Self-ratings of Health and Change in Walking Speed Over 2 Years: Results From the Caregiver-Study of Osteoporotic Fractures

doi:10.1093/aje/kwq445

Am J Epidemiol. 2011 April 15; 173(8): 882–889.

Published online 2011 February 25. doi: 10.1093/aje/kwq445

PMCID: PMC3105256

Copyright American Journal of Epidemiology © The Author 2011. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Self-ratings of Health and Change in Walking Speed Over 2 Years: Results From the Caregiver-Study of Osteoporotic Fractures

Jeffrey M. Ashburner, Jane A. Cauley, Peggy Cawthon, Kristine E. Ensrud, Marc C. Hochberg, and Lisa Fredman^*

^*Correspondence to Dr. Lisa Fredman, Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 02118 (e-mail: lfredman/at/bu.edu).

Received August 24, 2010; Accepted November 19, 2010.

Abstract

Although poorer self-rated health (SRH) is associated with increased mortality, less is known about its impact on functioning. This study evaluated whether poorer SRH was associated with decline in walking speed and whether caregiving, often considered an indicator of chronic stress, modified this relation. The sample included 891 older US women from the Caregiver-Study of Osteoporotic Fractures. SRH was assessed at the baseline Caregiver-Study of Osteoporotic Fractures interview, conducted in 1999–2001, and was categorized as fair/poor or excellent/good. Rapid walking speed over 2, 3, or 6 m was measured at baseline and 2 annual follow-up interviews. Respondents with fair/poor SRH walked significantly slower at baseline than those with excellent/good health (mean = 0.8 (standard deviation, 0.3) vs. 1.0 (standard deviation, 0.3) m/second, P < 0.001). In adjusted linear mixed models of percentage change in walking speed, respondents with fair/poor SRH experienced a greater decline in walking speed than those with excellent/good SRH (−5.66% vs. −0.60%, P = 0.01). Caregivers with fair/poor SRH declined more than noncaregivers (−9.26% vs. −4.09%). High-intensity caregivers had the largest decline (−12.88%), whereas low-intensity caregivers in excellent/good SRH had no decline (2.61%). In summary, poorer SRH was associated with decline in walking speed in older women, and the stress of caregiving may have exacerbated its impact.

Keywords: aged, caregivers, gait, health, self report, walking

Older adults who rate their own health as poor consistently have a greater risk of mortality, even after adjustment for other variables (1–3). Furthermore, those with poor self-ratings of health have an increased decline in self-reported measures of functional ability after adjustment for baseline disability (4–6). This decline in function is strongly associated with mortality (7), so it may operate as an intermediary in the relation between self-rated health and mortality. Objective measures of lower-extremity dysfunction, such as walking speed, are highly predictive of subsequent disability related to mobility and activities of daily living (ADL) (8, 9). Thus, since self-rated health is associated with functional decline, it may also be associated with the lower-extremity dysfunction that may precede it (10). While elderly, informal caregivers report poorer self-rated health than older noncaregivers (11), it is unknown whether the association between self-rated health and lower-extremity dysfunction differs in older caregivers and noncaregivers.

To date, the one study known to examine the relation between self-rated health and change in walking speed among older individuals found that those with fair or poor self-rated health were more likely to transition from normal to slow walking speed, and they took longer to walk a short distance over time compared with those with excellent or good self-rated health (10). These results suggest that self-rated health is associated with lower-extremity dysfunction.

Elderly, informal caregivers are more physically active than elderly noncaregivers (12, 13), and higher levels of physical activity and leg strength are associated with a slower rate of mobility decline (14). The physical demands of caregiving may allow caregivers to stay active, which may enable them to maintain their physical health and functional ability (15). In addition to these potential benefits, feelings of usefulness to others and altruism are associated with lower morbidity and mortality in older adults (16, 17). Thus, although older caregivers report poorer self-rated health than older noncaregivers do (11), the physical and psychosocial benefits of caregiving may protect them from functional decline.

Research evaluating the influence of caregiving on morbidity and mortality has produced inconsistent results (11). While some studies have shown modestly elevated rates of mortality for caregivers (13, 18), others have found that providing care may be more beneficial than receiving it (19, 20). In a study evaluating functional decline in caregivers, older women who performed more caregiving activities experienced the least functional decline compared with those who performed fewer or no caregiving activities (21). To date, no known study has evaluated the association between self-rated health and change in walking speed among elderly, informal caregivers.

