In this cohort of 1154 ambulatory community-dwelling elders without dementia, history of stroke or Parkinson’s disease, baseline global cognition was associated with incident mobility impairment. This association did not vary by sex or race and persisted even after adjustment for body composition, physical activity, chronic vascular diseases and vascular risk factors. Five different cognitive abilities including episodic memory, semantic memory, visuospatial abilities, perceptual speed and working memory were also related to incident mobility impairment. In subsequent analyses, similar associations were observed with respect to global cognition and five cognitive subscales and severe mobility impairment i.e. loss of the ability to ambulate. Lower levels of global cognition and 4 of 5 cognitive abilities at baseline were related to a more rapid decline in a continuous measure of mobility function. Finally, the rate of change of global cognition and 5 cognitive abilities were related to the rate of mobility decline. These findings suggest that there may be common pathophysiological processes contributing to both cognitive and mobility decline in the elderly. However, the current cohort study cannot exclude the possibility that lower levels of cognition cause mobility impairments.
Mobility is a complex behavior that involves dissociable neural systems which control gait initiation, planning and execution and the adaptation of these movements to meet motivational and environmental demands.29–32
These complex interactions underscore why a variety of psychosocial and environmental factors and resources may be employed to compensate for mobility impairments and prevent the clinical transition to disability. Cognitive abilities are crucial for ongoing planning, decision-making and monitoring of movements necessary for successful locomotion.31, 33
Cross-sectional clinical studies have shown that level of cognition is related to mobility disability and mobility function. 16, 17, 20
However, there are few longitudinal studies assessing the temporal relationship between cognition and declining mobility function in elders,18–20
and the association of specific cognitive abilities with mobility remains unclear. Thus the current study fills an important gap by showing that there is a temporal relationship between low levels of a wide range of cognitive abilities and the subsequent development of mobility impairments. Furthermore, in contrast to prior longitudinal studies which have only studied the association of cognition with gait speed,19, 20
the current study provides evidence that a wide range of cognitive abilities are associated with the rate of decline in a composite measure of mobility based on several gait and balance performance measures.
Some prior studies have suggested that executive cognitive function may be preferentially related to mobility.19, 20
A novel feature of the current study is the availability of a detailed battery of 18 cognitive tests summarized either as global cognition or grouped into five different constituent cognitive abilities. There has been increasing use of composite measures in aging research since these measures have metric properties which reduce random error, minimize floor and ceiling effects, and tend to be normally distributed making them well suited for longitudinal analyses. These advantages may account in part for the stronger association between global cognition with mobility decline as compared to the other cognitive abilities constructed from fewer tests ().34
All five cognitive abilities were related to incident mobility impairment and loss of the ability to ambulate and 4 of 5 abilities (except visuospatial abilities) were associated with the rate of declining mobility. These results provide some support for the association of executive cognitive abilities and declining mobility, as perceptual speed is a well-recognized component of executive function; further, semantic and working memory, which support executive function, also were associated with mobility impairments. However, episodic memory, the hallmark and often the earliest sign of Alzheimer’s disease, was also related to declining mobility, as well as mobility impairments. In contrast visuospatial ability was related to incident mobility impairment but not declining mobility. This lack of association may derive from the fact that our cognitive measures for this domain may not be as sensitive as the other cognitive abilities or that these abilities are involved in more complex visuospatial transformations which may not be employed during the mobility performances examined in the current study. Overall, these findings suggest a more generalized association between cognition and mobility in old age.
The present study suggests that a lower level of cognitive function is associated with incident mobility impairments and declining mobility, but the biology of this association remains unclear. Although cognitive function may represent a true risk factor for mobility impairments, causal inferences from this cohort study are limited. While environmental enrichment studies in animals suggest improved motor function, we are unaware of extant human intervention studies showing that cognitive enrichment improves mobility. Our results showing that the rate of cognitive decline was correlated with declining mobility suggests that both cognitive and mobility decline in old age may share a common eitopathogenesis. Recent work in this cohort suggests that several other genetic and experiential risk factors for cognitive decline are also associated with motor decline.35, 36
Furthermore, risk factors for cardiovascular disease (e.g., diabetes) and common vascular diseases (e.g., congestive heart failure, brain infarcts) have been related to both declining mobility and cognition. However, in the current study, controlling for vascular diseases and risk factors had little effect on the association of cognition and mobility. Recent work37
raises the possibility that subclinical neuropathologic changes of AD in cortical and subcortical motor regions may account, in part, for progressive decline in motor function in elders. Finally, AD pathology in cognitive systems which are now recognized to play an important role in mobility may contribute to declining cognition and mobility.38
The results from the current study have important translational implications and suggest that interventions to improve cognition may decrease the development of mobility impairments and thereby reduce the burden of mobility disability in elders.
The current study has some limitations. Although we adjusted for vascular risk factors and diseases, there is a possibility that other subclinical diseases may also contribute to incident mobility impairment and mobility decline. Finally, our results are based on selected cohorts that may differ in important ways from the general population, which underscores the need to replicate these findings in other cohorts. However, confidence in these findings is enhanced by several factors. Participants included a large number of community-dwelling elders initially free of dementia, history of stroke or Parkinson’s disease. Detailed cognitive testing allowed for composite measures of global cognition and five cognitive abilities, and annual testing with little missing data allowed for complementary analyses of both incident mobility impairments and mobility decline.