Why your knees cave — and what actually fixes it
Knee valgus under load is rarely a knee problem. It's a hip and foot problem showing up at the joint in between.
Watch almost anyone squat heavy or land from a jump and you'll see it: the knees drift inward. It's called dynamic valgus, and it's one of the strongest predictors of knee injury we have — particularly ACL tears, most of which happen with no contact at all.
The knee is the victim, not the culprit
The knee is a hinge. It's built to bend and straighten, with very little rotation available. When it collapses inward, that's almost never the knee failing on its own — it's the hip above failing to control the femur, or the foot below collapsing and taking the shin with it.
The usual suspects: a weak gluteus medius that can't hold the thigh out, an overactive TFL doing the glute's job, and a foot arch that flattens and rotates the whole leg inward.
Test yourself
Stand on one leg and do a slow quarter squat while watching your knee in a mirror. Does it track over your second toe, or drift inside your big toe? Now check your pelvis: does the opposite hip drop? A dropping hip is a gluteus medius that isn't holding.
Do both sides. Asymmetry between them tells you more than either one alone.
The fix
Strengthen the hip abductors directly — banded lateral walks, side planks. Build single-leg control and foot strength so the base stops collapsing. Then, crucially, practise the pattern under load: cue yourself to spread the floor and drive the knees out during squats, so the new strength gets wired into the movement that actually needs it.
Strength on its own doesn't transfer automatically. You have to rehearse the corrected pattern in the lift.
Knees caving under load is the MCL and ACL asking for help from muscles that aren't answering.