A Biomechanical Analysis of the Squat: Valgus Collapse (Knees Caving In)
Written by: Kevin Cann
The knees caving in on the squat is a very common technical breakdown of the lift. It is also why the “knees out” cue is a very popular cue on the squat. How much do the knees caving in (valgus collapse) even matter? Does it increase our risk of injury or decrease our performance?
We typically only see valgus collapse on the squat. It is very rare we see it on a conventional deadlift, and those that pull sumo tend to have an easier time keeping the knees out on the deadlift than they do on the squat. Why is this?
A big difference between the squat and the deadlift is the eccentric portion. The squat has one, but the deadlift does not. We need to control gravity during the eccentric portion of the squat to give us the best chance of lifting the weight.
Gravity is pulling us into hip adduction and hip internal rotation. This is also what happens when we see the knees caving in. We need to counter this with hip abduction and hip external rotation. This is done by our glute medius along with some other muscles of the hip.
The problem with this is that the glute medius tends to be weak in a lot of people because we sit down for long periods of the day. At the bottom of the squat, the glute medius has the least amount of leverage to abduct and externally rotate the hip. However, with a little bit of internal rotation, they regain some leverage.
This may seem great, but regaining leverage comes at a cost. The internal rotation at the hip actually gives our glute max less leverage to extend the hip. However, this may not be a big deal biomechanically and there may be a reason why we see some of the best squatters on the planet demonstrate a valgus twitch when they squat.
Our adductor magnus is the largest of the hip adductor group. It is the second largest muscle in the human body behind the glute max. The adductor magnus has a strong hip extension moment arm. The adductor magnus has an even stronger hip extension moment arm than glute max when the hips are flexed. That is right, in the bottom of the squat the adductors have more leverage to extend the hips than the glutes.
They also have more leverage in this position than the hamstrings. Remember from a previous article that a hard contraction from the hamstrings at full depth makes it harder on the quads to extend the knee. The adductor magnus only crosses the one joint, making it more effective because there is no cost associated with contracting it.
When we see the valgus twitch we may just be seeing the adductor magnus acting as the primary hip extensor over the glute max. Oftentimes people will attempt to make their squat stance narrower to fix this issue. However, research doesn’t agree with this change.
Studies have looked at stance width, and even wearing a belt for a squat, and effect on adductor magnus activation. Neither stance width nor wearing a belt changed the activation of the adductor magnus. This makes sense, as the adductor magnus does not cross from one hip to the next. Perceived tightness in the adductors is usually caused by some type of motor control issue and not a short and tight issue.
There are some anatomical variations that make the valgus collapse more prevalent. Firstly, being female makes keeping your knees out more difficult. This is due to the increased Q angle. The Q angle is the measurement between the anterior superior iliac crest (ASIS) and the patella tendon. The greater the angle, the more difficult it is to stabilize the knee joint. This is why women are more prone to knee injuries than men.
Pelvic width and how the femur sits in the acetabulum of the pelvis also can make our ability to keep the knees out more difficult. There is nothing you can do to correct these anatomical variations. You just need to focus harder on strengthening the important musculature that we have mentioned.
To correct this valgus twitch we need to strengthen the glutes in hip flexion. Exercises like good mornings and reverse hypers are great for this. Also strengthening glute medius in this position is important as well. I have added in clamshells with various degrees of hip flexion into my warmups as well as the warmups of a few of my lifters.
You may be wondering if this valgus twitch is dangerous. In most cases a little valgus twitch is not dangerous. In order for injury risk to increase with valgus collapse we need the knee to collapse inward further than the hip. This is where risk of ACL injury increases.
This is why assessing jump landing in athletes is important to gauge injury risk. A famous example is Robert Griffin III during the NFL combine. Upon landing from a broad jump he experienced significant valgus collapse. He then suffered some major knee injuries in the NFL.
Here is a still frame of his landing. You can see the knees in valgus collapse beyond his hip
This is also why I believe that squats are important for knee health. There are still people out there that believe that squatting is bad for their knees. This could not be further from the truth. I would argue that not squatting is bad for knees.
Every time that our foot lands we need to counter gravity pulling us into hip adduction and hip internal rotation. This increases by 4-6 times our bodyweight when we run. This is even more important to know as a female because of the greater Q angle. Those forces are more difficult for females to control.
Getting stronger in the squat can help teach us to counter gravity and keep us safe during other activities. Those coaches that do not squat with their athletes are missing a critically important piece to keeping them resilient to injury.
This does not have to be a bar on the back and squatting. However, progressively loading some variation of the squat is very important to preventing knee injuries. What if someone already has a current knee injury?
I still stand by my decision to squat. We can change the mechanics a bit to make it a little more knee friendly. Box squats are one way. Really pushing the hips back and keeping a vertical shin are a great way to limit shear force on the joint. Also, the box helps cue people to push their knees out over their toes.
We can limit depth on the squat to just above where the person experiences pain. 90 degrees of knee flexion puts the most stress on the knee joint. Raising the squat a tad higher than this can help someone squat pain free. Over time lower the squat to strengthen the knee through the full range of motion. Let pain be the guide here.
Valgus twitching is not inherently dangerous, however, it may be a sign that the adductors are stronger than the glutes at extending the hips in hip flexion. Strengthening the glutes will add pounds to the squat and help increase overall muscle mass and resiliency to injury. If your knees are caving in past your hips you are at a higher risk of injury. Learning to squat properly and strengthening the pattern can help alleviate those risks. If you have knee pain currently, find a squat variation that is pain free and train it. This is the fastest way to a healthier knee joint.
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