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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 61:1201-1205 (2006)
© 2006 The Gerontological Society of America

Tiredness and Subsequent Disability in Older Adults: The Role of Walking Limitations

Kirsten Avlund, Taina Rantanen and Marianne Schroll

1 Department of Social Medicine, Institute of Public Health, University of Copenhagen, Denmark.
2 Research Centre for Prevention and Health, Copenhagen County, Denmark.
3 Finnish Center for Interdisciplinary Gerontology, University of Jyväskylä, Finland.
4 Department of Geriatric Medicine, Bispebjerg Hospital, University of Copenhagen, Denmark.

Address correspondence to Kirsten Avlund, PhD, DMSc, Department of Social Medicine, Institute of Public Health, 5 Øster Farimagsgade, P.O. Box 2099, 1014 Copenhagen K, Denmark. E-mail: k.avlund{at}socmed.ku.dk


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Background. The purposes of the present study are to analyze whether tiredness in daily activities is associated with subsequent disability among nondisabled older adults and whether this association is mediated by walking limitations.

Methods. The investigation is based on baseline and follow-up data on 419 nondisabled 75-year-old persons in Jyväskylä, Finland, and Glostrup, Denmark. Onset of disability was measured by a validated scale based on seven items: combing hair, washing upper body, washing lower body, using the toilet, dressing upper body, dressing lower body, and cutting fingernails. Tiredness was measured by a validated scale based on the following items: using the toilet, washing and dressing lower body, and cutting toenails. Maximal walking speed was assessed by a 10-meter test.

Results. Tiredness in daily activities was significantly associated with subsequent disability when adjusted for walking limitations.

Conclusion. The present findings indicate that it is important to take it seriously when older persons complain about tiredness, as these people are at higher risk of onset of disability.

IT may be useful in primary prevention to identify individuals at high risk for functional decline before it actually occurs by characterizing early functional states that are associated with later disability. Recent studies based on data from the Nordic countries have shown that tiredness in daily activities is an indicator of an early stage of disablement. It predicts onset of functional limitations (1) and disability (2,3), use of social and health services (4), and mortality (5). The predictive value of tiredness has been shown among the young-old (5,6) and the old-old (7), among men and women (3,5), and among older adults in different countries (1,2).

Previous research showed that the associations between tiredness in daily activities and onset of disability was not fully explained by sociodemographic factors, pathology, and impairments. Tiredness in nondisabled older adults is a result of multiple potentially modifiable factors, for example, comorbidity, cognitive decline, and depressive mood (8). Tiredness may thus be seen as a subjective indicator of frailty, defined as a physiologic state of increased vulnerability to stressors that results from decreased physiologic reserves and even dysregulations of multiple physiologic systems (9).

Another early indicator of disablement is walking limitation. Several studies during the last 10 years have shown that poor walking performance is strongly related to subsequent functional decline (10–14). According to the theoretical pathway from disease to disability described by Verbrugge and Jette (15), poor walking performance can be considered a functional limitation, proximal to disability, which represents the interaction of individuals' abilities and environmental requirements (12). Tiredness in daily activities correlates with walking limitation (16). However, it has not yet been studied whether the association between tiredness and onset of disability is mediated by walking limitations.

The purposes of the present study are to analyze whether tiredness in daily activities is associated with subsequent disability among nondisabled older adults and whether this association is mediated by walking limitations.


   METHODS
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 Abstract
 Methods
Results
 Discussion
 References
 
The data for this study were collected as part of the Nordic Research on Aging Study (NORA) (17,18). The baseline study, performed 1989–1990, included all citizens born in 1914 in Jyväskylä, Finland, and a random sample of persons born in 1914, living in Glostrup, a suburban area of Copenhagen (n = 836). At 5-year follow-up, 213 participants had died and 96 persons did not want to participate in the follow-up study (n = 527). To select an initially nondisabled cohort, we excluded 103 persons reporting need of help in at least one of the following daily activities at baseline: combing hair, washing upper body, washing lower body, using the toilet, dressing upper body, dressing lower body, and cutting fingernails (n = 424). The present analyses are based on data from the 419 nondisabled participants, with completed tests and questionnaires on all included variables.

