Some 20 years ago our laboratory suggested that one of the most important factors influencing the quality of life in very old people was the maintenance of sufficient aerobic power to allow independent living. It was argued that a progressive age-related deterioration in various aspects of physical fitness brought most sedentary elderly people to the point where they lacked the necessary functional capacity somewhere between 80 and 90 years of age.1 Depending on the living environment and the degree of social support that was available, the limiting maximal aerobic power was estimated to be 12–15 ml/[kg.min]. It was suggested further that an appropriate progressive programme of endurance training could increase the maximal aerobic power of a middle-aged adult by 5–10 ml/[kg.min];2 this would counteract the anticipated age-related loss of 5 ml/[kg.min] per decade1 and reduce a person’s effective biological age by ⩾10 years. It was argued that this in turn should allow a corresponding prolongation of the individual’s independence.3

The purpose of the present short review is to re-examine evidence on several issues relating to maximal aerobic power and independent living, as follows:

• Does the well documented loss of maximal oxygen intake that occurs between the ages of 20 and 60 years continue at a similar rate during retirement?

• Is a decrease in maximal oxygen intake a common reason for loss of independence among elderly people?

• How far can the average elderly person prevent and/or reverse this loss?

• How large an effect does aerobic training have on an individual’s risk of becoming dependent?

• Does such training reduce the likelihood of a loss of independence from causes other than a decrease in maximal aerobic power?

RATE OF LOSS OF MAXIMAL OXYGEN INTAKE IN ELDERLY PEOPLE

Some authors have argued that maximal oxygen intake should be expressed in absolute units (l/min) rather than relative to body mass (ml/[kg.min]), thus avoiding the influence of changes in fat, muscle and bone mass on the reported score. However, given that the cost of most daily activities is closely related to an individual’s body mass,4 the traditional relative units (ml/[kg.min]) seem the most appropriate basis to describe functional losses with ageing.

Longitudinal studies are theoretically advantageous but, in practice, much of the available data has been collected over too short a period to calculate reliable slopes of ageing and findings have been vulnerable to short-term changes in physical activity. In some reports maximal oxygen intake is said to have remained unchanged with age while in other papers improbably large losses have been described.1 Cross-sectional studies are vulnerable to cohort effects, including inter-generational differences of body dimensions and possible secular changes in habitual physical activity, but nevertheless the estimates of loss are more consistent than for longitudinal data. In a typical sedentary man, cross-sectional observations show maximal aerobic power decreasing fairly steadily from perhaps 45 ml/[kg.min] at the age of 20 to about 25 ml/[kg.min] at the age of 60 years, a loss of about 5 ml/[kg.min] per decade.5 In women the deterioration of aerobic power begins around the age of 35 years, scores decreasing from the young adult value of perhaps 38 ml/[kg.min] to around 25 ml./[kg.min] at an age of 60 years; again, this approximates to a loss of about 5 ml/[kg.min] per decade.2 6 In one US study an accumulation of body fat and a decrease in habitual physical activity accounted for about a half of the age-related decrease in maximal oxygen intake.7 Perhaps for this reason, the rate of loss reported for continuing athletes (commonly around 3–4 ml/[kg.min] per decade) is somewhat less than in sedentary individuals.5

Until recently, relatively few investigators have studied the loss of aerobic power during the retirement years. Immediately after leaving work a person’s habitual physical activity may increase because more leisure time is available8 but, in later old age, social constraints such as lack of a partner or problems of transportation could limit a person’s opportunities for physical activity. The main analytical problem in both cross-sectional and longitudinal studies of elderly people is that data are usually collected on healthy survivors, particularly those who have conserved sufficient fitness to live independently and retain an interest in helping experimental studies. Curves fitted to cross-sectional peak oxygen intake data for some 400 adults aged 55–85 years who were living independently in London, Ontario took an exponential form. There was an apparent loss of 16% per decade, and the minimal values seen in those who continued to live independently were 18 ml/[kg.min] in men and 15 ml/[kg.min] in women.9 10 A longitudinal analysis of the same group of subjects (conducted between the average ages of 62 and 72 years) showed a 10-year loss of 4.3 ml/[kg.min] in men and 1.9 ml/[kg.min] in women. In men the rate of loss seemed relatively consistent from the age of 55–85 years but, perhaps because of a healthy survivor effect, the oldest cohort of women showed little loss over the decade of observation.11 A 6-year longitudinal study of 253 men and 339 women initially of median age 70 years estimated somewhat larger losses: 6.9 ml/[kg.min] per decade in men and 3.9 ml/[kg.min] per decade in women.12 The Baltimore study of 375 men and 425 women obtained longitudinal data over an average interval of 7.9 years; it noted an accelerating loss of function amounting to 5% per decade in young adults and 20% per decade in the middle-old and old-old age categories.13 A 20% loss would equate to around 4 ml/[kg.min]. There is thus some inter-observer disagreement on the rate of deterioration during the retirement years, but it seems reasonable to postulate that a loss of at least 4–5 ml/[kg.min] per decade continues into advanced old age.

