The deltoid muscle is the

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

forming the rounded contour of the

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...

shoulder The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder mak ...

. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic

cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...

. Anatomically, the deltoid muscle appears to be made up of three distinct sets of

muscle fibers A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mus ...

, namely the # anterior or clavicular part (pars clavicularis) # posterior or scapular part (pars scapularis) # intermediate or acromial part (pars acromialis) However,

electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

suggests that it consists of at least seven groups that can be independently coordinated by the

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

. It was previously called the deltoideus (plural ''deltoidei'') and the name is still used by some anatomists. It is called so because it is in the shape of the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

capital letter delta (Δ). Deltoid is also further shortened in

slang Slang is vocabulary (words, phrases, and linguistic usages) of an informal register, common in spoken conversation but avoided in formal writing. It also sometimes refers to the language generally exclusive to the members of particular in-gr ...

as "delt". A study of 30 shoulders revealed an average mass of in humans, ranging from to .

Structure

Previous studies showed that the insertions of the tendons of the deltoid muscle parts formed three discrete sets of muscle fibers, often referred to as "heads": # The

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

or clavicular fibers arise from most of the anterior border and upper surface of the lateral third of the clavicle. Retrieved January 2012. The anterior origin lies adjacent to the lateral fibers of the ''

pectoralis major The pectoralis major () is a thick, fan-shaped or triangular convergent muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor, ...

'' muscle as do the end tendons of both muscles. These muscle fibers are closely related and only a small chiasmatic space, through which the cephalic vein passes, prevents the two muscles from forming a continuous muscle mass. #*The anterior deltoidsAnatomy, Shoulder and Upper Limb, Deltoid Muscle - NCBI Bookshelf
/ref> are commonly called front deltsPick up your delts
from Muscle and Fitness: "target point: front/middle delts" for short. # Intermediate or acromial fibers arise from the superior surface of the

acromion In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The ...

process of the

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...

. #*They are also commonly called lateral deltoid.Lateral deltoid
/ref> This muscle is also called middle delts, outer delts, or side delts for short. #*They are also mistakenly called medial deltoid, which is wrong, as their origin is the least medial portion of the deltoid. # Posterior or spinal fibers arise from the lower lip of the posterior border of the spine of the scapula. #*They are commonly called posterior deltoid or rear deltoid (rear delts for short). Fick divided these three groups of fibers, often referred to as parts (Latin: ''pars'') or bands, into seven functional components as did Kapandji and Sakoma Y et al. : the anterior part has two components (I and II); the lateral one (III); and the posterior four (IV, V, VI, and VII) components. In

standard anatomical position The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...

(with the upper limb hanging alongside the body), the central components (II, III, and IV) lie lateral to the axis of abduction and therefore contribute to abduction from the start of the movement while the other components (I, V, VI, and VII) then act as adductors. During abduction most of these latter components (except VI and VII which always act as adductors) are displaced laterally and progressively start to abduct.

Insertion

From this extensive origin the fibers converge toward their insertion on the

deltoid tuberosity In human anatomy, the deltoid tuberosity is a rough, triangular area on the anterolateral (front-side) surface of the middle of the humerus. It is a site of attachment of deltoid muscle. Structure Variation The deltoid tuberosity has been re ...

on the middle of the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

aspect of the shaft of the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...

; the intermediate fibers passing vertically, the anterior obliquely backward and laterally, and the posterior obliquely forward and laterally. Though traditionally described as a single insertion, the deltoid insertion is divided into two or three discernible areas corresponding to the muscle's three areas of origin. The insertion is an arch-like structure with strong anterior and posterior fascial connections flanking an intervening tissue bridge. It additionally gives off extensions to the deep brachial fascia. Furthermore, the

deltoid fascia The deltoid muscle is the muscle forming the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat. Anatomically, the deltoid muscle appears to be made u ...

contributes to the brachial fascia and is connected to the medial and lateral intermuscular septa.

Blood supply

The deltoid is supplied by the thoracoacromial artery (acromial and deltoid branches), the circumflex humeral arteries, and the profunda brachii artery (deltoid branch). (Standring, 2005).

Nerve supply

The deltoid is innervated by the axillary nerve. The axillary nerve originates from the anterior rami of the cervical nerves C5 and C6, via the superior trunk, posterior division of the superior trunk, and the posterior cord of the brachial plexus. Studies have shown that there are seven neuromuscular segments to the deltoid muscle. Three of these lie in the anatomical anterior head of the deltoid, one in the anatomical middle head, and three in the anatomical posterior head of the deltoid. These neuromuscular segments are supplied by smaller branches of the axillary nerve, and work in coordination with other muscles of the shoulder girdle include pectoralis major and supraspinatus. The axillary nerve is sometimes damaged during surgical procedures of the axilla, such as for

breast cancer Breast cancer is cancer that develops from breast tissue. Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, milk rejection, fluid coming from the nipple, a newly inverted nipple, or ...

. It may also be injured by anterior dislocation of the head of the humerus.

