Hamstring-ACL
Interaction
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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