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GUEST EDITORIAL |
Geriatrics and Clinical Gerontology Program, National Institute on Aging National Institutes of Health, Bethesda, Maryland
FINDINGS of the LIFE-P (Lifestyle and Independence for Elders Pilot) study, a pilot study for a clinical trial to test the efficacy of a physical activity intervention to prevent loss of the ability to walk a modest distance (1), have recently been published in this Journal (2).
This study demonstrated success in sustaining adherence to a structured program of walking, resistance exercises, and balance training for 1 year in older persons at high risk for loss of the ability to walk 400 meters. It also found that the intervention has significant beneficial effects on physical performance measures that predict subsequent loss of this ability. These results support the idea that a full-scale trial, designed for adequate power to test this intervention's efficacy in maintaining the ability to walk 400 meters in a population at high risk for losing it, is feasible and offers good prospects of a positive result.
The design and findings of this pilot trial result from the fruitful confluence of several complementary lines of clinical aging research, including validation of physical performance tests that can be applied to large numbers of persons to predict risk for subsequent disabilities, and the evaluation of exercise interventions with regard to their effects on these predictors. The use of findings in these domains in LIFE-P illustrates their value in designing trials of interventions against physical disabilities, and in analyzing their results: Performance on the Short Physical Performance Battery (SPPB) and 400-meter walking speed predict risk for subsequent loss of the ability to walk 400 meters (3) as well as for other types of disabilities (4,5). SPPB performance was used as a LIFE-P eligibility criterion to select a trial population at appropriately high risk. In evaluating the trial's results, the intervention's positive effects on SPPB score and 400-meter walking speed provide evidence that it could prevent loss of 400-meter walking ability in a longer full-scale trial. Substantial recent progress in a third research area, the development of behavioral strategies for long-term adherence to physical activity interventions for older persons (6), was also crucial for the design and results of LIFE-P.
In regard to loss of the ability to walk 400 meters as a primary outcome for a disability prevention trial, it is worth noting the distinction (which is sometimes obscured) between its importance as a functional outcome in its own right, and as a surrogate marker or predictor of various other broader disability outcomes or adverse events. Certainly, the fact that inability to walk 400 meters predicts incidence of several such outcomes (5) adds considerable import to the findings that could come from a trial of an intervention to preserve this ability, even though it might not have adequate power to test effects on these outcomes directly. However, preserving the ability to walk 400 meters would be highly meaningful even if it did not prevent any other adverse outcome. For the wide variety of daily and social activities that customarily involve walking this modest distance, the loss of this ability forces persons to confront the loss of a function that they have likely taken for granted throughout their lives, and either to resort to compensatory strategies that pose social or financial burdens, or to restrict their lives to a narrower scope. The impact of this loss suffices to establish its importance as a target for prevention. The large number of older persons who suffer such a loss adds to its public health significance.
The implications of a trial of an exercise program to preserve walking ability are, of course, also relevant to more general public health issues, in particular the benefits of physical activity and the prevention of disability in elderly persons. In considering the results of such a trial in relation to these broader public health concepts, potential pitfalls in interpretation can result from the contrast between the high specificity of the trial in regard to its intervention, primary outcome measure, and population, and the global way in which these public health concepts are often considered.
The LIFE protocol employs a specific physical activity intervention (a structured center-based and home-based program of walking combined with selected strength and balance training exercises) targeted at a specific type of mobility limitation (inability to walk a moderate distance) in a specific older population (elderly individuals at high risk of losing the ability to walk a moderate distance). In contrast, global concepts such as "physical activity," "disability," and "the elderly" each embrace a heterogeneous variety of specific activities, performance abilities, or populations. "Physical activity" includes both structured exercise programs and recreational physical activity. Differing types and intensities of physical activity and exercise have differing effects on differing outcomes in differing populations, over differing time scales. Similarly, the concept of "mobility disability" is applied to a variety of disabilities in functions requiring mobility, such as household tasks, getting in and out of a bed or chair, climbing stairs, and walking across a room, as well as walking a moderate distance. In addition, the populations at risk for differing types of mobility disability occur also differ, for example, those at risk of losing the ability to walk a moderate distance versus those at risk for losing the ability to walk across a room. In view of these differences, the most effective prevention strategies for each are likely to vary, and some may be inapplicable to specific populations. [The presence of differences across a variety of trials of differing physical activity interventions in differing populations of older persons in the specific physical functional outcomes that have, and have not, been improved is consistent with this view (7).]
