Questioning the Order of Events in Knee OsteoarthritisScience has answered many questions about knee osteoarthritis (OA). Here's a brief list of what is known:
Questions loom, however. For example, does the muscle weakness occur because of reduced muscle mass? And how does pain relate to the muscle's ability to work?
These are two questions studied by researchers at the Department of Sports Medicine at a medical school in Turkey. Researchers studied 18 women with OA in both knees. Tests used to measure function included walking, rising from a chair, and going up and down stairs. A special computerized muscle testing machine was also used to measure strength. In this study, computed tomography (CT scans) was used to show the size of the knee muscles.
Researchers found that certain muscle contractions were more likely affected by age than by the OA.
For example, when going down stairs, the quadriceps muscle along the front of the thigh started in a shortened position and worked toward a lengthened position during the motion. This is called an eccentric contraction. The muscle takes all the force and keeps it from overloading the bones. If muscle weakness occurs, there's more stress on the joint. Overloading the joints may be what leads to the changes and pain of OA in the knee.
The authors of this study report that changes in muscle size don't really explain the loss of strength that occurs with OA. Researchers think it's possible that muscle weakness is due to poor function of the muscle, not actual weakness. But which comes first? The OA, the pain, or the muscle dysfunction? The authors of this study suggest a variety of factors. Decreased size of muscles, pain, and force of muscle fiber contraction may all add up to muscle weakness in patients with OA. It may even be based on how actively or poorly muscle fibers contract.
Here is a summary of the information from this study:
Hakan Gur, MD, PHD, and Nilgun Cakin, MD. Muscle Mass, Isokinetic Torque, and Functional Capacity in Women with Osteoarthritis of the Knee. In Archives of Physical Medicine and Rehabilitation. October 2003. Vol. 84. No. 10. Pp. 1534-1541.
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