Prospective study of physical activity and physical function in early old age

Article Outline

• Abstract
• Introduction
• Methods
  • Study population
  • Measures
  • Analysis
• Results
  • Discussion
• Conclusions
• Acknowledgment
• References
• Copyright

Background

In the elderly, higher levels of physical function have consistently been associated with higher levels of physical activity. In this study, we test the hypothesis that physical activity earlier in the life course preserves high physical function over an extended period of time, before the onset of major age-related declines in physical function.

Methods

A cohort study with an average of 8.8 years of follow-up (1991–1993 to 2001). Logistic regression analyses were conducted adjusting for long-standing illness, baseline physical function, smoking, body mass index, and employment grade. Participants were 6398 London-based civil servants aged 39 to 63 years at baseline, 90% of whom were working. The main outcome measure was physical function measured by the Short Form (SF-36) General Health Survey.

Results

Relatively fit and healthy, mainly working, middle-aged men and women who were physically active at recommended levels, were more likely to report high physical function at follow-up, compared to their sedentary counterparts (odds ratio 1.63, 95% confidence interval 1.32–2.00). The association between initial level of physical activity and high physical function at follow-up remained after adjustment for baseline level of physical function and the presence of long-standing illness.

Conclusions

Participation in a physically active lifestyle during mid-life appears to be critical to the maintenance of high physical function in those who are fit and well enough to work and do or do not report any long-standing illness.

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Introduction 

A key aspect of successful aging is the maintenance of physical function.¹ Typically, physical function declines with age, although there is considerable variation in older adults.² A number of risk factors are associated with declines in physical function, including low physical activity level.³ Regular physical activity is associated with significant health benefits,⁴ and the United Kingdom government has set a target for 70% of the population to be physically active by 2020.⁵ Current national and international guidelines recommend that adults undertake ≥30 minutes of “moderate-intensity” (5.0 to 7.5 kcal per minute) physical activity on ≥5 days of the week.⁶, ⁷ In England, just 25% of women and 37% of men, aged ≥16 years, were active at these levels in 1998, with proportions declining with age.⁸

Reviews of studies of the association between physical activity and physical function have shown that a physically active lifestyle reduces the risk of low physical function in the elderly.⁹, ¹⁰ These studies involve participants aged ≥65 years, many of whom already have chronic disease. What has not been clearly demonstrated is the value of physical activity earlier in the life course. In this study, we tested the hypothesis that physical activity around age 50 years is associated with the maintenance of high physical function about 9 years later, in a high-functioning, mainly working, group of men and women with and without long-standing illness.

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Methods 

Study population 

The target population for the Whitehall II study included all London-based office workers aged 35 to 55 who were working in 20 civil service departments.¹¹ With a response rate of 73%, the final cohort consisted of 10,308 participants (6895 men and 3413 women) at the first phase of data collection between 1985 and 1988. The screening at baseline (Phase 1) involved a clinical examination and a self-administered questionnaire containing sections on demographic characteristics, health, lifestyle factors, work characteristics, social support, and life events. Since baseline screening, further data collection rounds have been completed. Successive phases alternated between collecting data by self-administered questionnaire only and collecting data via a clinical screening in addition to questionnaire completion. The Short Form 36 Health Survey (SF-36)¹² was first introduced at Phase 3 (1991 to 1993, n =8548), and repeated at the most recent phase of data collection (Phase 6, n =7357), completed between 2000 and 2001. A total of 6398 men and women (average age of 50) completed the physical activity questions at Phase 3, and the Short Form 36 (SF-36) Health Survey at Phases 3 and 6. Phase 3 rather than Phase 1 was used as the baseline, and Phase 6 will be referred to as the follow-up. The average length of the follow-up period was 8.8 years (standard deviation, 0.4).

Measures 

Physical activity was self-reported, and participants were asked about their frequency of participation in “mildly energetic” (e.g., walking, weeding, general housework); “moderately energetic” (e.g., dancing, cycling, leisurely swimming); and vigorous physical activity (e.g., running, hard swimming, squash) in the previous 4 weeks. In addition, the average number of hours per week spent in such sports and activities was reported. Three categories of physical activity were computed that aimed to reflect existing public health recommendations as closely as possible⁴: “sufficiently active,” ≥2.5 hours per week of moderate physical activity or ≥1 hour per week of vigorous physical activity, Category 1; “sedentary,” less than once per week of moderate or vigorous physical activity and less than three occasions per week of mild physical activity, Category 3; and “insufficiently active,” Category 2 (i.e., neither Category 1 nor Category 3).

