The general term 'shoulders' can usually be taken to include the deltoid muscle group, the rotator cuff, and even the upper trapezius. Each of these muscles moves the arms in a specific way, and therefore requires different motions to stimulate growth. Shoulder workouts generally involve working one of these muscle groups in isolation, or performing compound movements that require the muscles to work synergistically. Thus, shoulder workouts are often classified as either compound, isolation, or rotator cuff exercises.
Deltoids, or shoulder caps, are actually three separate muscles. The anterior deltoid sits at the front of the body, tying in with the upper pectorals. Resting at the top and sides of the shoulder, the medial deltoid enhances the broad-shouldered look associated with physical development. The posterior deltoid is perhaps the trickiest to work. Sitting at the back of the shoulders, it is less involved during compound shoulder workouts than the other three, and sometimes needs to be worked in isolation to be properly stimulated.
Compound exercises are those that require movement of more than one skeletal joint. Consequently, these movements often involve a number of muscle groups, allowing increased load and intensity. As a result, compound exercises are usually used when an increase in muscle mass, or heightened caloric expenditure, is the goal of the workout.
Overhead shoulder presses are an example of compound exercises. While performing the movement, both the shoulder girdle and the elbows are called into play. This activates the deltoids and trapezius as the primary movers, but also numerous other muscles as stabilizers and synergists. The biceps, the triceps, the pectorals, and the upper back all contribute to the shoulder workout, increasing the amount of weight carried by the deltoids themselves. Overhead shoulder presses can be performed using dumbbells, a barbell, or specialized machines, but the basic motion remains the same.
Conversely, isolation exercises only involve the movement of one body joint, and generally target specific muscles or small muscle groups. Shoulder workouts that involve isolation exercises add tone and definition to the muscle, and allow each of the shoulder muscles to be worked individually. Examples of isolation shoulder exercises include front raises, lateral raises, bent-over raises, and shrugs. A typical shoulder workout might employ a single set of a compound exercise to warm-up, or pre-exhaust, the muscle, followed by a combination of isolation exercises.
The rotator cuff is a small group of muscles that sits inside the shoulder socket and attaches to the humerus. These muscles are responsible for rotation of the arm, as when washing windows or throwing a ball. The mechanical design of the shoulder makes it among the most injury-prone joints in the human body, and the rotator cuff is a common site. Specialized shoulder workouts to strengthen the rotator cuff are often used during physiotherapy, when injury has already occurred, but these same exercises are equally beneficial when used as preventive maintenance.