Football, of course, is frequently the source of an ACL tear. Football combines the activity of planting the foot and rapidly changing direction AND the threat of bodily contact. Downhill skiing is another frequent source of injury, especially since the introduction of ski boots that come higher up the calf. These boots move the forces caused by a fall to the knee rather than the ankle or lower leg. The ACL injury usually occurs when the knee is forcefully twisted, or hyperextended.
Many patients recall hearing a loud pop when the ligament tears, and feel the knee give away.
There has been a dramatic increase in the number of females who suffer an ACL tear. This is in part due to the rise in women's athletics, but studies have shown that female athletes are more likely to suffer this injury when compared to their male counterparts. It is uncertain why this is the case. Initially, it was thought that females were at higher risk because of differences in training intensity. But more evidence suggests that there may be a difference in the anatomy of the female knee, or the female ligament may not be as strong due to the effects of the female hormone estrogen.