Motor Function Is Associated With Incident Disability in Older African Americans

Abstract

Background: Disability in older African American adults is common, but its basis is unclear. We tested the hypothesis that the level of motor function is associated with incident disability in older African Americans after adjusting for cognition.

Methods: A prospective observational cohort study of 605 older community-dwelling African American adults without dementia was carried out. Baseline global motor score summarized 11 motor performances, cognition was based on 19 cognitive tests, and self-reported disability was obtained annually. We examined the association of motor function with incident disability (instrumental activities of daily living [IADL], activities of daily living [ADL], and mobility disability) with a series of Cox proportional hazards models which controlled for age, sex, and education.

Results: Average follow-up was about 5 years. In proportional hazards models, a 1-SD increase in baseline level of global motor score was associated with about a 50% decrease in the risk of subsequent IADL, ADL, and mobility disability (all p values < .001). These associations were unchanged in analyses controlling for cognition and other covariates. Further, the association of global motor score and incident ADL disability varied with the level of cognition (estimate -5.541, SE 1.634, p < .001), such that higher motor function was more protective at higher levels of cognition. Mobility and dexterity components of global motor score were more strongly associated with incident disability than strength (all p values < .001).

Conclusions: Better motor function in older African Americans is associated with a decreased risk of developing disability. Moreover, the association of motor function and disability is stronger in individuals with better cognitive function.

Keywords: African Americans; Aging; Cognition; Disability; Motor function.

Figure 1.

Cognition modifies the association of global motor scores and subsequent risk of developing disability in activities of daily living (ADL). To illustrate how the association of global motor scores and incident ADL disability varies with cognition, three pairs of hypothetical average participants with their estimated risk of developing ADL disability during the course of the study are shown. The illustration is based on a model including an interaction term between global cognition and global motor scores, with all the cases analyzed in this study. Model derived risk of developing ADL disability are illustrated for average participants with low (solid line, 25th percentile) and high (dashed line, 75th percentile) cognitive function with three levels of motor function: low (−1 SD below the mean global motor score; black), median (mean global motor score: Red), and high (+1 SD above the mean global motor score; blue). At both levels of cognitive function, the blue line (high motor function) is below the red line (median motor function) and both are below the black line (low motor function); the risk of developing ADL disability is lower in individuals with high motor function versus low motor function. Furthermore, it is evident that the distance between the black and blue lines (low and high motor function) is much wider for the dashed lines (high cognitive function) as compared with the solid lines (low cognitive function); thus cognitive function strongly modifies risk of developing ADL disability in individuals with high motor function (blue) with only a negligible effect on low motor function (black).