Diagram of posterior muscles.

Diagram of Posterior Muscles of the Human Body

The deltoid muscles (sometimes called the deltoideus muscles) are thick triangular muscles that cover the shoulder joints. There is one deltoid muscle over each shoulder joint.

Each deltoid muscle can be described in terms of three parts that originate from three distinct sets of fibres or 'heads'. They are:

  • Anterior Part: Originating in the anterior or 'clavicular' fibres, also known as the anterior deltoid or the front delts.
  • Lateral Part: Originating in the lateral or 'acromial' fibres, also known as the lateral deltoid.
  • Posterior Part: Originating in posterior or 'spinal' fibres, also known as the called posterior deltoid, or the rear deltoid or rear delts.

Origin, Insertion and Actions of the Deltoid muscles:

Points of Origin: Anterior Part:
  • Lateral part of the clavicle ('collar') bone
Lateral Part:Posterior Part:
Point of Insertion:
  • Deltoid tuberosity of the humerus (upper-arm) bone
Actions: The deltoid muscles have several actions that can be summarized as:
  • abducts the arm at the shoulder joint
  • flexes the arm at the shoulder joint
  • extends the arm at the shoulder joint
  • rotates the arm at the shoulder joint

Descriptions of the actions of the parts of the deltoid muscles vary. Which parts of the deltoid muscles perform which functions depends partly on the initial position of the body and on the stage of the movement (if movement is in progress). The different parts of the deltoid muscle work in co-operation with other muscles to perform certain movements. In some cases the relevant fibres of the deltoid muscles make only a small contribution to the force applied to produce the movement. Examples of some actions of the parts of the deltoid muscles include:

Anterior Part:
  • Protracts (draws forward) the humerus / upper-arm
  • Medially rotates the humerus / upper-arm.
Lateral Part:
  • Abducts the humerus at the shoulder joint - continuation of movement initiated by the supraspinatus muscle
Posterior Part:
  • Retracts (draws forward) the humerus / upper-arm
  • Extends the humerus / upper-arm.
  • Laterally rotates the humerus / upper-arm.

The deltoid muscles are labelled on the diagrams of the human anterior muscles and the human posterior muscles.

More about Muscles:

The following are some popular pages in the section about the human muscular system:

There are also more pages about the words used to describe muscles (terminology), the anatomy and physiology of muscles, muscular disorders and associated topics. See also books about sports medicine.

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