Perceived physical function was assessed using the SF-36, and scored with the Medical Outcomes Study scoring system.¹³ The scale has high internal consistency.¹⁴ Participants were asked to provide the extent to which their health limited their ability to perform ten physical activities (in the SF-36) ranging from “vigorous activities such as running, lifting heavy objects, participating in strenuous sports,” to “bathing and dressing yourself.” Possible responses for each of the ten items were “a lot,” “a little,” “not at all.” The scores from the ten items were summed and transformed to range between 0 (limited a lot in performing all ten types of physical activities) and 100 (performs all ten types of physical activity without limitations).¹³

Data on age, gender, employment grade (a sensitive measure of socioeconomic status),¹¹ cigarette smoking (n =764), access to a car (n =5568), and retirement (n =659) were collected at baseline by questionnaire. Body mass index (BMI)—defined as weight (kg) divided by height squared (m²)—was recorded at the screening clinic. The following mainly self-reported measures of long-standing illness related to physical function ¹⁶ were recorded at baseline and follow-up (numbers in parentheses show the prevalence of long-standing illness at baseline and then follow-up): diabetes (self-report or oral glucose tolerance test, n =142/323); claudication (self-report, n =446/362); respiratory illness (self-report, n =295/422); coronary heart disease (self-report or clinical test, n =533/903); cancer (self-report or medical documentation, n =22/147); musculoskeletal disorder (self-report, n =721/945). Overall, 26.1% of men and 32.5% of women reported one or more of the above conditions at baseline.

Analysis 

The distribution of physical function scores at baseline and follow-up was skewed to the left, with many participants achieving the maximum score. Participants were therefore categorized as high physical function or not at follow-up, defined as a score in the highest gender-specific quintile, equivalent to 100 for both genders. Baseline physical function score was treated as a covariate. Other covariates included long-standing illness, BMI, smoking, and employment grade. A series of logistic regression models was computed to examine the relationship between baseline physical activity and high physical function at follow-up. Analyses were carried out using Stata, version 7 (Stata Corp, College Station TX, 1997). The statistical significance level was based on likelihood ratio tests. Data were analyzed from the end of 2003 to early 2004.

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Results 

There were 451 deaths between baseline and follow-up. Missing cases who were alive at Phase 6 (n =740) had a baseline physical function score three units lower, were more likely to be sedentary, and less likely to report a long-standing illness. Of the 7357 participants who did return a questionnaire, 959 had incomplete data at baseline or follow-up, leaving 6398 for analysis. The median age of participants at baseline was 49 years (range, 39 to 63) for men, and 50 years (range, 39 to 63) for women, with a mean of 8.8 years between baseline and follow-up. Some 49% of men and 28% of women were sufficiently active at baseline, and 26.1% of men, and 32.5% of women reported one or more long-standing illnesses at baseline (Table 1).

Table 1. Distribution of selected baseline characteristics by baseline physical activity level and gender

Baseline characteristic	Sedentarya(%)	Insufficientlybactive (%)	Sufficientlycactive (%)
Men	n=642	n=1640	n=2208
Any longstanding illness	31.9	26.3	24.2
Lowest tertile of physical function	50.2	37.3	28.3
Obese (BMI ≥30)	14.8	11.0	8.9
Current smoker	14.3	10.7	10.0
Employment grade (row %)
Administrative	11.2	36.0	52.8
Professional and executive	16.2	37.1	46.7
Clerical	28.9	36.2	34.9
Women	n=483	n=883	n=542
Any longstanding illness	34.4	35.2	26.4
Lowest tertile of physical function	42.2	33.2	21.6
Obese (BMI ≥30)	24.0	17.4	13.1
Current smoker	17.2	13.1	14.2
Employment grade (row %)
Administrative	27.1	45.8	27.1
Professional and executive	23.0	45.0	32.0
Clerical	27.5	48.3	24.2

BMI, body mass index.

