If you have been around weight lifting or strength training for any length of time and have been exposed to the squat in any way, there’s a high chance that you have heard the statement ‘don’t let your knees go past your toes.’
The origin of this antiquated cue is largely unknown, but it continues to persist into coaching and therapy today. Is this claim founded on sound evidence? Or is it a similar narrative along the lines of you shouldn’t deadlift because it is bad for your back?
Keep reading to find out what the research has to say, what my personal beliefs are, and what the real-world looks like regarding this topic.
Background
As far as I’m aware, the reasoning behind telling someone to not let their knees go out in front of their toes is to limit shear forces and joint stress that will lead to 'injury'. This thought process might have come from a purely anatomical place and what the ligaments look like from a cadaveric standpoint. At the time that this cue started to become mainstream, the technology to actually test the stresses on the ligaments and the knee joint itself was not available.
Some people in the field attribute this cue’s origination from the cue to ‘keep the knees over the second toe’ which is regarding the side-to-side movement of the knee - that is, you don’t want the knee collapsing inward under load.
Regardless of where it came from, we now have the technology to understand these forces and how all of these tissues and the joint interact synergistically during the complex movement of the squat.
Let’s dive into some of the research.
The Research
Ligaments
The first article to discuss is one that looked at the kinetics and kinematics of the squat. This review included 79 studies that analyzed the joint kinematics and kinetics at the ankle, knee, hip, and spine. For this review, there was not a standard established as to how the squat would be performed, but due to the high variance in squat technique the researchers opted for analyzing the data across all of these studies to average out the findings and generalize the information a bit more. That being said, there is a lot to unpack here since this article goes through all the major joints involved in a squat, however, for the purposes of this blog article, we will focus on their findings for the knee. Going back to the claim that anterior translation of the knee increases shear forces on stabilizing ligaments, let’s look at the numbers that this review found.
First, let’s talk about the anterior cruciate ligament (ACL). This is the most common ligament that people express concern with when it comes to the squat - and probably for a good reason since the ACL is responsible for most of the stabilization coming from ligaments. The research found that peak ACL shear forces occur between 15-30° of knee flexion during the squat - which is a pretty small range of knee flexion. At peak shear forces, the ACL is only under 6% of the maximal load it can take in a young, healthy person’s ACL. I think we can all agree that 6% percent is not a lot as far as numbers go, so based on that, you might reason that there really isn’t that much shear force on the ACL during the squat.
Now, there are many outside factors to consider such as a prior history of knee injuries - ACL rupture, meniscal injuries, etc. and this study uses the phrase “young, healthy person’s ACL” which we could say doesn’t apply to most people.
I would agree with that, it certainly does not apply to most people, but with the insignificant amount of shear force found in this research, it stands to reason that someone would have to have an extremely deficient ACL to sustain an injury when compared to control. We will further discuss some of the real-life examples of this later on in the article.
Now, on to the posterior cruciate ligament (PCL). This article found that peak shear force on the PCL is roughly 50% of the maximal force a young, healthy person’s PCL can handle - the same rules apply here as with the ACL.
While this number is higher than that of the ACL, 50% still leaves a lot of room to work with before injuries and disruption of the PCL occur during squatting.
The other thing to keep in mind is that, much like muscle tissue, connective tissues such as the ACL and PCL (and other ligaments) adapt favorably to progressive resistance training, by increasing their tolerance to stress.
Patella
Another common concern with anterior knee movement during the squat is the stress/force that is placed on the patellofemoral joint (the kneecap and its relation to the tibia and femur). Many people link increased stress on this joint to degenerative changes in the cartilage and knee pain in the long-term. The next study addresses the stress placed on the patellofemoral joint and the possibility of degenerative changes due to this stress.
What the researchers found is that forces on the patellofemoral joint were greatest at 90° of knee flexion. After 90° of flexion, what the researchers call the “wrapping effect” occurs, and the posterior thigh and proximal calf come into contact with one another and distribute these forces more evenly throughout the knee.
What can be taken away from this is that patellofemoral stress does not occur more or less due to the anterior translation of the knee, but instead, occur due to the amount of knee flexion. That being said, with some anterior translation of the knee, you will reach - and exceed - 90° of knee flexion sooner which means that you begin to take advantage of that wrapping effect that the researchers described.
Furthermore, when it comes to degenerative changes, the researchers found that this additional stress of the patellofemoral joint does not contribute to an increase in degenerative changes. The caveat being that a proper training progression is utilized and that other major faults in technique are not present.
Knee and Hip Torques
The last point I want to address has to do with torque forces at the knees and hips during the squat. Torque in this context can be thought of as overall forces/stress throughout a joint. A research study that examined the differences between a restricted squat - the knees were blocked by a wooden wall and not allowed to go past the toes vs. an unrestricted squat where the knees were allowed as much anterior translation as naturally occurred for each of the test subjects. Let’s look at the numbers.
