The one-arm dumbbell press is a compound strength exercise that mainly targets your shoulders, but also hits muscles in your upper back and arms. You can do this exercise standing or seated, but if you are standing, you will also work some of your core muscles, due to the instability induced by holding a dumbbell only on one side.
The Movement
Hold a dumbbell in your right hand at shoulder-level with your palm facing forward. Press the dumbbell overhead until your arm is fully extended. Slowly lower back to the starting position and repeat. You can use different hand positions to change the feel of the exercise slightly. Try the one-arm dumbbell press with your palm facing in toward you or add a twisting motion, starting with your palm facing you and ending with it facing forward at the top of the press.
Shoulder Muscles
Any overhead pressing movement, including the one-arm dumbbell press, targets your shoulder, or deltoid, muscles. The shoulder comprises three deltoid heads: the anterior deltoid in the front, the lateral deltoid on the side and the posterior deltoid on the rear. The one-arm dumbbell press mainly targets the anterior and lateral deltoid heads. The supraspinatus, which is part of the rotator cuff group of muscles, activates during the one-arm dumbbell press to help stabilize the shoulder joint.
Synergists
The one-arm dumbbell press is a compound exercise, meaning the movement occurs at two joints: the elbow and shoulder. Several muscle groups are involved during a compound exercise. The triceps muscle isn't the primary mover, but it assists the shoulder muscles in pressing the dumbbell overhead. Several of your upper back muscles, including the lower and middle parts of the trapezius muscle, also assist the shoulder muscles during the one-arm dumbbell press.
Core Muscles
Your core muscles include the muscles of your abdomen and lower back. These muscles are not primary movers during the one-arm dumbbell press, but they engage to help stabilize your torso against the instability of holding a dumbbell in just one hand. When you do the one-arm dumbbell press while standing, your core muscles work harder than when you are seated, since sitting down provides more stability for your torso.
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