Sedentary participants tended to report more long-standing illness, poorer physical function, more obesity, and more smoking at baseline. However, 25% of participants classified as sufficiently active at baseline also reported prevalent long-standing illness and low physical function. Men employed in administrative jobs were more likely to be sufficiently active than those in clerical jobs. This trend was not observed for women.

The prevalence of maximum physical function scores (100) declined between baseline and follow-up in both men and women (38.7% vs 22.3%, and 26.4% vs 15.9% for men and women, respectively; p <0.001).

The proportion of participants with a physical function score equal to 100 at follow-up declined markedly with age. Baseline level of physical activity, physical function, and the presence of any long-standing illness were important predictors of physical function at follow-up (Table 2).

Table 2. Proportion of participants with high physical function (score=100^a) after 8.8 years of follow-up

	n	High physical function at follow-up (%)
Men	4490	22.3
Age group (years)
39–44	1205	30.7
45–49	1219	26.3
50–54	896	19.5
55–63	1170	11.6
Baseline physical function score
0–90	1558	6.0
91–99	1193	16.4
100	1739	40.9
Longstanding illness at baseline
Yes	1171	11.9
No	3319	26.0
Baseline level of physical activity
Sedentary	642	14.0
Insufficiently active	1640	18.5
Sufficiently active	2208	27.5
Women	1908	15.9
Age group (years)
39–44	458	24.7
45–49	467	16.5
50–54	400	16.0
55–63	583	8.6
Baseline physical function score
0–84	614	2.8
85–99	790	12.2
100	504	37.9
Longstanding illness at baseline
Yes	620	6.5
No	1288	20.5
Baseline level of physical activity
Sedentary	483	11.4
Insufficiently active	883	14.8
Sufficiently active	542	21.8

Tests for interactions at the 5% significance level indicated that the effect of physical activity level on high physical function at follow-up did not differ by baseline physical function, long-standing illness, employment grade, age, or gender. Results of the logistic regression models presented in Table 3 combine men and women, and adjust for gender and age in single years.

Table 3. Odds ratios (95% CI) for high physical function (score=100) at follow-up

Model	Baseline level of physical activity
	Sedentary (n=1125)	Insufficiently active (n=2523)	Sufficiently active (n=2750)	pvalue for trend
Age and gender adjusted	Reference	1.37(1.11–1.68)	2.29(1.87–2.79)	<0.001
Model 1	Reference	1.31(1.06–1.62)	2.19(1.78–2.68)	<0.001
Model 2	Reference	1.11(0.89–1.38)	1.63(1.32–2.00)	<0.001
Model 3	Reference	1.08(0.87–1.35)	1.58(1.27–1.95)	<0.001
Model 4	Reference	1.10(0.88–1.37)	1.64(1.33–2.04)	<0.001
Model 5	Reference	1.10(0.88–1.36)	1.60(1.30–1.98)	<0.001

Notes:Model 1 adjusted for age, gender, and the presence of long-standing illness at baseline or follow-up including: diabetes; self-reported cancer; self-reported claudication; self-reported musculoskeletal disorder; self-reported respiratory illness; coronary heart disease. Model 2 adjusted for covariates in Model 1 and baseline physical function. Model 3 adjusted for covariates in Model 2 and body mass index. Model 4 adjusted for covariates in Model 3 and smoking. Model 5 adjusted for covariates in Model 4 and employment grade categories.

CI, confidence interval.

In gender- and age-adjusted analysis, the baseline level of physical activity was related to report of high physical function at follow-up (odds ratio [OR]=0.22; 95% confidence intervals [CI]=0.17–0.28). In addition, low physical function (lowest tertile) at baseline, and prevalent or incident long-standing illness, reduced the likelihood of maintaining high physical function at follow-up (OR=0.41; 95% CI 0.35–0.48). Adjustment for long-standing illness at baseline or follow-up (Table 3, Model 1) slightly diminished the odds ratios for high physical function, but they remained significant for the insufficiently and sufficiently active groups. In Model 2, adjustment for baseline physical function reduced the odds ratio of the sufficiently active group by 41% from Model 1, and 18% for the insufficiently active group (based on β coefficients). Additional adjustments for BMI, smoking, and employment grade had little effect on the odds ratios (Models 3 to 5). Further adjustment for occupational stress, car ownership, and retirement status (results not shown) did not change the trend or the magnitude of the association between exercise and high physical function.