First, the knee: In the unrestricted squat, the torque forces were higher when compared to the restricted squat: 150.1 +/- 50.8 Nm vs. 117.3 +/- 34.2 Nm, respectively. First glance at this and you are probably thinking, “well that says it all, there is more stress on the knee when it moves anteriorly past the toes,” however, this isn’t the whole picture. When we look at the torque forces at the hip, the results might surprise you. When comparing the unrestricted squat to the restricted squat, the torques during the unrestricted squat were significantly lower than in the restricted squat: 28.2 +/- 65.0 Nm vs. 302.7 +/- 71.2 Nm, respectively.
What do you think now?
At face-value, the knee forces DO increase during the unrestricted squat. But at what cost? The torque forces at the hip during the restricted squat increase by almost 1,100%!
And that’s not it, with the restricted squat, forward trunk lean increased placing additional shear forces on the joints of the spine. So if you want to decrease the forces placed on the knee at the cost of putting more stress on both the hips and the spine, then keeping the knees behind the toes is the squatting method for you. However, if you want a more evenly distributed squat - that is generally more comfortable and allows the lifter to move through a greater range of motion - then allowing some anterior translation of the knees serves as the preferred squatting method.
Discussion
Now that you have been presented with what the research says regarding anterior translation of the knees, what do you think?
Keep in mind that there is certainly more research out there on the subject but for the purposes of this article, I chose to only discuss the articles mentioned above in detail.
If you want more research and details regarding this subject, I highly recommend checking out this article from the Strength and Conditioning Journal (a pretty good place to discuss something such as how to squat, I’d say).
To sum up, the squat technique that they recommend is as follows:
“A full-depth squat, with a natural foot position, approximately shoulder-width apart, with unrestricted anterior movement of the knee, an upright trunk, with a forward and upward gaze is recommended.”
Personally, I think their recommendation when it comes to anterior displacement of the knee is spot-on and leads to a better overall squat. Me, being me, I don’t necessarily agree with their recommendation as far as “forward and upward gaze”, but that’s for another time. So we’ll just stay focused on the knee.
Real-Life Squatting
If the research still doesn’t have you convinced, then I felt that it would be beneficial to include some real-life examples of squatting so that you can see for yourself what the knee does in high-level athletes.
Powerlifting
If you are able to attend a powerlifting meet or look at videos/pictures of powerlifters in competition - squatting the most weight that they can at that point in time - you will likely see a variety of squat techniques, anterior knee movement, and a lot of strong men and women.
These are some of the strongest people in the world and they are squatting with anterior knee movement while under serious amounts of load. They didn’t get to that point overnight, they have had to train for a long period of time before getting to the point where you get to see them execute a competition squat. Meaning, their knees have moved anteriorly during every rep of squat in their training. Yet, they don’t usually have ruptured/compromised ACLs or PCLs, and for the most part, they don’t have complaints of pain due to degenerative changes.
Here is a picture of Layne Norton, PhD in nutrition science and former powerlifting competitor performing a squat:
Olympic Lifting
Same as above, if you are ever able to observe an olympic lifting meet, you will most certainly see anterior knee translation when the athletes are squatting - during a back squat in their training, or catching a snatch or a clean.
Here is a picture of Dmitry Klokov, an elite-level olympic weightlifter. If you don’t know who Dmitry is, you should look up some of his videos on YouTube - you will be impressed.
The anterior knee translation you will see when olympic weightlifters catch a snatch or a clean is much larger than that you will see in someone squatting during strength training or at a powerlifting meet. The same cases still stand, these athletes are not ACL/PCL deficient due to their training and are able to train at a consistently high-level without knee pain.
Personal Experience
Once I started really observing and looking at how all of these powerlifters and elite athletes move and how to be more efficient in the squat. I realized that what their knees were doing was much different than what I had been taught my entire life. This led me down the path of researching if it was okay to let your knees travel that far forward during squat. After finding out what the research had to say, and what popular strength training/powerlifting camps were teaching as far as squat technique, I learned that letting the knees travel forward past the toes can actually help my squat.
Fast forward to now, roughly 8 years later, and I still squat every week, multiple times per week with my knees going over my toes. And I have not had any additional knee issues since then. In fact, my knees hurt less than they did prior and I am much, much stronger in my squat.
On top of all of that, I had an ACL reconstruction, meniscus repair, and meniscectomy (meniscus are the cartilage ‘shock absorbers’ that sit on top of the tibia and provide cushion for the femur) in my right knee before changing my squat technique. I didn’t start letting my knees translate more anteriorly until after I had this major knee surgery.
Conclusion
Now, there are many nuances that go along with knee pain (most of us currently have, or have experienced knee pain in the past) and weightlifters are no exception. But this discussion is for an entirely different article, the purpose of this article was to dispel the myth that anterior knee translation during the squat is bad for you - and I feel that we have done just that.
I have full confidence that even if someone has had a previous knee injury, or has experienced or is experiencing knee pain, and wants to get into squatting at their highest level, they can do so with a quality squat technique with knees going past the toes and not worry about causing harm to ligaments in the knee, or other connective tissues.
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