If you want to build shoulder muscles, you need to understand how the shoulder works. One of the most complex joints in the body, the shoulder is the junction of three major bones—the clavicle (or collarbone), scapula and humerus—which together articulate into the glenohumeral, sternocalvicular, and acromioclavicular joints.

All these mechanics help facilitate a large range of motion, so the shoulder can abduct, adduct, flex, extend and rotate in the sagittal plane. The downside? All these moving parts make the shoulder extremely susceptible to injury. Therefore, when building shoulder muscles, I divide lifts into three categories: mass muscle builders, performance enhancers and muscle stabilizers. Each is important for building strength and preventing injury. (See Build Balanced Shoulder Strength.)
To build mass, you must overload the deltoid with a weight that you can lift for 10 to 12 repetitions but no more.
To stabilize the joint (a major key for injury prevention), you have to work the muscles of the rotator cuff and the rear deltoids. The rear deltoids need to be equally developed in order to prevent overuse injuries and imbalances. For the muscle stabilizers, you should perform 12 to 15 reps.
Perform the following workout to build your shoulder muscles, and finish it off with stabilizing exercises. In any workout, always start with the largest muscles before moving to the smaller stabilizer muscles. The small muscles help balance out the shoulder, keeping you injury free for the entire season.

Mass Builders

Seated Arnold Press

Sets/Reps: 4×12

• Sit upright holding dumbbells in front of body with palms facing your face
• As you push the weights above your head, externally rotate your shoulders until your palms are facing away from your body

Bent Arm Raises

Sets/Reps: 4×12

• Assume athletic stance with knees slightly bent
• Begin holding dumbbells in front of body
• Raise weights out to side while keeping elbows flexed
• When you finish, your forearms should be parallel to the ground

Performance Enhancers

Clean and Jerk

This traditional Olympic movement builds explosive power in the hips. To build shoulder muscles, you have to do a Clean and Jerk as part of your shoulder routine. It enhances performance in almost every sport. Breaking the movement down, it combines a Shrug with a Push Press, both of which elevate the scapula and work the shoulders. Limit repetitions to 6.
Sets/Reps: 4×6

Push Press

Sets/Reps: 4×6

• Hold a pair of dumbbells with a neutral grip with palms facing each other
• Bend knees and push the weights above your head, extending from knees, hips and ankles
• Finish strong with weight extended over your head

Muscle Stabilizers

Reverse Flys

Sets/Reps: 3×15

• From seated position, bend forward at waist until stomach rests on thighs
• Hold dumbbells behind legs
• With elbows slightly flexed, raise dumbbells out to sides
• Do not allow torso to rise up off legs

Internal/External Rotations

Sets/Reps: 3×15
Use a cable machine for this exercise.

• Stand perpendicular to cable machine
• Hold handle with forearm parallel to ground and elbow tucked in to ribs
• Externally or internally rotate from the shoulder without allowing elbow to leave ribs

LYTW

Sets/Reps: 3×15

• Lie face down on stability ball with stomach on ball and feet planted firmly on ground
• Hold light dumbbells at side
• Lift dumbbells to form the letters
• Hold at top of each exercise for at least a half second

Learn more about how to care for this vital joint: How to Keep Your Shoulders Healthy: Part 1: Mid-Back Mobility, Part 2: Shoulder Stability and Part 3: Rotator Cuff Strength.