When executing the barbell back squat, beginners typically misinterpret the “flat back” cue and try squatting with their back/torso perpendicular to the ground. Ain’t gonna work. This flawed and unfortunately common approach not only keeps them from lifting as heavy and safely as they’re actually capable, but in the process it wreaks havoc on their knees.
When squatting, a flat back does not mean a vertical spine. Yes, your back/torso is flat when viewed from the side but relative to the ground, your torso is actually angled about 45-degrees (as shown in above photos). That is, keeping your back flat — i.e., chest up, spine extended — you bend at the hip as you squat and “shove your butt in the hole.”