Its the knees for me, 450 lb box squat to almost parallel . I can't go parallel because of my injuries. You should try Power Cleans, I am training for my first games in April and have been doing 5x5 at 180 on Mondays and Fridays. Cleans will help you explode like crazy, old football lift to help linemen

http://www.youtube.com/watch?v=lF6SntdVvjw