Hip flexion, hip internal rotation, ankle dorsiflexion, and thoracic extension are the typical joint movements we look at when it comes to executing a good squat. There's a movement that I have admittedly overlooked for a long time that's just as important as the above listed.
The ability to flex the knee plays a huge role in how our squat looks and more importantly, feels. You'd be surprised how many people can't flex their knee much deeper than 90 degrees. We can collectively blame that on our environment: desk chairs and cars have taken a toll on the majority of American's spines, hips, and knees.
If limited, we need to improve the active range of motion at our knee in order to prevent injury under load when squatting. A great way to do this is through half-kneeling knee flexion PAILs/RAILs. This is a great way to target the closing angle of the knee in order to learn how to stabilize in shortened ranges of motion. Check out the video below to learn more: