Question

In absence of deltoid, trick abduction movement of shoulder joint is performed by

Answer 1

The trapezius - or traps, as commonly referred to - are large, paired muscles on the dorsal side of the body (that's the back side). The traps have multiple origins, including the skull and the vertebral column. They insert on the clavicle and scapula of the pectoral girdle. Contraction of the trapezius muscles results in elevation and retraction of the shoulders, as well as extension of the neck. All of these things help to hold the shoulder in position or stabilize the shoulder.

If we remove the trapezius, we can see the deeper levator scapulae, which, as its name suggests, elevates and stabilizes the scapula. Now, this muscle originates on the cervical vertebrae and inserts on the scapulae - that's plural for scapula. Inferior to the levator scapulae are the rhomboids, which originate on the thoracic vertebrae and insert on the scapula. They retract and adduct and stabilize the shoulders.

We can see two major shoulder stabilizers on the ventral, or front side, of the body. Let's take a look. The pectoralis minororiginates on the superior ribs and inserts on the scapula. Therefore, it depresses and protracts, or abducts the shoulder. The serratus anterior originates on the ribs, courses around the thoracic cavity and inserts on the scapula, thus helping with stabilization and protraction of the shoulder. Let me make a side note. The serratus anterior is named, in part, because of its appearance. Like a serrated knife, this muscle has a serrated appearance on the front surface of the body as you see it moving in towards the ribs.