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Age and Ageing 2005 34(6):594-602; doi:10.1093/ageing/afi188
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© The Author 2005. Published by Oxford University Press on behalf of the British Geriatrics Society. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Mobility disability in the middle-aged: cross-sectional associations in the English Longitudinal Study of Ageing

David Melzer1, Elizabeth Gardener2 and Jack M. Guralnik3

1 Department of Epidemiology and Public Health, Peninsula Medical School, RD&E Wonford Site, Barrack Road, Exeter EX2 5DW, UK
2 Department of Public Health and Primary Care, University of Cambridge, Institute of Public Health, Forvie Site, Robinson Way, Cambridge CB2 2SR, UK
3 Laboratory of Epidemiology, Demography and Biometry, National Institute on Aging, 7201 Wisconsin Avenue, Bethesda, MD 20892, USA

Address correspondence to: D. Melzer. Tel: (+44) 01392 406751. Fax: (+44) 01392 406767. Email: david.melzer{at}pms.ac.uk


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 
Background: mobility (locomotor) disability is an early marker of disability progression, health care utilisation and institutionalisation in older people. Whether mobility disability has different causes in the middle-aged has received limited attention.

Objectives: to examine associations of mobility disability with sociodemographic, behaviour and disease status and to contrast these with associations in older groups.

Design: cross-sectional interview data from the 2002 English Longitudinal Study of Ageing. Mobility status based on reported difficulty walking a quarter of a mile (402 m).

Participants: a total of 11,392 community-living respondents aged 50 years and over.

Results: in the middle-aged, 8% (95% CI 7–9%) of women and 9% (95% CI 8–11%) of men reported having much difficulty or being unable to walk a quarter of a mile, equating to 787,000 (95% CI 700,000–831,000) people in England. Factors which at least doubled odds of mobility disability in the middle-aged were chronic obstructive lung disease, angina, stroke, recently treated cancer, comorbidity, lower limb and back pain. Factors associated with mobility disability in older groups were similar. Thirty-eight per cent of mobility disability in the middle-aged population was related to high levels of lower limb pain and 15% to high levels of back pain.

Conclusions: mobility disability in the middle-aged is relatively common. The associated conditions in the middle-aged are similar to those in older people. Lower limb and back pain make dominant population contributions to mobility disability. Prevention of later disability progression may require more attention being paid to mobility difficulties and its causes in the middle-aged.

Keywords: chronic disease, disability, elderly, leg, pain, walking


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 
Walking from one place to another is a fundamental component of everyday activities. Mobility (locomotor) disabilities in older people are well established as early markers of the disablement process, being predictive of severe disability, institutionalisation and mortality [1, 2]. However, mobility difficulties are not restricted to the elderly: Iezzoni et al. [3] described that 9% of non-institutionalised people aged 50–69 years in the US reported moderate or major mobility difficulties, on the basis of difficulties moving unassisted around communities, walking a quarter of a mile or walking up ten steps.

A wide range of factors have been linked to the development of disability in older people, including gender, social position, specific conditions including cardiovascular disease and arthritis, as well as the total numbers of comorbidities [4]. However, there has been limited research on the overall causes of mobility disabilities in the middle age, although the role of, for example, musculoskeletal conditions [5, 6] has received attention. An overview of the causes of mobility disability in middle-age is potentially important in delaying disability progression later in life. Early-onset disabilities are also of crucial importance for policies on retention of older workers and the discouragement of premature retirement, which are forming an increasingly important element of pension policy.

In this article, we report an analysis that aimed to estimate prevalence and identify the factors associated with mobility difficulties in a nationally representative study of people aged 50 years and over. Specifically, we explore whether the factors linked to mobility in those aged 50–64 years (referred to here as ‘middle-aged’) differ from the factors involved in older people. The English Longitudinal Study of Ageing (ELSA) [7] provides a rare opportunity to explore this issue, as few studies of ageing include middle-aged samples.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 
This article uses data from the 2002 wave of the ELSA. The ELSA aims to provide data on the dynamic relationships of health and functioning, social networks, resources and economic position, as people plan for, move into and progress beyond retirement [7]. ELSA derives its sample from the Health Survey for England (HSE), an annual government-funded general health survey. The sample includes those who were living in a responding HSE survey household in the years 1998, 1999 or 2001, aged 50 years and over and still living in a private residential address in England. Original sampling was based on identifying households from postal code sectors representative of the general population. The face-to-face interviews for ELSA 2002 were undertaken using structured computerised questionnaires. The health part of the interview covered diagnosed disease, cardiovascular symptoms, respiratory symptoms, disability and lifestyle risks.

