One of the biggest misconceptions with regard to resistance training that won’t die on the Internet is the mistaken belief that muscle groups fire all or nothing. Essentially, the argument goes that you can’t zero in on a particular area of a body part, say the upper chest or lower abs, because the movement enervates all the muscle fibers. That thinking suggests, then, that you’d need to do only a single exercise in your body-part workouts because, in essence, they’re all the same, activating every muscle fiber.
Of course, empirically we know that’s not true, but theoretically the confusion arises because muscle fibers – not groups – fire all or nothing.
Trainers manipulate angles in their workouts to better focus on (notice I didn’t say isolate) a given area. For example, on a hanging leg raise, the fibers on the lower end of the rectus abdominis undergo a greater degree of muscle shortening than the fibers on the upper end. That gives this exercise the label of a lower ab exercise.
You can increase the focus on a given area of the muscle, like doing close-grip benches that emphasize the inner pecs and triceps, but you can never truly isolate a particular area.