Function

When all its fibers contract simultaneously, the deltoid is the prime mover of arm abduction along the frontal plane. The arm must be medially rotated for the deltoid to have maximum effect. This makes the deltoid an antagonist muscle of the ''pectoralis major ''and ''

latissimus dorsi The latissimus dorsi () is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: ''latissimi dorsorum'') comes from ...

'' during arm adduction. The anterior fibers assist the ''pectoralis major'' to flex the shoulder. The anterior deltoid also works in tandem with the subscapularis, pecs and lats to internally (medially) rotate the humerus. The intermediate fibers perform basic shoulder abduction when the shoulder is internally rotated, and perform shoulder transverse abduction when the shoulder is externally rotated. They are not utilized significantly during strict transverse extension (shoulder internally rotated) such as in rowing movements, which use the posterior fibers. The posterior fibers assist the ''

'' to extend the shoulder. Other transverse extensors, the ''

infraspinatus In human anatomy, the infraspinatus muscle is a thick triangular muscle, which occupies the chief part of the infraspinatous fossa.''Gray's Anatomy'', see infobox. As one of the four muscles of the rotator cuff, the main function of the infraspin ...

'' and '' teres minor'', also work in tandem with the posterior deltoid as external (lateral) rotators, antagonists to strong internal rotators like the pecs and lats. An important function of the deltoid in humans is preventing the

dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...

of the humeral head when a person carries heavy loads. The function of abduction also means that it would help keep carried objects a safer distance away from the thighs to avoid hitting them, as during a

farmer's walk Strength athletics, also known as Strongman competitions, is a sport which tests competitors' strength in a variety of non-traditional ways. Some of the disciplines are similar to those in powerlifting and some powerlifters have also successfully ...

. It also ensures a precise and rapid movement of the

glenohumeral joint The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint ...

needed for hand and arm manipulation. The intermediate fibers are in the most efficient position to perform this role, though like basic abduction movements (such as lateral raise) it is assisted by simultaneous co-contraction of anterior/posterior fibers. The deltoid is responsible for elevating the arm in the scapular plane and its contraction in doing this also elevates the humeral head. To stop this compressing against the undersurface of the

the humeral head and injuring the supraspinatus tendon, there is a simultaneous contraction of some of the muscles of the

rotator cuff The rotator cuff is a group of muscles and their tendons that act to stabilize the human shoulder and allow for its extensive range of motion. Of the seven scapulohumeral muscles, four make up the rotator cuff. The four muscles are the supraspi ...

: the

and subscapularis primarily perform this role. In spite of this there may be still a 1–3 mm upward movement of the head of the humerus during the first 30° to 60° of arm elevation.

Clinical significance

The most common abnormalities affecting the deltoid are tears, fatty atrophy, and

enthesopathy An enthesopathy refers to a disorder involving the attachment of a tendon or ligament to a bone. This site of attachment is known as the enthesis (pl. entheses). If the condition is known to be inflammatory, it can more precisely be called an ...

. Deltoid muscle tears are unusual and frequently related to traumatic shoulder dislocation or massive rotator cuff tears. Muscle atrophy may result from various causes, including aging, disuse, denervation,

muscular dystrophy Muscular dystrophies (MD) are a genetically and clinically heterogeneous group of rare neuromuscular diseases that cause progressive weakness and breakdown of skeletal muscles over time. The disorders differ as to which muscles are primarily af ...

cachexia Cachexia () is a complex syndrome associated with an underlying illness, causing ongoing muscle loss that is not entirely reversed with nutritional supplementation. A range of diseases can cause cachexia, most commonly cancer, congestive heart fa ...

and iatrogenic injury. Deltoideal humeral enthesopathy is an exceedingly rare condition related to mechanical stress. Conversely, deltoideal acromial enthesopathy is likely a hallmark of seronegative spondylarthropathies and its detection should probably be followed by pertinent clinical and serological investigation.

Other animals

The deltoid is also found in members of the

great ape The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the ...

family other than humans. The human deltoid is of similar proportionate size as the muscles of the rotator cuff in apes like the

orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genu ...

, which engage in

brachiation Brachiation (from "brachium", Latin for "arm"), or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb ...

and possess the muscle mass needed to support the body weight by the shoulders. In other apes, like the

common chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative t ...

, the deltoid is much larger than in humans, weighing an average of 383.3 gram compared to 191.9 gram in humans. This reflects the need to strengthen the shoulders, particularly the rotatory cuff, in knuckle walking apes for the purpose of supporting the entire body weight. The deltoid muscle is a main component of both the bat and

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

wing musculature, but in crown-group birds it is strongly reduced, as they favour sternum attached muscles. Some Mesozoic flying theropods, however, had more developed deltoideus.

References

* Standring, S. (2005). Gray's Anatomy: The Anatomical Basis of Clinical Practice (39th ed.). Elsevier Churchill Livingstone. {{DEFAULTSORT:Deltoid Muscle Muscles of the upper limb Shoulder abductors Shoulder flexors Shoulder medial rotators Shoulder extensors Shoulder lateral rotators