Mobility disability in 2002 was assessed using self-reported ability to walk a quarter of a mile (402 m). If participants reported much difficulty or that they were unable to walk a quarter of a mile, they were classed, for the purpose of these analyses, as having mobility disability.

The initial ELSA sample consists of 11,392 participants, and of these 9,413 (83%) were classed as having no mobility difficulty and 1,803 (16%) as having mobility disability. One hundred and seventy-six participants (2%) had missing information on difficulty walking a quarter of a mile; 158 of these were proxy interviews.

Age was grouped into 50–64 years, 65–79 years, 80 years and above. Social class was grouped into managerial and professional, intermediate occupation, routine and manual, other. Education was grouped into ‘degree/higher’ (National Vocational Qualification NVQ4/NVQ5/degree or equivalent), ‘intermediate’ (higher education below degree, NVQ3/GCE A-level equivalent, NVQ2/GCE O-level equivalent, NVQ1/CSE other grade equivalent or foreign/other), ‘no qualifications’. Smoking status classed people as ‘never smoker’, ‘ex-smoker’, ‘current smoker’. Alcohol consumption was grouped into ‘not at all’, ‘less than twice a month’ (‘special occasions only’ or ‘once or twice a month’), ‘at least once a week’ (‘once or twice a week’, ‘daily or almost daily’ or ‘twice a day or more’). Wealth was measured as net total non-pension wealth and divided into quintiles (using data from the whole ELSA sample).

Eyesight and hearing were self-assessed; although originally on a five-point scale, they were re-grouped as ‘excellent or very good’, ‘good’, ‘fair or poor’. If participants spontaneously declared that they were registered or legally blind, they were put into the ‘fair or poor’ classification. For the other medical conditions, participants were asked: ‘Has a doctor ever told you that you have (or have had) any of the conditions on this card?’ Conditions were: high blood pressure or hypertension; angina; a heart attack (including myocardial infarction or coronary thrombosis); congestive heart failure; a heart murmur; an abnormal heart rhythm; diabetes or high blood sugar; a stroke (cerebral vascular disease); any other heart condition; chronic lung disease such as chronic bronchitis or emphysema; asthma; arthritis (including osteoarthritis or rheumatism); osteoporosis, sometimes called thin or brittle bones; cancer or malignant tumour (excluding minor skin cancers); Parkinson’s disease; any emotional, nervous or psychiatric problems; Alzheimer’s disease; dementia/organic brain syndrome/senility or any other serious memory disability. Cancer was only included if participants also reported receiving treatment for cancer within the last 2 years.

‘Number of diseases’ was derived from the number of nine disease areas in which a respondent had a condition: ‘cardiovascular’, ‘lung’, ‘bone’, ‘cognitive’, ‘diabetes’, ‘stroke’, ‘Parkinson’s disease’, ‘cancer if treated within the last 2 years’ and ‘sensory difficulties’.

‘Pain in lower limb when walking’ splits respondents into four groups ‘no pain or never walks’, ‘symptomatic intermittent claudication’ (based on the Edinburgh claudication questionnaire [8]), ‘pain in hip, knee or feet (1–5)’, if they were not classed as having intermittent claudication but reported pain on a scale of 1 to 10 to be between 1 and 5 inclusive and ‘pain in hip, knee or feet (6–10)’, if they were not classed as having intermittent claudication but reported pain on a scale of 1 to 10 to be between 6 and 10 inclusive. ‘Pain in back when walking’ was classed as ‘no pain or never walks’, ‘low pain’ if they reported pain on a scale of 1 to 10 to be between 1 and 5 inclusive and ‘high pain’ if they reported pain to be between 6 and 10 inclusive.

