• WordNet 3.6
    • n trapezius either of two flat triangular muscles of the shoulder and upper back that are involved in moving the shoulders and arms
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Century Dictionary and Cyclopedia
    • n trapezius A large superficial muscle of the back of the neck and adjacent parts. It arises from the external occipital protuberance, the inner third of the superior curved line of the occipital bone, the ligamentum nuchæ and the spines of the last cervical and of all the thoracic vertebra, and is inserted into the outer third of the clavicle and the acromion and spine of the scapula. Each trapezius is triangular, and with its fellow of the opposite side forms a somewhat diamond-shaped figure, little like the trapezium of geometry. Also called cucullaris and cowl-muscle or shawl-muscle. See cut under muscle.
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In literature:

The next muscles to become stiff and painful are those of the neck, especially the sterno-mastoid and trapezius.
"Manual of Surgery" by Alexis Thomson and Alexander Miles
C. Clavicular attachment of the trapezius muscle.
"Surgical Anatomy" by Joseph Maclise
In time other muscles compensate in part for the loss of the trapezius.
"Manual of Surgery Volume Second: Extremities--Head--Neck. Sixth Edition." by Alexander Miles
An analogous region, but of only slight depth, exists in quadrupeds; its borders are formed by the mastoido-humeral and trapezius muscles.
"Artistic Anatomy of Animals" by Édouard Cuyer

In news:

John A Jane, the actor's attending physician, said Mr Reeve 's ability to flex his right trapezius, a shoulder muscle in the back, and to make some movement on his left side "means that impulses are getting through the area of injury.".
Orioles right fielder Chris Davis scratched with right trapezius strain.
Estée Lauder Has Robbed Carolyn Murphy of Her Trapezius .
Model Carolyn Murphy's left trapezius appears to be in order, but her right one appears to have taken a vacation.
Estée Lauder Has Robbed Carolyn Murphy of Her Trapezius.
Although there are exercises (such as trapezius shrugs ) and programs such as Muscle Activation Techniques, which may help, the true "cause" lies within your posture and becoming aware of it.
Switch to this exercise for greater trapezius growth.