Why your knees cave in when you squat

Knee valgus is the inward collapse of the knees during a squat. Instead of tracking roughly over your toes, the knees drift toward each other, often right as you drive up out of the bottom where the demand is highest. A tiny amount of inward movement in a maximal lift is not automatically dangerous, but a visible, repeated cave is a signal that something in the chain is not keeping up.

The important idea is that the knee is caught in the middle. It does not have much 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 itself. It is at one or both of its neighbours.

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. Reviews of the research consistently link hip abductor weakness with greater knee valgus during squatting and landing tasks.

Limited ankle mobility

To squat deep with an upright torso, your shin has to travel forward over your foot, which needs ankle dorsiflexion. 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 knee caves not because the hip is weak but because the 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 test: if your knees track fine with bodyweight or a light bar but cave under heavy loads, the issue is load and control, not mobility.

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. Use this simple sequence.

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 work like banded lateral walks, clamshells, single leg squats, and hip abduction. Building the gluteus medius gives you something to actively hold the knees out with. This is the highest yield fix for most people.
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, deep goblet squat holds. In the meantime, squatting with slightly raised heels or lifting shoes is a valid bridge.
3. Cue the knees out. A simple, powerful cue: actively push your knees outward, in line with your toes, throughout the 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.
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 knee tracking, then progress slowly. Grooving the right pattern light beats reinforcing a bad one heavy.