The ACL is the ligament that helps keep your knee stable.
Now the American Academy of Pediatrics has some proven ways athletes can reduce their risk of injury in the first place.
It has happened to the best, Lindsey Vonn, Misty May Treanor and Rebecca Lobo, and many more young girls as young as 8 are tearing their ACL's, totally blowing out their knees.
Caitlin Young, who played Lacrosse at UNC, tore her ACL after college and cannot believe how many young girls get hurt so severely.
"Does it break your heart? Yes!! So upsetting to see it happen and to girls so young -- just saw it happen to a freshman," she said.
Experts say this is happening for two reasons. More girls are playing sports and staying with the sport year round instead of cross training.
"The way the girls jump and land is different from the way the guys land -- that' a big reason we are seeing the injuries," said Dave Flower with Chelsea Piers in Connecticut.
Dave Flower says strengthening the entire body, core and the backs of legs is the key. Number 1, learn how when jumping to land straight down do for a minute. Number two, do one leg squats for one minute each side, slowly and with good form without knees going out over feet. Number 3, practice walking lunges again one minute.
This is bounding knees up -- go for strength and control. Then do lifts, squeezing from bum to knee.
Chelsea Piers, Connecticut has just started a program to prevent ACL tears. Make sure trainers and coaches near you are doing the same.