When a woman becomes fatigued, as during an athletic event, the angle on landing becomes more pronounced, further increasing the chance of injury.
A band of fibrous connective tissue called the anterior cruciate ligament (ACL) passes through a “notch” in the lower end of the thighbone that forms part of the knee. The ligament is about the same size in women and men. But the notch is narrower in women. “That makes the woman’s ligament more susceptible to tearing,” Evans says.
A woman’s hamstring muscles, at the back of the thigh, are often weak compared with her quadriceps, the muscles at the front of the thigh. The quadriceps pull the bones of the lower leg forward, and the hamstrings pull them back. Hamstring muscles help protect the ACL from injury. When the pulling power is out of balance, the knees suffer.
The muscle imbalance tends to be far worse in women. “The imbalance may begin to happen during childhood if girls engage in less physical activity,” Slenker says. “Exercise can help overcome the problem.”
Risk for injury to the ACL appears to be higher during ovulation, when estrogen levels are highest.