the deeper you go the more the lower body is worked in my opinion. The quads are worked more because they have to work through a bigger ROM even if the concentration isnt on them at the deepest part and the hams and glutes are called into play more because as most people know, in the bottom of the squat alot of the force is from the hams and glutes. Besides for me is feels wierd stopping half way down. But when I do a full powerlifting meet you will see alot of parrallel squats come into my training journal leading up to it...