Did you know that the strength of the muscles around the knee have a LOT to do with helping you prevent an ACL tear? Let us go a little into the anatomy and you’ll understand why.
The knee is an inherently unstable joint.. it’s basically a femur (thigh bone) stacked on top of the tibia (main lower leg bone)… and that’s about it. Just kidding… kind of. That really is the basic structure. The fibula is another lower leg bone that has a ligament attachment, but is not weight bearing at the knee, and the patella (knee cap) has benefits for strength mechanics, but doesn’t provide any structural support. There is something called a “screw home” mechanism, which occurs when the knee is completely straight and the tibia locks into the femur, but the only structural advantage to that is when the knee is straight.
You may be wondering how the knee just doesn’t randomly fall apart at this point (with good reason!), and that is where the ligaments and muscles come into play to prevent an ACL tear.
The major ligaments:
- ACL: anterior cruciate ligament. Simply, it goes from the back of the femur to the front of the tibia and it is the main ligament to prevent the tibia from gliding too far forward on the femur.
- PCL: posterior cruciate ligament. Simply, it goes from the front of the femur to the back of the tibia and it is the main ligament to prevent the femur from gliding too far forward on the tibia.
- MCL: medial collateral ligament. This lies on the medial aspect of the joint from the femur to the tibia and supports the inner knee.
- LCL: lateral collateral ligament. This lies on the lateral aspect of the joint from the femur to the fibula
- Medial meniscus: a medial cushion between the femur and the tibia
- Lateral meniscus: a lateral cushion between the femur and the tibia
The major muscles:
- Quadriceps: 4 muscles – rectus femoris, vastus lateralis, vastus medialis, vastus intermedius. They generally attach from the front of the femur to the patella (then the patella attaches to the tibia. Together, they extend the knee; the vastus lateralis helps to stabilize the knee laterally. Without support from the posterior structures, they would pull the tibia forward on the femur.
- Hamstrings: semimembranosus, semitendinosis, biceps femoris. They attach from the back of the femur to the back of the tibia. Together, they flex the knee. Without support from the anterior structures, they would pull the tibia back on the femur.
- Gracilis: runs from the pelvis to the front of the tibia. It flexes the knee the internally rotates the tibia
- Gastrocneumius: it runs from the back of the femur to the calcaneus (the heel bone). It assists the hamstrings with knee flexion.
As we’re sure you can tell, without the ligaments or the muscles, the knee would not be much of anything. Also! With too much from certain muscles groups and not enough from others, you might be able to see how imbalanced the knee can get. Case in point: strong quads and weak hamstrings can lead to such a gross imbalance that the ACL is often overly used to prevent the anterior translation of the tibia on the femur. What do we mean by that? Essentially, because of the line of pull of the quadricep (and it’s tendency to be the strongest of the major muscle players, it can pull the tibia forward on the femur. If you remember, the ACL’s job is to prevent this forward translation, but it is only designed to perform about 85% of the work. That other 15%, in theory, should come from the hamstrings. If the hamstrings are weak, which they often are, that means that poor little ACL has to do more work than it’s actually designed for! Imagine that you and your coworkers are only designed to do your specific jobs… if someone at work left and your company decided that instead of hiring a replacement, they would give all of you and your coworkers additional work that you weren’t designed for… over time, you would all feel stressed and overworked and may just snap – that’s EXACTLY what can happen to an ACL if it is overly taxed and doesn’t have the correct support system.
Moral of the story: strengthen your hamstrings to prevent an ACL tear!
-Dr. Alexandra Terpos-