A version of this story originally appeared on Shape According to a 2010 study published in the North American Journal of Sports Physical Therapy , women who practiced "jumping and pivoting" sports (like running) were four to six times more likely to end up tearing their ACLs (the ligament that runs through the middle of your knee) than men. This is a pretty dramatic difference, right? Shape thought so, too.To better understand why women are at such a greater risk of ACL injuries, Shape spoke with Armin Tehrany, MD, orthopedic surgeon, shoulder and knee specialist, and founder of Manhattan Orthopedic Care Related: 7 Workout Routines Secretly Causing Knee Pain For starters, Dr. Tehrany said that this is an unfortunate result of most women's anatomy, namely wider-set hips. "The alignment of a man's leg is straight, because they have a narrower pelvis. A woman has a wider pelvis, so the knee is no longer aligned straight," he explained. This is important, because a wider pelvis creates a bigger angle between the thigh bone and the shin bone, which puts the ACL at a greater risk of being twisted or torn.Related: Exercise and Your Menstrual Cycle: What Your Period Means for Your Workout Schedule This one's a little less obvious — Dr. Tehrany said that our menstrual cycles may also be to blame. During your period, your joints and ligaments loosen, thanks to shifting levels of hormones: "Women usually have looser joints and ligaments than men to begin with. Then, the menstrual cycle can make them get even looser, which means that during pivoting sports or shifting of the knee, the ACL is under more risk of tearing." He added that you should feel free to wear a knee brace on your runs at any point during your cycle.Luckily, this doesn't mean women need to stop doing activities that involve these movements. It only means that it might be extra important for women to cross-train to keep those quads and hamstrings strong. Dr. Tehrany explained that keeping your strength in these two muscle groups relatively balanced will reduce your risk of an ACL injury."Women should seek out a professional — either a physical therapist or personal trainer — for advice on whether or not they have stability between the quad muscles in the front of the knee, versus the hamstring muscles on the back of the knee."Related: 10 Knee-Friendly Lower-Body Toners