Running and the Development of Disability with Age

The development of musculoskeletal disability with age frequently decreases quality of life. As the general population ages, it is ever more important to identify those risk factors that precede and accompany musculoskeletal disability, thereby suggesting ways to decrease the magnitude of this problem. Musculoskeletal disability among older persons is commonly linked to increased prevalence of osteoarthritis but is also potentially associated with senescent changes in soft tissues, including deconditioning, and with other chronic diseases. Physical activity, particularly aerobic physical activity, decreases mortality rates [1-3]. Its effects on morbidity and disability, however, have not been shown as clearly. On the one hand, vigorous physical activity (such as aerobic running) could accelerate the development of osteoarthritis and disability, and repetitive injuries to soft tissues might increase disability as a result of cumulative trauma [4-6]. On the other hand, increased fitness and training, leading to increased cardiovascular reserve, increased bone density, and increased strength from regular exercise could delay or prevent disability [7-13]. The effect of exercise on the development of disability is thus an important public health issue, but clinical studies in this area are difficult to design and perform. Experimental trials comparing exercise with no exercise over prolonged periods are not practical. Cross-sectional studies may be inconclusive because increased physical activity could be the self-selected result of good health, rather than good health the result of the physical activity. In longitudinal studies, on the other hand, we expect that if a self-selection bias is present, a baseline difference in disability between exercising and nonexercising groups would narrow with time, and, if physical activity leads to development of disability, that the trend lines eventually would converge. For the past 9 years, the Stanford Arthritis Center has conducted prospective longitudinal studies of the effects of long-distance running, a popular form of vigorous physical activity, on patient outcomes in 537 members of a runners' club with participants at least 50 years old and 423 community controls. In addition to longitudinal study, our controls for self-selection bias included analysis of multiple variables at baseline and during the study, including family history of arthritis, body mass index, weight, history of arthritis, cigarette smoking, history of fractures, and history of lower-extremity abnormality. We have also studied presence of comorbid disease, progression as shown on x-ray films, comparisons of progression between hand and knee on x-ray films, bone density, and frequency of fractures. We performed longitudinal analyses with intent to treat analyses in which initial group membership is retained although individual members may have decreased the level of or stopped running. Preliminary results have indicated, at baseline and at 2 years, that running was associated with substantially reduced disability, a marked increase in lumbar bone density, and no differences in the development of radiologic osteoarthritis [14-16]. At 5 years, radiologic osteoarthritis was developing with similar frequencies in running and nonrunning groups. Effects on associated lumbar bone density persisted for 5 years but were sensitive to changes in running, increasing in those who increased activity and decreasing in those who decreased activity [17-20]. Fracture rates were more common, however, in the running groups, with the increase seen in sports-related and fall-related fractures; fractures of wrist, hip, and spine were more common in nonrunners [21]. Presence of osteoarthritis was nearly identical in dominant and nondominant hands, supporting the argument that greater use is not necessarily associated with increased radiologic change [22]. Extensive validation studies showed absence of differential self-report bias in runners and nonrunners [14]. We report the results of our 8-year longitudinal study of progression of disability scores between runners and nonrunners, with additional analyses by age and sex. Methods Sample Selection The 50+ Runners Association, with members across the United States, provided access to many long-distance runners who were 50 years or older. Runners' club members voluntarily joined an organization that has, as one of its purposes, participation in studies of the health effects of long-distance running. The typical runners' club member had run 16 869 miles during an average of 12.4 years before entering the study. The Lipid Research Clinic study, begun in 1972, provided access to a sample, similar in age, from the Stanford University community [23]. In January 1984, study descriptions were sent to and received by 1345 runners and control participants. Those persons who met age (50 years or older), education (high school graduate or additional education), and language requirements (English as their primary language) were sent questionnaires and consent forms. The 537 runners and 423 controls returning initial questionnaires were enrolled in the study. All participants provided information concerning exercise history, musculoskeletal injuries, medical history, dietary history (especially calcium and fat), and other variables. They also completed the Stanford Health Assessment Questionnaire. Since then, similar questionnaires have been sent to participants each year. At 8 years, 84% of the runners' club members and 78% of the community controls continue to participate in the study. Drop-outs closely resembled those in their groups who continued in the study for nearly all variables. Users of alcohol in all groups tended to drop out at a higher, but nonsignficant, rate compared with those who did not use alcohol. Initial disability in runners' club completers was 0.026 (0.004) and 0.029 (0.01) in drop-outs (P = 0.77) and 0.079 (0.01) in control completers compared with 0.152 (0.02) in drop-outs (P = 0.008). Exercise levels were similar in completers and in drop-outs, as were age, sex, body mass index, and education level. Eight runners' club members (1.5%) and 30 community controls (7%) have died. Deaths in the relatively few participants lost to follow-up were estimated using the National Death Index. The Health Assessment Questionnaire has been the subject of multiple validation studies and is used widely [24-29]. The self-administered questionnaire assesses functions in eight areas: dressing and grooming, arising, eating, walking, hygiene, reach, grip, and activities over four categories scored from 0 to 3 (no difficulty, some difficulty, much difficulty, and unable to perform, respectively). Three components are evaluated for each area: 1) ratings of the degree to which daily functions are difficult to perform; 2) reported use of special aids or devices; and 3) activities for which assistance of another person is required. The scores on each of the 8 functional areas are averaged to obtain the disability index (0 to 3 units). For reference, a typical patient with symptomatic osteoarthritis has a score of 0.6 to 0.8. Statistical Analysis We compared differences in participants at onset and during follow-up using chi-square and t-tests. We analyzed survival using the Kaplan-Meier method. We adjusted for covariates using analysis of covariance and Cox proportional-hazards models. Principal longitudinal analyses focused on the 330 community controls and 451 runners' club members who remained in the study through 1992. We maintained participants in their initial groups (formed in 1984), regardless of subsequent change in running status. We developed groups of ever-runners and never-runners from the entire group because many participants in the control group also exercised vigorously. These groups were formed on the basis of responses to the question, Have you ever run for exercise for a period greater than 1 month? Because the ever-runner group thus includes runners who discontinued this activity before study onset, we hypothesized that if self-selection bias were operating, differences between these groups would be less striking than with the parent groups. This reclassification shifted 83 persons to the ever-runner group from the community controls group used in the intent-to-treat analysis (see Table 1 for group sizes). Table 1. Characteristics of Participants in 1984 (1992 Participants)* We calculated the rate of change of disability for each participant during follow-up. We adjusted crude rates for age, baseline disability, body mass index, education level, smoking, history of arthritis, and comorbid conditions. Results Table 1 lists characteristics of study participants at study onset in 1984; these participants were classified as runners' club members and community controls, and also as ever-runners and never-runners. We found substantial differences between the participant cohorts. The running groups are younger by approximately 3 years, had substantially lower disability scores at study onset, have lower body mass indices, are less frequently smokers, and, of course, exercise much more vigorously and run many more miles per year. Runners have lower systolic and diastolic blood pressures, visit their physicians less frequently, and use fewer medications. The runners have a less frequent history of arthritis and fewer other major medical problems. Runners report less frequent chronic joint swelling or chronic joint pain. On the other hand, runners report more fractures. We found no differences between groups in education level, alcohol intake, or history of other major musculoskeletal injury. These differences, at the onset of longitudinal study, may be alternatively interpreted as representing self-selection into the runner group or as representing beneficial effects from the average 12 years of running before the study onset, or both. These alternatives cannot be separated b