MAXIMAL OXYGEN INTAKE AND INDEPENDENCE

Our original hypothesis that independence would be challenged when the maximal aerobic power had dropped to 12–15 ml/[kg.min]1 was based on a study that demonstrated an exponential relationship between the duration of aerobic exercise and the relative intensity that was tolerated.14 Fatigue was likely if effort over an 8-hour day demanded more than 40–50% of an individual’s maximal aerobic power. Thus, if the maximal aerobic power of an elderly person decreased to 4 metabolic equivalents (METS) (14 ml/[kg.min]), he or she would be able to sustain only a very limited range of activities requiring an energy expenditure of less than 2 METS.

Fleg and associates13 also reasoned that, as retirement progressed, elderly people would reach an age where the decrease in maximal aerobic power made dependence almost inevitable. Likewise, Weiss and associates found that, in their oldest subjects, even standing demanded half of their maximal oxygen intake of 13 ml/[kg.min].15

The lowest values of maximal oxygen intake that Paterson and colleagues observed in cross-sectional studies of free-living elderly people were 18 ml/[kg.min] in men and 15 ml/[kg.min] in women.10 A follow-up of 297 subjects with an initial average age of 70 years found that 43 of the group became dependent over an interval of 8 years. A logistic regression that included age, disease, body size, maximal oxygen intake, joint flexibility, walking pace and habitual physical activity found that, at least in this initially healthy sample, the significant predictors of subsequent dependency were age, maximal oxygen intake and disease.16 A 14% increase in the risk of dependency was estimated for each 1 ml/[kg.min] fall in maximal aerobic power.16 Likewise, Woo and associates17 found that a slow walking speed and a short stride length predicted the 36-month likelihood of institutionalisation in Chinese men, and others have also noted an influence of cardiorespiratory fitness or lower extremity function on subsequent disability.18^–21

It remains difficult to assess the population attributable risk of dependence arising from a suboptimal level of aerobic fitness because, even where investigators have begun with a randomly selected or representative population sample, only a small fraction of those thus identified (probably biased towards the healthy and those with an interest in exercise) have completed the investigation.

AEROBIC TRAINING RESPONSE OF ELDERLY PEOPLE

Early reports suggested that the response to progressive aerobic training was as large in elderly people as in younger adults, at least if expressed in percentage terms.22 Nevertheless, it is not easy to determine the trainability of older people. Caution of the subjects or those conducting the tests may lead to less than maximal effort during an initial evaluation; if caution is reduced following training, the individual concerned may appear to have made a very large gain in maximal oxygen intake. Alternatively, the exercise programme that is provided or undertaken may lack challenge, underestimating the individual’s ability to enhance his or her aerobic power. If data are expressed in traditional units (ml/[kg.min]), the issue may be further confused by a loss of body fat over the course of training.

Table 1 summarises information on the response of elderly people to programmes of aerobic training published since 1990, making an attempt to exclude overlapping studies. The data show a weak trend for a greater response in studies of longer duration, from a weighted average gain of 12.9% with an 8–10 week programme to 14.1% with 12–18 weeks of training and 16.9% with 24–52 weeks of conditioning. Three reports show a below average response (table 2); the primary factor in these studies seems to have been a relatively low intensity of training. Likewise, seven reports show a greater than average response; here, common factors seem a high intensity of training and a relatively long period of training. The response of this last group, a gain of almost 25%, would be equivalent to an increase in maximal oxygen intake of 6 ml/[kg.min]; this is equivalent to a decrease in biological age of about 12 years and, on the basis of studies by Paterson et al,16 could delay the onset of dependency by a corresponding margin.

AEROBIC TRAINING AND INDEPENDENCE

In principle it would seem possible to prevent and/or reverse the age-related decrease in aerobic power by an appropriate programme of aerobic training. Pertinent questions include whether elderly people retain benefit from exercise earlier in adult life and whether current training makes a clinically useful contribution to the delay in dependency.

