Pectoral girdle myalgia in women: a new function for pectoralis major?

Brassière straps transfer breast weight from pectoral fascia to the shoulder region where upper trapezius supports it. It is apparent that upper trapezius also has a posterior component because of its enthesis from the vertebral spines. Contraction of upper trapezius should, therefore, cause retraction of the pectoral girdle over the thorax; but this movement does not occur. Pectoralis major, because of its anterior enthesis, opposes this posterior component of trapezius, thereby maintaining the pectoral girdle in its normal position during upper trapezius contraction. This action of pectoralis major has not yet been described by anatomists and is an original observation.