Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament, or “ACL”, is one of the four major ligaments that support the knee joint. The knee join is the junction between the thigh bone, (femur) and then shin bone (tibia).

The characteristics of a ligament are twofold:

  • It needs to be elastic so that the bones can move apart to allow the different knee functions
  • It needs to be strong to prevent excessive motion which can cause pain, increased wear and tear, or improper alignment

Ligaments should not be confused with tendons, which attach muscle to bone. We can control the tightness of a tendon by stretching or shortening the muscle to which is attached. We do not have control over ligaments. They are short elastic tissues, which connect one bone to the next. When there is excessive quick motion at the joint, the ligament is stressed.

In this case, the ligament may not be able to hold. Any injury to a ligament is classified as one of the three types: *These types are only guidelines

  • Grade I – Stretching the ligament. This is treated with rest, ice and proper strengthening.
  • Grade II – Tearing of a significant number of ligament fibers. Present studies indicate that surgery may be the preferred therapy if the patient wishes to return to sports.
  • Grade III – Complete rupture of the ligament. This would require surgical repair for the person to return to high-level activity.

The function of the ACL in particular is to prevent the shinbone from moving too far forward. If this happens, it feels as if the knee is slipping, sliding or bucking. It is important to avoid overstressing an injured ligament. Jumping, quick changes of direction, sprinting, or twisting all put increased stress on the ACL. If the feet and lower leg are planted and the upper body is twisted or has sufficient momentum, the force can “pry” the bones apart and rip the ACL. A second way of injuring the ACL is by kicking or using the quadriceps muscle of the thigh forcefully.

This muscle attaches to the front of the shin bone and therefore, a forceful contraction of this muscle can pull the bone forward. The forward movement of the bone stresses the ACL and can result in a stretching, partial, or complete tear of the ligament. This situation is especially stressful when the leg is approaching a straight position (as at the end of the forward kick of the Nautilus leg extension machine).

Therapy for ACL injuries is aimed at restoring strength to the knee without stressing the ligament until it is sufficiently healed. Movement early in rehabilitation is important in order to:

  • Decrease Swelling
  • Decrease pain
  • Increase flexibility
  • Increase strength
  • Increase circulation and nutrition to cartilage and knee ligaments for development It is crucial, however to follow guidelines set up by trained personnel.

Pain cannot always be the guide. In many cases, people progress too quickly because the knee feels good. This may be overstretching the ligament, which can lead to looseness of the joint, which may in turn lead to the possibility of surgery. If the quadriceps muscle of the thigh pulls in the direction, which the ACL is trying to prevent, then there must be another muscle which pulls along with the ACL. These are the hamstring muscles in the back of the thigh.

For this reason, a strong hamstring is very important because it will help to check the excessive motion which cannot be handled by the injured ACL. Hamstring strengthening can be done early in the rehabilitation process because it supports and does not stress the ACL. In the normal knee, the quadriceps muscle (at the front of the thigh) is usually 33% stronger than the hamstring (at the back of the thigh).

In a person with ACL damage, however, the strength of the hamstring should be close to equaling that of the quadricep. This decreases the looseness of the knee joint. In summary, remember the following guidelines when you have injured the ACL:

  • Avoid jumping, twisting, turning, quick changes of direction or kicking motions for a sufficient length of time.
  • If you have been immobilized, stick to guidelines and do not move the knee into restricted positions even if you can do so without pain.
  • Early movement and exercises for the other structures in the leg will promote better recovery but must be done within specific guidelines.
  • Hamstring strengthening can be done early and will always be an important factor.
  • Improper movement of weak muscles can lead to increased looseness of the knee joint which can in turn lead to future ligament damage or cartilage irritation (an “arthritic knee”).