except when you pre-exhaust the quads, its the glutes, hips and hams that do most of the work in the squat .. so you actually work the target muscle LESS because the other muscles have to do the work, with a reduced load.
Pre exhaust is so retarded I can't even be bothered to explain all the reasons it sucks.. just pyramid up the weight on your compound leg exercise and do 10-20 reps instead of badly performed sets of 5 with too much weight