Characteristics of study participants by group membership (runners vs. controls) are listed in . Values for all subjects at study inception are listed in the first columns. Baseline values (1984) for those subjects in each group who continued to participate through 2005 (completers) and for those who did not complete the follow-up period (non-completers) are listed in subsequent columns. The last columns list characteristics of the completers at 21-years of follow-up (2005). After 21-years of follow up, 284 runners and 156 controls remained in the study. Annual attrition rates among living subjects were approximately 3% for runners and 6% for controls. Differences between groups were observed both at baseline and at 21 years. Runners were younger, leaner, tended to be male, smoked less, and exercised more than controls. Mean education level and alcohol intake were statistically similar between the two groups. Both groups had little disability at the baseline, but runners had lower HAQ-DI scores and were more likely to have a HAQ-DI=0. Analysis of completers and non-completers showed that, among controls, completers tended to be younger (p<0.001), run more (p<0.001), have less baseline disability (p<0.05) and were more likely to have a baseline HAQ-DI score of 0 than non-completers. Among runners, the only statistically significant difference was the two-year age disparity between completers and non-completers (p<0.001).
Cohort Demographics (Runner’s Club vs. Community Controls)
Similar findings were observed when participants were divided into ever-runner and never-runner groups ().
Cohort Demographics (Ever vs. Never Runners)
shows the progression of average disability by year.. Mean disability increased with time in all groups. shows mean annual disability levels for runners compared to controls; illustrates disability curves for Ever-Runner versus Never-Runner groups. Separate curves are shown for means computed using data available for all 961 initial study participants and for means computed for only those 440 subjects who completed 21-years of observation.
Members of the running groups had significantly lower mean disability levels at all time points. Members of both running groups had nearly identical mean disability levels irrespective of completer status, indicating few differences in disability between completers and those who dropped-out or died.
In contrast, there were significant differences in disability levels between the inception cohort and completers among the control groups in both figures. Baseline disability levels in 1984 were statistically lower when computed for completers than when all initial study participants were included. This disability pattern among completers in the control groups continued at almost all time points, indicating differential drop out of the subjects with higher disability among the control groups, creating a potential bias towards lower disability in the observed control groups. However, even when restricting the cohort to completers, runners had significantly lower disability than controls and disability curves continued to diverge at 21 years of follow-up. Analyses by Ever- versus Never-runners showed comparable results.
Average disability levels for completers in each group (runners and controls) are separated by gender in . Both male and female runners maintained low disability levels at all time points, significantly lower than controls. The difference between runners and controls was most striking for females. Male controls had higher disability levels than male runners at all time points except the initial few years of the study. Few differences existed between male and female runners.
Figure 2 Mean disability levels by year separated by gender. Mean and standard deviation for runners (solid lines) and controls (dashed lines) by year from 1984 through 2005. Male subjects are depicted with open triangles and females with solid triangles. Only (more ...)
The rate of progression of disability over 21 years of observation using general linear mixed models is shown in . The rate of progression of disability was significantly lower for runners (0.007 units/year) compared to controls (0.016 units/year, p<0.001).
Figure 3 Progression of Disability. Linear mixed models of progression of disability and postponement of disability (PD). Regression lines are derived from linear mixed models and adjusted for the following covariates: age, sex, BMI, smoking, and initial disability (more ...)
The time required to reach specified levels of disability was significantly longer for runners than for controls. The mean time to reaching a HAQ-DI of 0.075 from study onset was approximately 2.6 years for controls, and 8.7 years for runners yielding a difference of approximately 6.2 years (95%CI: 3.9–8.9). Similarly, the time to reach a HAQ-DI of 0.10 was 8.2 years later (95% CI: 5.1–11.7) and to reach a HAQ-DI of 0.15 was 12.1 years later (95% CI: 8.1–18.3) for runners. These data illustrate that the slower rate of progression of disability continued to increase over time among runners through at least the 21 years of observation.
Results of multivariable Cox regression analyses using time-dependent covariates are shown in . The final model for disability outcomes (HAQ-DI=0.5 and HAQ-DI=1.0) included the following variables: group membership, age (year), gender, BMI (lagged by 1 year), and weekly vigorous exercise minutes (lagged by 1 year). These analyses were restricted to participants (completers and non-completers) with a baseline HAQ-DI=0. For the outcome of HAQ-DI=0.5, runners had a hazard ratio of 0.62 (95%CI 0.46–0.84) compared to controls. Analysis of covariates showed that greater BMI within 1 year was associated with an increased hazard (HR 1.09, 95%CI 1.05–1.13), as was age (HR 1.07, 95% CI 1.05–1.09), but male sex was associated with decreased risk (HR 0.63, 95% CI 0.47–0.85). Weekly vigorous exercise from all activities was marginally significant (HR 0.96, 95% CI 0.91–1.00, p=0.05). Nearly identical results were obtained for the outcome of HAQ-DI of 1.0.
Multivariable Cox regression analyses for disability and mortality
By the end of 2003, 81 (15%) of the runners and 144 (34%) of controls had died. The Kaplan-Meier plot of survival estimates for each group () shows that runners had a significant reduction in early mortality that was maintained, or increased, over the study period (p<0.001).
Figure 4 Kaplan-Meier unadjusted survival curves for all cause mortality in Runners Club members and Community Controls from study onset through 19 years of follow-up. All 941 subjects at study inception are included. The difference between groups remained significant (more ...)
Multivarible Cox proportional hazard models generated to adjust for other variables at the baseline that were associated with survival () found that runners continued to demonstrate a significant survival advantage (HR 0.61, 95% CI 0.45–0.82). As expected, older age (HR 1.12, 95% CI 1.10–1.14), male gender (HR 1.52, 95% CI 1.12–2.07), and initial HAQ-DI level (HR 1.16, 95% CI 1.07–1.25) were associated with increased hazard for mortality. BMI, smoking, and baseline exercise did not meet sufficient significance to be included in the final model.
Causes of death are summarized in . A total of 225 deaths (23% of all study participants) were seen over 17,201 person-years of observation. Rates of death were increased in controls compared to runners not only for cardiovascular outcomes as anticipated, but also for nearly all identified causes.
Causes of Death since study inception (1984)