Dude-Man and Warrior are on the money. A good analogy I once heard is working a certain part of a muscle is like trying to stretch only a certain part of a bungee cord while holding it from both ends. Can't do it. I would suggest making the most out of your contractions while increasing your range of motion, this might help recruit fiber over a broader range of the muscle. Other than that, just focus on overall mass by using compound movements instead of "shaper" excercises that IMO are a waste of time. To me it seems like those excercises fatigue you more than recruit muscle fiber.