Questions based on the CES-D questionnaire [9] were used to detect depressive symptoms. The number of negative statements agreed with or positive statements disagreed with was totalled. A score of 3 or more was classed as depressive syndrome.

Statistical methods
Three separate analyses (for each of the three age groups) were performed with all of the socioeconomic and behavioural measures and health conditions being fitted in a logistic regression model with disability as the dependent variable. Population attributable risk (PAR) and 95% confidence intervals were estimated for each risk factor of interest from the model using Aflogit in Stata/SE 8.2. The PARs take into account the strength of the association between the risk factor (exposure) and disease and the prevalence of the risk factor. Because we cannot assume a causal relationship between risk factor and disability and the removal of one risk factor may have an impact on the distribution of the other risk factors [10] we use the PARs to give an indication of the proportion of disability that is related to exposure and to establish which factor would have the largest impact on the population were causality later established. The sum of PARs does not represent the PAR of risk factors in combination, as risk factors may overlap or interact. A fourth, overall model was fitted to all the data. The same variables were included as factors, with an age-group interaction being fitted to the health measures to compare the effects on disability between age groups. The age-group interactions compared to the 50- to 64-year age group.

The results for the sociodemographic and behavioural factors are not presented as they were included only as potential confounding factors; results are available from the authors upon request. Owing to non-response in at least one of the questions, 195 participants were not included in the model, leaving 11,020 participants in the model.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 
Proportions of participants reporting mobility disability were substantially higher as age group increased (Table 1). Similar percentages of females and males reported mobility disability in the younger two age groups (8 versus 9% for 50–64 years and 20 versus 17% for 65–79 years), but the percentage of females with mobility disability was higher than males in the ≥80-year olds (47 versus 36%).


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  		Table 1.. Sample numbers and the percentage within groups having mobility limitation for socioeconomic and behavioural factors by age group

 

Social class, qualification and wealth all showed similar patterns on their relationship with mobility disability; generally the higher the occupational social category, the more education and the higher the wealth quintile, the lower the proportion of participants reporting mobility disability, with very large differences present in the youngest group. Current smokers reported more mobility disability than the ex- and never-smokers in the 50–64 and 65–79 age groups (14 versus 8% and 6% for 50–64 years and 25 versus 19% and 15% for 65–79 years), though there was no difference in the ≥80 years group (44 versus 42% and 43%). The more frequent drinkers of alcohol reported less mobility disability.

The frequency distribution of each diagnosis of medical condition and other medical measures by age group is given in Tables 2 and 3, in the ‘sample total’ column. Of interest, however, is the percentage of participants within each of these condition groups who reported having mobility disability (presented in the second columns for each age group). Increased mobility disability was associated with diagnosis of each medical condition, poorer eyesight and hearing and increasing number of conditions. Participants who had symptoms of intermittent claudication had proportions of mobility disability similar to those with low levels of pain in their lower limbs when walking. Mobility disability was much more common in the group reporting high levels of non-claudication lower limb pain: in the youngest age group 40% of those rating their leg pain as 6 or more out of 10 reported mobility disability, compared to only 9% of those with claudication. Mobility disability was also more common in those with higher levels of back pain and with symptoms of depression.


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  		Table 2.. Sample numbers and the percentage within groups who had mobility limitation, for physician-diagnosed conditions by age group

 

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  		Table 3.. Sample numbers and the percentage within groups who had mobility limitation, for eyesight, hearing, pain and depression score by age group

 

Multivariable associations in the middle-aged
The presence of a medical condition generally increased the odds of reporting difficulty walking a quarter of a mile (Table 4). In the middle-aged, chronic lung disease, arthritis, recently treated cancer, angina, heart attack and stroke were all found to significantly increase the odds of mobility disability. The other conditions were not found to be associated (Table 4). The estimated odds of mobility disability increased with the presence of any condition, compared to no conditions present.


