Baseline Global Motor Function
There were 906 persons in these analyses with a mean follow-up of 4.9 years (SD, 2.21; range 2, 11 years), Baseline motor function ranged from -2.1 to 2.1 (mean, -0.02; SD, 0.57). Global motor function was inversely related to age (r = -0.45. p<0.001), positively associated with education (r=0.19, p<0.001), and men had higher levels of global motor function (mean, 0.25; SD, 0.58) than women (mean, -0.11; SD, 0.54) [t  = -8.69, p<0.001]. As expected, global motor function was associated with other activity measures, disability, cognition, depressive symptoms, vascular diseases and joint pain ().
Correlation of Global Motor Function and Social Activity and Other Covariates*
Social Activity and Change in Global Motor Function
Baseline social activity scores were approximately normally distributed (mean, 2.6; SD, 0.58; skewness, -0.18). Scores ranged from 1.00 to 4.17 with higher values indicating more frequent participation in social activity. Social activity was inversely related to age (r = -0.17. p<0.001), positively associated with education (r = 0.14, p<0.001), and women had higher levels of social activity (mean, 2.6; SD, 0.57) than men (mean, 2.5; SD, 0.61), [t  = 2.23, p=0.026]. Social activity was associated with global motor function, activity measures, disability, cognition, and depressive symptoms ().
Participants who reported low social activity at baseline were older, more likely to be male, less educated, reported less frequent participation in physical and cognitive activities, reported more disability, had lower cognitive function, and were more likely to have lower BMI and diabetes than those who reported high social activity ().
Demographics of the Cohort at Baseline*
We used a generalized estimating equation model to test the hypothesis that more frequent participation in social activity is associated with a slower rate of decline in global motor function. On average, global motor function declined at a rate of about 0.05 unit/year (Time, ). Baseline frequency of participation in social activity was associated with both baseline level of global motor function (Social Activity, ) and the rate of change in global motor function (Social Activity*Time, ). That is, for each point below the mean social activity score at baseline, the average rate of decline in global motor function was 33% more rapid (Time, ). Since age was also related to the rate of global motor decline, we can compare the amount of global motor decline associated with increased age with the amount of motor decline associated with social activity. For each additional year of age, global motor function declined an additional 0.003 standard units (Age*Time, ). In contrast, each point decease in social activity, global motor function declined an additional 0.016 standard unit (Social Activity*Time, ). Thus, in terms of declining motor function, a 1-point decrease on the social activity scale was equivalent to being about 5 years older at baseline.
Association of Social Activity with Change in Motor Function*
The association of social activity with motor decline did not vary along demographic lines (results not shown). In a sensitivity analysis, we excluded participants who were unable to ambulate at baseline and the association was unchanged [Estimate, 0.017; 95% CI (-0.004, 0.030), p<0.010].
To illustrate the findings with a common measure, we used a similar model to examine the relationship between social activity and the rate of change in walking speed. In the average participant, walking speed at baseline was about 65cm/s and declined at about 2cm/sec/year. In contrast gait speed in a person with high social activity (score=3.3, 90th percentile) declined by about 1.5cm/sec/year versus 2.6cm/sec/year for a participant with low social activity (score=1.8, 10th percentile).
Social Activity, Other Covariates and the Rate of Change in Global Motor Function
Next we examined a number of covariates which might affect the association of social activity with change in motor function. None of these additional analyses altered the estimate of the association (). First, we adjusted for cognitive and physical activity (, Model 1). Next, we added terms for baseline disability using the Katz, Rosow-Breslau and IADL scales (, Model 2). We next adjusted for baseline global cognition and depressive symptoms (, Models 3 & 4). Then we examined a number of health-related covariates including body composition, vascular risk factors, vascular disease burden and joint pain (, Model 5). Finally all of the above covariates were included in a single model and social activity remained associated with the rate of motor decline (, Model 6).
Late-Life Social Activity, Other Covariates and the Rate of Change in Motor Function*
Clinical Significance of Change in Global Motor Function
To determine the clinical significance of the amount of change in global motor function associated with social activity identified in the analyses above, we constructed Cox proportional hazards models examining the association of change in motor function with death and disability and subsequently estimated the hazard ratios associated with a change of 0.16 unit/year, i.e., the amount of change in global motor function associated with a 1-point decrease on the social activity scale. From these models (data not shown), we calculated that a mean annual change in motor function of 0.16 unit/year (, Social Activity × Time) is associated with a more than 40% increased risk of death (Hazard Ratio: 1.44; 95% CI: 1.30, 1.60); 65% increased risk of incident Katz disability (Hazard Ratio: 1.65; 95% CI: 1.48, 1.83) and 34% increased risk of incident Rosow Breslau disability (Hazard Ratio: 1.34; 95% CI: 1.18, 1.52).
Components of Social Activity and Change in Global Motor Function
In a series of exploratory analyses, we examined the relation of each social activity index to rate of global motor decline. Three of the six activities were related to motor decline: unpaid volunteer or community work [Estimate 0.006; 95%CI (0.001, 0.021), p=0.027]; visiting friends or relatives [Estimate, 0.012; 95%CI (0.005, 0.019), p<0.001] and attending church or religious services [Estimate, 0.011 95%CI (0.0007, 0.0018), p=0.027].
