ACL PART 1: MECHANICS

Moving Your Knee: The Muscles

The hamstrings, the "quads", and the gastrocnemius (the bulky calf muscle at the back of the lower leg) are the primary movers of the knee joint.

There are, of course, other muscles that work at the knee joint to help rotate it, bend it, and prevent the joint from moving out of place [Figures 1,2]. Muscles like the popliteus at the back of the knee that rotate the knee [the femur (thighbone) outwards or the tibia (shinbone) inwards], the gracilis* and sartorius muscles on the inside of the knee that help to bend it, and the tensor fascia latae muscle operating at the outside of the knee by way of its long iliotibial tract or band to help stabilize the knee when it's straight.
*May also rotate the lower leg inward.

Keeping Your Knee In Place: The Ligaments

There are four main ligaments in the knee. Among the main four - the Anterior Cruciate Ligament (ACL), Posterior Cruciate Ligament (PCL), Medial Collateral Ligament (MCL), and Lateral Collateral Ligament (LCL) - the two most likely to become damaged in sports like football are the ACL inside the knee and the MCL on the inner side of the knee.The other two ligaments, the PCL located inside the knee behind the ACL and the LCL, a rounded linear band on the outer side of the knee, are injured less frequently.

These are, however, not the only ligaments located in or at the knee. There are also ligaments that attach to the meniscus like the meniscofemoral ligaments (from the lateral meniscus to the thigh bone) and meniscomeniscal ligaments (attaching one meniscus to the other meniscus).Some scientists even say that there are small ligaments, meniscotibial or coronary ligaments, that attach the menisci to the tibia bone (the larger of the two leg bones).

The Quadriceps: Tendon or Ligament?

The "tendon" of the quadriceps muscle group ("quads") is a tissue with many names [Figure 3].

- Above the kneecap it is consistently called the quadriceps tendon, which attaches from the quadriceps muscle group to the kneecap (patella). A tendon is a tissue that attaches muscle to bone.

- After it passes the kneecap, it is most often labeled the patellar ligament -- as it extends from the kneecap to a bump at the front of the tibia called the tibial tuberosity. That's because many believe it attaches from one bone, the patella, to another bone, the tibia. A ligament attaches one bone to another bone (though as we saw above there are some exceptions).

- Others, however, believe that the kneecap is a "floating" bone that lies within the tendon, acting as a fulcrum point (like at the center of a seesaw) to improve the quadriceps ability to straighten the knee. Without it, the muscles cannot straighten the knee all of the way.* These people often call this extended segment of the quadriceps tendon from the kneecap to the tibial tuberosity, the patellar tendon. This location is where players can get what we call patellar tendonitis (inflammation of the patellar tendon).

* Some people can actually straighten the knee past neutral into what is called hyperextension.

- At the same time, there are people who just refer to the whole tissue from the quadriceps muscles to the tibial tuberosity as either the quadriceps tendon or the patellar tendon ... it can get confusing!

The "Double Pop" Mechanism

Studies have shown that 75% of players who suffer an ACL tear do so during what are called non-contact injuries (injuries that happen without the player coming in contact with another player or object). Experts, in fact, find that the ACL is damaged most often as a player lands on one leg or when he or she changes directions on the pitch.

In the picture above, as the player plants his foot on the ground, the knee quickly collapses inward toward the other knee (within 40 milliseconds of the foot hitting the ground) while the tibia rotates inward at the knee in relation to the femur. This is where the ACL is believed to tear. Afterwards, the knee often rotates outward (in the opposite direction) at the tibia, which may happen because the knee suddenly becomes unstable after the ACL is torn. This pattern may help explain the feeling or sound of a "double pop" common to ACL injuries.*

*see Michael Owen's Injury at http://www.youtube.com/watch?v=WU2I4OlAZzA

Smack!: Collision Injuries

Players can also injure an ACL through contact with another player or through unchecked contact with the ground. While it may occur with the knee in any position, overly bent and overly straight knees are the most common. Uncontrolled rotation of the knee, too far in either direction, will also have an effect.

One contributing factor to ACL trauma may be the absent protection from the surrounding muscles. During contact injuries, muscles are often either relaxed or working in another fashion when the injury happens (let's say, trying to pivot the leg on the ground rather than helping to protect the ACL). This can allow contact to tear the ligament, unopposed.

Other times the forces that are placed on the knee are simply too great and the ACL tears just because it happens to be in the way.

"I Just Tore My ACL": What to Expect

When an ACL tears, there may be immediate pain or just a feeling that something "very wrong" just happened, like the knee moved in a way that it shouldn't move.

Blood vessels that surround the ACL are usually torn as well.This can cause significant swelling and releases blood into the knee leading to the discoloration that may show up on the skin surface like a bruise, hours or even days after an injury.

In complete ACL tears, it's also common to feel that your knee is now unstable, though muscles that start to guard the knee after a few hours may mask the sensation. Over time, however, the knee may start to buckle with even simple activities, like when you're turning around in the kitchen or walking down a flight of stairs. These motions often do not require a lot of muscle use at the knee to perform and instead rely on the passive tissues (like ligaments) to support the knee (i.e. when walking down stairs, some people let the thigh muscles relax, letting gravity take hold of the descent). If the ACL is torn, that passive support is not there and that's why you'll notice a problem. Yet when you run in a straight line, the muscles work better to control the knee (and the ligament - even if it had not been completely torn -- does less work in this motion) so you may not notice any instability at all.

The ACL Injury: It Might Not Be Alone

Not all knee injuries result in damage to the ACL. Yet knee injuries that do involve the ACL often involve other knee tissues at the same time. The most common tissues injured with the ACL are the MCL (Medial Collateral Ligament) on the inside of the knee and either of the two menisci (the dense C-shaped pads of cartilage) on both sides of the internal knee joint. In other words, ACL injuries do not always happen alone; other tissues may also be damaged.

Is The ACL Tear A Female Injury?

Well, no. But it does appear to be more common in female players than in male players. Still, the numbers seem to vary among sources. Current indications are that the number is somewhere between 4-8 times more often in female than male football players.

ACL Treatment Options (PDF): CLICK HERE>>

Part 2: What To Do

In ACL Part 2: Prevention we will discuss the areas players should address in their workouts that appear to help prevent ACL tears. We will also provide you with resource information for proven exercise programs that coaches, trainers, and players can start using right away—to help put players on the desired course toward injury prevention.

ACL Part 2: Prevention: CLICK HERE >> 

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