Variables
Disability was measured by a validated scale (the PADL-H Scale) showing need of help in combing hair, washing upper and lower body, using the toilet, dressing upper and lower body, and cutting fingernails. Reliability tests on the PADL-H Scale showed agreement percents from 98.1 to 1.0 and kappa values from 0.73 to 1.0 for the included items on intrarater and interrater tests (19). Item bias analyses showed that all participants in the baseline study at a given index score on the PADL-H Scale had the same probability of scoring on an item, regardless of gender and geographic locality (p <.01) (20). Onset of disability was defined as the onset of need of help in at least one activity during follow-up. This cut-point was chosen because of the clinical relevance for older adults of being able to manage without help.

Tiredness in daily activities was defined by the old participants themselves. We asked whether the participants felt tired after performing the following activities: 1) using the toilet, 2) washing lower body, 3) dressing lower body, 4) cutting toenails, 5) combing hair, 6) washing hair, 7) cutting fingernails, 8) washing upper body, and 9) dressing upper body. The questions were introduced by the following paragraph: "Many elderly people feel that they get tired when they have performed certain activities. We are going to ask the following questions in order to learn whether you get tired when you have performed the individual activities." Tests by the Rasch model for item analysis showed that the included items could not validly be combined into one scale, but that they formed two scales: The Lower Limb-Tiredness Scale (items 1–4) and the Upper Limb-Tiredness Scale (items 5–9) (21,22). Further item bias analyses on the present data showed that all participants in the baseline study at a given index score on the Lower Limb-T Scale had the same probability of scoring on an item, regardless of gender and geographic locality (p <.01) (20), but there was strong item bias on most items in the Upper Limb-T Scale with regard to locality (20). Consequently, we used the Lower Limb-T Scale for the present analyses. The scale counts the number of activities managed without tiredness. We distinguished persons who felt tired in one or more activities and persons who did not feel tired. Reliability tests on the Lower Limb-T Scale showed agreement percents from 92.6 to 1.0 and kappa values from 0.39 to 1.0 for the included items on intrarater and interrater tests (19).

Walking limitation was measured by maximal walking speed assessed by a 10-meter test done in the laboratory corridor and timed with a stopwatch (23). The participants were allowed to use walking aids if they usually did so. They started walking about 5 meters before the first line in order to achieve maximal speed before timing started, and continued walking for a couple of meters after the second line. For analytical purposes the measure was divided into tertiles using gender specific cut-off values (Men: slow = <1.6 m/s, medium = 1.6–2.0 m/s, quick = >2.0 m/s); women: slow = <1.4 m/s; medium = 1.4–1.7 m/s, quick = >1.7 m/s).

Covariates
Income: low (only compulsory old age pension), high (both compulsory old age pension and other income sources).

Number of chronic diseases was measured in connection with the medical examination by an open-ended question: "Do you suffer from any longstanding illness ...?" If the answer was positive, further information was collected on the medical diagnosis, type, duration, and localization of the disease. The answers were checked during the medical examination by a physician. Based on this information, the physician coded relevant diagnoses for each participant, in accordance with World Health Organization's International Classification of Diseases, 8th Edition (ICD). The main groups used for the present article were: musculoskeletal diseases (ICD 710–849), mental, nervous, and sensory disorders (ICD 290–389), disorders of the circulatory system (ICD 390–458), respiratory disorders (ICD 460–503), disorders of the digestive system (ICD 520–577), endocrinological and metabolic disorders (ICD 240–279), and various tumors (ICD 140–239).

