Walk Faster, Live Longer?

A very interesting article in JAMA a number of years ago requires the application of critical thinking to avoid making a misinterpretation.

 A team of scientists put together a summary of nine studies over the years totaling almost 35,000 subjects over the age of 65, residents of various nursing facilities . Each study was trying to find out whether measuring a person’s walking speed could help predict how much longer they would live.

The data came from the various researchers measuring how fast people walked over a short measured course, between 8 and 20 feet long, using a stopwatch and converting the speed to feet or meters per second. They then followed up years later to see how well the measured speed at the time of the test correlated with how much longer the individuals lived. (The authors refer to it as “gait speed.”)

The published conclusion, when they put the information from all the studies together: “In this pooled analysis of individual data from 9 selected cohorts, gait speed was associated with survival in older adults.” The graphs accompanying the article show this quite clearly, and the findings held true for both men and women and for people of different races. The top group walked the course at 1.6 meters per second, which converts to about 3.6 miles per hour. (For a longer distance this would mean a pace at which someone could walk a mile in about 17 minutes.)

How is this useful? “First, gait speed might help identify older adults with a high probability of living for 5 or 10 more years, who may be appropriate targets for preventive interventions that require years for benefit.” It could help assess whether a particular medical procedure would be worth the risk – there is always a risk, especially for older people. “Second, gait speed might be used to identify older adults with increased risk of early mortality, perhaps those with gait speeds slower than 0.6 m/s.” It is a simple and informative way for “assessing expected survival to contribute to tailoring goals of care in older adults.” Furthermore, it is a simple measurement that can be easily done in a nursing home corridor by non-professionals.

How is this not useful? Correlation is not causation. Someone seeing this article and not using critical thinking might conclude that beginning to walk faster was a way to guarantee longer life. After all, the graphs on the page clearly show that those who walked faster lived longer. Except for the fact that exercise is good for anyone, this conclusion is not necessarily so; it’s not what the analysis shows.

The authors theorize that the correlation exists because, walking “requires energy, movement control, and support and places demands on multiple organ systems, including the heart, lungs, circulatory, nervous, and musculoskeletal systems.” A slower walking speed could indicate some damage or a larger than normal energy requirement for other reasons. Both could be a general indicator of lessened vitality. It makes sense that those who were going to live longer were just capable of walking faster, and did so.

Unfortunately, scientists did not prove that increasing our walking speed will increase our life expectancy. (In fact, people walking faster while texting could easily decrease their life expectancies.) Getting more exercise, including walking, and generally taking care of ourselves will. There is rarely one easy answer. The real secret to good health appeared in this space over five years ago and, unfortunately requires some degree of discipline.