A new study shows that millions of Americans with painful knee osteoarthritis (OA) do not get the recommended amount of exercise, but it appears that pain is not a major cause of this inactivity. The findings were published recently in Arthritis & Rheumatism.

Exercise has many established benefits, especially for those with knee OA, including reducing knee pain and improving the ability to move. It also helps to keep weight in check, which in turn creates less stress on painful knee joints.

So why don’t more people with knee OA exercise? Researchers wanted to know if it is due to pain – as is widely believed – or if it is because arthritis itself makes it difficult to reach a certain level of activity and maintain it for a certain amount of time.

“I wanted to better understand the relationship between disease and knee pain with physical activity in people with knee OA,” says study co-author Daniel K. White, a research assistant professor at Boston University. “In particular, I wanted to find out how knee OA and knee pain were related to meeting recommended levels of physical activity.”

For the study, which was partially funded by the Arthritis Foundation, White and his colleagues looked at more than 1,750 people who had knee OA or who were at high risk for it. They measured participants’ physical activity for seven days, then determined the percentage of those who met the 2008 Physical Activity Guidelines for Americans set by the Department of Health and Human Services (HHS). According to the guidelines, Americans should exercise for at least 150 minutes a week. The guidelines note that that the physical activity should last for at least 10 minutes at a time.

Among study participants with knee OA, 7.3 percent of men and 6.3 percent of women met the HHS guidelines. But the numbers weren’t significantly different for those participants who were at risk of knee OA but didn’t have it – only 10.1 percent of men and 7.8 percent of women met the guidelines.

There also were no significant differences in the percentage of people meeting the guidelines when the researchers broke it down by moderate/severe pain versus no pain – 12.9 percent with moderate/severe pain met guidelines compared to 10.9 percent with no pain among men, and 6.7 percent with moderate/severe pain compared to 11 percent among women.

In other words, the study showed few people got enough exercise, according to the guidelines, and it suggests pain was not a factor for the inactivity.