By Gavin Buehler
Ski season is upon us, and unfortunately every season we see knee injuries. A torn Anterior Cruciate Ligament (ACL) is an especially popular injury among skiers, so this month we’ll look at how we can reduce the risk of the infamous ACL injury.
In nearly every ACL rupture case I’ve seen (with the exception of people having their legs swiped out from under them by another object or body) there has been a severe strength imbalance between the quadriceps (front thigh muscles) and hamstrings (back thigh muscles). Studies have shown that ACL injuries are far less common in athletes that have a strength ratio where the hamstrings have 80% or greater strength of the quadriceps. Below that ratio the injury incidents escalate and become greater as the strength difference between the two muscle groups increases. Studies have also recommended that for day-to-day activities a minimum of 50% hamstring to quadriceps strength is needed to maintain a healthy ACL ligament. Of course, skiing or any sport for that matter, even if you do them daily, are not considered day-to-day activities. Personally, I believe 50% to be grossly low for any population as it leaves no margin for error. If you are active in any sport, I’d highly recommend getting your hamstrings up to at least 80% of the strength that your quadriceps have.
“But how can we find out our ratio?”
The easiest to it yourself method is to go to your local gym that has a leg extension and leg curl machine. Find out how much you can lift with your leg extension, and then do the same for your leg curl. Divide your hamstring weight into your leg extension weight. If your number is between 0.80 and 1.00, you’re doing well. Unfortunately, most of you won’t be in that range.
Tips on this method… I recommend doing it one leg at a time. You should also be able to keep your toes pointing straight up or down for each movement. With the leg curl, if your toes rotate out as you curl, you are dominant with your lateral hamstring (biceps femoris). Lower the weight so that you can keep your foot straight. (We’ll touch on how this affects things a little later.)
Why is this ratio so important? Well, your quadriceps cross over your knee joint in the front and attach to the front of your tibia (shin bone). Your hamstrings cross over the back of your knee joint but only the semimembranosus and semitendinosus (medial or closer to the inner thigh) muscles attach to the tibia, while the biceps femoris (lateral or outer thigh) hamstring attaches to your fibula (the smaller bone on the side of your shin). These muscles counter act one another and help the ACL to keep that shin bone aligned with the thigh bone. In a bent knee position such as in skiing if your quadriceps over power your hamstrings the shin bone will glide forward putting shear force on the ACL. With skiing, the strength of the quadriceps is amplified due to the fact that your foot is anchored down to your ski, and you can lean back and create more force from the leverage. With this much strain on your ACL a small catch of your edge or hitting an unexpected rut can push it past its limits. But here’s another factor to consider. Most people tend to be dominant with their biceps femoris (outer) hamstring, which doesn’t attach to the shin bone. This one-sided dominance now creates a twist on top of the shear force putting even more strain on the ACL.
In this video I explain how this muscular imbalance impacts the knee while providing a little more of a visual.
As always, this video is for educational purposes only. Please consult a health professional before attempting new exercises, as the following suggestions may or may not be appropriate for you.