On skis, women are believed to be at more than twice the risk that men are of an ACL injury. That is a LOT more. It is appropriate for women to be aware that they are more at risk, and many (most?) women already know it, and often seek ways to reduce that risk. There is no single “smoking gun” answer to this. Rather, a number of factors may contribute:
1. “Q-Angle” – a woman’s hips are wider in relation to their ankles, with significant differences in the related bones and muscles. Men’s hips are narrower, forming a straighter connection at the knee (generally about a 15% angle), while women’s knees generally form wider angles (generally about 20%). This could increase risk, but no one is sure.
2. Muscle Differences – Women have proportionally less leg muscle mass – muscle that may help stabilize and protect the knee in some situations.
3. Muscle Response - There is some evidence that women do not have the same neuromuscular control and speed as that men do. Studies have shown, for example, that a female quadriceps does not “fire” as quickly as a male’s.
4. Intercondylar Notch - The ACL passes through and over bone. In a woman’s knee, the bone is formed with a tighter, sharper opening than in a man’s. This is, perhaps, the most widely believed reason that female ACL injuries are more likely to be complete, rather than partial, tears. It is so widely believed that some female athletes have had “preventative” surgery to “round off” this sharper bone to reduce the chances of injury.
5. Genetic Material – the actual ACL material in a woman is not genetically identical to the ACL material in a man. No one knows for sure why it evolved differently.
6. ACL Size – a female ACL is not as thick as a male ACL, in proportion to muscle mass and body size. Female athletes appear able to create muscle mass that disproportionately larger than their ACLs compared with male athletes.
7. Menstruation/Hormones – Women generally have more flexibility than men, which may contribute to the risk. In addition, many believe that menstruation and other hormonal influences such as birth control pills can accentuate this by relaxing and expanding joints even further. One study – conducted some fifteen years ago – found that women are more likely to be injured just before or just after their cycle than during it. It has been theorized by some that menses causes physical changes that put women more at risk, but also that women may subconsciously change their behavior during the peak of their cycle, thereby reducing their risk.
It is almost certain that the increased risk for women is multi-factored – that the above reasons combine in various ways to create increased risk. None of these is a proven, single cause. However, they may contribute to the increased likelihood of an injury during a “phantom foot” event.