Both high and low levels of exercise can damage knee in middle aged people, says a new study.

According to estimates by Centers for Disease Control and Prevention, by 2030, about 67 million Americans over the age of 18 will have some form of diagnosed arthritis.

In the study, researchers from University of California in San Francisco (UCSF) used magnetic resonance imaging (MRI)-based T2 relaxation times to track the degeneration of the knee cartilage in adults over the years. The study included 205 patients aged 45 to 60 who were asked to fill out a questionnaire regarding their lifestyle and levels of physical activity.

"T2 relaxation times generated from MR images allow for analysis of the biochemical and molecular composition of cartilage. There is increased water mobility in damaged cartilage, and increased water mobility results in increased T2 relaxation time," said Wilson Lin, B.S., research fellow and medical student at UCSF.

Results of the study showed that people who had higher levels of physical activity, especially the high impact ones like running, had more accelerated knee cartilage degeneration compared to other people. Researchers say that these people had a higher risk of developing osteoarthritis.

"When we compared the scores among groups, we found an accelerated progression of T2 relaxation times in those who were the most physically active. Those who had very low levels of activity also had accelerated progression of T2 values. This suggests that there may be an optimal level of physical activity to preserve the cartilage," said Thomas M. Link, M.D., professor of radiology and chief of musculoskeletal imaging at UCSF.

Dr. Link said that the use of T2 measurements in the study show that this method can be used to detect knee cartilage degeneration early, when the condition may still be reversible.

Researchers say that people who have high risk of osteoarthritis (people who have family history of the condition) need to maintain healthy weight and avoid high-impact activities.

"Lower impact sports, such as walking or swimming, are likely more beneficial than higher impact sports, such as running or tennis, in individuals at risk for osteoarthritis," Dr. Link said in a statement.

The study was presented at the annual meeting of the Radiological Society of North America.