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  		Table 4.. Logistic regression models of health measures associated with mobility status, by age group, adjusted for socioeconomic and behavioural factors

 

Only the most impaired classification of eyesight (fair or poor) was found to significantly increase the odds of mobility disability compared to excellent or very good eyesight, while hearing was not a significant factor.

Pain when walking was found to be an important factor for mobility disability. Although having symptoms of intermittent claudication was not found to be significantly related to mobility disability (OR = 1.58; 95% CI 0.77–3.24), low and high levels of pain in leg, knee or foot and not attributable to claudication did significantly increase the odds of mobility disability, with an OR = 6.17 (95% CI 4.33–8.79) for those reporting pain of severity 6 or more out of 10, compared to having no pain. High levels of back pain when walking were also associated with mobility disability (OR = 2.58; 95% CI 1.83–3.63), although no significant difference was found between low levels of back pain and when none was reported.

Symptoms of depression also associated with mobility disability (OR 2.03; 95% CI 1.57–2.62).

Multivariable associations comparing middle-aged and older age groups
A key question is whether the factors associated with mobility disability in the middle-aged differ from those in older groups. The direction and magnitude of the effects in the model fitted for the older age groups were generally similar to the 50- to 64-year olds (Table 4). Estimates of effects tended to be smaller as age increased, with decreased precision for estimates and less variability in outcome being explained by the factors in the models (pseudo R2 were 41, 30 and 26% for 50–64, 65–79 and ≥80-year-old models, respectively).

To explore this further, a multiple regression model including all the three age groups was developed (data available from authors). Interaction terms were then tested between age group and each factor (see Methods) to establish whether there were significant differences between age groups in the role of associated factors. From the overall model which adjusted for age and fitted age-group/medical condition interactions (with 50–64 years being the baseline), the odds of mobility disability were found to increase with age; OR = 3.25 (95% CI 1.71–6.19) and OR = 16.83 (95% CI 8.26–34.29) for the 65- to 79-year olds and ≥80-year olds compared to 50- to 64-year olds, respectively. Five interactions were found to be significant. An interaction between osteoporosis and ≥80 years (OR = 2.50; 95% CI 1.17–5.34, P = 0.018) indicated a stronger association at older ages. (The apparent increase in odds linked to stroke in the oldest age group was not significant as an interaction term.) An interaction between abnormal heart rhythm (cardiac arrhythmia) and ≥80 years (OR = 0.46; 95% CI 0.23–0.91, P = 0.025) indicated a decrease in strength of association with mobility disability in the oldest age group. Similarly, there was an interaction between more severe levels of pain in lower limb when walking compared to no pain in the 65- to 79-year olds (OR = 0.57; 95% CI 0.36–0.91, P = 0.018). Both the 65- to 79- and the ≥80-year interactions were significant for angina (OR = 0.40; 95% CI 0.25–0.65, P<0.001 for 65–79 years and OR = 0.40; 95% CI 0.23–0.72, P = 0.002 for ≥80 years), both indicating decreased odds at the older ages.

The population impact of the associated factors in the middle-aged
When considering the proportion of mobility disability in the population related to reported disease diagnosis in the middle-aged, arthritis was found to have the greatest impact (PAR = 15%; 95% CI 5–24%), with the attributable risk for other conditions ranging from 1% for cancer which has been recently treated to chronic obstructive lung disease (7%) (Table 5). However, pain in the lower limb when walking and reporting of any medical condition were found to have the largest potential impact on mobility disability in the 50- to 64-year-old population (44%; 95% CI 34–52% and 40%; 95% CI 10–60%, respectively). All pain when walking (jointly considering back and lower limb pain) had the largest potential impact on mobility disability in the 50- to 64-year-old population with a PAR of 53% (45–59%) (data not shown). In the older age groups, the same factors emerged as relatively important in terms of PARs, but the absolute size of these risks was smaller.


