Each part of the muscle is emphasized with different movements. The anterior deltoid is emphasized during flexion, which is when your shoulder moves forwards. The middle deltoid is emphasized with abduction, which is when the shoulder moves out to the side, and the posterior deltoid is emphasized with horizontal abduction, which occurs when the arm moves backwards in the horizontal plane.
It is every easy to emphasize different parts of the deltoids with dumbbells. Front raises are the first exercise, and they will emphasize the anterior deltoid. Simply stand with two dumbbells in your hands with your palms facing down. Then, exhale as you lift your arms in front of you to about 90 degrees.
Although you can isolate the anterior delt with front raises the anterior delt also works during chest exercises, such as push ups, bench press, and chest flies. Since the anterior portion of the deltoid gets a lot of work during chest exercises, some people choose not to isolate it with front raises, but if you really want to focus on building the shoulders, front raises are a great way to build up the anterior delt.
A second exercise with dumbbells for the deltoids is side lateral raises. All 3 parts of the shoulder will work during lateral raises, but the middle deltoid is emphasized. To perform lateral raises you simply stand with dumbbells in your hands and your palms facing the sides of your body. Exhale, and raise your arms out to the side to about 90 degrees.
During lateral raises it is important not to shrug up your shoulders. When the shoulders are weak, the tendency is to cheat by shrugging the shoulders up towards the sky. Move in a slow and controlled motion to decrease the change of momentum taking over.
The posterior deltoid is probably the most under-worked part of the deltoid muscle, but it may be the most important part. The posterior deltoid works to balance out the strength of the anterior deltoid. And because the anterior deltoid works so hard during chest exercises, it often gets much more developed than the posterior deltoid.
Reverse flies with dumbbells are a great dumbbell exercise to focus on the posterior deltoid. You can lie face down on a bench or you can stand up and hinge forward from your hips. When you do reverse flies from a standing position, you must be mindful of the position of your lower back because your lower back muscles will stabilize your body. When you lie on a bench to perform reverse flies, there is much less pressure on your lower back.
These exercises are all pretty simple to perform but they are all great to build up your shoulders. If you want to develop your shoulder muscles, try adding front raises, lateral raises, and reverse flies into your workout routine.