Exercise earlier in adult life

The cross-sectional study of Shephard and Montelpare23 showed an association between physical activity in middle age and the likelihood of continued independence in old age. Christensen and associates24 also noted a positive effect of physical activity at the age of 70 upon functional ability 5 years later but, after allowing for this, they did not detect any additional benefit from physical activity which had been undertaken at an earlier age.

Current training and dependency

Spontaneous variations in physical activity complicate the detection of relationships between current training and dependency. A Scandinavian 8-year follow-up study of 542 healthy people initially >70 years of age underlined the considerable variability in functional loss; 50% of their sample showed no change of function over the 8 years of observation and in 9% it actually improved.25

Some cross-sectional studies have found no relationship between current leisure activity and subsequent dependence.16 26 27 Presumably, the volume of spontaneous habitual activity encountered in such studies has been insufficient to enhance aerobic power; however, the risk of functional loss was smaller in those who took deliberate exercise or walked at least 1.6 km once a week.28 29 A prospective study of 549 74-year-old subjects noted that, relative to individuals who had decreased their habitual physical activity or had remained sedentary, those who maintained or increased their habitual activity had a much lower risk of developing disability as determined by the Health Assessment Questionnnaire Disability Index.30 Other authors have shown a beneficial effect of exercise programmes on various measures of disability. The LIFE study31 evaluated a 12-month programme of moderate intensity physical activity in a randomised sample of 424 subjects aged 70–89 years; the intervention countered at least one index of deteriorating aerobic function (an inability to walk a distance of 400 m) relative to control subjects. Likewise, a controlled trial of a home-based exercise programme for frail individuals aged 75 years and older showed improved performance of such critical indices as gait speed and a timed chair stand.32 Physical activity and a greater sense of self-efficacy were also seen as important factors in the recovery of independence in a sample of 420 newly disabled individuals over the age of 70 years.33

AEROBIC TRAINING AND OTHER CAUSES OF DEPENDENCY

There seems good evidence that the conservation of maximal oxygen intake increases the likelihood that the healthy elderly person will retain functional independence. It is less clear what proportion of the overall elderly population loses independence not through a decrease of aerobic fitness but through some other cause including chronic disease or a medical catastrophe. From the practical point of view, we may note that a progressive aerobic exercise programme is likely to be effective not only in enhancing various components of personal fitness but also in preventing many other conditions that are liable to precipitate dependency.

Regular exercise substantially reduces the risks of obesity, maturity onset diabetes mellitus, hypertension, myocardial infarction, some forms of stroke, several forms of cancer and osteoporosis, not only in middle age2 34 35 but also during the retirement years.1 36 37 It is also helpful in rehabilitation following such critical incidents as a myocardial infarction or congestive heart failure. Regular aerobic exercise may have some impact on the likelihood of becoming blind because of a reduced risk of maturity onset diabetes mellitus, and catastrophic falls are less likely if regular aerobic exercise maintains muscle power, balance and coordination. The value of aerobic exercise in preventing cognitive problems remains controversial,38^–40 although some benefit seems likely both immediately (through increased cerebral blood flow and the social interactions associated with group exercise) and more long-term (through a decreased risk of atherosclerosis). Finally, participation in group exercise programmes may counter the social isolation that encourages some elderly people to seek institutionalisation.

FUTURE RESEARCH NEEDS

One critical gap in current knowledge is the relative importance of the various causes of institutionalisation in a representative sample of the general population. How large a fraction of such admissions are attributable to a deterioration of aerobic fitness? And, if such fitness is restored, how likely is independence to be restored? Given substantial differences in the rates of institutionalisation between developed countries, what are the reasons for such differences—geographical, socio-cultural or the availability of support services? What domestic services and changes to the home environment are most effective in postponing institutionalisation in the face of declining function, and how cost-effective are such measures relative to the protection afforded by an increase in aerobic fitness?

CONCLUSIONS

The progressive deterioration of aerobic fitness observed in middle-aged adults appears to continue unabated during the retirement years. In initially healthy adults, loss of independence is likely when maximal oxygen intake has dropped below a threshold of 18 ml/[kg.min] in men and 15 ml/[kg.min] in women. A regular exercise programme can slow or reverse the loss of aerobic fitness, reducing the individual’s biological age and prolonging independence. There remains a need to clarify the importance of deteriorations in fitness relative to other potential causes of dependency but, from the practical viewpoint, regular aerobic activity can address many of the issues of both functional loss and chronic disease.