Capping off your upper arm is the deltoid muscle, the main visible muscle that’s associated with the shoulder region. The deltoid muscle is a large three headed triangular muscle which originates from the clavicle and the scapula at the rear of the shoulder and extends down to the upper arm.
The main function of the shoulder muscle is to rotate and lift the arms. The anterior (front) deltoid lifts the arm to the front. The medial deltoid lifts the arm to the side, and the posterior (rear) deltoid lifts the arm to the rear.
Anterior Deltoid Muscle
The anterior deltoid is located at the front of the shoulder muscle region, near the clavicle and is responsible for movements in shoulder abduction when it is externally rotated. The anterior shoulder muscle is also the secondary muscle used in a bench press movement. The head of this shoulder muscle actually originates at the collarbone area.
You would associate the anterior deltoid muscle with movements that are away and to the front of your body. For example, these kinds of movements might include pushing a swing or a bench press movement.
Posterior Deltoid Shoulder Muscle
The posterior deltoid muscle lies along the lower section of the border of the scapula and the spine. These muscles are the furthest back on this triangular set of muscles. The posterior shoulder muscles originate at the scapula and are associated with movements away from the shoulder and to the rear of the body.
Medial or Lateral Deltoid Muscle
The medial deltoid originates from the collarbone like the anterior deltoid and is associated with movements that are up and to the side (lateral movements). For example, raising your arms to the side requires the medial shoulder muscles.
While it might not seem like it, the deltoids are associated with a number of everyday movements that you might not even think about. From getting out of bed to washing the car or typing at your desk, your shoulder muscles are either directly involved or helping other muscles in order to complete these kinds of simple movements.
However, because the shoulder muscles are an important part for a number of movements, they are often prone to injury. Proper warm ups and stretches before any movements or exercises is always recommended to minimize injury.
Shoulder Muscle Development
Your shoulder width is largely determined by skeletal structure and genetics - something you're born with. However, with proper development of the side deltoids you can achieve a fuller and wider look. With a fully developed side shoulder muscle, you can achieve an impressive look when they are flexed.
The lateral deltoid is only one aspect of a complete shoulder development. For thickness, you need to develop your front and rear deltoids. Anterior, or front delts typically get worked along on chest days, where the front deltoids are secondary muscles used in a variety of chest exercises such as flat bench press, incline bench press, dumbbell presses, and so on.
Shoulder Muscle Training
There are two basic types of exercises for shoulder muscles: 1) shoulder presses, and 2) raises. Presses include exercises such as military press, seated dumbbell press, standing shoulder press, arnold presses and so on. Pressing movements involve the triceps, and is a compound movement where multiple muscles are involved. You are able to lift heavier with presses.
Raises include exercises such as front raises, lateral raises, bent over dumbbell laterals and so on. Raises involve lifting your extended arm upward in a wide arc, which works to isolate your shoulder muscles. Doing raises, the triceps are not involved, and the shoulders are performing isolated movements; however you will not be able to lift as much.
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