What happens to contortionists' bones as they age?

I recently saw a circus act with contortionists folding themselves in amazing ways.

C.S.

Arlington

A. As contortionists age, they stop being contortionists, says Dr. William Mitchell, an orthopedic surgeon with the sports medicine service of New England Baptist Hospital.

"Contortionists are actually remarkably young, as in their early teens," says Mitchell, who has also served as the orthopedic surgeon who treats the performers with Cirque du Soleil.

Contortionists, often called "human pretzels," have the rare ability to twist themselves into unnatural positions because of their hyperflexibility. This ability is due not to the performers' bones, says Mitchell, but to muscle and tendon strength and to joint structure. Contortionists who are double- jointed have ligaments around their joints that are more elastic than normal.

And just as circus performing runs in families, so does hyperflexibility. ''It's a heritable trait," says Mitchell. "It's a function of how your body makes collagen, a protein found in connective tissue such as ligaments."

Extra flexibility is also found in gymnasts, says Mitchell, but to a lesser extent than contortionists.

Alas, when the joints and tendons and muscles of contortionists fully mature, the performers become less flexible and are unlikely to continue performing, says Mitchell.

But contortionists are not likely to become injured on the job, says Mitchell, because their bodies are tolerant of the stretching their performance requires. "It's the more dynamic performers like tumblers who get injured," he said.

Mitchell also said he did not anticipate contortionists having any specific problems with their bones as they age.

But like anyone who is aging, he adds, former contortionists can help offset arthritis or osteoporosis if they supplement their flexibility with exercise that includes aerobic conditioning and strengthening.