Deltoid muscle

In human anatomy, the **deltoid muscle** is the muscle forming the rounded contour of the shoulder.

It was previously called the **Deltoideus** and the name is still used by some anatomists. It is called so because it is in the shape of the Greek letter Delta (triangle).

The deltoid is a frequent site to administer intra-muscular injections.

## Origin

It arises in three distinct sets of fibers:

* Anterior fiber: from the anterior border and upper surface of the lateral third of the clavicle
* Middle fibers: from the lateral margin and upper surface of the acromion
* Posterior fibers: from the lower lip of the posterior border of the spine of the scapula, as far back as the triangular surface at its medial end

## Insertion

From this extensive origin the fibers converge toward their insertion, the middle passing vertically, the anterior obliquely backward and laterally, the posterior obliquely forward and laterally; they unite in a thick tendon, which is inserted into the V-shaped deltoid tuberosity on the middle of the lateral aspect of the shaft of the humerus. At its insertion the muscle gives off an expansion to the deep fascia of the arm.

## Innervation

Axillary nerve (C5,C6) and, to a lesser extent, lateral supraclavicular nerve.

## Action

The **anterior fibers** are involved in shoulder abduction when the shoulder is externally rotated. The anterior deltoid is weak in strict transverse flexion but assists the pectoralis major during shoulder transverse flexion / shoulder flexion (elbow slightly inferior to shoulders).

The **posterior fibers** are strongly involved in transverse extension particularly since the latissimus dorsi is very weak in strict transverse extension. The posterior deltoid is also the primary shoulder hyperextensor.

The **lateral fibers** are involved in shoulder abduction when the shoulder is internally rotated, are involved in shoulder flexion when the shoulder is internally rotated, and are involved in shoulder transverse abduction (shoulder externally rotated) -- but are not utilized significantly during strict transverse extension (shoulder internally rotated).

## Training

 File:Bench press.gif
The deltoid muscle has three sections, the front head, the side head and the rear head. These heads have different actions and so need different strength training exercises to completely train the whole of the muscle.

* The *front* (anterior) deltoid is used in all pressing movements, such as the bench press, and can be trained adequately without any isolation movements. However for isolation front dumbbell raises are the most commonly used.

* The *middle* (medial, lateral) head of the deltoid is the main abductor at the gleno-humeral joint (shoulder) and is commonly trained using side lateral raises (also known as the shoulder fly. The middle head also benefits from overhead pressing movements.

* The *rear* (posterior) head of the deltoid is best trained with bent-over rows, or to use less weight, dumbbell laterals (an inverted fly)

## Structure

The Deltoid is a classical example of a multipennate muscle.

The middle fibres of the muscle arise in a bipenniform manner (like a bird's feather) from the sides of the tendinous intersections, generally four in number, which are attached above to the acromion and pass downward parallel to one another in the substance of the muscle. The oblique fibers thus formed are inserted into similar tendinous intersections, generallyree in number, which pass upward from the insertion of the muscle and alternate with the descending septa. The portions of the muscle arising from the clavicle and spine of the scapula are not arranged in this manner, but are inserted into the margins of the inferior tendon.

## Additional images

Gray410.pngSuperficial muscles of the chest and front of the arm.Gray200.pngLeft clavicle. Superior surface.Gray203.pngLeft scapula. Dorsal surface.Gray207.pngLeft humerus. Anterior view.Gray411.pngDeep muscles of the chest and front of the arm, with the boundaries of the axilla.Gray412.pngMuscles on the dorsum of the scapula, and the Triceps brachii.Gray521.pngThe scapular and circumflex arteries.Gray523.pngThe axillary artery and its branches.Gray525.pngThe brachial artery.Gray576.pngThe veins of the right axilla, viewed from in front.Gray808.pngThe right brachial plexus with its short branches, viewed from in front.Gray810.pngSuprascapular and axillary nerves of right side, seen from behind.Nerves of the left upper extremity.gifNerves of the left upper extremity.Gray818.pngThe suprascapular, axillary, and radial nerves.Gray1211.pngSurface anatomy of the back.
## References

1. ↑ Mnemonic at medicalmnemonics.com *3558*

## External links

* *-691011507* at GPnotebook
* LUC *delt*
* SUNY Labs *03:03-0103*
* *Muscles/DeltoidPosterior at exrx.net*
* *Muscles/DeltoidAnterior at exrx.net*
* *Muscles/DeltoidLateral at exrx.net*

ar:عضلة دالية
ca:Deltoide
cs:Deltový sval
de:Musculus deltoideus
es:Deltoides
fr:Muscle deltoïde
hr:Deltoidni mišić
id:Otot deltoideus
it:Deltoide (muscolo)
he:שריר הדלתא
la:Musculus deltoideus
hu:Deltaizom
nl:Musculus deltoideus
ja:三角筋
pl:Mięsień naramienny
pt:Músculo deltóide
ru:Дельтовидная мышца
sl:Deltasta mišica
fi:Kolmipäinen hartialihas
sv:Deltamuskel
th:กล้ามเนื้อเดลทอยด์
zh:三角肌