Figure 1 shows that participants who reported a higher level of physical activity at baseline tended to have higher physical function scores at follow-up in the presence or absence of any long-standing illness at baseline. To put the scores in context, the prevalence of reported limitations (either limited “a little” or “a lot”) in climbing stairs is zero for a score of 100, 37% for a score of 90, and 75% for a score of 80.

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• Figure 1. 

  Median (25th and 75th percentiles) physical function score at follow-up by baseline longstanding illness and baseline physical activity level. The line in the middle of each box represents the median, and the box extends from the 25th to the 75th percentile. The lines emerging from the box represent the 25th and 75th percentiles ±1.5 × the interquartile range.

Discussion 

This study shows that regular physical activity appears to be critical to preserving high physical function in relatively fit, healthy, middle-aged men and women. Even in a group that is healthy enough to work at baseline, physical activity adds to health as measured by physical function. Study participants who were physically active at recommended levels⁶, ⁷ were more likely to report no limitations in physical function after 8.8 years of follow-up, compared to their sedentary counterparts. The effect of physical activity on physical function remained after taking into account baseline physical function and long-standing illness. Further adjustment for BMI, smoking, and socioeconomic status made little difference to this effect.

Compared to other studies of the relationship between physical activity and physical function, Whitehall II study participants are younger and healthier. Almost all were working at baseline (10% retired). In addition, all important covariates were collected both at baseline and follow-up. Further, the average follow-up of 8.8 years in this study is long compared to the 2 to 4 years typically reported in other studies, and the sample is one of the largest.¹⁷, ¹⁸, ¹⁹

Previous studies have repeatedly shown that physical activity in the elderly (usually aged >65 years) reduces the risk of functional limitations.¹⁷, ¹⁸, ¹⁹, ²⁰, ²¹ This study has shown that physical activity is equally important for maintaining high function much earlier in the life course. High physical function can be maintained over a long period of time (8.8 years) by engaging in a physically active lifestyle, regardless of baseline function, in a sample of people less at risk of functional limitations due to age or chronic disease. One previous study included middle-aged participants, but focused on the risk of functional limitations rather than the maintenance of high function. Importantly, it was unable to take account of the level of physical function at baseline.²²

The three-way relationship among physical activity, physical function, and physical disease is complex. Here, all three baseline measures were associated with high physical function at follow-up. Controlling for prevalent or incident long-standing illness only slightly altered the odds of high physical function at follow-up for insufficiently and sufficiently active levels of physical activity (Table 3, Model 1). When baseline physical function was controlled for, the odds of high physical function at follow-up among participants sufficiently active at baseline remained significant but were reduced by 41% (Table 3, Model 2). Lower physical function or long-standing illness does not, in general, preclude participation in physical activity at recommended levels. In this study, 29.0% of those in the lowest tertile of baseline physical function were sufficiently active, compared to 54.3% in those with maximum function and 37.9% of those who reported some long-standing illness were sufficiently active compared to 45.0% who did not.

It might be that physical activity is simply a surrogate for good overall health and high physical function rather than an independent protective factor of physical function. This possibility was tested by restricting the prospective analysis (data not shown) to people with maximum physical function scores at baseline who did not report any long-standing illness. The association between baseline physical activity and physical function at follow-up was similar in this subgroup for insufficiently active (OR=1.21; 95% CI=0.86–1.70) and sufficiently active (OR=1.74; 95% CI=1.26–2.41) people.

In the disablement process framework described by Nagi²³ and Verbrugge and Jette,²⁴ the main pathway to disability is via pathology, impairment, and functional limitations. This study shows that physical inactivity can lead to loss of function even in the absence of long-standing illness or disease. Although median physical function scores at follow-up tended to be lower among those reporting long-standing illness compared to those who did not (Figure 1), the effect of baseline physical activity on physical function at follow-up was not significantly modified by prevalent or incident long-standing illness (χ²=3.65, p =0.16). Loss of function in those who do report long-standing illness appears not to be inevitable, and can be avoided or at least delayed by being sufficiently active.

