Marlene Fransen, lead researcher, and her co-workers studied 152 men and women over 60 with chronic osteoarthritis. The research team randomly assigned participants to hydrotherapy classes, Tai Chi classes or a waiting group. The classes, one hour in length, were offered two times per week.

After 12 weeks, both groups showed significant improvements on scores measuring pain and physical function. At 24 weeks, researchers found that the improvements had been sustained, with the greatest improvement measured in the hydrotherapy group.

"Hydrotherapy classes appeared to be more acceptable (higher attendance), appeared to provide greater relief of joint pain, and resulted in larger improvements in objective measurements of physical performance," wrote Fransen.

Patients assigned to the hydrotherapy group were more likely to attend classes than those assigned to Tai Chi, which the researchers noted may be due to a lack of familiarity with Tai Chi. Eighty-one percent of participants in the hydrotherapy group attended at least half of the classes offered, compared to 61 percent of Tai Chi participants.

According to the Arthritis Foundation, arthritis is the leading cause of disability in the United States and affects 46 million Americans. Osteoarthritis, often referred to as "OA," is also called osteoarthroses or degenerative joint disease, and is diagnosed in 21 million people in the U.S. Typical onset is after age 40.

On May 3, 2007, the Center for Disease Control (CDC) released a statement that the prevalence of all types of arthritis will likely surge in the coming decades, reaching as many as 67 million Americans by 2030. In the statement, the CDC acknowledged that weight control and exercise are vital to arthritis prevention.