Cognitive performance was measured by a trained psychologist. The test battery included the following intelligence and memory tests: Visual Reproduction, Digit Symbol, Raven's Progressive Matrice, and Word Fluency (24). These tests were included as they measure cognitive performance "to the limit" by stressing speed, accuracy, and span. A factor analysis was made for each locality separately in order to analyze patterns of intercorrelations among the cognitive tests, hereby testing whether these variables form reasonably similar patterns in the two localities (25). The factor loadings were relatively high and alike for all tests (0.59–0.84) in the two localities. This indicates that the four cognitive variables weigh similarly for the purpose of adding them into one variable. The present analyses are thus based on the combined variable "cognitive performance," divided into tertiles.

Depressive symptoms were measured by the Center for Epidemiological Studies Depression Scale (CES-D) (26) based on 20 items, each rated from 0, indicating rarely or none of the time, to 3, indicating most of the time. The total score is a sum of all items, ranging from 0 to 60. The internal consistency for the CES-D scale using Cronbach's alpha was above 0.80 in both localities and across genders (27). A cut-off score of 16 was used to distinguish individuals with and without depressive symptoms (26,27).

Statistical Analyses
The first step was a logistic regression analysis of tiredness in daily activities against walking speed. The second step was a series of logistic regression analyses with onset of disability against tiredness in daily activities. The first models were adjusted by sex, locality, income, number of diseases, cognitive performance, and depressive symptoms, one by one. The next model included the covariates, which attenuated the association between tiredness and onset of disability more than 20%. Finally, we included walking speed to test its potentially mediating role. The SAS procedure LOGISTIC was used for all logistic regression analyses (28).


   RESULTS
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 Methods
 Results
Discussion
 References
 
In total, 8% (n = 34) of the nondisabled study participants had onset of disability during the 5-year follow-up. Table 1 shows the baseline characteristics of the nondisabled study participants. Seventy percent had at least one disease, the most common ones being musculoskeletal and cardiovascular diseases.


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  		Table 1. Characteristics of Baseline Sample (N = 419).

 
The analyses for tiredness in daily activities by walking speed showed strong and significant associations. The odds ratio (OR) for tiredness by slow versus quick walking speed was 2.89 (95% confidence interval [CI] 1.24–6.76); and the odds ratio for tiredness by medium versus quick walking speed was 3.39 (95% CI 1.46–7.84).

Table 2 shows the multivariate logistic regression analyses of the association between tiredness in daily activities and onset of disability, which demonstrated an odds ratio of 3.61 (95% CI 1.76–7.42). The estimate was nearly the same when adjusted by sex and locality (OR = 3.63 95% CI 1.75–7.52). The models 1–4 show the adjustments by income, number of diseases, cognitive performance, and depressive mood, one by one. Model 5 includes the variables, which attenuated the association between tiredness and onset of disability more than 20% (number of diseases and depressive mood). This attenuated the estimate, but the association between tiredness and onset of disability was still strong and statistically significant. In the final model, we included walking speed to test its potentially mediating role. Compared with model 5, the effect of tiredness was only mildly attenuated, so not much of the effect of tiredness on subsequent disability was mediated by walking speed.


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  		Table 2. Tiredness in Daily Activities at Age 75 as Predictor of Onset of Disability at 5-Year Follow-Up: Results of Logistic Regression Analysis (N = 419).

 
We repeated the logistic regression analyses using different cut-points for onset of disability (Table 3). The odds ratios for tiredness on onset of disability were high, independent of the cut-point used.


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  		Table 3. Odds Ratios for Onset of Disability at 5-Year Follow-Up by Different Cut-Points for Tiredness in Daily Activities (N = 419).

 

   DISCUSSION
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 Methods
 Results
 Discussion
References
 
We found that tiredness in daily activities was significantly associated with subsequent disability when adjusted by walking limitations. The present study thus underlines findings from other studies about the importance of tiredness in daily activities (1–5) for the development of disability.

The association between tiredness in daily activities and onset of disability is not mediated by walking limitations. The results (Table 2) also showed that the odds ratios for tiredness on onset of disability decreased when adjusted for comorbidity, cognitive performance, and depressive mood, but also that the estimates stayed significant. It is plausible that tiredness captures initial dysregulations across a multiplicity of physiological and psychological systems and may be a subjective indicator of frailty. This fits well with the definition of tiredness in daily activities as a biologic syndrome of decreased reserve capacities and decreased resistance to stressors (29), resulting from cumulative declines across multiple physiologic systems—declines that have not yet caused frank disability. This is also in line with the definition of the clinical frailty syndrome, which includes fatigue and weakness in the multiple characteristics of frailty (9).

