The deltoid muscle is the muscle that gives the shoulder its rounded shape. Many mammals have this muscle, as the design is very efficient, and this muscle is responsible for the abduction of the shoulder and some of the rotation of the arm or limb. In humans, the deltoids are a common target of weight training, with people engaging in activities like presses to develop and strengthen them. In people with very well defined musculature and low body fat, the deltoid is often highly visible on the shoulder and upper arm.
This muscle has three heads. The posterior deltoid originates at the scapula, while the anterior and lateral deltoids connect with the collarbone. The other end of the muscle attaches to the humerus, the armbone. Structurally, the muscles resemble triangles, or the Greek letter Δ (delta).
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The primary function of this muscle is to abduct the shoulder. When people rotate a shoulder, the deltoid muscle does much of the work, guiding and stabilizing the movement. Outward rotation of the arm also involves the muscle. In a classic example of this muscle at work, people who swing their arms while walking are relying heavily on the it for the swinging motion.
The axillary nerve, which originates in the brachial plexus, a tight bundle of nerves near the neck, is responsible for innervating the deltoid muscle. Problems with the function of the muscle can arise both as a result of damage to the axillary nerve and as a result of damage to the muscle itself. Strain is a common problem with the shoulder muscles, and can be caused by overextension or lifting too much weight. The symptoms of strain include the sudden onset of pain and soreness, as well as stiffness in the muscle and a limited range of movement.
For mild strains, ice and rest are often enough to allow the muscle to recover. Some patients also find the application of hot compresses beneficial. More severe strains may require treatment, such as physical therapy or medication, to manage pain while the muscle heals. If the axillary nerve is damaged, patients may lose muscle control or notice tingling and painful sensations. Nerve damage may resolve if it is mild, or be retained if it is more severe. A neurologist can conduct an examination to determine whether or not the nerve is really damaged and to determine the severity of the problem.