This is a tough question for me, because what I believe isn't always what's best for the goal. IMO, and take that for what it's worth, a safe squat is ALWAYS a better squat. To me, a safe squat is one where the shins remain vertical, and the spine stays in a position designed to bear compressive force. If you are doing that, then it will hit hams and glute most, also the quads, and parts of the upper body (as a result of supporting the bar). If you asked me what the primary movers are in a squat I'd say hips or hamstrings (depending on the width of the feet and depth of the squat). The quads get hit, and probably get hit quite enough for growth, but I don't see them as primary movers. I don't associate "squat" with "quad." Generally, I hollar at people for "quad squatting" because I don't want them to get hurt, and I want them to be able to move more weight in the future. I get hollared at for this occassionally too..........still. I feel like front squats hit my quads more, and abs too, but I still sit back and go deep on those too...shins vertical. It's like this, you're trying to use a compound move to isolate a muscle. Why not do the compound move as safely as possible, and then use an isolated move (like extensions) to really hit that one particular muscle. That's what I used to do. I did my squats, then seated leg extensions. One heavy set, with slow eccentric. Compound moves I do to "form failure" and on the isolated moves, if you want, you can go to "muscle failure." I think that's safer. By form failure, I mean you can't do another rep with proper form. I personally don't go to failure on my accessory work either. I come pretty close, but stop short of it. That's how I can get by with training each muscle twice a week.
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