Deltoid muscle

The deltoid is a muscle on your shoulder.

It makes your shoulder look round. It is shaped like a Greek letter. This muscle helps you lift your arm. It also helps you move your arm fast.

The muscle has three parts. These parts work together to move you. It can even help you carry heavy things. This keeps the weight away from your legs. It is a very busy muscle!

The deltoid is the muscle that makes your shoulder look round.

Its name comes from a Greek letter. That letter is shaped like a triangle. This muscle has three main parts. The first part is in the front. It is called the anterior deltoid. The middle part is the acromial part. The last part is in the back. It is called the posterior deltoid.

These parts help you move your arm in many ways. When all parts work together, they lift your arm out to the side. This movement is called abduction. The front part helps you lift your arm forward. The back part helps you move your arm backward. The muscle also helps you rotate your arm.

Your body uses a nerve called the axillary nerve to control it. This nerve sends signals to the muscle. The deltoid also helps keep your shoulder joint safe. It stops the top of your arm bone from slipping out of place. This is helpful when you carry heavy things. It keeps the weight away from your legs.

The deltoid muscle creates the rounded shape of your shoulder.

It is often called the common shoulder muscle. This name is used for animals like the domestic cat. The name comes from the Greek letter delta. That letter looks like a triangle.

Some people even call it a "delt" for short. Scientists have studied how much this muscle weighs. In humans, the average mass is between 150 and 250 grams. This muscle is very important for moving your arm.

The muscle works in three main parts. The anterior part is in the front. The intermediate part is in the middle. The posterior part is in the back.

These fibers start at the collarbone and the shoulder blade. They all meet at a spot on your upper arm bone. This bone is called the humerus. The fibers attach to a place called the deltoid tuberosity. This structure helps the muscle pull on the bone. It works like a strong bridge for your arm.

Your nervous system tells the muscle how to move. It uses a specific path called the axillary nerve. This nerve comes from the C5 and C6 nerves.

The nerve can be hurt during some surgeries. It can also be hurt if your shoulder pops out of place. There are actually seven small groups of muscle fibers. These groups can work on their own. They work together with other muscles like the pectoralis major. This teamwork allows for very precise arm movements.

Moving your arm happens in many different ways. When all the fibers pull at once, you lift your arm out to the side. This is called abduction.

The front part helps you lift your arm forward. The back part helps you move your arm backward. The muscle also helps rotate your arm. It even helps keep your arm bone from slipping out of the joint. This is very useful when you carry heavy loads. It keeps the weight away from your thighs.

Many different animals have this muscle too. Chimpanzees have a much larger deltoid than humans. Their muscle weighs about 383.3 grams on average. This helps them support their weight while walking on their knuckles.

Bats and pterosaurs also used this muscle to fly. Some birds have a much smaller version of it. The deltoid is a key part of how many creatures move. It shows how much we share with the animal world.

The deltoid muscle, or musculus deltoideus, is the muscle that creates the rounded contour of the human shoulder.

It is often referred to as the "common shoulder muscle," a term also applied to animals like the domestic cat. The name is derived from the Greek capital letter delta (Δ), which matches the muscle's triangular shape. In casual settings, people often shorten the name to "delt." In humans, the average mass of this muscle typically ranges from 150 to 250 grams. This muscle is essential for a wide variety of complex arm movements.

Anatomically, the deltoid is composed of three distinct sets of muscle fibers. The anterior or clavicular part (pars clavicularis) originates from the lateral third of the clavicle, which is the collarbone.

This part sits adjacent to the pectoralis major muscle, separated only by a small space where the cephalic vein passes. The intermediate or acromial part (pars acromialis) arises from the superior surface of the acromion process on the scapula. Finally, the posterior or spinal part (pars scapularis) originates from the lower lip of the posterior border of the scapular spine.

These various fibers converge toward a single insertion point on the humerus, the bone of the upper arm. They attach to a specific area called the deltoid tuberosity, located on the middle of the lateral shaft of the humerus.

While traditionally viewed as one attachment, the insertion is actually divided into two or three distinct areas. These areas correspond to the three different origins of the muscle fibers. The insertion forms an arch-like structure with strong connections to the surrounding fascia. This includes the deep brachial fascia and the medial and lateral intermuscular septa.

Movement of the deltoid is controlled by the nervous system through the axillary nerve. This nerve originates from the anterior rami of the cervical nerves C5 and C6. It travels through the superior trunk and the posterior cord of the brachial plexus to reach the muscle.

While the muscle is described as having three anatomical heads, electromyography suggests it contains at least seven neuromuscular segments. These segments include three in the anterior head, one in the middle, and three in the posterior head. These segments allow the nervous system to coordinate the muscle with incredible precision.

When all the fibers contract at the same time, the deltoid acts as the prime mover for arm abduction. Abduction is the process of moving the arm away from the midline of the body along the frontal plane.

The muscle also assists in several other directions depending on which fibers are active. The anterior fibers help the pectoralis major to flex the shoulder and assist in internal rotation. The intermediate fibers perform basic abduction and transverse abduction. The posterior fibers assist the latissimus dorsi to extend the shoulder and act as external rotators.

Beyond simple movement, the deltoid plays a critical role in joint stability. It helps prevent the humeral head from dislocating when a person carries heavy loads. This is useful during activities like a "farmer's walk," where it keeps objects at a safe distance from the thighs. During arm elevation, the deltoid also lifts the humeral head. To prevent this head from compressing the supraspinatus tendon against the acromion, the rotator cuff muscles must contract simultaneously. This coordination ensures the joint moves smoothly without causing injury.

Because of its location, the deltoid is a common site for intramuscular injections. Medical professionals usually inject into the lower half of the muscle to avoid the axillary nerve. This nerve winds around the surgical neck of the humerus and could be damaged by an incorrect injection. Other clinical issues include muscle tears, which often follow shoulder dislocations. Atrophy, or muscle wasting, can also occur due to aging, disuse, or nerve injury. Doctors often test deltoid strength by asking a patient to move their arm against resistance.

Comparing humans to other animals reveals how the deltoid adapts to different lifestyles. In chimpanzees, the deltoid is much larger than in humans, averaging 383.3 grams. This extra mass supports the weight of the animal during knuckle walking.

In great apes like orangutans, the deltoid is proportionate to the rotator cuff to support brachiation, or swinging through trees. The muscle is also a key component of the wing musculature in bats and pterosaurs. However, in most modern birds, the deltoid is significantly reduced because they rely more on muscles attached to the sternum.