Well, as you descend into the bottom of the squat or lunge (in hip flexion and knee flexion), the rectus femoris is trying to lengthen at the knee but shorten at the hip, and ends up staying roughly the same length. Then as you ascend (performing hip extension and knee extension), the muscle is trying to shorten at the knee but lengthen at the hip, and again ends up staying about the same length. (3)

all exercises that involve free weights and cables have a point within the range of motion (ROM) where the exercise is hardest on the muscles involved, and another point where the exercise is the easiest based on the lever-arm (i.e. moment arm) involved.

Horizontal pressing exercises like bench presses and push-ups effectively strengthen the pecs when they're in their mid to lengthened range. However, they don't strengthen the muscles when the pecs are in their shortened range, like when the arms are straight out from the torso

An upright torso places much more emphasis on the quads while a forward trunk lean can increase the recruitment of the glutes and hamstrings