The anterior cruciate ligament (ACL) is one of a pair of ligaments in the center of the knee joint that form a cross,
and this is where the name "cruciate" comes from. There is both an anterior and a posterior cruciate ligament (PCL).
Both of these ligaments function to stabilize the knee from front-to-back.

In medical terms, the ACL is the primary restraint to anterior displacement of the tibia on the femur. This means that
when the ACL is injured, the shinbone can slide forward on the thighbone, causing the knee to "give way".

What happens when the ACL is torn Most people who rupture their ACL can recall the exact moment at which they felt
it pop. This usually happens when changing direction quickly in pivoting or cutting sports like soccer, landing from a
jump in sports such as basketball, or falling while skiing. The four "classic" symptoms that people may feel when they
tear their ACL are:

- They hear a "pop" from inside the knee.
- They feel the knee give away at the time of injury.
- They develop a swollen knee immediately, or within a few hours.
- The pain is bad enough that they can not continue play that day.

Most ACL injuries are non-contact related: running and cutting sharply in a more erect posture than in men, landing from a jump
without bending the knees sufficiently, and playing on surfaces with a high coefficient of friction. The level of skill and
experience of the athlete is always of significance. Of course, contact injuries like a direct blow to the knee from the rear or side
can cause a tear in the ACL. The tear usually occurs in the mid-portion of the ligament which is the weakest part.

Commonly, the athlete runs, suddenly stops, and then turns, thereby causing a deceleration of the lower limb, a forced
hyperextension of the knee, or a forced tibial rotation. Other mechanisms include an internal rotary force applied to a femur
on a fixed weight-bearing tibia, an external rotation force with a valgus [outward] force, or a straight anterior force applied to the
back of the leg, forcing the tibia forward relative to the femur.

An audible pop often accompanies this injury, which often occurs while changing direction, cutting, or landing from a jump
(usually a hyperextension/pivot combination). Within a few hours, a large hemarthrosis develops. Patients usually are unable to
return to play, secondary to pain, swelling, and instability or giving way of the knee.

When the ACL ligament is stretched too far and ruptures, the blood vessels inside the ligament rupture as well, and this blood
fills the knee joint, causing the knee to swell. While the ACL injury itself is usually not terribly painful, the swelling that results
from blood inside the knee prevents most people from returning to the game or the athletic activity they were involved in at the
time of injury. Examining an athlete immediately after they have had a knee injury can sometimes be difficult and it is often not
possible to tell what structures, if any, have been injured. This is because the quadriceps and hamstring muscles "guard"
the knee, preventing the examiner from testing the ligaments accurately. A better examination can usually be obtained after the
swelling has gone down and the pain from the injury has subsided. Although accurate diagnosis of the exact injury can be difficult,
it is relatively certain that an athlete who develops a swollen knee immediately after an injury should not return to play and should
seek medical evaluation.

Most surgeons prefer to wait until at least three months have passed before allowing their patients to return to agility training
exercises. When to return to competitive or recreational sports after an ACL reconstruction is a difficult decision. The timing of
your return will depend upon how well you have done with your rehabilitation, how solid your knee feels to you, and what you and your
surgeon decide is reasonable for you. Even after you return, it is important that you pay special attention to keeping your quadriceps
and hamstrings in good shape, as this is considered an essential part of protecting the ACL.

KNEE INJURY PREVENTION
1. Proper leg muscle strength and flexibility training as well as core training.
2. Proper neuromuscular (balance and speed) training. A proper training program would include squats, lunges, stiff-leg deadlifts,
hamstring curls, and different types of jumping movements. These jumping movements would progress from:

-Slow jumps landing on two legs in one plane of motion.
-Slow jumps landing on two legs in multiple planes of motion (front to back, side to side, and rotation).
-Slow jumps landing on one leg in one plane of motion.
-Slow jumps landing on one leg in multiple planes of motion.
-faster jumps landing on one or two legs in multiple planes of motion.

3. Proper coaching on jumping and landing and avoiding any straight knee landing.
4. Proper footwear and orthotics if necessary.