You may be asking yourself, how in the world do bodybuilders get these ridciulous quadricep muscles when you can't get any definition in your legs at all. First and foremost, you need to squat and do a lot of isolation movements specific for targeting the quads, but the order of doing it is very important. Most of the time, your hamstrings or glutes give out before your quads do. This is a key indicator that you need to pre-exhaust your quads before you get to the heav y compound lifts that utilize your hamstrings and glutes. So in order to bring up a stubborn pair of quadriceps, you need to pound your quad muscles with more fury during your leg workouts; that means getting them fatigued before you even start your squats so that they can fatigue at about the same time your hamstrings or glutes do.

Here is a routine that will have your quads screaming at the end of them and bust them through a new phase of growth you're not used to:

Leg Extension- 5 Sets x 12-15 reps
Back Squat- 3 Sets x 10 Reps
Dumbbell Stationary Lunge- 3 Sets x 15 Reps
Leg Press- 3 Sets x 12 Reps

Remember to take each set to failure, in order to recruit as many muscle fibers as possible and so that you can stimulate your growth by achieving a full muscular pump.