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GUEST EDITORIAL |
Physical Activity and Health Branch, Division of Nutrition and Physical Activity, Centers for Disease Control and Prevention, Atlanta, Georgia.
Address correspondence to David M. Buchner, MD, MPH, CDC/NCCDPHP, MS K-46, 4770 Buford Hwy. NE, Atlanta, GA 30341-3717. E-mail: dbuchner{at}cdc.gov
IN 1845, Alexander Cartright codified rules for the game of baseball. In a decision sometimes referred to as "divinely inspired," he set the distance between bases at 90 feet. The result was that plays in baseball are often "close calls" and are truly influenced by the ability of the players, for example, how fast a batter can run versus how fast and accurately an infielder can throw.
About 150 years later, researchers studying the disablement process began proposing various distances for performance tests of walking ability. One commonly used distance is 400 meters. Of course, 400 meters is a standard distance for one lap around an outdoor track. World class athletes run the distance in less than 50 seconds. Older adults commonly walk the distance in less than 6 minutes.
There are different protocols for measuring the ability to walk 400 meters. The protocol in a large cohort study (Health, Aging and Body Composition [Health ABC]) involved a 2-minute warm-up, followed by a 400-meter walk test in a hallway on a 20-meter-per-segment course for 10 laps (1). Study participants were instructed to "walk as quickly as you can, without running, at a pace you can maintain." In the Lifestyle Interventions and Independence for Elders pilot (LIFE-P) study, participants walked 400 meters at their usual pace and were allowed to stop and rest. Performance on a 400-meter walk is commonly scored either as walking speed (a continuous outcome) or ability to walk the entire distance (a dichotomous outcome). For example, the LIFE-P study defined major mobility disability as inability to complete the 400-meter walk test within 15 minutes (2).
So will 400 meters be to disability research what 90 feet is to baseball—a wise choice that will stand the test of time? Even if the 400-meter walk test is a marvelous test, are we comfortable limiting the definition of major mobility disability to the ability to walk a set distance? A study in this issue suggests that exercise intervention studies would have to randomize several thousand older people for sufficient statistical power to detect an intervention effect on ability to complete a 400-meter walk (3). What is the importance of discussing an outcome measure that can be used in so few studies?
It is easy for me to accept that a person who cannot walk several hundred meters has a major mobility limitation. Accomplishing many everyday tasks requires the ability to walk this far, such as shopping for groceries, visiting a neighbor down the street, walking to a seat in a theater or sports arena, and using mass transit. It is clearly useful to have a standardized distance for a walking test, even if the distance is somewhat arbitrary. Having a standardized distance and protocol greatly facilitates comparison of results across studies.
The use of the 400-meter walk in LIFE-P illustrates the consensus for its use as a measure of disability mobility. LIFE-P also demonstrated the feasibility and usefulness of this distance—older adults randomized to exercise significantly improved their 400-meter walking speed (2), increasing the plausibility that the LIFE-P intervention could significantly prevent mobility disability in a large definitive trial. The distance is long enough that test performance depends on aerobic fitness. One study reported a correlation of R = –0.79 between 400-meter walk time and peak VO2max (4). Data on aerobic fitness provide valuable information about health status, such as risk of premature mortality (5).
A study by Espeland and colleagues (3) in the November 2007 issue argued that the test has another important advantage in large exercise trials. Compared with three other possible outcome measures (a 4-meter walk test, the Short Physical Performance Battery, and the Self-Reported Disability Scale), the 400-meter walk was the most efficient. That is, the study reported that the sample size required to detect an intervention effect on the 400-meter walk is smaller than that required for the other tests.
However, I think it is premature to regard the 400-meter walk test as the universally preferred measure of major mobility disability. The concept of mobility is broader than walking speed and distance, and includes other dimensions such as stair climbing, lateral mobility, and risk of falls. Researchers should continue theoretical work on the concept of mobility disability, and explore alternatives for measuring it. We need more research on composite disability performance measures that aggregate performance across a few or several mobility tasks (including walking). It is possible that such a measure could have higher sensitivity to change and greater efficiency.
We also need more information about measures of mobility disability. For example, to what extent do the data on the 400-meter walk from LIFE-P generalize beyond the study sample to adults with different types and severities of mobility impairments? To what extent do the efficiency and sensitivity of a disability measure depend on the intervention being studied? Although it makes sense that a walking test is efficient when studying an exercise intervention that includes walking, it may not be as efficient when studying other interventions.
In one important respect, I believe we are on the right track in using a walk test as a measure of disability mobility. As a previous editorial in this journal noted (6), ultimately we seek to increase the levels of physical activity among all Americans including older adults. For this to happen, it is necessary (but not sufficient) for good news about benefits of physical activity to be communicated beyond the scientific community to policy makers, health professionals, the public, and others. A walk test is relatively easy to understand, which will facilitate communication. My impression is that people intuitively understand the importance of having the ability to walk 400 meters to the independence and health of older adults.
How does this discussion of outcome measures fit into the broader context of addressing the public health problem of inactivity? The success of the LIFE-P study means that we have arrived at the doorstep of large, definitive randomized trials of exercise interventions. These studies are needed to demonstrate and quantify some very important potential benefits of physical activity in older adults. In addition to the LIFE-P study, I can imagine a large trial addressing whether physical activity prevents or delays cognitive impairment in older adults. Given the high prevalence of overweight and obesity among older adults, the effect of physical activity on obesity and associated functional limitations is of importance. Given the evidence that physical activity has a broad range of health benefits, it is quite plausible that such trials will demonstrate benefits. We should therefore be well-prepared to disseminate the results of positive trials, in ways strategically designed to help increase people's levels of physical activity. In addition to identifying the scientific issues that affect the selection of outcome measures in a large trial, we seek outcome measures that maximize the ability of the trial's result to positively influence physical activity levels in the population.
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The opinions expressed in this article are those of the author, and do not necessarily represent positions or policies of the Centers for Disease Control and Prevention, nor of the Department of Health and Human Services.
References
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