ACL injuries don't just sideline pro-athletes, they can happen to almost any active person. All it takes is a sharp turn, a quick stop, or a missed-step to do damage. Your ACL, or anterior cruciate ligament, runs through the middle of your knee joint. Its job is to connect the upper part of your leg to the lower part. More specifically, it stabilizes your knee and keeps your tibia or shin bone from moving out in front of your femur or thigh bone and hurting your joint. When your knee is forced in an UNNATURAL position, it puts pressure on the ligament that it's NOT designed to handle. Sometimes this can cause a small ACL tear...or, in more severe cases, the ligament will break COMPLETELY in half or pull away from the bone--taking part of the bone with it. The telltale sign of an ACL injury is the POPPING sound it makes as the ligament tears. This is usually combined with INTENSE pain and instability of the knee to the point where it feels as if it's giving out. It'll become almost impossible to stand. For hours afterward, swelling in your knee will continue to grow. If you suspect you've hurt your ACL, do NOT try to move your leg. It can only do more damage. Instead, splint your knee to keep it from bending until you've see a doctor for X-rays. For minor injuries, treatment includes ice packs, painkillers, and knee splinting. You'll usually need physical therapy to re-strengthen the knee muscles and to regain full range of motion. Surgery and rehab is almost always necessary in severe cases. Because the ACL can't be stitched back together, a small piece of tendon from another part of your leg is used to replace it. The full healing process can take 4 to 6 months or even up to a year for some people. While you can't protect yourself 100 percent from an ACL injury, if you're an athlete or an avid exerciser, it's worth altering your training to include leg muscle building and balance exercises. To learn more about other common joint injuries, watch the rest of the videos in this series.