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Train Smart continued...

Then you have to punish your skeleton a bit. Low-impact activities such as walking, swimming, and cycling are good for your heart and muscle tone, but they don't do much for your bones. High-impact activities such as running and weightlifting build bone. "I think strength training is the key here," DiNubile says.

Moderation is important, too. "It's exercise as medicine," DiNubile says. All medicines must be taken at the right dose. Too little has no effect; too much causes awful side effects.

Women who overdo it may develop a condition called amenorrhea, which is triggered when overall body fat drops below 15%-17%. This is basically exercise-induced menopause. An amenorrheic woman stops menstruating and her estrogen levels take a dive. With low estrogen levels, bone loss speeds up, just like it does in post-menopausal women. Many female athletes, DiNubile says, are pleased with their lean, cover-girl bodies, "Yet they're not healthy. They've got disease.

"You need to train smart," he says. You should maintain a healthy weight -- not too plump, but not too hard -- and allow your bone enough time between workouts to heal and grow.

Good Vibrations

Clinton Rubin, PhD, a scientist at the State University of New York in Stony Brook, thinks he may have found another way to build bone: vibration. His experiments have shown that gentle, high-frequency vibrations greatly increase bone growth. In a recent study, he had sheep stand on a vibrating platform for 20 minutes, five days a week, over the course of one year. The density and volume of their leg bones increased by more than 30%.

Rubin's research is funded by NASA's National Space Biomedical Research Institute. NASA is interested because bone loss is a bane to astronauts. According to Rubin, we lose about 2% of our bone per decade here on Earth. Astronauts lose about 2% of their bone per month in space. Under no stress from gravity, bone dissolves. "It's doing exactly what it thinks it should do," Rubin says. That is, a weightless body doesn't really need bones. It's responding to the environment.

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