In this study of more than 2,700 non-demented women in their 80's, objectively measured daytime wrist movement was associated with higher levels of cognitive function and lower risk of cognitive impairment. This association between daytime movement and cognitive function was independent of age, race, education, medical comorbidities, physical function and other healthy behaviors, and it was similar across different subgroups of the study population. Although objectively measured daytime movement and self-reported walking were correlated, both measures were independently associated with better cognitive function and lower risk of cognitive impairment in these older women.
We also found that the association between daytime wrist movement and cognitive function was stronger for Trails B than for MMSE. This is probably explained by ceiling effects and the lack of variability in MMSE scores (range 24−30), which make it difficult to detect small differences in cognition with this test. In addition, prior studies have found that the effects of physical activity are most pronounced in the frontal lobes and, therefore, are best detected with measures of executive function such as Trails B.(27
The results of this study provide objective evidence to support our prior finding that women in this study population who reported engaging in more physical activity in their 60's and 70's were less likely to experience cognitive decline over 6 to 8 years of follow-up.(29
) Our current findings build upon this by using an objective measure of daytime movement and by showing that the association between daytime movement and cognitive function was present as these women entered their 80's.
Our findings also are complimentary to other observational studies in humans, which have found that elders who report leading more ‘active’ lifestyles have higher levels of cognitive function and lower risks of cognitive impairment and dementia.(1
) Some of these studies have found that mental activities may be most important,(2
) while others have found independent effects for physical, mental and social activities.(1
) Together, these studies have provided support for the cognitive reserve hypothesis,(33
) which holds that leading a more active lifestyle may help to build a buffer against the clinical manifestations of diseases such as dementia. Our study provides objective evidence to support this hypothesis, although longitudinal studies will be required to clarify whether more daytime movement leads to better cognitive function or vice versa.
There are several potential mechanisms by which more daytime movement could result in better cognitive function. Our analysis pointed to self-reported difficulty performing IADL—such as walking, climbing, cooking, cleaning and shopping—as a potential mediator. Adjustment for IADL difficulty attenuated our findings by about one-third; however, the association between daytime movement and cognitive function remained statistically significant and was similar in women who did and did not report IADL difficulty. Taken together, these findings suggests that difficulty performing IADL activities may explain some, but not all, of the association between daytime movement and cognitive function. However, it also remains possible that our IADL measure did not fully capture participants’ true functional capacity and that the relationship we observed between daytime movement and cognitive function may have been attributable to residual confounding.
It also is possible that daytime movement enhances cognitive function through a vascular mechanism. It is well-established that physical activity reduces the risk of vascular diseases and vascular risk factors such as hypertension, diabetes and stroke(34
) which, in turn, are associated with increased risk of dementia.(35
) However, as with our analysis of IADL, we found that the association between daytime movement and cognitive function was similar in subjects with and without these conditions, suggesting that they also may explain some, but not all, of the association.
There is growing evidence from animal studies that greater activity may lead directly to enhanced neuronal health and function, including reduced brain damage following injury, angiogenesis (formation of new blood vessels), neurogenesis (formation of new neurons), synaptogenesis (formation of new synapses) and increased levels of neurotrophic factors.(38
) In one study, mice that were raised in an ‘enriched’ environment—which involved cages with more animals (social activity), colorful toys and tunnels (mental activity) and a running wheel (physical activity)—experienced enhanced neurogenesis and synaptic plasticity in the dentate gyrus of the hippocampus and learned to perform a water maze task more quickly.(39
) In another study using a transgenic mouse model, mice raised in an enriched environment experienced decreased deposition of amyloid-β, which is a pathological hallmark of Alzheimer's disease.(40
) It is possible that, through these and other mechanisms, greater activity may enable elders to build a cognitive reserve that enhances current cognitive function and lowers risk or delays onset of cognitive impairment and dementia.(33
Strengths of our study include the large study population of non-demented women in their 80s. Few studies of cognitive function have focused specifically on this age group, which is currently the fastest growing segment of our population.(9
) In addition, we used actigraphy to objectively measure women's daytime movements, rather than relying on self-report. This is important, because it enabled us to minimize recall bias and to incorporate those activities that may not be adequately captured in questionnaires (e.g., movements around the house).
This study also has several limitations. First, it is cross-sectional, so we cannot determine the direction of the association between daytime movement and cognitive function. Second, the study population was restricted to women; additional studies should determine whether a similar association is present in men. Third, the cognitive test battery was somewhat limited; future studies should include a broader range of cognitive tests. Fourth, because the actigraph is worn on the wrist, it may not accurately capture lower body movements, which might be better measured by a device worn at the waist or leg. Finally, actigraphy does not provide information about the effects of specific activities; rather, it provides an integrated measure of daytime wrist movement that captures a wide range of activities. Future studies should determine which specific daytime activities are most strongly correlated with actigraphically-measured daytime movement.