The gender divide

The anterior cruciate ligament may be an Achilles' heel for younger female athletes, but exercise is protective against several conditions that disproportionately affect women.

Most studies show that men and women benefit equally from physical activity, but there are several notable differences. For example, women typically don't get quite the same boost in their "good" HDL cholesterol levels that men do when they exercise because, on average, they start out with higher HDL levels. Women metabolize more fat and less carbohydrate and protein than men during endurance events. And they're more vulnerable to concussions. In every high school and collegiate sport played by both sexes, with the exception of lacrosse, female athletes have a higher rate of concussion.