A relatively small amount of physical activity reduces the risk of arthritis-related disability in middle-aged and older adults, according to a study published online Monday in the American Journal of Preventive Medicine.
The study found that meeting a modest physical activity threshold — just an hour a week of brisk walking — was a strong predictor of whether people who have or are at risk of osteoarthritis of the knee would remain disability-free over a four-year period.
“This is less than 10 minutes a day for people to maintain their independence,” said Dorothy Dunlop, the study’s lead author and a professor of preventive medicine at Northwestern University, in a released statement. “This minimum threshold may motivate older adults to begin their path toward a physically active lifestyle with the wide range of health benefits promoted by physical activity.”
An estimated 20 million Americans over the age of 45 have knee osteoarthritis, which is caused by a breakdown of cartilage between the bones in the joints of the knee. Many factors put people at risk for the degenerative condition, including age, genetics, joint injury or overuse, and being overweight.
Regular physical activity can help prevent disability in people who have or who are at risk for knee osteoarthritis, previous research has shown. The current study was undertaken to see what might be the minimum amount of exercise needed to achieve that important goal.
How the study was done
For the study, Dunlap and her colleagues analyzed four years of data collected from a group of 1,564 American adults, aged 49 to 83, who participated in the Osteoarthritis Initiative, a large, multicenter study on knee osteoarthritis.
At the start of the study, in 2008, all the participants were at risk for developing knee osteoarthritis, based on X-rays, family history, past injuries and other factors. Each had pain, aching or stiffness in their hip, knee, ankle or foot joints, but none were considered disabled.
The study defined “disability-free” as being able to walk at a speed of at least one meter (3.2 feet) per second and not reporting limitations in the ability to do routine daily activities, such as getting dressed or walking across a room.
To get an objective measure of their physical activity at the start of the study, the participants’ were monitored with accelerometers for a week. After two years and then again after four years (at the end of the study), the participants’ walking speed was re-tested. They were also interviewed at each of these follow-up sessions to determine if their daily activities had become limited.
At the end of the four years, 89 percent of the participants remained free of a mobility disability and 84 percent remained free of an activities-of-daily-living disability.
The study also found, however, that the risk of developing both these types of disabilities was associated with a minimum level of physical activity — at least 56 minutes a week of some kind of moderate to vigorous exercise.
People in the study who did not meet that minimum threshold were eight times more likely to have a mobility disability (24 percent versus 3 percent) and almost twice as likely to have an activities-of-daily-living disability (23 percent versus 12 percent) than those who did.
Those differences in risk held even after the researchers adjusted the data for other factors related to disability, such as age, gender, body mass index (BMI) and the presence of knee osteoarthritis.
Limitations and implications
The research comes with caveats. Most notably, as the researchers themselves stress, causation cannot be inferred from this observational study. Although all participants were free of disability when they entered the study, it could be that their physical activity level at that time was a proxy for their general health.
Also, the accelerometers used to assess physical activity level may have underestimated — or not even captured — certain types of exercise, such as cycling and swimming. That factor may have affected the results.
Still, the findings support other research by the same group of researchers that has suggested only 45 minutes a week of moderate to vigorous activity may be sufficient for maintaining physical function.
That doesn’t mean that everyone should aim only for an hour a week of exercise. Federal guidelines recommend — and plenty of research supports — a higher level of physical activity to protect against chronic diseases, including heart disease, type 2 diabetes and certain cancers. That recommendation is for 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity physical activity each week.
Yet, only about half of American adults get that amount of exercise. And the figure is even smaller — only 20 percent — among adults with knee osteoarthritis or other problems affecting the joints of the legs.
“We hope this new public health findings will motivate an intermediate physical activity goal,” said Dunlop.
“One hour a week is a stepping stone for people who are currently inactive,” she added. “People can start to work toward that.”
FMI: You’ll find an abstract of the study on the website for the American Journal of Preventive Medicine, but the full study is behind a paywall.