Thus it is hazardous, when assessing the potential impact of a study such as LIFE, to consider its specific intervention, outcome, and population as "representing" all of the heterogeneous elements subsumed within their global conceptual counterparts: "physical activity," "mobility disability," and "the elderly." A parallel may be drawn with the global concept of pharmacotherapy, which includes a multitude of drugs. Just as we do not dismiss the need for a trial of a new drug for heart failure simply because "we already know that drugs can improve health," it would be unreasonable to dismiss the need for a trial of a specific exercise intervention against a specific functional limitation, based solely on the fact that there is substantial evidence for other benefits of exercise. [Indeed, the need for information about physical activity's effects on a broader range of disability measures was noted in a recent report by a Centers for Disease Control panel (8).] Correspondingly, a positive result from any such trial should not be interpreted too broadly as evidence that "exercise" prevents "mobility disability" in general, and a negative result should not be a basis for dismissing public health recommendations for physical activity in older persons that are based on current evidence for its other benefits.
A positive outcome from a trial in a large and diverse population of at-risk older persons of a structured physical activity program to maintain the ability to walk 400 meters would be important evidence supporting the adoption of the program by individuals, health care organizations, and fitness centers, as well as stimulating further studies to extend knowledge of its effectiveness and applicability in other populations and diverse settings. An effective program, if widely used, could lead to better quality of life for very many older persons, as well as possibly diminishing incidence of a variety of other adverse functional and health outcomes. It is also worth noting that a negative result from such a trial, though disappointing, would nonetheless provide useful information. If the trial achieved good adherence to its intervention, but did not significantly decrease loss of walking ability, this would, of course, not imply that current evidence-based recommendations for physical activity for older persons should be diminished, but it would clarify what can and cannot be expected from specific types of physical activity for specific functional outcomes in specific populations. Though such knowledge would not be a basis for changing opinions about the desirable level of physical activity, it would allow better-informed decisions by older persons and health professionals about the types of physical activity that they choose or recommend. In addition, a negative trial result could modify unsupported expectations by some older patients, caregivers, and public health practitioners about the benefits of a physical activity program such as that used in LIFE for maintaining mobility, and heighten their attention to other preventive interventions (including nonexercise interventions) with proven effectiveness against other mobility disability risk factors. For researchers on disability prevention strategies, findings from a negative trial would be useful in designing other approaches whose efficacy against loss of walking ability could be tested, for example, targeting other risk factors that are not strongly influenced by the LIFE strength, balance, and walking interventions, or targeting these interventions to an earlier stage in the progression to loss of walking ability.
Regardless of positive or negative results of a trial of an intervention such as the LIFE exercise program in maintaining walking ability, there would be many other important questions about physical activity and maintenance of physical function in older persons remaining to be addressed (8). The range of these questions is far too great for any single trial to answer. In this regard, it is worth considering how a trial design such as the one planned for LIFE relates to other possible research designs for addressing some of these issues.
The LIFE design is based on classic strategies for maximizing the likelihood that a clinical trial of a preventive intervention can answer an important question about its efficacy, given a limited time span and limited study population size: selecting a population at high risk for suffering a specific adverse event within a reasonably short time, and providing a highly targeted intervention designed to influence the risk factors for this event, which are elevated in this population. Other important questions about the role of physical activity in preventing physical disabilities in older persons may require different types of study designs to answer. For example, physical activity during youth and middle age, as well as in old age, may substantially lessen risk for mobility disabilities in old age by lessening the risk, or slowing the progression, of chronic conditions such as vascular diseases and diabetes, which contribute to mobility disability directly or through disabling events to which they predispose. A variety of types of physical activity, suiting individuals' preferences, might achieve such protective effects. Such hypotheses can be tested with a variety of intervention study designs, but these would likely require very long intervention and follow-up periods, with the practical and methodologic challenges that these entail. Another set of questions stems from the fact that the contributors to many disabilities in functions that require physical abilities (e.g., shopping) are not confined to physical performance deficits, but also include environmental, behavioral, and other factors (7). The potential for preventing or ameliorating such disabilities by multifactorial strategies that include interventions against such risk factors as well as physical activity interventions can be tested in clinical trials, including algorithmic approaches to target individual modifiable risk factors (9).
The breadth of potential strategies for preventing disabilities though physical activity should not, however, distract from the importance of the LIFE pilot study, nor of the full-scale trial that could ensue from it. Exploring other strategies will require extensive planning and preliminary study, analogous to that done by LIFE. The results of the LIFE pilot illustrate that this groundwork can contribute to the design of a trial of great public health significance. The quest for better ways to prevent losing our abilities to walk as we grow old has faced challenging problems along the way, and will continue to do so, but the goal is worth the journey. LIFE constitutes a large step forward on this path.
References
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