Knee Cave: is it Really a Death Sentence?
“You’re going to hurt yourself!”
“Do you ever do any work for your Gluteus Medius?”
“You should put a band around your knees, then they wont go in like that”
“You should lighten the load and focus on form”
Look at the comment section of any video of a lifter (especially a female lifter) performing a set of squats with even a minuscule amount of the dreaded “knee cave”, and you are bound to see at least a few of the above gems. Ever since we all first picked up a barbell, we have been coached not to permit our knees to drift inward when we squat. Yes—there is definitely a good reason and even better intentions behind this line of thinking and lots of research that seems to back this up. But the purpose of this article is to propose that maybe, just maybe, inward motion of your knees in your squat or jumps does not spell the end of your athletic career, and is actually part of a larger biomechanical strategy to produce more force more efficiently.
Conventional Wisdom
Most of the preconceived notions surrounding knee cave come from the idea that it can lead to serious knee injury. This idea can largely be attributed to the wealth of research surrounding ACL injury, which is prevalent in field sports, and women’s teams in particular. Here it has been demonstrated, time and again, that knee valgus is a potential mechanism of injury for ACL tears. But why?
The anterior cruciate ligament acts to prevent anterior translation of the tibia on the femur. The orientation of the ACL means that it is particularly susceptible to rotational forces that are placed upon it (this is common among various structures within the knee, as the knee joint as a whole is not designed to rotate). These rotational forces are not uncommon in field sports and it comes as no surprise that the vast majority of non-contact ACL injuries come when athletes are attempting to change direction. A femur rotating at high speed over a planted foot/tibia creates extremely high rotational forces on what is in reality not a very big or strong structure, and can lead to an ACL tear. So really, the more prevalent mechanism of injury isn’t simply the state of knee valgus alone, but rather the often-associated rotational forces that are also seen under these circumstances.
Does this mean that we should discourage the knees from ever coming closer to the midline of the body under any circumstances? I don’t think so. I think that in many movements such as bilateral jumping and squatting, this is simply the body’s natural way of using its energy in the most efficient way possible to maximize force production. At the end of the day, someone’s online weekend personal training certification does not know better than your own human body.
Success leaves clues. Look at almost all elite-level powerlifters or Olympic weightlifters (who are an even better example than powerlifters because they squat to a greater degree of hip flexion… more on this later) and you will find a lot of knees that seem to come inward as they squat. This pattern is also pervasive in even the highest levels of sports such as basketball and track where explosive jumping is necessary. Are all of these professional athletes doing it wrong? Or, have we maybe been lead to some false conclusions? I know which one I think is more likely.
Part of a bigger picture
The human body naturally uses cyclical movement to store and use elastic energy to move more efficiently and maximize force. There are examples of this everywhere, but an easy place to spot this is in the gait cycle.
Without taking too much time to break down the different phases of gait extensively, suffice it to say that each time you take a step, your foot and ankle naturally pronate, and your hip internally rotates, adducts, and extends. Each time you pick up your leg afterwards, hip flexion, abduction, external rotation, and supination occurs.
In other words, every action in the body has an equal and opposing reaction that allows motion to occur in a natural spiral pattern. Each time you flex your hip while you are walking, for example, you pre-stretch the muscles of extension for when this movement gets reversed. When the stretched tissues then act to extend the hip, the muscles of flexion then begin to stretch, setting up an energy-recycling machine that pre-loads itself before its next expenditure.
This same phenomenon occurs in squatting, even though it is a different movement pattern and your feet never leave the ground. As we descend into the squat, we naturally see people open up at the knees. What they are actually doing is externally rotating and abducting the hip as it goes through flexion during the eccentric portion of the lift. Guess what this sets them up for?
Drum roll please…
You guessed it: internal rotation and adduction working in conjunction to extend the hip during the concentric portion of the squat (or a jump for that matter). If you are going through these motions in your head, you should have noticed that this equates to a knee traveling inward towards the midline of the body on the way out of the hole… in other words, a knee cave.
So in reality, training your knees to externally rotate out of the bottom of a squat with a mini band may be fighting your natural biomechanical instincts. While the glutes play an undeniably important role in hip extension in the squat, the adductors may arguably be even more important in getting out of a position of deep hip flexion, and they are definitely trained far less than the glutes.
The adductors are an interesting group. Most of these muscles function to flex the hip, especially when it is already above 90 degrees of flexion. Interestingly, the adductions unique orientation means that they externally rotate the hip while the foot is off the ground, but internally rotate the hip while the foot is planted (such as in a squat). Interestingly, in a deep squat position, Adductor Magnus has a greater moment arm for hip extension than either Gluteus Maximus or the hamstrings. That’s right: your adductors might contribute more to getting you out of the hole than your glutes and hamstrings, and that knee cave might just be your adductors doing their job.
Why Women?
Females are at a much higher risk for ACL tears than their male counterparts (some estimates range from 3-6 times as likely). Women also seem to exhibit knee cave far more often than men. This must mean that knee cave leads to ACL injury, right? Not necessarily. These situations may be correlative, but I would argue that they are not causative.
There are many reasons that females are at a higher risk for ACL injury, and the discrepancy is not fully understood. One possible explanations is that females have relatively less muscle mass and strength than men, and must rely more on the ACL (a ligamentous structure) to hold the knee in place than on the muscles like the hamstrings, which also prevent anterior translation of the tibia. Another is that there is less space between the femoral condyles in females, which limits ACL movement.
The factor that relates to both knee cave and a higher incidence of ACL injuries, however, is the higher Q-angle that females posses. The Q-angle is in effect a measure of the inward slope of the femur as it exits the acetabulum. Because females tend to have wider hips, they tend to have a greater Q-angle. Because they have a greater Q-angle, their knees naturally appear to come inward.
While this may present itself as knee valgus in some situations, it is important for a coach or trainer to remember that for someone with a large Q-angle, “anatomical zero” looks different on them than on someone without much angle at all. If a female with a relatively large Q-angle squats with her toes forward and her feet just outside shoulder width—a perfectly comfortable stance for her—there is a very good chance that her knees will appear to be in valgus the entire time. In reality, this is just the natural angle of her femurs leaving her pelvis, and her knees are not in danger.
A Few Caveats.
Now, I am not giving my blessing to all forms of knee valgus, nor am I saying that it is always safe. Yes, knee valgus has been linked to ACL injury. BUT, the more prevalent injury mechanism is the dissociation between the tibia and femur that comes from them rotating in opposing directions. If the knee is moving inwards but the femur and tibia are rotating together, I personally don’t see this as much of an issue, especially in a relatively low-speed situation such as squatting. In a high-speed situation, such as when an athlete plants their foot forcefully and rotates their femur over the tibia in an unnatural way while attempting to change directions… now we have a problem.
As far as the weight room, the most important thing to take away from this article is that valgus movementdoes not necessarily even mean the knee has entered a valgus position, just that the knee has moved closer to the midline than where it was before. Much of the time, “knee valgus” in the weight room is not actually valgus at all, just internal hip rotation in disguise. The problem arises when the femur and the tibia are not operating in synch.
Use your best judgment. As an experienced coach, if something doesn’t look right, it probably isn’t. On the other hand, your knees coming in slightly as you come out of the bottom of your squat or counter-movement jump?
Probably not as big of a deal as everyone thinks.
