Author: Randeep Singh / go to all Science of Yoga articles
The three prominent, finger like undulating parallel bulges,
runing from over the shoulders to the lateral side of the upper arms,
never go unoticed as symbols of muscular strength,
which are responsible for imparting the exaggerated,
rounded outline to the shoulders are deltoid muscles.
These muscles take their name from the
Greek letter delta as both resemble the shape
of a eqvilateral triangle. Though, at first glance, the shape of the deltoid muscle evokes in the mind the shape of the petal of a flower with its broader base glued transversely along the border of the shoulder with the torso and its pointed end pressed over the center of the upper arm.
Deltoid mucles were earlier known as Deltoideus, or even Delts. These muscles can be easily seen or felt as they are located just under the skin of the shoulders.
Structural Anatomy of Deltoid Muscle
Deltoid muscle, along with other straited skeletal muscles, ioriginates out of the mesodermic (middle) layer present in the foetus during pregnancy. This middle layer contains myoblasts, the cells from which originate the muscle fibers as the foetus develops in the womb. Muscle fibers of the deltoid in particular develop from the myoblasts present in the upper backside (dorsal) of the body.
Deloid muscle, like any other skeletal muscle, is attached to any two given points on the bones of the skeleton: origin ( which remains immobile during an action) and the insertion ( which moves during an action) points. Also, like all other skeletal muscles deltoid muscle fibers do not directly attach to these pints on the respective bones; the muscle fibers attach to tendons which in turn attach to the bones.
In order to understand the origin of the deltoid muscle one need to first understand the dtructure of the bones which form the shoulder joint. Two nearly linear bone structures, joiined together at the ends form a bony strap which begin from the center front of the upper rib cage ( sternum), turns around the latera; side of the rib cage and ends fixed on the back side of the rib cage.
The end of one part of this bony strap around the ribcage reamins fastened ( with the help of ligamenst and muscles) on to the center of the upper sternum, the other end of this semi curved, strip like bone reaches upto just short of the top of the rib cage on one side. This part of the shoulder strap is known as clavicle, or the collar bone since it exactly run under the path of the collars of the shirt when worn by the individual.
Thus, there are two clavicular bones running from the center of the upper sternum toward each shoulder forming a semicircular ring running across the base of the neck as seen from the front of the body. Clavicular bones can be easily felt by hand as strisp of bones running transversely to the rib cage, encircling the neck line. The second part of the shoulder strap begins as a triangular plate of bone which remains fastened to the uacromion pper back of the rib cage.
This bony plate with three sides is known as the scapula. The edge of the scapula which is towards the spine rises into a ridge as it moves up from its lower end, this raised ridge of bone from the scapula (spine of the scapula) turns over the upper edge of the scapula away from the spine and then turns into a flat rectangular strip like structure which leaves the body of scapula, moves around the outer, upper edge of the rib cage and joins the free edge of the clavicular bone coming from the sternum.
The portion of the strip like projection coming from the scapula which turns around the lateral edge of the upper rib cage and joins the collar bone, in the front, is known as the acromion process of the scapulla. The clavicular bone acromion process of the scapula, and the ridge of the scapula (raised bony line) form a continous line for the broader edge of the deltoid muscle to remain latched on to.
The three bands of the Deltoid Muscle
From each of these three parts of the bony strap of the shoulder girdle, the collar bone, the acromion, and the Ridge (spine) of scapula, originates three bands of muscle fibers fastened to the bones with the help of tendons. So, the origin of the deltoid muscle is much broader from over the shoulder, where as the insertion is a pointed other end into which the muscle fibers of each of the these bands converge, entwine, along with the end tendons, to form a single insertion poin located some midway on the later shaft of the humerous (bone of the upper arm) known as the deltoid tuberosity. The ghenohumeral joint ( joint where the head of the humerous (upper arm muscle) attaches to the shoulder girdle) lies below the deltoid muscle.
The muscle fibers which originate from the front part, and lateral third of the clavicular bone form the clavicle band portion, known as the anterior (front) deltoids. The band of muscle fibers which originates in tandem with the corresponding tendond from the upper and lateral acromion region is known as the acromial band or the lateral deltoids, and the band of muscle fibers which attaches along the ridge, spine of the scapula as its origin are known as the posterior deltoids.
This muscle is peculiar in its structure as compared to other skeletal muscles by way of oblique arrangement of the muscle fibers in the acromial band (central) in contrast to the arrangement of muscle fibers in the other two bands. In case of posterior, and anterior bands of the deltoid the muscle mass is composed of long muscle fibers placed parallel to each other as bundles along the length of, and forming the body of the muscle, tendons are attached at the two ends of the muscle bands which bind them to their corresponding points of attachment.
