These are the infamous muscles known to all body builders and athletes. Developing these muscles help you attain the v shape that all women crave. But there are far more interesting, intricate details about these muscles that body builders don't know. We'll be going through some of those in this article.
Latismus Dorsi
Latismus dorsi is the widest muscle in the human body. It covers the majority of the dorsum of the torso. Well-developed latismus dorsi muscles, most notably seen on Bruce Lee, extend and towards the arms when flexed, resembling wings of a bat. Perhaps Bruce Lee is the real bat man.
Attachment
It originates from the spinous processes of the lower six thoracic vertebrae, and the interspinous ligaments. The muscle origin extends below, down to the lumbar vertebrae. A broad aponeurotic layer is called the thoracolumbar fascia connects the latismus dorsi to the spinous processes of the lumbar vertebrae and its related interspinous ligaments. The thoracolumbar fascia also connects the latismus dorsi to the iliac crest. Sometimes it is also attached to the lower 3 or four ribs. Fibres arising from this broad area of origin, converge and insert into the floor of the intertubercular sulcus.
Innervation
The latismus dorsi is innervated by the thoracodorsal nerve (C6, C7, C8).
Actions
The latismus dorsi can adduct the humerus at the shoulder joint, it can medially rotate the arm and also extend the arm at the glenohumeral (shoulder) joint. Hence, wide grip pull-ups, which adduct the arm, and barbell rows and one arm row exercises which extend the arm at the shoulder joint are the exercises of choice to develop the latismus muscle. Oarsmen and swimmers are known for having significantly developed latismus dorsi muscles. Although rare, posterior shoulder displacements are more commonly seen in these sports because of the increased backward force on the shoulder due to the over developed latismus dorsi muscle.
Trapezius
The trapezius is a large muscle of the back, that also involves the scapula, hence the movement of the upper limb. The supraspinatus, levator scapulae, rhomboid major and rhomboid minor muscles are all completely covered by the trapezius muscle.
Attachments
Proximally, the muscle originates from the superior nuchal line and external occipital protuberance of the skull, the medial margin of the ligamentum nuchae, the spinous processes of C7 to T8 and their supraspinous ligament. The ligamentum nuchae is the continuation of the supraspinous ligament above the C7 vertebral level.
Distally it gets inserted into the superior edge of the crest of the spine of the scapula, the acromion process, and the posterior border of lateral on-third of the clavicle.
Innervation
This muscle is innervated by a cranial nerve, not by a spinal nerve; which means this nerve doesn't arise from the spinal cord. It arises directly from a part of the brain stem and comes out through a foramen of the skull. The Cranial nerve 11 or CN XI has two part. Spinal part and a cranial part. The spinal part is what innervates the trapezius muscle.
Action
The fibres above the spine of the scapula elevate the scapula. This is the muscle involved in shrugging. The middle fibres can retract the scapula. The inferior fibres can depress the scapula. The superior and inferior fibres together can rotate the scapula in abduction of the shoulder joint beyond 90 degrees.
Deltoid
The deltoid is a bulky multi-pennate muscle. This muscle is responsible for the circular bulge on the shoulder seen in swimmers and gymnasts.
Attachments
The deltoid originates from the inferior edge of the crest of the spine of scapula, the lateral margin of the acromion process, the anterior border of the lateral one third of the clavicle. (the posterior border of which is attached to the trapezius). Distally, the deltoid attaches to the deltoid tuberosity of the humerus, which is located in the mid-shaft.
Innervation
The deltoid muscle is innervated by the Axillary nerve. Its nerve supply is also shared by the teres minor. Damage to the axillary nerve in anterior dislocation of the shoulder joint paralyses the deltoid muscle and causes numbness on the regimental badge area.
Action
The deltoid is the main abductor of the arm at the shoulder joint. Its mechanical advantage for abduction is from 15 degrees to 90 degrees. The abduction up to that point is done by the supraspinatus muscle. The abduction of the humerus ate the glenohumeral joint is not possible beyond 90 degrees because of the acromion process, which lies above the glenohumeral joint. However, abduction of the arm beyond 90 degrees is possible. This is because of the rotation of the scapula itself, which is done by Trapezius and Serratus anterior. Therefore, abduction of the arm beyond 90 degrees is not an action of the deltoid muscle.
Pectoralis Major
This is the muscle that Dwayne Johnson (the rock), and Terry Crews flexes when they twerk their chests. This is a muscle that deserves its own separate article.
attachments
At its origin, it has two heads. The clavicular head and the sternocostal head. The clavicular head arises from the anterior surface of the medial half of the clavicle. The sternocostal head of the pec-major originates from the anterior surface of the sternum, the first seven costal cartilages, and the sternal end of the 6th rib. The sternocostal head is also attached to the aponeurosis of the external oblique muscle, inferiorly.
These two heads converge into one tendon which inserts onto the lateral lip of the intertubercular sulcus of the humerus. The fibres insert in an inverted U shape. The fibres of sternocostal head are twisted in such a way that the top most fibres are attached most inferiorly at the insertion and vice versa.
Innervation
This muscle is innervated by the medial and lateral pectoral nerves. The clavicular head is innervated by the nerve roots C5 and C6, while the sternocostal head is innervated by the nerve roots C5, C6, C7, C8 and T1. This means that damage to the spinal cord at C7 level leaves the clavicular head of the pectoralis major intact, while the tone of the sternocostal head will be affected.
Action
The pectoralis major flexes, adducts and medially rotates the arm at the shoulder joint. The clavicular head is responsible for the flexion of the extended arm. The sternocostal head is responsible for the extension of the flexed arm.
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