Of special concern for girls is damage to their anterior cruciate ligament, or A.C.L., the tiny muscle in the knee that connects the two halves of the leg. Female athletes are four or five times more likely than male athletes to have A.C.L tears, says Dr. William Levine, the director of sports medicine at Columbia University and the head physician for its varsity teams.

As Dr. Levine explains, once girls begin to menstruate, they become more "quadricep-dependent" than males, and that thick slug of muscle in the middle of the thigh then works against the A.C.L., sometimes causing tears. "Female athletes jump and land in a more erect posture, which puts increased stress on their A.C.L," he says.