The tendons do not run along the mass of the body of the muscle band. In the case of the middle band, lateral deltoid, the tendons run into, and along the length of the muscle mass from one end to the other, and the shorter muscles fibers are attached obliquely between two adjacent tendons,instead of runing along the entire length of the muscle band. This type of arragement of the muscle fibers on the tnedons is known as multipennate arrangement.
This perculiar arragement of muscle fibers in the lateral band of the deltoid muscle imparts it a coarse structure and more strength to its function.
Function of this Upper Arm Muscle
Owing to the location of the deltoid muscle, over the shoulder joint, the main functions of this muscle can be adduced as helping in the movement and protection of the shoulder joint. As per Fick, the three main bands of deltoid muscle can be divided into seven functional units. The anterior deltoid has 2 components (1,2), the lateral deltoid has one functional component (3), and the posterior deltoid can be divided into 4 functional components ( 4, 5, 6, 7).
Deltoid Muscle Actions – Abduction
All the seven segmentations of the deltoid muscle play different roles in abduction and adduction of the arm and keeping the shoulder joint stabilized during these movements. The lateral band of the deltoid plays a major role in enabling abduction of the corresponding arm only after the arm has been initially lifted (abducted) to a minimum of 15 degrees. The lateral deltoid is ineffective in abducting the arm when placed at an angle of less than 15 degrees as its axis of contraction (which creates the lift) remains parallel to the axis of abduction of the arm in this position.
Thus, for giving the arm an initial threshhold lift the functional component 2 ( located in anterior deltoid band) and the functional component 4 ( located in the posterior deloid band) work with the lateral deltoid since the axises of contraction of these two additional muscle bands are lateral to the axis of abduction of the arm. Rather, its only the functional components 6 and 7 (located in the posterior deltoid) that always act as adductors, all the other functional components act as abductors – as they shift laterally – in various degrees during the arm’s lateral lift.
Supraspinatus muscle, also a rotator cuff muscle, is located transversely in the upper back region over the scapula also works in tandem with the deltoid muscle to abduct the arm to its intial 15 degrees after which the acromial band takes over. In order for the lateral deltoid to effectively aid in abduction of the arm the arm must remain internally rotated as it is then that the deltoid becomes the antagonistic muscle to the two muscles: Pectoralis Major which covers one half of the chest like a fan in the front; and Latissmus Dorsi muscle whic is its counterpart in the upper back region.
Deltoid Muscle Actions – Adduction, Flexion, and Extension
The anterior deltoid in conjunction with the Pectoralis Major muscle help keep the the shoulder, and thus the arm, adducted inwards (internally rotated) or in the anatomically neutral position of the arm. This helps in the flexion of the shoulder joint when one attempst to reach something located in the front.
The posterior deltoid muscle band acts with Lattismus Dorsi muscle to extent, rotate the shoulder joint, and the arm outwards and backwards. Such a motion of the arm is frequently seen while when one tries to reach the upper back while dressing or get into the arms position as in Gomukhasana.
Another of a very significant functin fo he deltoid muscle is to stabilize the shoulder joint, glenohemural joint, while its various actions are underway. Any imbalance in the action fo the muscles surrounding the shoulder joint while the movement at the joint is underway can cause the joint to dislocate. Anatomically the shoulder joint is not a very stable and strong joint compared to other major joints in human skeleton.
The reason for this is that unlike other joints (hip joint) the upper head of the arm bone (humerus) doesn’t remains inserted into any cooresponding socket in the shoulder girdle, rather, the head of the humerous just remains attached to a the surface of contact located in the scapula with the help of ligamenst and the supporting muscles. Due to which this joint doesn’t remain confined to any socket (grove within which the movement can remain confined), while executing the motion of abduction the head of the humerus gets elevated towards under the acromion projection of the shoulder girdle.
This can cause detrimental compression to the acromion region of the shoulder girdle. In order to prevent this elevation of the humerus beyond a safer 1-3 mm the posterio band of the deltoid muscle in tandem with other rotator cuff muscles contract adequately during abduction.
Deltoid Muscle – Blood and Nerve Supply
The Thoracoacromial branch of the Axillary artery is the main blood channel to the deltoid muscle. It emerges from the second Axillary artery located behind the Pectoralis Minor muscle in the front. Posterior Circumflex artery, and the branches of the Profunda Bracii are minor contributors to the deltoid muscle.
These arteries pass through the space between the deltoid musle and the humerus bone making them susceptible to damage in case of any injury to the shoulder joint. Cephalic vein located in the delto pectoral groove is the main venous drainage pathway for the deltoid muscle.
Deltoid muscle receive nerve signals, nervous coordination, from the Axillary nerve which emerges from the front junction of the C5 and C6 nerves emerging from the cervical region of the spine. Similar too the seven functional sections present within the entire shoulder spread of the deltoid muscle seven neuromuscular segments have been discovered over it.
Three of these segments are located int he anterior band, one in the lateral band, and three in the posterior band of deltoid muscle fibers.