The present study examined the direction of the association between cognitive and physical function in a large sample of middle aged British civil servants. Over an average period of 5 years, we found that cognitive performance at baseline predicted physical function at follow-up, while physical function at baseline did not predict cognitive performance at follow-up. Our findings suggest that the direction of any causal association is more likely to be from cognitive to physical function, rather than the reverse.
Our results indicate that poor cognitive function may play a role in the initiation and the progression of poor physical function. However, the size of the regression coefficients between baseline cognitive function and physical function at follow-up would indicate that the contribution of cognitive performance to physical performance is rather small in this middle-aged population. Our results seem not to support the possible reverse proposition that poor physical function leads to poor cognitive function.
Our findings are in agreement with earlier studies demonstrating a correlation between low cognitive function and later declines in physical functioning [2
]. Most of these studies were on the elderly (aged > 65 years) and physical function was assessed by asking about activities of daily living or basic/instrumental activities of daily living. Our results add to these findings by showing that there is an association already in middle age using measures of physical function appropriate for middle aged individuals. It has been shown that declines in cognitive function predict decrements in both demanding and routine physical tasks in the elderly, suggesting that cognition plays an integral role in the execution of most physical tasks and the maintenance of physical functioning [28
The mechanisms underlying the association between cognitive function and physical function require further investigation. It is possible that factors that influence both these outcomes are driving the association observed in this study. One potential candidate is physical exercise that has repeatedly been shown to be a predictor of improved physical functioning. Similarly, many longitudinal intervention studies have shown that regular physical exercise and related physical health status predict improved mental functions and slower cognitive decline in elderly subjects (for a meta-analysis, see [29
]). Positive effects of regular physical exercise for some, but not all cognitive tasks, have been found also among younger subjects [30
]. It has also been found that leisure time activities is associated with cognition in middle age [31
]. In our study, the relationship between cognitive and physical function was, however, quite robust to adjustment for physical exercise level measured by MET hours at baseline. This suggests that physical exercise is not a major driving force for the association found in our study.
Another potential underlying factor is depression. In previous studies, depression has been shown to predict impairment in the physical components of health-related quality of life (HRQoL, [33
]). However, our results did not offer support for this hypothesis because adjusting for depressive symptoms had little effect on the results. Similarly, there is evidence in the literature supporting the view that a considerable proportion of cognitive decline in the elderly population is vascular in origin even when there is no sign of specific physical symptoms. However, adjustment for prevalent CHD or hypertension had hardly any effect on our results. Finally, socioeconomic position is likely to be a potential confounder of the association observed here. However, this appeared not to be the case as adjustment for employment grade reduced the size of cognitive performance effects only slightly.
This study has a number of strengths and limitations. It benefits from data from the Whitehall II study, a well-characterized cohort with sufficient power to detect effects, and repeated measures of the key factors: cognitive and physical function. Furthermore, cognitive function was tested using a wide variety of well characterized validated tests. However, the participants were mainly white women and men from white-collar occupations, thus results may have limited applicability to other ethnic groups and occupations. Nonetheless, given the increase in the percentage of workers in affluent societies employed in white-collar occupations, our sample is largely representative, although observed associations are likely to be smaller than in the overall population because of the healthy worker effect. Future research in more diverse samples should confirm the generalizability of our findings. Similarly, our findings need to be replicated in studies with a longer follow-up time with several measures of both cognitive and physical functioning in order to draw robust conclusions on the magnitude and direction of the association. However, the five year follow-up used in the present study is comparable to those used in previous studies on cognitive function and later functional abilities among elderly populations [34
]. Availability of long-term repeated data from more than two occasions would help to reduce some of the potential limitations of this study and further illuminate the longer term associations between cognitive ageing and decline in physical functioning.
In this study, physical function among adults was assessed using the SF-36, a questionnaire which measures health functioning and is among the most widely used quality of life measures in studies of patients and the general population [23
]. The SF-36 physical composite score (PCS) measures capacity for performing physical activities (vigorous, moderate and light), with one question related to activities of daily living (dressing and bathing). In previous studies, physical functioning among older adults has mostly been measured using information on activities that require extremely light physical efforts, such as activities of daily living or instrumental activities of daily living[34
]. Such measures can not be used as indicators of physical function in a middle-aged population due to higher mean levels of physical functioning leading to ceiling effects on instruments designed for older adults. However, the argument for a life-long view of ageing is increasing being made [35
], making it important to examine the determinants of ageing earlier than has been done previously. Ageing is characterised by increasing heterogeneity [13
], our analysis attempts to add to the understanding of the divergence in ageing trajectories.
In summary, findings over a 5 year period from a large-scale prospective British middle-aged cohort support the view that cognitive function predicts future physical function, but that there is little evidence for the reverse. Extensive covariate adjustment did not remove the associations observed in this study. This study suggests that interventions to improve cognitive functioning in middle age can have long-term effects on physical functioning, and they may therefore constitute important, indirect tools for health promotion and the prevention of physical decline.