Originally Posted by
Scott Hanson
These really don't look too bad, based on the suboptimal viewing angle. The biggest problems I see are that you relax your lumbar as you hit bottom (lower back flattens, then rounds), and your knees are sliding forward a bit; they never really stop moving forward. Cue yourself "big chest", especially as you approach the bottom. Get your knees out over your toes in the first third of your descent, then keep them there. Don't slow down so much as you approach the bottom; stay tight and rebound. These look okay for depth, but some of your depth is achieved by relaxing your lumbar at the bottom. Stay tight. A video from the rear at about 45 degrees is preferable for squat form checks.
As far the bar slipping on your back, we can't see if the bar is in the right position. This could be caused by a slick shirt material, lack of knurling on the center of the bar, or the bar being a little too low. It should feel secure there and not move around.