Why your knees cave in when you squat and how to fix it

Knee valgus is the inward collapse of your knees during a squat. Instead of tracking roughly over your toes, they drift toward each other, most often right as you drive up out of the bottom where the demand is highest. A small amount of inward movement in a near maximal lift is common and not automatically dangerous. A visible, repeated cave is a signal that some part of the chain is not keeping up with the load.

The key insight is that the knee is caught in the middle and has very little say of its own. What the knee does is largely decided above it, at the hip, and below it, at the ankle and foot. So when the knee caves, the answer is almost never at the knee. It is at one or both of the joints around it.

Weak or underactive hip muscles

This is the most common driver. The muscles on the outside of your hip, the abductors and especially the gluteus medius, are what pull your knees outward and hold them there under load. When they are weak or not firing well, nothing resists the inward pull and the knees collapse. A scoping review of the research found that most studies link gluteal strength deficits with greater dynamic knee valgus during squatting, landing, and running tasks.

Limited ankle mobility

To squat deep with an upright torso, your shin has to travel forward over your foot, which needs ankle range. If your ankles are stiff, your body finds range elsewhere: the foot rolls inward, the arch flattens, and the knee follows it inward. Here the cave is not a hip problem at all. The stiff ankle sent the collapse up the chain.

Too much weight, too soon

Sometimes the hips are strong enough and the ankles are fine, but the load is simply heavier than you can currently control. Under a weight near your limit, the body takes the path of least resistance and the knees cave. This one is the easiest to spot: if your knees track fine with bodyweight or a light bar but cave under heavy loads, the issue is load and control, not mobility or strength.

Before you throw drills at it, spend five minutes working out where your cave comes from. The fix is completely different depending on the cause, so guessing wastes months. Run this simple sequence, ideally filming yourself from the front.

Most people benefit from a combination, but lead with whatever your testing pointed to. Here is how to attack each cause.

1. Strengthen the hip abductors. Add banded lateral walks, clamshells, single leg squats, and hip abduction work. Building the gluteus medius gives you something to actively hold the knees out with. This is the highest yield fix for most people, so start here if in doubt.
2. Free up the ankles. If heels on a wedge cleaned up your cave, work ankle dorsiflexion: loaded knee over toe stretches against a wall and deep goblet squat holds. In the meantime, squatting with slightly raised heels or lifting shoes is a valid bridge, not cheating.
3. Cue the knees out. A simple, powerful cue: actively push your knees outward, in line with your toes, through the whole squat, especially as you drive up. Screwing your feet into the floor helps you feel it. Sometimes the muscles are strong enough and just need the instruction to fire.
4. Drop the weight and rebuild the pattern. If the cave only shows under heavy loads, back off to a weight you can squat with clean tracking, then progress slowly. Grooving the right pattern light beats reinforcing a bad one heavy, every time.

For most healthy people, knee cave is a technique and strength issue to improve, not an emergency. But a few situations deserve more care.

• Pain, not just poor tracking. Aching, sharp, or swelling knees during or after squats is a reason to back off the load and speak to a clinician, rather than pushing through.
• A big side to side difference. If one knee caves far more than the other, or one leg is clearly weaker, that asymmetry is worth addressing with single leg work and, if it persists, a professional assessment.
• A history of knee injury. If you have had an anterior cruciate ligament (ACL) injury or knee surgery, follow the guidance of your clinician or physiotherapist over any general cue, since your rehab needs may be specific.