Knees caving in when you squat? Look at the feet first.

Usually a few things at once, from the ground up:

• Feet turned out, so the shin starts the squat already rotated.
• An arch that drops under load, so the shin rolls in with it. The flat feet guide covers this.
• Stiff ankles, so the knee dives in to find depth when the shin can't travel forward.
• A sleepy side glute and weak inner thighs, so nothing holds the thigh on its line.

Stiff ankles are easy to miss. If your heels lift or your feet turn out as you go down, read the ankle mobility exercises guide.

Both ends, with the knee in the middle. The knee is a hinge: it bends and straightens well, but it doesn't like to twist. In a good squat, the arch lifts, the thigh turns out a touch, and the knee stays over the second toe while the twist runs through the glute.

When the arch drops, the shin rolls in from below. When the glutes are switched off, the thigh rolls in from above. Fix both. The guide to glutes switched off from sitting covers the top half.

Slow down and give the knee a target:

• Squat to a box with the feet straight, big toe down and arch lifted. 3 sets of 8, three seconds down.
• Watch the knees in a mirror or film from the front. Each one should point over the second toe.
• A light band just above the knees can remind the glutes to push out. Take it off once the pattern holds.
• Slow split squats and lunges: knee over the second toe the whole way.
• Side glute work: side-lying leg lifts or banded side steps, 2 sets of 12.

The From the Feet Up app reads six photos of how you stand and finds what to fix first. Its new movement scan goes a step further: it watches a squat and a lunge and checks whether each knee stays over the second toe, side by side. Most people have one knee that drifts more than the other, and that tells you which side to work on first.

If your knees ache as well as cave, start with the wider knee pain guide.