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Gender Differences

It is well known that women are more susceptible to ACL injury than men; the reasons for this disparity however are unclear. In some cases, ACL injury is ten times more common in women than men when performing the same action (9). This is a very hot topic for research and consequently a number of theories, which attempt to explain this phenomenon, have been produced. Unfortunately, no one theory seems to clarify why women are more prone to ACL injury. Most likely, many factors play a role in women's greater susceptibility to ACL rupture.

Hamstring and Quadriceps Strength

A study by Patricia Buchanan and Vassilios Vardaxis explores the differences in hamstring and quadriceps strength of boys and girls from ages 11-17 (10). It was determined that boys, ages 15-17, had significantly greater quadriceps strength than 15-17 year old girls. What is more alarming is that quadriceps strength did not differ between the 11-13 and 15-17 year old groups of girls. For the boys, quadriceps strength increased 60% when comparing the age groups 11-13 and 15-17. It is clear that the quadriceps muscles play an important role in knee joint stiffness, which limits the movement of the joint and ultimately protects it against injury. Now that this difference in quadriceps strength has been realized it is important to understand why such a discrepancy exists. Again there is controversy surrounding any explanation, but Buchanan and Vardaxis believe that lack of strength training for girls in competitive sports is a main component of insufficient quadriceps strength. There is evidence suggesting that simply participating in sports may not sufficiently strengthen the quadriceps. As females are becoming more competitive, at increasingly younger ages, trainers need to stress strength training just as much as they do for male athletes (10).

Men are thought to play sports in a more crouched position whereas women tend to assume a more upright athletic position. Because ACL injuries most often occur when an individual plants or pivots when the knee is near extension, women may be more susceptible to ACL injury.


Role of Menstrual Cycle and Hormones An obvious difference between men and women are the type of hormones produced within the human body. There is evidence that hormonal differences could be the cause of increased joint laxity in females and subsequent likelihood of ACL injury. It was documented that 26 of 37 athletes tore their ACL during the follicular phase of the menstrual cycle (11). This stage is the first few days of the menstrual cycle when the egg is maturing within the ovary and the hormone levels of estrogen and progesterone are low. Although researchers do not know why ACL injuries occur more frequently early in the menstrual cycle, this evidence suggests that there may be a hormonal component to ACL injury. Currently, it is hypothesized that estrogen and progesterone may cyclically alter the expression of certain genes that play a role in tissue remodeling and repair. Fluctuating levels of these hormones could render the female more susceptible during certain stages of the menstrual cycle (12).

	Number of Injuries

			Day of the Menstrual Cycle

		This data was taken from the research of Slauterbeck et al. (11)

Structure of Women's Body Shaped Differently Additionally, women are slightly structurally different than men. For this reason, it is theorized that a female's susceptibility to ACL injury may be due to the unique alignment of their skeletons. In the knee, the femur meets the tibia at an angle Q. This angle is directly related to the size of the pelvis. Since women have a wider pelvic area than men their Q angle is greater. An increase in the Q angle has been shown to place more force on the ACL, leading to higher incidence of injury in women (6). Also the cross-sectional area of a women's ACL is slightly smaller on average than a male's (9). Researchers believe that this size difference may relate to a discrepancy in ACL strength. In addition, a female's intercondylar notch appears to be smaller and less round than the notch in males. It is predicted that a smaller, sharper notch could more easily shear a female's ACL (13).

	Hughston Sports Medicine Foundation