Deltoid muscle

Previous studies showed that the insertion of the intramuscular tendons of the deltoid muscle formed three discrete sets of muscle fibers, often referred to as "heads":[3]

1. The anterior or clavicular fibers arise from most of the anterior border and upper surface of the lateral third of the clavicle.[4] The anterior origin lies adjacent to the lateral fibers of the pectoralis major 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.[5]
   • The anterior deltoids[6] are commonly called front delts[7] for short.
2. Lateral or acromial fibers arise from the superior surface of the acromion process of the scapula.[4]
   • They are commonly called lateral deltoid.[8] This muscle is also called middle delts,[7] outer delts,[9] or side delts[8] for short.
   • They are also mistakenly called medial deltoid,[10] which is wrong, as their origin is the least medial portion of the deltoid.
3. Posterior or spinal fibers arise from the lower lip of the posterior border of the spine of the scapula.[4]
   • They are commonly called posterior deltoid[11] or rear deltoid[12] (rear delts for short[13]).

Fick[14] 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. :[15][16] 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 (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.[15]

Side.

Front.

Back.

Animation.

Deltoid muscle.

  Anterior part of deltoid (arises from most of the anterior border and upper surface of the lateral third of the clavicle.)

  Lateral part of deltoid (arises from the superior surface of the acromion process.)

  Posterior part of deltoid (arises from the lower lip of the posterior border of the spine of the scapula.)

Deltoid muscle with superior limb in abduction

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.[23] This makes the deltoid an antagonist muscle of the pectoralis major and latissimus dorsi 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.[24]

The lateral 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.[25]

The posterior fibers assist the latissimus dorsi to extend the shoulder. Other transverse extensors, the infraspinatus 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.[26]

An important function of the deltoid in humans is preventing the dislocation 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. It also ensures a precise and rapid movement of the glenohumeral joint needed for hand and arm manipulation.[2] The lateral 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.[27]

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 acromion the humeral head and injuring the supraspinatus tendon, there is a simultaneous contraction of some of the muscles of the rotator cuff: the infraspinatus 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.[2]

The most common abnormalities affecting the deltoid are tears, fatty atrophy, and enthesopathy. Deltoid muscle tears are unusual and frequently related to traumatic shoulder dislocation or massive rotator cuff tears. Muscle atrophy is the result of various causes, including aging, disuse, denervation, muscular dystrophy, cachexia 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.[28]

The deltoid is also found in members of the great ape family other than humans. The human deltoid is of similar proportionate size as the muscles of the rotator cuff in apes like the orangutan, which engage in brachiation and possess the muscle mass needed to support the body weight by the shoulders. In other apes, like the common 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, in knuckle walking apes for the purpose of supporting the entire body weight.[2]