The main muscle at the shoulder joint is the deltoid muscle. This muscle is divided into three segments: the anterior (front) section, the acrominal (middle) section and the posterior (back) section. The deltoid muscle is involved in practically all movements involving the shoulder joint and the upper arm.
The anterior section is used in the raising and lowering of the arm. The acrominal segment is a powerful abductor. Finally, the posterior section is used in any movements requiring the arm to go backwards.
Shoulder joints are very susceptible to injuries. For this reason ensure that a thorough warm up has taken place and avoid any sudden or jerky movements while carrying out the exercises. Another major injury worry is the lower back. Many people attempt to complete an extra repetition be swinging the back or moving it away from its support (the bench), this puts extra stress on the area and can cause injury. Lower the weight instead.
Exercises:
Shoulder Press Machine
Smith Military Press
Shoulder Press Dumb-Bells
Arnie Press
Side Lateral Raises
Front Raises
EZ Bar Front Raises
Dips
Bent Over Lateral Raises
Shrug
Upright Row