   
The Rhomboids are the sacrum of the shoulder girdle.
What a weird concept.
Even though one is a muscle and the other bone, they are responsible for connecting the shoulder girdle and the pelvis to the spine.
The pelvis is made up of three bones—two hip bones and the sacrum.
The shoulder girdle is made of four bones—two collar bones and two shoulder blades.
The pelvis connects at the front of the body when the two hip bones meet at the pubic symphysis.
The shoulder girdle connects at the front of the body where the clavicle meets the sternum, the bone at the center of the chest.
The pelvis is solid to allow for weight transfer and locomotion.
The shoulder girdle is much less solid to allow for greater range of motion in the arms.
Our shoulders and hips are both ball and socket joints with a great deal of freedom but because we are upright and the arm is no longer weight bearing it has/needs a lot more freedom to move.
The legs are not afforded the same range of motion because the solid pelvis only allows them to move so far away from the spine.
This is because the pelvis is connected to the spine at its back by the solid sacrum wedged in between the hip bones. The rhomboids play the role of the sacrum in the upper body, connecting the shoulder girdle to the spine, but they lack the solidity that the sacrum provides the lower body.
This is one reason why the arms have access to so much range of motion but also a reason why the shoulder is so vulnerable to injury.
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