In order to have the most mechanically optimal squat, you need stability both at the knees AND feet.
Your squat may be unstable due to ‘knee valgus’ aka knees that buckle-in. Knee valgus can be cause by a number of factors which include having flat feet or having a weak glute medius, which results in internal rotation of the knee and hip (refer to the picture).
So How can we increase stability in both the knees and feet? We can do this by adding torque. Here’s an excellent lifting cue that will support your stability…
KNEES – ??♂️“Drive your knees out to the sides”
FEET – ?“Screw your feet out”