The Hamstrings and Anterior Cruciate Ligament (ACL) are extensively intertwined and very dependent upon each other, when the body is functioning correctly. Research has demonstrated that athletes who are quad dominant are at much higher risk for an ACL rupture. This leads us to training the hamstrings to unload the ACL and protect it from injury.
To understand how this all works, we first need to remind ourselves what the primary role of the ACL is. The job of the ACL is to keep the Tibia in the lower leg from translating anteriorly (forward). If the tibia translates anteriorly past the point at which the ACL can control it, the ACL ruptures, leaving the athlete debilitated and in need of surgery.
When the quads fire, they tend to cause anterior translation, especially when the knee is bent. If you think about the origin and insertion of the quads, it makes perfect sense. The quads originate on the pelvis and Femur and come down to insert on the anterior aspect of the tibia at the tibial tubercle. When the muscle shortens with the knee bent, it wants to pull the tibia forward, thus stressing the ACL. This is why the vast majority of orthopedic surgeons prohibit their patients who have had their ACL reconstructed from using leg extension machines following surgery.
Conversely, the hamstrings can protect the ACL by firing at the right time to prevent anterior translation of the tibia. The hamstrings originate on the Ischial Tuberosity on the posterior (back) of the pelvis. They follow the backside of the femur and insert on the tibia in the lower leg. When the hamstrings fire and shorten, they pull the tibia posteriorly and back into the knee joint, thus unloading the ACL.
Ligament vs. Muscle Dominance
Research has described some athletes as being ligament dominant vs. muscle dominant. Athletes who are ligament dominant depend almost solely on the ligaments in the joints for stability. It is much more advantageous for the long term health of the athlete to be muscle dominant. These athletes depend upon the muscles surrounding their joints for stability and integrity, thus leaving their ligaments as a backup to provide stability in a worse case scenario.
The key to training to prevent Quad dominance is teaching the hamstrings to fire at the proper time and in the proper sequence. This is accomplished by teaching proper squatting technique, proper landing from a jump, and not overtraining the quads. For more information on this visit our page on injury prevention.
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