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Q: Is cracking your knuckles really bad for you?

. . . My mother always told me it can cause arthritis. Is that true?

M.K., Cambridge

No. In fact, it's not dangerous at all, said Dr. Jonathan Kay, clinical director of the rheumatology unit at Massachusetts General Hospital.

"The cracking sound you hear when you pull on your fingers is caused by the release of negative pressure within the joint," he said. Technically, this is called cavitation. Inside every joint in the body, a gooey substance called synovial fluid coats the surfaces of cartilage to protect them. Just as two discs coated with mineral oil stick together and can only be pulled apart with a bit of force, so it is inside the joint. When you pull, you break the vacuum seal created by the synovial fluid and the breaking of that seal causes the popping sound -- from the bursting of bubbles of gases dissolved in the fluid.

Curiously, it takes about 20 minutes for the seal to re-establish itself. So crack your knuckles all you want. You might stretch a few ligaments, but you will not give yourself arthritis. But it'll be no fun unless you wait 20 minutes between cracks.

JUDY FOREMAN

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