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.   Basically, a tunnel comprised of hard tissue (the rib), bound by a variety of other tissues (muscles, fibrous strands, and blood vessels), supplies the innervation for the upper quadrant of the body. An abnormal increase in factors that cause further irritation - such as a bony callus from a fractured rib or, more likely, collarbone; pumped-up muscles from weight lifting; or traumatized muscles from activities such as competitive volleyball - can compromise this space and irritate these large nerve bundles. Abnormalities in the blood vessels, such as clots or dilatations, can produce the same effect. Movement of the shoulder and arm changes all of these relationships. If one imagines a triangular pyramid with the corners of the base at the spine, at the chest bone, and deeper at the first rib, and the apex at the shoulder, one can see that elevating the tip of the pyramid brings the adjacent sides closer together, squeezing the contents. Over-the-shoulder movements, hunched backs, or reaching all produce this constriction. Once these large nerves become compromised, constriction further aggravates their irritability and increases symptoms. To complicate matters, in the vast majority of cases, the site of injury is not easily determined since it resides inside the nerve trunks.   .