This study tested the hypothesis that poorer self-rated health is associated with higher rates of decline in walking speed over 2 years among older women, and that this association would be less pronounced among caregivers compared with noncaregivers. We examined this question utilizing a subsample of the Study of Osteoporotic Fractures (SOF).

MATERIALS AND METHODS

Source population

Subjects included in these analyses were enrolled in the SOF study (22). This prospective cohort included 9,704 women aged 65 years or older recruited between 1986 and 1988 from population-based listings in Baltimore, Maryland; Minneapolis, Minnesota; Pittsburgh, Pennsylvania; and Portland, Oregon. Women were excluded if they had a history of hip fracture or could not walk without assistance, as were African-American women because of their low incidence of hip fracture. However, 662 African-American women who were aged 65 years or older with similar functional characteristics were enrolled in 1996–1997. Approximately every 2 years, SOF participants undergo a comprehensive clinical evaluation.

Caregiver-SOF subsample

The aim of the Caregiver-SOF study was to compare health outcomes in older caregivers and noncaregivers. The study sample was identified in 2 phases. The first phase involved an interviewer-administered caregiver screening questionnaire to 5,952 SOF participants who participated in the sixth biennial SOF examination between 1997 and 1999. Each respondent was asked whether she gave unpaid help to a relative or friend with any of 7 ADLs (walk across a room, groom, transfer from bed to chair, eat, dress, bathe, use the toilet) (23) or 7 instrumental activities of daily living (IADL) (use the telephone, get to places out of walking distance, shop, prepare meals, manage medications, manage finances, do heavy housework) (24) because that person was unable to perform the activity on his or her own. Respondents who helped one or more persons with at least one ADL or IADL were categorized as caregivers, whereas those who did not provide ADL or IADL help to anyone were categorized as noncaregivers.

The second phase, beginning in 1999, consisted of readministering the screening questionnaire to all caregivers and a subset of noncaregivers. Respondents who were currently caregiving were invited to participate in Caregiver-SOF, while those who had stopped caregiving were excluded (n = 493). Each caregiver participant was matched with 1 or 2 noncaregivers by SOF site, age, race, and zip code, resulting in a final sample of 375 caregivers and 694 noncaregivers.

Data collection

Data were collected via face-to-face interviews conducted in the respondent's home within 2 weeks of the readministered screening questionnaire and at 2 annual follow-up interviews (2002–2004). This study was approved by the institutional review boards at each SOF site and the Boston University Medical Center. All participants provided written informed consent prior to the Caregiver-SOF baseline interview.

Measures

Caregiver status.

Participants were classified as caregivers if they reported assisting someone with at least one ADL or IADL. Caregiver status was reassessed at each interview.

Exposure: self-rated health.

This measure was assessed at baseline only by asking participants, At the present time, would you say your health is excellent, good, fair, or poor? These responses were collapsed into a dichotomous variable: excellent/good and fair/poor.

Outcome: percentage change in walking speed.

To assess walking speed, participants were instructed to walk a 2-, 3-, or 6-m course at a fast, but safe pace (25). The time to complete this task was measured in seconds and converted to walking speed (meters/second). Percentage change in walking speed was calculated as the difference between walking speed at each annual follow-up interview and the baseline interview, divided by baseline walking speed. A negative percentage change reflected a decrease in walking speed, whereas a positive percentage change reflected an increase in walking speed.

Covariables.

Demographics.

Information was collected on age at baseline, self-reported race (non-Hispanic white vs. other), marital status (married vs. other), and highest educational level (high school graduate or higher vs. less than high school graduate).

Health-related variables.

Body mass index (weight in kilograms divided by height in meters squared) was calculated at the baseline and first follow-up interviews using the participant's current measured weight and height assessed at her first SOF visit. Number of chronic conditions was based on the participant's self-report of the following: arthritis, emphysema/chronic bronchitis/asthma, high blood pressure, heart trouble (myocardial infarction, congestive heart failure), stroke, diabetes, cancer, Parkinson's disease, and Alzheimer's disease. Depressive symptoms were measured using the 20-item version of the Center for Epidemiologic Studies Depression scale (26). We used the traditional cutpoint to classify respondents as having high or low depressive symptoms (Center for Epidemiologic Studies Depression scale score ≥16 or <16, respectively). Total number of ADL (0–7) limitations was based on the participant's self-reported ability to perform each of the ADLs referenced above.

Physical activity.

Participants were categorized as physically active, moderately active, or sedentary based on their levels of leisure-time exercise (classified as active if they reported walking one block or more without stopping or engaging in any regular exercise program other than walking at least once/week) and daily physical activity (classified as walking ≥4 blocks/day or climbing ≥6 flights of stairs/day).

