| Deltoid muscle | |
|---|---|
 Deltoid muscle | |
 Muscles connecting the upper extremity to the vertebral column  | |
| Details | |
| Origin | The anterior border and upper surface of the lateral third of theclavicle,acromion,spine of thescapula |
| Insertion | Deltoid tuberosity ofhumerus |
| Artery | Thoracoacromial artery,anterior andposterior humeral circumflex artery |
| Nerve | Axillary nerve |
| Actions | Shoulderabduction,flexion andextension |
| Antagonist | Latissimus dorsi |
| Identifiers | |
| Latin | musculus deltoideus |
| MeSH | D057645 |
| TA98 | A04.6.02.002 |
| TA2 | 2452 |
| FMA | 32521 |
| Anatomical terms of muscle | |
Thedeltoid muscle is themuscle[1] forming the rounded contour of thehuman shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domesticcat. Anatomically, the deltoid muscle is made up of three distinct sets ofmuscle fibers, namely the
The deltoid's fibres arepennate muscle. However,electromyography suggests that it consists of at least seven groups that can be independently coordinated by thenervous system.[2]
It was previously called thedeltoideus (pluraldeltoidei) and the name is still used by some anatomists. It is called so because it is in the shape of theGreek capital letterdelta (Δ). Deltoid is also further shortened inslang as "delt".
A study of 30 shoulders revealed an average mass of 192 grams (6.8 oz) in humans, ranging from 84 grams (3.0 oz) to 366 grams (12.9 oz).[3]
From this extensive origin the fibers converge toward their insertion on thedeltoid tuberosity on the middle of thelateral aspect of the shaft of thehumerus; 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 thedeep brachial fascia. Furthermore, thedeltoid fascia contributes to the brachial fascia and is connected to themedial andlateral intermuscular septa.[6]
The deltoid is supplied by thethoracoacromial artery (acromial and deltoid branches), the circumflex humeral arteries, and theprofunda brachii artery (deltoid branch).[7]
The deltoid is innervated by theaxillary nerve.[8] The axillary nerve originates from theanterior rami of thecervical nerves C5 and C6, via the superior trunk, posterior division of the superior trunk, and the posterior cord of thebrachial plexus.[9]
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.[10] 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.[10]
The axillary nerve is sometimes damaged during surgical procedures of theaxilla, such as forbreast cancer. It may also be injured by anterior dislocation of the head of the humerus.[11]
Structures under deltoid[12]
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.[13] This makes the deltoid an antagonist muscle of thepectoralis majorandlatissimus dorsi during arm adduction. The anterior fibers assist thepectoralis major to flex the shoulder. The anterior deltoid also works in tandem with thesubscapularis, 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 thelatissimus dorsi to extend the shoulder. Other transverse extensors, theinfraspinatus andteres 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 thedislocation of thehumeral 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 afarmer's walk. It also ensures a precise and rapid movement of theglenohumeral joint needed for hand and arm manipulation.[3] 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.[14]
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 theacromion the humeral head and injuring thesupraspinatus tendon, there is a simultaneous contraction of some of the muscles of therotator cuff: theinfraspinatus 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.[3]
The most common abnormalities affecting the deltoid are tears, fatty atrophy, andenthesopathy. 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,cachexia andiatrogenic injury. Deltoidal 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.[15]
The Deltoid Muscle is tested by asking the patient to abduct the arm against resistance applied with one hand, and feeling for the contracting muscle with the other hand.
Site of the intramuscular injection in deltoid: The intramuscular injections are commonly given in the lower half of the deltoid to avoid injury to the axillary nerve, which[16] winds around the surgical neck of the humerus.
The deltoid is also found in members of thegreat ape family other than humans. The human deltoid is of similar proportionate size as the muscles of the rotator cuff in apes like theorangutan, which engage inbrachiation and possess the muscle mass needed to support the body weight by the shoulders. In other apes, like thecommon chimpanzee, 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, inknuckle walking apes for the purpose of supporting the entire body weight.[3]
The deltoid muscle is a main component of both thebat andpterosaur wing musculature,[17] but in crown-group birds it is strongly reduced, as they favour sternum attached muscles. Some Mesozoic flying theropods, however, had more developed deltoideus.[18]