These decreased reserve capacities include age-related declines in muscle strength in older adults (30), which are influenced by an age-related decrease in muscle mass caused by loss of motor neurons, lower levels of steroid hormones, a reduction in dietary protein, and a decreased level of physical activity (31). This is very likely to lead to tiredness as older humans are required to use a greater percentage of their reduced maximal strength for activities of daily living, so that their rate of recovery may be impaired following such tasks (32). However, other age-related declines may also play a role, for example, the age-related declines in the nervous system, sensory system, and in the immunological system (33), or tiredness may reflect subclinical infections, which are regarded as biological responses of the immune system to a number of different stimuli (34). In addition, tiredness may reflect the cumulative impact across multiple life domains such as a cumulative negative impact of having a low social position or stressful social relations or lack of social relations throughout the life span (35). Tiredness may then be seen as a kind of chronic reaction to the accumulated chronic strain of dealing with the effects of cumulated social stress throughout life in addition to the physiological and biological decline that characterizes aging.

Some methodological aspects should be considered in the interpretation of our results. The study is restricted to a nondisabled population. There is a risk of selection bias during follow-up, because participants who felt tired at baseline had a significantly larger risk of dying during follow-up and of not participating in the 5-year follow-up compared with persons who did not feel tired at baseline (36). However, as the associations between tiredness and onset of disability are in the same direction as the associations between tiredness and nonparticipation and mortality at follow-up (36), we do not think that this caused any serious risk of selection bias. The selective drop-out is more likely to have caused an underestimation of the results.

The validity of the results is supported by the strong associations between tiredness and onset of disability, when other cut-points were used for the outcome measure as indicators of various levels of severity of disability. Strengths of the study are that the analyses are based on well-validated measures of tiredness in daily activities (The Mob-T Scale) and of disability (The PADL-H Scale) (20) and objective measures of diseases, cognitive performance, and walking performance. These measurements were done by trained professionals in standardized conditions.

It weakens the interpretations of the results that tiredness and walking limitations are measured at the same point in time. Thus it is not known whether individuals developed tiredness first and then walking limitations, or vice versa, or whether they developed them simultaneously. It may also be regarded as a weakness that onset of disability is measured over a 5-year period. Hardy and colleagues (37) demonstrated that brief (1–2 months) disability episodes represent the majority of disability episodes among older persons. Rudberg and colleagues (38) showed an extremely heterogeneous pattern of transitions between different levels of functional ability in a study using four waves of data over a 6-year period. These different patterns of transitions will obviously be missed by studying assessment intervals of 5 years as in the present study. However, a study of self-reported difficulty in functioning among women with data obtained weekly over a 6-month period showed significant consistency between responses after 1 week and after 24 weeks (39). In spite of these uncertainties, the findings presented here are supported by earlier studies that showed that tiredness in daily activities is also predictive of onset of disability measured at 1.5-year follow-up (3), 10-year follow-up (6,7) and 15-year follow-up (6).

The present findings indicate that it is important to take seriously complaints of tiredness from older persons, as these individuals are at higher risk of onset of disability.


   Acknowledgments
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 Methods
 Results
 Discussion
 References
 
Support for this research was provided by a research grant from The Danish Medical Research Council, the Danielsen Foundation, the Wedell-Wedellsborg Foundation, Academy of Finland, the Ministry of Education in Finland, the Ministry of Social Affairs and Health in Finland, the Social Insurance Institution in Finland, and the City of Jyväskylä.


   Footnotes
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Decision Editor: Luigi Ferrucci, MD, PhD

Received February 20, 2006

Accepted July 12, 2006


   References
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 Abstract
 Methods
 Results
 Discussion
 References
 

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