In a cohort of more than 900 older persons free of dementia, stroke or Parkinson's disease at baseline, we found that a lower frequency of participation in social activity was associated with a more rapid rate of motor decline. The effect size was equivalent to about 5 years of age; an amount of change associated with more than 40% increased risk of death and more than 65% increased risk of developing disability. Moreover, the association of social activity was robust to a wide range of potential confounding variables and remained unchanged after controlling for disability and excluding persons unable to ambulate at baseline reducing the potential for reverse causality. These findings expand upon the accumulating literature showing that participation in a broad spectrum of late-life activities are associated with positive health outcomes in old age and suggest that more frequent participation in social activity may be protective against motor decline in older persons.
It is widely recognized that increased levels of physical activity are associated with a slower rate of motor decline and a reduced risk of other adverse health outcomes.10-13
However, emerging data suggest that physical activity is only one component of an active and healthy lifestyle.16
For example, increased cognitive and social activities in the elderly are associated with increased survival and a decreased risk of dementia.37-41
In addition, a number of studies have reported a link between social activity and disability or functional status.25, 42
The current study extends these previous studies by showing that late-life participation in social activity is related to the rate of change in motor function based on objective quantitative measures. Further, the association persisted even after controlling for the frequency of participation in physical and cognitive activities. These findings may be particularly relevant for intervention strategies designed for older adults, for whom participation in physical activities may be constrained because of underlying health problems. Furthermore, these results have important translational implications because they suggest that public health interventions using a broader range of leisure activities might increase the efficacy of efforts to decrease the burden of age-related motor decline.
The basis for the association between social activity and motor decline is uncertain. Emerging evidence suggests that efficient goal-directed movement requires the orchestration and integration of a wide range of sensory, motor and cognitive functions43, 44
Human social interaction is complex and social behavior is generated in the brain through interconnected brain structures which process different elements of sociocognitive and socioaffective information which are eventually integrated and translated into action.45
Thus, both successful social and motor behavior depend on the structural and functional integrity of neural systems that integrate the varied inputs needed for planning and execution of behavior. For example, mirror neurons are thought to play important roles not only for generating movement but also for a wide range of activities essential for social interaction including self-awareness, empathy and language.46, 47
Recent work with mirror neurons suggest that social and motor behavior may be linked not only at the neural-system levels but also at the level of single neurons.46, 47
Moreover, mirror neurons discharge not only when a particular motor act is being performed but also when we observe the same movement being done by others. Although the functional and structural links between social and motor behavior do not explain how higher levels of social activity qre related to motor decline, it is noteworthy that physical activity in humans is thought to contribute to improved motor function by increasing neuronal plasticity and protecting against ischemic or neurotoxic damage.48-50
Animal studies suggest that physical activity may be associated with improved function through changes in brain plasticity.17
Our study has some limitations. Most importantly, inferences regarding causality must be drawn with great caution from observational studies. While the findings were robust to potential confounding variables and sensitivity analyses, the potential for reverse causality cannot be excluded. Further, it is possible that residual confounding from an unmeasured latent variable is related to both social activity and motor decline. Other limitations include the selected nature of the cohort, the self-report chronic diseases, in addition to self-report social, physical and cognitive activities. The combination of diaries and devices which provide quantitative measures of activity such as actigraphy would provide more accurate information about the duration of activity and energy expenditure. Death as informative censoring is also problematic in studies of aging.
However, several factors increase confidence in our findings. Perhaps most importantly, the study enjoys high follow-up participation reducing bias due to attrition. In addition, social activity was assessed among persons without dementia based on a detailed clinical evaluation and motor function was evaluated as part of a uniform clinical evaluation and incorporated many widely accepted and reliable strength and motor performance measures; strength testing was done in all four extremities, and motor performances were tested in both the arms and legs. The aggregation of multiple measures of motor function into a composite measure yields a more stable measure of motor function and increases statistical power to identify associations. In addition, a relatively large number of older persons representative of the general population were studied, so that there was adequate statistical power to identify the associations of interest while controlling for several potentially confounding demographic variables.
Decline in motor function is a common condition with adverse health outcomes including death, disability, and the development of other conditions. Thus, it is increasingly being recognized as a major public health problem. Yet little is known about risk factors for motor decline which could translate into potential public health or clinical interventions. These data raise the possibility that social engagement can slow motor decline and possibly delay adverse health outcomes from such decline. Further work is needed to ensure that this is a causal relationship. First, the findings will need replication in other cohorts. Second, intervention studies may be needed. In fact, demonstration projects are already underway that may inform on the potential value of interventions. For example, in a novel translational study, a randomized trial of participation in Experience Corps is underway in Baltimore. 51
Participants are randomized to volunteering in elementary schools which serves as a rich source of cognitive, physical, and social engagement, vs. being on a wait list. Second, preclinical animal studies potentially could be used to determine whether different types of activities work through a common biologic mechanism. Finally, additional knowledge of the biology, in particular the neurobiology, of motor decline is needed. In this study, we excluded clinical stroke and Parkinson's disease. However subclinical manifestations of these or other conditions, in addition to non-neurologic conditions are responsible for motor decline. Very little is known about biology of motor decline. Such information would allow for much more refined hypotheses regarding the mechanisms underlying the association which will be important for the design and execution of potential interventions.