Cognitive and physical functions as determinants of delayed age at onset and progression of disability.

Background: This study examined the association of cognitive and physical functions with age-related transition and progression of activities of daily living (ADL) disability in a population-based longitudinal cohort of nondisabled older adults.

Methods: A longitudinal population-based cohort study of 5,317 initially nondisabled older adults with an average age of 73.6 years of an urban Chicago community were interviewed annually for up to 8 years from 2000 through 2008. Cognitive function was assessed using a standardized global cognitive score and physical function using a combination of measured walk, tandem stand, and chair stand. A novel two-part model was used to access the relationship between cognitive and physical functions and age at onset and progression of ADL disability.

Results: The sample consisted of 5,317 participants, 65% blacks, and 61% females. Twenty-five percent reported an onset of ADL disability during follow-up. After adjusting for confounders, lower cognitive and physical functions were associated with an increased risk for lower age at onset. Lower cognitive function was longitudinally associated with increased rate of progression of disability after onset. However, lower physical function did not alter the rate of progression of ADL disability.

Conclusions: Cognitive and physical functions were associated with age at onset. However, only cognitive function was associated with the rate of progression of ADL disability.