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  		Table 5.. Population attributable risks (PAR) from logistic regression models of health measures associated with mobility status, by age group, adjusted for socioeconomic and behavioural factors

 

Sensitivity analyses
To investigate whether arthritis and a measure of pain when walking in the same model could cause collinearity problems, models were run removing each term. For all models, when either of these factors was removed, the estimated odds of the other factor/components of factor increased slightly, though the overall significance and precision did not change. Similarly this also increased for physician-diagnosed condition ‘emotional, nervous, psychiatric problems’ and symptoms of depression. For all models, when the condition ‘emotional, nervous, psychiatric problems’ was removed, the estimate of odds for depression decreased slightly, though the overall significance and precision did not change, and when depression was removed the estimate of odds for ‘emotional, nervous, psychiatric problems’ increased slightly, though the overall precision remained the same, although the effect in the ≥80-year age group no longer reached significance.

When the analyses were repeated using the non-response weights for the ELSA 2002 study [7], no substantial changes in estimates were obtained.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusions
 Key points
 Conflicts of interest
 References
 
Mobility disability is often thought of as a problem of the elderly, but this study has shown that in the middle-aged, 8% (95% CI 7–9%) of women and 9% (95% CI 8–11%) of men in England reported having much difficulty or being unable to walk a quarter of a mile. This implies that an estimated 787,000 (95% CI 700,000–831,000) middle-aged people in England have mobility disabilities. Bearing in mind that a quarter of a mile (402 m) would easily be covered in, for example a typical visit to a supermarket, this level of disability is substantial.

As expected, several physician-diagnosed conditions were associated with markedly higher rates of mobility disability in this middle-aged group, including chronic obstructive lung disease, angina, stroke and recently treated cancer, as well as comorbidity (having more than one condition). In addition, more severe pain in the lower limb and back and a high depression score were also associated with mobility disability in this middle-aged group. The similarity between the factors in young and older people was striking. Indeed, on formal testing, there were few interactions of the more common factors with age. Also striking was the pattern of PARs of the various factors, with leg and back pain, depression and comorbidities having the largest impacts.

In interpreting these results, a number of limitations to this study need to be considered. The data presented are cross-sectional and assume the relationships in the Nagi [11] model of disablement, i.e. that the symptoms and diseases result in the disability. The sample is restricted to the community-living populations, but rates of institutionalisation are very low in middle-age and should not have introduced major biases.

Mobility disabilities are only one of several aspects of disability [12] or health-related quality of life. Of those with mobility disability studied, 70% had difficulty on at least one of six activities of daily living (including ‘dressing including putting on shoes and socks’, ‘walking across a room’, ‘bathing or showering’, ‘eating such as cutting up food’, ‘getting in or out of bed’ and ‘using the toilet including getting up or down’). This mobility-disabled group is therefore of considerable importance in terms of population morbidity and potential need for clinical care.

Much of the analysis is based on physician diagnoses as reported by patients. Recent work comparing such diagnoses in disabled older women [13] has shown high rates of agreement with clinical notes for most of the major disabling conditions, although with notable exceptions including osteoporosis, arthritis and lung disease. Thus, in this analysis, we have included both the reported diagnosis of arthritis, as well as lower limb pain on walking. Similarly, depression has been independently rated using the CES-D, in addition to considering histories of physician diagnosis. Future work should examine the role of lung disease with more direct measures of diagnosis and disease severity.

In addition to the limitations, the strengths should also be considered, including the national representative, large sample size and the relatively rare opportunity to examine mobility difficulty in the middle-aged as well as the older populations in the same study.

The overall prevalence of mobility limitation in this middle-aged group is consistent with previous estimates. For example, Bajekal et al. [14] reported responses to the following question in the HSE 2001: ‘what is the furthest you can walk on your own without stopping and without discomfort’. In the 55- to 64-year olds, 16% of men and 13% of women reported distances of 200 m or less. Iezzoni et al. [3] also reported significant levels of mobility difficulty in the middle-aged in the US.