Caregiving variables.

Caregivers were classified as “high intensity” or “low intensity” based on the number of IADL and ADL tasks performed for their care recipient. High-intensity caregivers helped with 6 or more IADLs or 2 or more ADLs, the median values for each scale; low-intensity caregivers did not meet either of these requirements.

Statistical analyses

Bivariate associations of baseline variables with self-rated health were assessed using chi-square tests for categorical variables and t tests for continuous variables. We tested our hypothesis using a repeated-measures linear mixed model adjusting for baseline walking speed. To assess effect modification by caregiver status, we evaluated the statistical significance of the interaction term (caregiver status × self-rated health) in a model that also included these independent variables. Furthermore, we conducted an analysis stratified by caregiver status and, among caregivers, by caregiving intensity. Potential confounders were selected from an a priori list of variables known to be associated with self-rated health and walking speed. We used change-in-estimate methods as a model-building strategy (27), retaining those variables that changed the difference in percentage change in walking speed between self-rated health groups by more than 5%. Variables considered time varying were caregiver status, caregiving intensity, body mass index, Center for Epidemiologic Studies Depression scale level, and physical activity. All analyses were conducted using SAS version 9.1.3 software (28).

RESULTS

The sample included 891 participants (mean age: 81.1 years, range: 70–95) with a self-rated health assessment at baseline and with measured walking speed at baseline and the first annual follow-up interview (average time from baseline to first follow-up interview: 1.04 years, standard deviation (SD), 0.16). Of these participants, 757 also had measured walking speed at the second annual follow-up (average time from first to second follow-up interview: 1.00 year, SD, 0.16). Compared with the 178 participants excluded because they were missing a self-rated health assessment at baseline (n = 1), had died before the first follow-up interview (n = 27), or were missing measured walking speed at baseline and/or the first follow-up interview (n = 150), respondents included in these analyses were younger; and they were more likely to have graduated high school, to be physically active, to be physically and psychologically healthier, and to be a caregiver at baseline. However, they did not differ regarding race, marital status, body mass index, or high-intensity caregiving status.

At baseline, 189 (21.2%) respondents reported fair/poor self-rated health (Table 1). Compared with the 702 (78.8%) respondents with excellent/good self-rated health, those with fair/poor self-rated health were more likely to be widowed, a high-intensity caregiver, and depressed and were less likely to be white, a high school graduate, and physically active. They also were older, had a higher body mass index, had more chronic conditions and ADL limitations, and had a slower baseline walking speed.

Table 1.

Characteristics, by Self-rated Health Status at Baseline, of 891 Older US Women Participants in the Caregiver-SOF Sample, 1999–2004

At baseline, respondents with fair/poor self-rated health had a mean walking speed of 0.84 m/second (SD, 0.26) (Table 1), whereas those with excellent/good self-rated health had a mean walking speed of 1.01 m/second (SD, 0.26) (P < 0.001). The mean difference between baseline and the first annual follow-up was a decline of 0.016 m/second (SD, 0.24) for subjects with fair/poor self-rated health and a decline of 0.020 m/second (SD, 0.25) for subjects with excellent/good self-rated health (P = 0.85). The mean difference between baseline and the second annual follow-up was a decline of 0.012 m/second (SD, 0.27) for subjects with fair/poor self-rated health and a decline of 0.008 m/second (SD, 0.29) for subjects with excellent/good self-rated health (P = 0.87). Walking speeds were slightly faster among participants who used the 6-m course instead of the 2- or 3-m walks, but this included only 11% of participants at the first annual follow-up and 23% of participants at the second annual follow-up and did not differ by self-rated health status (P = 0.36 at first annual follow-up, P = 0.48 at second annual follow-up).

Multivariable longitudinal analyses

In multivariable models, fair/poor self-ratings of health were associated with a decline in walking speed among all participants (P = 0.01) (Table 2), and this decline in walking speed was greater for caregivers compared with noncaregivers. Additionally, baseline walking speed, age, marital status, body mass index, physical activity, and depressive symptoms were significantly associated with a decline in walking speed among all participants: caregivers or noncaregivers.

Table 2.

