For many years, doctors have advised their patients with osteoarthritis (OA) of the knee to keep moving and stay active despite the pain -- based on the notion that inactivity would make their condition worse. Now, there's scientific proof to back up this recommendation.

Researchers studied 107 people with OA of the knee, average age 69. They were asked to what degree they avoided activity during a painful flare.

The researchers found that the people who avoided activity were more likely to be disabled than people who continued on with simple activities or used rest in between activities to make it through the day.

"In the short term, pain can be reduced by avoiding physical activity. In the long term, however, low activity levels will result in a deterioration of physical condition, especially in muscle weakness," says study author Martin P. M. Steultjens of the Netherlands Institute of Health Services Research Center in Utrecht, in a news release.

"Due to this muscle weakness, joints become less stable and their ability to carry a load is reduced. This results in increased disability," says Steultjens. "Consequently, the patient avoids activity even more, thus entering a downward spiral toward increasing physical disability."

Once differences in muscle strength were taken into account, the study found avoidance of activity still accounted for much -- but not all -- of the differences in disability levels. Researchers say that means there may also be a second way in which inactivity affects disability aside from just decreasing muscle strength.

Part of the problem may be that inactive people develop doubts about their capabilities -- which causes them to avoid certain everyday tasks even with no clear physical reason for this, says Steultjens.

Researchers say their findings highlight the need for patients with OA to remain active and perform routine exercises to maintain muscle strength and mobility.

The study is published in the July issue of Arthritis and Rheumatism.