mobility impairments in this age group. For older ages, arthritis becomes increasingly important; its share of the mobility limitation impairments rises from 40 percent at ages 45-69 to 58 percent at ages 85 and above. These increasing percentages are applied to an increasing base of people with mobility limitations, so the prevalence of people with limitations caused mainly by arthritis increases more than 20-fold from 4.6 per 1,000 at ages 18-44 to 109.3 per 1,000 at ages 85 and older.

The importance of chronic diseases (circulatory, respiratory, cancer, diabetes, etc.) as conditions causing activity limitation also increases with age. Taken together, the prevalence of limitation with a main cause in this group increases 10-fold from 15.5 per 1,000 at ages 18-44 to 156.2 per 1,000 at ages 85 and above. Diseases of the heart and circulatory system are the major contributors to this category, increasing from two-thirds of the category at ages 45-69 to three-fourths over age 85.

Verbrugge and colleagues (1989) note that the aggregate measures cited here are actually a function of two components: a condition's prevalence, and whether the condition becomes a disability. These aggregate measures are appropriate measures of public health impact. When comparing the causes of disability, however, it can be helpful to look at each condition's

FIGURE 2-8 Percentage distribution of main causes of activity limitation, by age, 1983-1985. Source: Calculated from LaPlante, 1988.