Repeated-Measures Linear Mixed Multivariable Model Results for the Association Between Self-rated Health and Change in Walking Speed Among All US Participants, Caregivers, and Noncaregivers in the Caregiver-SOF Sample, 1999–2004

The association between self-rated health and percentage change in walking speed remained consistent over the follow-up periods because the interaction between time and self-rated health was not significant in any model. Respondents with fair/poor self-rated health experienced a greater decline in walking speed compared with those who reported excellent/good self-rated health (−5.66% vs. −0.60%, difference = −5.06%, P = 0.01) (Table 3). Caregiver status was an effect modifier of this relation (P = 0.03). When stratified by caregiver status, walking speed decreased in all groups except for caregivers with excellent/good self-rated health. Caregivers with fair/poor self-rated health experienced the greatest decline in walking speed (mean change = −9.26%), which differed significantly from caregivers with excellent/good self-rated health, whose walking speed increased by 1.75% (P = 0.002). Noncaregivers experienced a modest decline in walking speed, regardless of self-rated health status (−4.09% vs. −1.59% for those with fair/poor vs. excellent/good self-rated health, respectively).

Table 3.

Longitudinal Association Between Self-rated Health and Change in Walking Speed Among All US Participants, Caregivers, and Noncaregivers in the Caregiver-SOF Sample, 1999–2004

Analyses within levels of caregiving intensity

In stratified analyses, although the interaction term between self-rated health and caregiving intensity was not significant (P = 0.55), walking speed declined in all groups of caregivers except for low-intensity caregivers with excellent/good self-rated health. High-intensity caregivers with fair/poor self-rated health showed the greatest percentage decline in walking speed, while high-intensity caregivers with excellent/good self-rated health had a slight decrease in walking speed (−12.88% vs. −1.51%, difference = −11.37%, P = 0.02) (Table 4). Low-intensity caregivers with fair/poor self-rated health decreased their walking speed and those with excellent/good self-rated health increased their walking speed (−3.12% vs. 2.61%, difference = −5.73%, P = 0.33).

Table 4.

Longitudinal Association Between Self-rated Health and Change in Walking Speed Among US Caregivers by Caregiving Intensity, the Caregiver-SOF Sample, 1999–2004

DISCUSSION

This longitudinal study found that respondents reporting fair/poor self-rated health at baseline experienced a greater decline in walking speed in subsequent years compared with those reporting excellent/good self-rated health independent of baseline walking speed and of demographic, physical, and psychological characteristics. Decline in walking speed was greatest among caregivers with fair/poor self-rated health, which differed significantly from the slight increase in walking speed among caregivers with excellent/good self-rated health. Moreover, poorer self-rated health was associated with the largest decline among high-intensity caregivers, whereas it had a more modest relation in other subgroups of caregivers. These results suggest that poorer self-rated health is associated with a decline in walking speed and that greater involvement in caregiving may worsen this decline.

Our overall findings add to the evidence that poorer self-rated health increases the risk of lower-extremity dysfunction (10). Other studies have established that lower-extremity dysfunction is a risk factor for future disability (8, 9). Our study results are consistent with the assertion that poorer self-rated health is not only associated with functional decline (4–6) but also contributes earlier in the pathway through its association with lower-extremity dysfunction (10).

We hypothesized that the act of caregiving would provide the caregiver physical and psychosocial benefits that would protect against the decline in walking speed for those with poorer self-rated health. Caregiving did appear to benefit those with excellent/good self-rated health because their walking speed increased, whereas noncaregivers with similar health ratings experienced a small decrease over time. However, the influence of poorer self-rated health on decline in walking speed was greater among caregivers than among noncaregivers. It may be that these female caregivers with poorer self-rated health continue their caregiving roles despite their own declining health, the high stress of caregiving, and less time to engage in preventive health behaviors (29, 30) while being less likely to utilize health care services (31). Caregivers with higher levels of self-rated health may benefit from their caregiving activities and the increased physical activity and leg strength compared with noncaregivers (12–14).

Furthermore, in stratified analyses, caregiving intensity modified the association between self-rated health and change in walking speed. High-intensity caregivers who rated their health as fair/poor experienced the greatest decline in walking speed. These results are consistent with the “wear and tear” theory of caregiving stress. That is, greater involvement in caregiving leads to more stress, resulting in poorer health outcomes. The high-intensity caregivers in our sample may have been more vulnerable to the burden of caregiving (32). By contrast, we observed an increase in walking speed among low-intensity caregivers with excellent/good self-rated health, suggesting that, when caregiving demands are minimal and caregivers perceive their health as good, these persons have positive functional outcomes. Our results are consistent with those of one study showing that elderly caregivers who were most intensely involved in caregiving had the highest rates of mobility limitation (13). However, our results differ from previous analyses of Caregiver-SOF that found high-intensity caregivers experienced the least decline in performance-based functioning (21). These differences may be due to different outcomes and analytic methods: the current study evaluated percentage change in rapid walking speed, whereas our previous analyses evaluated mean change in a composite measure comprising usual walking pace, chair stand speed, and grip strength.

