ACL Injuries in the Female Athlete

Physical therapist becomes board certified

By Dr. Chad M. Pens

With the ever increasing participation in women’s sports, there has been an rising incidence of knee injuries. Specifically, anterior cruciate ligament (ACL) injuries are reported to be approximately 2,200 per year in females alone. The incidence of serious knee injuries are 1 in 100 for the female athlete. The question that so many athletes, parents, and coaches are asking is, why? Are women built differently that predispose them to injuring their ACL?

A brief overview of the structures of the knee is important to understand why females have a higher predisposition to knee injuries than males. The knee joint is comprised of the long bone of the thigh (femur), the shin bone (tibia), and the knee cap (patella). The knee has four major ligaments that stabilize the knee itself. Ligaments are connective tissue bands that connect bone to bone. The medial collateral ligament (MCL) provides stability to the inside of the knee. The lateral collateral ligament (LCL) stabilizes the outside of the knee. The posterior cruciate ligament (PCL) stabilizes the tibia from shifting backwards on the tibia, whereas the anterior cruciate ligament (ACL) stabilizes the tibia from moving forwards on the femur. The three major muscle groups that cross the knee are the quadriceps (front of thigh), the hamstrings (back of thigh), and the gastrocnemius (calf). These muscles are the primary active stabilizers of the knee, which means their muscle action holds the femur and tibia in alignment. The quadriceps pull the tibia forward, the hamstrings pull the tibia backward, and the gastrocnemius holds the femur backward. Other muscles also cross the knee but they are beyond the scope of this article.

There are differences between males and females that lead to an increased likelihood of ACL injuries for women. Specifically, the incidence of occurrence of an ACL injury when looking at numerous different sports has been reported to be 4.6 to 1, female to male. The first difference is anatomical, whereby the female skeleton is built to have a wider pelvis, resulting in an alignment issue at the knee. This alignment issue forces the knees to be slightly “knocked kneed.” Inside the knee joint itself there is a notch at the end of the thigh bone (femur), which has been found to be narrower. This narrowed notch can lead to rubbing on the tendon against the bone forcing it to fray over time and eventually give way.

When it comes to muscle control about the knee, women have been found to have less developed muscles of the thigh. The hamstring muscles (back of the thigh) should be at least 66% the strength of the quadriceps (front of the thigh) in order for muscle balance to exist. Some suggest that the hamstring to quadriceps strength ratio to be as high as 75% in order to prevent injury. Studies have demonstrated that most women are around 45%, but have been reported as low as 25%. Also, women have demonstrated a slower contraction of the muscles about the knee.

Another factor that increases the injury likelihood in the female athlete is the way women land from a jump. Women tend to demonstrate a pattern of being more upright when jumping. The knees and hips do not bend as much to absorb shock when landing. Men have been shown to have three times more bend in the knees and hips to absorb the shock when landing from a jump. This biomechanics of jumping that is commonly found in females leads to an altered pattern of muscle stabilization about the knee that places more stress on the ligament. Landing in a position where the knees and hips are fairly straight leads to increased quadriceps activity and decreased hamstring activity. These unbalanced forces about the knee can place increased stress on the ACL.

A final gender difference to note when dealing with ACL injuries is the female menstrual cycle. Due to the fluctuations in hormone levels in the female body related to the menstrual cycle, the ligament stiffness actually changes. Normally the male ligament is almost twice as stiff as the female. During the first nine days of the cycle, the ligament has been shown to be 150% more loose than normal. Yet more injuries are expected to occur between day 10 and 14 because of a surge in estrogen and the hormone Relaxin. The specific role of this hormone is to relax ligaments to prepare the body to accept a new life. These changes in the ligament stiffness return to normal around day twenty-four of the average twenty-eight day cycle.

What all of this means is that the female athlete has to be handled differently from the perspective of strength and conditioning, injury prevention, as well as rehabilitation from an injury or following surgery. Early sport-specific training programs have been found to be effective in developing proper muscle balance, flexibility, and motor control. This type of program can be developed by a physical therapist, either in prevention of an injury or in recovery following an injury. By doing so flexibility, muscle imbalances, and biomechanical faults can be identified and addressed. When it comes to the rehabilitation of a female’s knee injury, the necessary strength and biomechanical concerns need to be addressed in order to prevent a re-injury on the same side or injuring to the other knee.