Doctors know that people with osteoarthritis in their knees tend to have weak muscles around the affected joint, and exercising to strengthen those muscles has been shown to reduce pain and improve mobility. But they've generally assumed that the muscle weakness develops because pain prevents the arthritis sufferer from fully using the joint.
Now a study of elderly people suggests that, at least for women, the muscle weakness may come first--or may at least develop very early in the disease. The weakness may then contribute to the progressive joint damage.
Osteoarthritis, a major cause of disability in older people, is degeneration that often occurs in joints that are overused or bear a lot of weight, particularly knees and hips.
Researchers at Indiana University School of Medicine studied 462 people over 65 to examine the relationship between muscle weakness and knee arthritis. Participants underwent X-rays of the knees and answered questions about pain and function. Muscle strength was also tested.
Thirty-one percent of participants had X-ray evidence of osteoarthritis. On tests of the quadriceps, the large muscle at the front of the thigh that straightens the knee, the group with arthritis was about 20% weaker than participants without arthritis. People who had joint pain were weakest.
However, among women who had X-ray evidence of arthritis, even those who had no joint symptoms were significantly weaker than women with no arthritis. Yet the arthritic women's total muscle mass was higher--indicating that their quadriceps muscles had not shrunk because of disuse.
Those findings suggest that weakness of the quadriceps occurs early in the development of knee arthritis, perhaps because of a problem in the muscle itself or because it's receiving inhibitory signals from nerves in the diseased joint, said Kenneth D. Brandt, who heads the university's rheumatology division. He said the weak muscles may fail to protect the knee from stresses of gravity, leading to joint damage.