Although these declines in walking speed may appear unsubstantial, even small declines increase the risk of mortality (33). A clinically meaningful change in walking speed of 0.10 m/second has been established previously (34), and a decline of this magnitude was observed in our sample among all subjects with fair/poor self-rated health at baseline and among all caregivers and high-intensity caregivers with fair/poor self-rated health at baseline. Additionally, if converted into distance walked over 30 seconds (the time often given to cross a street), then high-intensity caregivers with fair/poor self-rated health would walk 6.93 m less in 30 seconds over 2 years of follow-up compared with baseline, while high-intensity caregivers with excellent/good self-rated health would walk 2.85 m less. This decline in walking speed may affect not only caregivers’ quality of life but also the ability to provide high-quality care to their care recipient.

This study had several potential limitations. We had only a single baseline assessment of self-rated health. Although repeated measures of self-rated health were used in a previous study (10), we evaluated baseline self-rated health as a potential predictor of functional decline over the next 2 years. In addition, we combined fair and poor self-rated health levels to increase statistical power. Whereas respondents who rated their health as fair or poor experienced declines in walking speed over time, the decline was greater among those with poor ratings. Furthermore, caregiving intensity was based on the median number of ADL and IADL tasks performed and not the number of hours per week spent performing caregiving tasks. ADL and IADL tasks may differ in terms of the amount of time, difficulty, and stress involved; however, number of caregiving tasks performed has been positively correlated with number of hours spent caregiving as well as stress (21, 35). In addition, our sample of elderly women was largely white and high functioning, so generalizability may be limited.

Our findings support those of a previous study that examined self-rated health and its relation with walking speed as a possible antecedent to ADL limitations in elderly men and women (10). Our study design enabled us to assess whether performing caregiving tasks modified this relation. A major strength of our study is assessment of walking speed at multiple follow-up points; other studies have relied on a single follow-up measure (6). Furthermore, our outcome was an objective, performance-based measure rather than a self-reported measure. An additional strength is that the Caregiver-SOF study is part of a large, multisite, community-based study of elderly women; its 2 annual follow-up interviews allowed for time-varying assessment of important covariables. Caregiving was reassessed at each annual interview, which minimized the likelihood of misclassification of caregiver status.

We chose not to adjust for number of chronic conditions and ADL limitations, as in a previous study (10). Our goal was to evaluate the association between subjective ratings of health and subsequent functional decline, and studies have found that self-rated health is strongly related to these objective measures of health (5, 36). Indeed, this finding was true because, when we added number of chronic conditions and ADL limitations to our model, the effect of self-rated health disappeared among noncaregivers and decreased substantially among caregivers (data not shown). It seems that self-rated health may act through the pathway of these objective measures of health.

In conclusion, we found that poorer self-rated health was associated with a decline in walking speed, a measure of lower-extremity dysfunction that may precede onset of ADL limitations in the established pathway between self-rated health and mortality. This decline in walking speed was most apparent in high-intensity caregivers. Given the increasing reliance of our health care system on the contributions of informal, elderly caregivers (37), targeting interventions toward caregivers with poorer self-ratings of health could help maintain their health and ability to continue providing essential care for their recipients.

Acknowledgments

Author affiliations: Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts (Jeffrey M. Ashburner, Lisa Fredman); Department of Epidemiology, University of Pittsburgh, Pittsburgh, Pennsylvania (Jane A. Cauley); San Francisco Coordinating Center, California Pacific Medical Center Research Institute, San Francisco, California (Peggy Cawthon); Division of Epidemiology and Community Health, University of Minnesota School of Public Health, Minneapolis, Minnesota (Kristine E. Ensrud); and Division of Rheumatology and Clinical Immunology, Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland (Marc C. Hochberg).

This work was supported by grants and contracts from the National Institutes of Health (AG18037 to L. F.; and AG05407, AR35582, AG05394, AR35584, and AR35583 to K. E. E., M. C. H., and J. A. C.).

The authors thank Aga De Castro, Grace Hsu, Dr. Alice Mark, Jessica Maxwell, Nora McElroy, Rose Radin, Craig Ross, Dr. Daniel Rubin, Sarah Stone, and Dr. Martha Werler for their contributions to this manuscript.

Conflict of interest: none declared.

Glossary

Abbreviations


ADL	activities of daily living

IADL	instrumental activities of daily living

SD	standard deviation

SOF	Study of Osteoporotic Fractures

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