Being super bent over in the lower back doesn't do anything good for injury risk but... it reduces your ability to produce force through the quads. The back becomes a limiting factor... you should at least have your spine relatively straight, brace nice and tight, keep your chest not necessarily super up but don't let it cave forward... At the bottom of a deep squat, gravity and body mechanics want you to fold over on yourself - just don't do it. Resist by maintaining upright posture... [with the] back either neutral or in a slight lordosis.
Renaissance Periodization