The deltoid muscle is a tripartite muscle mass that connects the humerus bone of the arm to the scapula and clavicle bones. When you raise your arms above your head, the deltoid muscle is at work. This muscle, also known as the common shoulder muscle, is seen in many different vertebrate species!
The deltoid muscles (on each shoulder) are one of the main muscles that help move the arms (or limbs) up, down, forward, and backward. Each deltoid consists of three parts: an anterior deltoid, a posterior deltoid, and an intermediate deltoid. This muscle connects to both the scapula and the collar bone, allowing the arm to be moved in many directions.
This muscle not only keeps the humerus bone engaged in the shoulder socket, but the muscle can move the arm up, down, forward and backward. While primates can use this muscle to help themselves swing through trees (or throw a ball), other animals have other uses. Bats, for example, use the deltoid as a primary flying muscle.
Where is the Deltoid Muscle?
The deltoid muscle has attachments on the humerus bone. These attachments are formed nearly halfway down the humerus, which allows these muscles to move the humerus upward when they are flexed.
If you feel your shoulder, nearly the entire muscle mass you can feel is your deltoid. On the front side, the deltoid muscle extends from the humerus to the clavicle (collar bone). You can feel it flex when you move your arm forward. On the backside, the deltoid extends between the humerus and your scapula (shoulder blade). You can feel this muscle flex when you move your arm backward.
If you raise your arm above your head, you can feel the mass of the deltoid muscle flex. It is the shortening action of this muscle that pulls the humerus into an upright position.
Each deltoid is broken up into three groups of muscle fibers. These acromial fibers include the anterior acromial fibers, posterior acromial fibers, and intermediate (or lateral) acromial fibers. All three of these fibers have an insertion point that is halfway down the humerus, giving the muscles enough leverage to move the limb.
The three parts of the deltoid muscle
However, each of these three deltoid parts connects to a different region in the shoulder. The anterior deltoid connects to the clavicle (the collar bone). The intermediate fibers connect the humerus to the acromion (a bony extension of the scapula that sits at the top of your shoulder). The posterior deltoid connects to the spine of the scapula. Together, these three separate fibers can give the arm a wide range of motion.
The deltoids are used in almost every arm movement humans can complete. They are a major source of force in arm movements, whereas other muscles like the teres minor muscle and trapezius muscle help refine and control these movements.
The deltoid muscle in action
Interestingly, the deltoid only functions well when the palms are facing downward. This is due to how they are connected to the humerus and shoulder bones. You can try this for yourself. If you try to lift something with your palms down, as in the picture above, your deltoid muscle will be highly active.
If you try to flip your palms over and complete the same task, your deltoids will become twisted and highly strained. Movements like these must be completed with the biceps, or damage may be done to the deltoids.