"Women have a greater tendency to be knock-kneed and have less muscle mass on the inner thighs than men," said Letha Y. Griffin, a staff physician at the Peachtree Orthopedic Clinic in Atlanta and clinical professor of orthopedic surgery at Emory University.

* Low-impact aerobics, which places less strain on the knees and is less likely to cause stress fractures. "But don't just jump into a class of people who are pretty well conditioned," [John Bergfeld] said. Try a beginning class, then slowly work up to the more rigorous workouts.

"Knee-cap pain is not uncommon in women who gain significant weight during pregnancy and try to return to vigorous exercise following delivery without first losing some of this weight and regaining muscle strength," Griffin said.