(10-12 minute read, 7 minutes for first part)
Whenever I come across an awkward squat, the first question I ask the squatter is, “Have you heard the rule that you shouldn’t allow your knees to go past your toes?” Most of the time, the answer is yes. When I left the insular world of the strength gym and began working in a large commercial gym again, I was surprised to find this cue alive and well. The general argument for it is that allowing the knees to travel past the toes puts one at increased risk for knee injury. But is this true? First we’ll go over some simple biomechanics. Then we’ll take a look at some research, and finally I’ll tell you how this cue came to be in the first place. If you’re a novice, you can just read the biomechanics portion. I’ve got an article coming up just for you guys on the best way to start squatting!
When you descend into a squat, your hip, knee, and ankle need to bend. The idea is that the knees-behind-toes cue is that joint stress will be decreased if we can minimize flexion (bending) of the knee. If only things were that simple. Unfortunately, in the squat, no joint works in isolation; if you don’t want to bend your knees or ankles very much, as is the case with the knees-behind-toes cue, you’re going to have to make it up elsewhere, i.e. in the hips and back. Gravity places another demand on the squat because in order to keep from falling, we must keep our center of gravityover our base of support, i.e. our feet. This is why when you see someone do a knees-behind-toes bodyweight squat, they’ll put their arms out as a counterbalance. Let’s take a look at what happens when you don’t allow the knee to travel forward. These photos are from a 2003 study that showed that although there was 28% more torque at the knee in squat A, there was almost ten times the torque in hips in squat B. Further, the study states that “these hip forces will ultimately be transferred through the lower back.”
Why does that happen? Well, you can see that in squat B the hips are having to flex more to make up for the lack of flexion at the knee and ankle. The torso angle is much closer to horizontal in squat B, which means the back is having to work a lot harder. This isn’t the end of the world. After all, conventional deadlifts use a very similar back angle.
But let’s look at the position of the barbell relative to the squatter’s base of support. In squat B, the bar is over the toes. Because the bar weighs about 45lbs, the hips can counterbalance it, but what happens when the bar is loaded? Suddenly keeping the barbell itself over the foot becomes paramount in order to maintain balance. To compensate, many people who practice knees-behind-toes squat will arch their backs, effectively shortening the back to bring the weight back over the foot.This not only places more demand on the back, it takes abdominal involvement out of the picture, which is rarely good. One of my top priorities with my lifters is teaching them how to maintain core integrity, meaning keeping the ribcage and the pelvis in a constant relationship. When people over-rely on their backs, they lose this relationship, and this can lead to all kinds of issues with the hips and shoulders down the line. (More on this in future articles.)
Also, notice how when the knee is restricted, the squatter’s torso is practically lying on the thighs by the time they reach parallel. Because the knees can't go any further forward, trying to drop down further will mean pulling that torso down and back along with the hips, putting him in a position to fall backwards. He's not going to be able to get much below parallel, and below parallel, with a lot of knee flexion, is where all the good stuff happens. You can see in this diagram that quadriceps (front of the thigh) and glute (🍑) activation are highest below parallel. And notice that the different shearing force occur above or at parallel. the more time you spend above parallel, the more shearing force occurs at the knee. It seems, then, that the most benefit would be derived from a squat that allows the lifter to get below parallel, i.e. a knees-forward squat.
Olympic-style weightlifters are the perfect population to study if you want to see what the long-term effects of knee-dominant squatting are. The sport demands that we be able to squat extremely low while keeping the torso upright and the knees way over the toes. You can see this in the photo below of me receiving a clean.
A 1980 study of degenerative joint disease in male weightlifters found significantly fewer issues in this group than in males of the same age in the general population, with joint degeneration present in 38% of the general population but present to the same degree in only 20% of the lifters studied.
A 1989 study found that experienced weightlifters and powerlifters had significantly tighter (more stable) knees than the control group of untrained college students. It should be noted that after eight weeks of squat training, the control group showed no appreciable changes in knee stability, but this is to be expected considering that ligaments change much more slowly than muscle.
An exhaustive 2010 research review by the famed Brad Schoenfeld, PhD concluded that a healthy person's anterior and posterior ligaments were more than capable of handling the forces involved in squatting.
There’s more research out there, but I think you get the point.
So where did this ill-begotten cue come from? In 1961, Karl Klein did a study on ligamentous laxity in the knee, comparing populations who performed deep squats and those who didn’t. He fitted the test subjects with a special brace that looks like armor, with a hinge at the knee. He then manually applied pressure in different directions, using made a special device that looks like armor, with a hinge at the knee that measured how the knee responded to the pressure.
From the results he obtained looking at the two groups, he concluded that too much knee flexion was dangerous to the internal structures of the knees. He mounted a crusade against knee flexion and a year later, the American Medical Association issued a caution against deep squatting. A few states even outlawed it in secondary schools. The trouble is, he asked the test subject whether they squatted deeply before testing, and some test subjects complained that he applied so much force that it caused pain. The test has since been roundly criticized and its results have never been replicated, but it appears that the effects of Klein’s campaign against knee flexion have stuck around.
There’s also the possibility, mentioned in this incredible video from Juggernaut, that geared powerlifting, popular in the 1980s, has had a lasting influence. In geared powerlifting, competitors wore stiff, springy suits which helped them bounce back up. They would squat back into the suit, using it like a trampoline. Needless to say, this technique has little transfer to the person wearing shorts or yoga pants or a singlet.
What if you already have knee pain when squatting? The knee is a very simple joint and even at peak forces, its ligaments are more than capable handling the stress. However, issues at the hip or ankle can lead the muscles that attach at the knee to work differently than they should, which can pull at the patella and cause pain. Rather than simply giving up on yourself and avoiding squats, however, which can be an amazing exercise for most people, I would suggest going to see a knowledgeable physical therapist or strength coach who can help you squat pain free. You’ll be giving yourself a lifelong gift of access to a wonderful, functional exercise. If you would like to set up a free consultation call with me to discuss your squat (or anything else), please email me at firstname.lastname@example.org I am also happy to refer you to a great physical therapist.
 - Center of gravity is different from center of mass in that it is dependent upon what gravitational forces you’re subjected to. The center of gravity is the point at which distribution of weight is equal in all directions. This point changes as the body changes position relative to the acting gravitational forces. So if I am on planet Earth, and I jump up in a perfectly vertical line, my center of gravity is the same because gravity is acting in a vertical line. However, if I put one leg out in front of me, my center of gravity will move forward, to my toe or beyond, unless I counterbalance it with my trunk and/or other limbs.
 - Powerlifters change the position of the bar (low-bar position), placing it below the upper trapezius muscles and on the actual shoulder blades. This allows the lifter to keep the bar over the foot while still staying in a relatively knees-behind-toes position. A powerlifting squat is sport-specific, however, and should only be executed to parallel, never beyond, because to do so would actually compromise their ability to lift the most weight. If we are looking to train the legs, however, as most non-competitive folks are when they squat, a low-bar squat is not the best tool. In a future article, I will go over the different advantages and disadvantages of the high-bar, low-bar, and front squat.