identified the study of change over time and transitions to different states of physical and social functioning as essential dimensions of disability and rehabilitation research. Indeed, the report defined rehabilitation research as “the study of movement among states [that is, pathology, impairment, functional limitation, and disability] in the enabling-disabling process” (IOM, 1997, p. 25).

Anyone can experience an injury or illness that unexpectedly and dramatically alters his or her life. Moreover, as described in Chapter 3, people who live long enough are very likely to develop disabilities, although the pathways of disabilities in late life may still be quite variable. Using data from the Women’s Health and Aging Study, Guralnik and colleagues (see, e.g., Ferrucci et al. [1996] and Guralnik et al. [2001]) have identified two distinct types of disability onset in later life: the sudden, catastrophic onset of multiple limitations (which often happens with a stroke or a traumatic injury) and the slow, steadily progressive decline in functioning (which often occurs with osteoarthritis or congestive heart failure). Other studies are revealing the diverse, recurrent, and episodic nature of many of the disabilities that occur in late life (see, e.g., Fried et al. [2001], Gill and Kurland [2003], Hardy et al. [2005], and Gill et al. [2006]). This variabil-

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