The deltoid, a muscle on the uppermost part of the arm and top of the shoulder, is basically responsible for the majority of all arm rotation. Properly training the deltoids helps to avoid arm dislocation and injury to the arm. It consists of three different parts - the anterior deltoid, lateral deltoid, posterior deltoid - each of which have different functions that work together.
The anterior deltoid rotates the shoulder joint by moving the arm inward. The lateral deltoid rotates the shoulder joint side to side, responsible for moving the arm outward. The posterior deltoid also rotates the shoulder joint side to side, but alternatively is responsible for moving the arm backward and outward. Due to how much it is responsible for, it is essential to focus on deltoids when lifting and it should not be overlooked during your pump session. Consistently training the deltoids using some of the exercises below has been shown to result in much stronger, muscular deltoids.
Anterior Deltoid Raise
Dumbbell Reverse Fly
One Arm Lateral Raise
Dumbbell Shoulder Press