The diagnoses identified as related to mobility limitation in this study are consistent with the substantial body of previous work on mobility limitation in the elderly [4]. However, the novel feature of this analysis is that these conditions emerge as being similarly important in the middle-aged, with only a limited number of interactions with age in formal testing. In the US study, Iezzoni et al. [3] examined the prevalences of self-reported causes of various grades of mobility difficulties in the 50–69 age group and the >70s, but did not undertake any modelling of conditions or symptoms. Thus, independent replication of our findings of the similarity of associates of mobility limitation in the middle-aged and the older samples should be a priority. The question that arises from this is where the extra mobility disability comes from in old age, if, as reported, the factors associated with mobility disability are similar across the age groups, with few statistical interactions between conditions and age groups. In our standard logistic regression models, the mobility risks associated with each condition are reasonably well modelled as being additive. What appears to drive the extra disability with advancing age is not generally greater than additive effects in older age groups, but predominantly the rising prevalence of most of the conditions studied. Thus, the prevalence of having two or more diseases rises from 29% in those aged 50–64 years (1,674 of the sample total of 5,792, in Table 2) to 55% (662 of 1,207) in the >80s. In general, this rising prevalence of conditions is enough to explain the rising net risks of mobility disability with advancing age, as we found relatively minor and few interaction effects.

A further striking aspect of the findings relates to the role of pain in the lower limb (hip, knees and feet) as a major factor in mobility limitation across the age range, but particularly strongly in the middle-aged. Adamson et al. [15] examined data from a survey of 858 people aged approximately 58 years living in Scotland and reported that the conditions most strongly associated with locomotor disability were musculoskeletal and cardiovascular conditions, but noted that reported frequency of pain was an important independent explanatory factor. The prominence of pain in the lower limb joints and back as causes of disability have also been reported in samples aged >65 years [16, 17]. These findings are also consistent with earlier work on mobility [18] and the impact of rheumatic disorders in the British population [19].

The finding that in the middle-aged, a very high proportion of mobility limitation in the population is attributable to pain in the lower limb (hips, knees and feet) or the back, is of considerable importance in identifying approaches to prevention, treatment and rehabilitation. In considering these PARs, it is important to note that the estimates should not be taken as implying complete causality of each factor and that the sum of attributable risks can exceed 100%, as the effects of different factors can overlap. Nevertheless, in relative terms, it is clear that pain plays a major role in mobility disability compared to each of the individual diagnoses. It should also be noted that depression and comorbidity are also major contributors to mobility limitations in the middle-aged population.


   Conclusions
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
Key points
 Conflicts of interest
 References
 
Substantial numbers of middle-aged men and women are unable to walk or have difficulty walking medium distances that would be covered in typical everyday activities outside the home. The diseases associated with mobility limitation in the middle-aged were similar to those present in older groups. Pain in the hip, knee or feet together with back pain and depression made a dominant population contribution to mobility limitation in the middle-aged. As several of the associated factors are amenable to intervention to prevent disability progression, further work is needed on the response of clinicians to pain and mobility difficulties in middle-age.


   Key points
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 Key points
Conflicts of interest
 References
 

  • What is already known on this topic: Mobility (walking) disability is an early sign of the disability process in older people. In older people, common causes include arthritis, cardiovascular disease and numbers of comorbidities.
  • What this study adds: We estimate that there are over three-quarters of a million middle-aged people in England with significant mobility disabilities. The causes in the middle-aged people are similar to those in the elderly. A dominant contributor is pain in the hip, knee or feet, followed by back pain. More clinical attention should be paid to mobility difficulties in middle-age, as several causes may be amenable to intervention.


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
References
 
There are no conflicts of interest.


   Acknowledgements
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 
E.G. was funded by a grant from the Health Foundation Ref: 543/2216. The authors’ work was independent of the funding agency (the funding source had no involvement). Ethics approval: IRB number for the ELSA study: IRB00002308 and the latest MREC approval for ELSA, Ref: MREC/04/006.