The most likely reason for loss of physical function apart from illness is sarcopenia, the progressive loss of muscle mass and strength with normal aging, which starts around age 30 years.²⁵, ²⁶ Loss of muscle mass is associated with physical fitness²⁷ (measured as maximal oxygen consumption), which also declines with age and is associated with physical function.²² At any age, both muscle mass and physical fitness can be increased with appropriate physical activity programs, and the sarcopenia and fitness-related loss of function can be partially prevented or at least delayed.²⁵, ²⁷ This study shows that physical activity in mid-life can preserve physical function, but there is a strong argument for promoting physical activity at all ages for the prevention and treatment of sarcopenia.

The potential for improving the proportion of people in early old age with high physical function is large, as the prevalence of inactivity in mid-life is high. In this study, 51% of men and 72% of women were not sufficiently active at baseline, similar to rates in the general population of similar age (39 to 63 years).⁸ The prevalence of early loss of physical function is also high. In this study, 78% of men and 84% of women reported at least some limitation in their physical function either at baseline or follow-up. Low physical function in the general population is common. Between one third and one half of adults aged 55 to 64 years (30% of men and 49% of women) cannot walk 1 mile at 3 miles per hour (5 km per hour) on the level without “severe exertion”²⁸ (a level of exertion that is certain to limit other activities of daily living).

The high prevalence of both physical inactivity and loss of function suggests that we should add to our existing efforts to prevent disability in older age, the preservation of physical function through adoption and maintenance of physical activity at all ages, in people with and those without long-standing disease. It has been proposed that physiologic aging and disuse (typically defined as a lack of physical activity) be added as points of entry into the disablement process as they can both lead to impairment independent of pathology.²⁹ Our data support this proposal as they show that physical activity, independent of long-standing illness, preserves physical function. Further, a physically active lifestyle not only preserves physical function, but reduces the risk of many of the chronic diseases associated with loss of physical function.⁴

A limitation of this study was that levels of physical activity, physical function, and long-standing illness were self-reported, with the exception of some cases of diabetes that were identified with a glucose tolerance test. Self-reported physical activity is likely to be linked to some misclassification. If this misclassification were purely random, then the observed association with physical function would be attenuated. If there was reporting bias, and, for example, participants who over reported physical activity at baseline tended to over-report physical function at follow-up, then the observed effect of physical activity would be inflated. The magnitude of this potential confounding is unclear. However, the observed relationship is consistent with cohort studies with objective measures of physical function, and experimental studies have shown that increased physical activity leads to improved physical function.¹⁹, ³⁰ Therefore, it is unlikely that reporting bias completely accounts for the observed relationship between physical activity and physical function. Further follow-up with improved measures of physical activity and objective measures of physical function such as walking speed and spirometry will produce more-accurate estimates of the benefits of physical activity in this age group (39 to 63 years).³¹ Bias caused by nonresponse at follow-up was likely to be conservative, as nonresponders were less active and had poorer physical functioning. Although fewer reported any long-standing illness, the association between baseline physical activity and long-standing illness was small (correlation coefficient, 0.08), and the effect of baseline physical activity on follow-up physical function was not significantly modified by long-standing illness.

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Conclusions 

Participation in a physically active lifestyle during mid-life appears to be critical to the maintenance of high physical function in people who are fit and well enough to work and do or do not report any long-standing illness. The preservation of physical function through adoption and maintenance of physical activity at all ages should be added to our existing efforts to prevent disability in older age in people with and without long-standing illness. Doing so would likely extend the years of independent living, and therefore lead to an improved quality of life in older age.¹⁵

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We are grateful to all participating civil service departments and their welfare, personnel, and establishment officers, the Occupational Health and Safety Agency, the Council of Civil Service Unions, all participating civil servants, and all members of the Whitehall II study team.

This study has been supported by grants from the Medical Research Council, Health and Safety Executive, and British Heart Foundation in the United Kingdom; National Heart Lung and Blood Institute (2 RO1 HL36310), Agency for Health Care Policy Research (5 RO1 HS06516), New England Medical Center, Division of Health Improvement, and The John D. and Catherine T. MacArthur Foundation Research Network on Successful Midlife Development (United States); and the Institute for Work and Health (Canada). MGM is supported by a Medical Research Council professorship.

No financial conflict of interest was reported by the authors of this paper.

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