What he said. The more upright the torso the more inherent quad activation you're going to get. In a low bar squat you have to start out with a certain degree of forward lean otherwise the bar will fall off your back. Do a low bar squat, then a high bar squat, then a front squat and note the differences. One fairly recent study compared half back squats, full high bar back squats, and full front squats and assessed how the gains translated into VJ gains - the front squats actually came out slightly ahead. IIRC the same sizes were fairly small, so more research needs to be done. That doesn't surprise me though considering those mirrored my own experiences. BUT, I still don't typically recommend the front squat unless someone just prefers doing it..it's too hard to load, too much technique, hurts too much - just not much of a fun movement IMO. And if someone prefers a low bar back squat over a high bar back squat I really have no problems with it in the grand scheme of things.