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Emphasizing vertical tibias during the squat requires less knee flexion and greater angles of hip flexion. In any heavy barbell lift, the bar must stay centered over the mid foot. As you attempt to pull your shins into a vertical position your knees move back. In order to keep the mass of the barbell centered over your feet, you must adjust the mass forward either by flexing more at the hips (resulting in a greater forward lean of the torso) or by shooting the hips forward and up, bringing the entire system slightly forward. Either way, vertical shins are going to result in greater torque at the hip and a less vertical torso. This leaves you with two options: work on hip flexion ROM and practice the motor pattern to create a back squat with a more horizontal torso or widen your stance. Widening your stance decreases the distance (from front to back) between your hips and feet and allows you to perform the lift with your torso a little more vertical. My advice would be to perform some of the hip flexion mobilizations on this site, and prioritize vertical shins from the beginning of the lift (it’s very difficult to recapture position after it is lost), and experiment with the width of your stance.