I started doing box squats about two months ago. My max squat has jumped up about 100 lbs. (from 315 to 415), seriously, I’m not kidding. However, I’m not entirely sure I have the best form on the box. I stand wide, but comfortable, point toes out a tad (due to poor hip flexibility), and attempt to sit back as much as possible. Here’s the thing. When I sit back far, I feel I have to lean over with the upper torso. I remain upright, no less than a 45 degree angle, but I saw a picture of guy, on this site, who was almost all the way upright when doing box squats. His torso was basically a convex shape. I assume I am forced to lean over due to two things: poor lower back flexibility, and poor lower back strength. Just wanted to see if I was right in my assumption. I have taken measures to improve in those areas. Thanks to all who reply.
In order to sit back far enough in the box squat you really need strong hams, low back, and abs. Really hammer those muscle groups with glute-ham reverse hypers, Good Mornings, and pull-throughs. Make sure to work on your hip flexibility too. Try doing some overhead squats as a warm-up to your box squat to help loosen up your hips.