The ACL measures between 31 to 35 mm in length and is believed to withstand almost 500 pounds of pressure before failure or tearing. Normal activities apply about 100 lbs of force on the ACL. The ACL can also withstand up to 25% of lengthening without failure in younger and healthy adults.
Hyperextension of the knee and the medial rotation of the tibia apply the greatest amount of force on the ACL. These same positions also place the ACL at highest risk for tearing or rupture. Hyperextension of the knee is the excessive straightening of the knee that forces the tibia excessively forward in relation to the femur. Medial rotation of the tibia is the excessive inward turning of the tibia.
Non-contact Injuries vs. Contact Injuries
When the ACL sustains forces beyond its capacity, tearing or rupture of the ACL occurs. The injury is commonly incurred from one episode, although repetitive strain to the ACL may also cause the injury. The ACL is injured mainly through two mechanisms of injury: non-contact and contact.
Non-contact injuries
The most common mechanisms of ACL non-contact injuries include:
1. Planting and cutting - abrupt or sudden alteration in direction or speed with the foot fixed on the ground
2. Straight-knee landing - the foot strikes the ground with the knee straight
3. One-step-stop landing with the knee in hyperextension - the leg abruptly stops as the individual with the knee in excessively straightened position
4. Pivot-shift - rapid slowing down followed by a quick turn in one direction with the feet securely fixed on the ground; the most common mechanism of injury
In these situations, the ACL injury results from the application of indirect forces to the knee. And so, these injuries are classified as non-contact injuries. Non-contact mechanisms occur in 70 to 80% of ACL tears. The pivot-shift mechanism is the most common non-contact injury. The incidence of non-contact ACL injuries in female athletes is between 4- and 6-fold higher than in male athletes.
Contact injuries
The ACL is also injured through direct force or blow to the front of the knee and through collisions that force the knee to hyperextend. Contact or traumatic mechanisms usually cause injuries of the surrounding structures. In addition to ACL, the medial collateral ligament and the medial meniscus are also damaged. Next to pivot-shift mechanism, contact injuries are the most common mechanism of ACL injuries.
Female Susceptibility
So why are women more likely than men to incur ACL injuries? Some of the reasons are biological: for example, a wider pelvis, less muscle strength, a shorter ACL, greater knee laxity; and some are simply that women are more vulnerable in executing the movements that typically lead to ACL injuries: for example, jumping and landing.
To mitigate these traits and increase knee strength, women should include agility drills and plyometrics in their training programs. Women should include exercises that strengthen the knee and the hamstrings, such as leg presses, squats and lunges. Learning to use the hamstrings instead of the quadriceps through muscle control techniques is also important for women. This can be done with OKC (open-kinetic chain) exercises.
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