   References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 Key points
 Conflicts of interest
 References
 

  1. Guralnik JM, Ferrucci L, Pieper CF et al. Lower extremity function and subsequent disability: consistency across studies, predictive models, and value of gait speed alone compared with the short physical performance battery. J Gerontol A Biol Sci Med Sci 2000; 55: M221–31.[Abstract/Free Full Text]
  2. Penninx BW, Ferrucci L, Leveille SG, Rantanen T, Pahor M, Guralnik JM. Lower extremity performance in nondisabled older persons as a predictor of subsequent hospitalization. J Gerontol A Biol Sci Med Sci 2000; 55: M691–7.[Abstract/Free Full Text]
  3. Iezzoni LI, McCarthy EP, Davis RB, Siebens H. Mobility difficulties are not only a problem of old age. J Gen Intern Med 2001; 16: 235–43.[CrossRef][Web of Science][Medline]
  4. Stuck AE, Walthert JM, Nikolaus T, Bula CJ, Hohmann C, Beck JC. Risk factors for functional status decline in community-living elderly people: a systematic literature review. Soc Sci Med 1999; 48: 445–69.[CrossRef][Web of Science][Medline]
  5. Woolf AD, Zeidler H, Haglund U et al. Musculoskeletal pain in Europe: its impact and a comparison of population and medical perceptions of treatment in eight European countries. Ann Rheum Dis 2004; 63: 342–7.[Abstract/Free Full Text]
  6. Woolf AD, Pfleger B. Burden of major musculoskeletal conditions. Bull World Health Organ 2003; 81: 646–56.[Web of Science][Medline]
  7. Marmot M, Banks J, Blundell R, Lessof C, Nazroo J. Health, Wealth and Lifestyles of the Older Population in England. The 2002 English Longitudinal Study of Ageing. London: Institute for Fiscal Studies, 2003.
  8. Leng GC, Fowkes FG. The Edinburgh Claudication Questionnaire: an improved version of the WHO/Rose Questionnaire for use in epidemiological surveys. J Clin Epidemiol 1992; 45: 1101–9.[CrossRef][Web of Science][Medline]
  9. Radloff LS. The CES-D scale: a self-report depression scale for research in the general population. Appl Psychol Meas 1977; 1: 385–401.[CrossRef]
  10. Benichou J. Attributable risk. In: Armitage P, Colton T, eds. Encyclopedia of Biostatistics. Chichester: Wiley, 1998.
  11. Nagi N. An epidemiology of disability among adults in the United States. Milbank Mem Fund Q Health Soc 1976; 6: 496–508.
  12. Fried LP, Ettinger WH, Lind B, Newman AB, Gardin J. Physical disability in older adults: a physiological approach. Cardiovascular Health Study Research Group. J Clin Epidemiol 1994; 47: 747–60.[CrossRef][Web of Science][Medline]
  13. Simpson CF, Boyd CM, Carlson MC, Griswold ME, Guralnik JM, Fried LP. Agreement between self-report of disease diagnoses and medical record validation in disabled older women: factors that modify agreement. J Am Geriatr Soc 2004; 52: 123–7.[CrossRef][Web of Science][Medline]
  14. Bajekal M, Primatesta P, Prior G. HSE 2001 – Disability: A Survey Carried Out on Behalf of the Department of Health. London: The Stationery Office, 2001.
  15. Adamson J, Hunt K, Ebrahim S. Association between measures of morbidity and locomotor disability: diagnosis alone is not enough. Soc Sci Med 2003; 57: 1355–60.[CrossRef][Medline]
  16. Ettinger WH Jr, Fried LP, Harris T, Shemanski L, Schulz R, Robbins J. Self-reported causes of physical disability in older people: the Cardiovascular Health Study. CHS Collaborative Research Group. J Am Geriatr Soc 1994; 42: 1035–44.[Web of Science][Medline]
  17. Leveille SG, Fried L, Guralnik JM. Disabling symptoms: what do older women report? J Gen Intern Med 2002; 17: 766–73.[CrossRef][Web of Science][Medline]
  18. Martin J, Meltzer H, Elliot D. OPCS Surveys of Disability in Great Britain. Report 1: The prevalence of disability among adults. London: HMSO, 1988.
  19. Badley EM, Tennant A. Impact of disablement due to rheumatic disorders in a British population: estimates of severity and prevalence from the Calderdale Rheumatic Disablement Survey. Ann Rheum Dis 1993; 52: 6–13.[Abstract/Free Full Text]
Received March 15, 2005; accepted in revised form August 17, 2005.


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