Watch a hard set of squats from the front and you'll see it on someone: the lifter hits the bottom, starts to drive, and both knees dive toward each other like they're keeping a secret. That inward collapse — valgus, if you want the term — is the squat's most common fault, and also one of its most fixable, because the knees aren't misbehaving. They're following orders from hips that haven't been given a better plan.
When it happens, and why
Knee cave has a favorite moment: the first third of the drive out of the bottom, right where the load is heaviest and the hips are working hardest. The knees drift in because inward is, briefly, mechanically easier — the hips borrow a little help from a position that costs the knees their alignment. It's usually asymmetric (one knee leads the dive) and it's always louder with fatigue: rep one holds the lane, rep five doesn't.
That last part matters for how you fix it. Cave that appears only on late reps isn't a technique hole — it's a set-management story. The pattern is fine; you kept renting it past the lease. End the set one rep before the knees stop listening and half the problem disappears without a single drill.
The camera that sees it — and how honest the read is
Here's the thing about knee cave: it is almost invisible from the side. Depth, back angle, balance — all side-view stories. The knee lane belongs to the front camera, full stop.
Film front-on and CoachVision measures your knee spacing against your ankle spacing at its narrowest moment on the way up. Below about 0.85 — knees pulled inside the line of the ankles — the knees are caving. And here's an honesty detail worth knowing: camera perspective tends to inflate the knee gap at the bottom of a squat, so this measurement under-calls the fault rather than over-calling it. Translation: if the number says your knees dove, they really dove — trust it. If the number reads clean, that's a good sign rather than a certificate. The keyframes tell you the rest, including which knee leads.
The fix — spread the floor
The cue that reaches most lifters: spread the floor. From the first inch of the drive, push the ground apart with your feet — as if the floor were two plates you're trying to slide away from each other. Knees track out over the toes the whole way up. You're not forcing the knees outward with willpower; you're recruiting the hip muscles whose job is to hold the lane.
Check your base while you're at it. A stance too narrow physically steals the room your knees need to track out. Around shoulder width or a bit beyond gives them a lane worth holding.
The drill — Band-Around-Knees Squats
A light band just above the knees, 2 sets of 10 at warm-up weight. Push out against the band the entire rep — down and up.
The band works because it gives the knees a physical target instead of a verbal one. The gentle inward pull is a question every inch of the rep, and your hips answer it hundreds of times until the answer becomes automatic. It's a warm-up staple, not a workout — two sets before every squat session.
Then verify: film a front-on set once a week and watch the knee-tracking number climb back over the line, rep by rep, week by week. Knees that hold their lane when you're fresh and when you're tired — that's the goal, and it's a band and a stopped-early set away.