NEW YORK (Reuters Health) - People with arthritis of the knee joint will have less pain and better physical function if they have strong thigh muscles, Mayo Clinic researchers have confirmed in a new study.

"Strong quadriceps muscles are, overall, good for the knees," Dr. Shreyasee Amin, the lead researcher on the study, told Reuters Health. What her study can't show, she added, is which exercises are best for strengthening these muscles. People with achy knees should not go out and do just any exercise for their quadriceps based on the findings, she cautioned. "Having a qualified physician or therapist work with you is important to do the exercises correctly and so you don't injure yourself in other ways."

Studies of how quadriceps strength affects arthritic knees have had mixed results, Amin and her team note in the journal Arthritis & Rheumatism. Some have found that weak quads mean worse pain and knee function, and one investigation suggested that strong quadriceps could actually accelerate the progression of arthritis, especially in people with misaligned knee joints.

To investigate, Amin and her team followed 265 people with arthritis of the knee for 30 months, testing their quadriceps strength at the beginning of the study and then assessing cartilage loss with MRI scans at the beginning of the study and again at 15 and 30 months.

Greater quadriceps strength at the beginning of the study seemed to protect people against loss of cartilage behind the kneecap, with individuals in the top third for quadriceps strength at 60 percent lower risk of losing cartilage. Strength wasn't related to cartilage loss between the femur and tibia, however. Study participants with stronger quadriceps also had better physical function and less knee pain.

The next step, Amin said, is to identify the exercises that are most effective in building quadriceps strength safely -- and finding exercises people will want to keep doing.

For now, she added, taking water aerobics classes or swimming are two great ways for people with knee arthritis to exercise safely. People with should avoid weight-bearing activities such as walking and running on dry land, she added.

SOURCE: Arthritis & Rheumatism, January 2009.