One of the 5 main muscles of movement.
Keeping it simple: trapezius.
Anatomy in detail.
The left and right trapezius muscles form the most superficial muscle layer from mid-back to the base of the skull.
Thin muscles, sculpted down the neck and towards the shoulders, attaching to both scapula (shoulder blade) and clavicle (collar bone) of each arm.
Wikipedia: Trapezius: from Late Latin trapezium, from Greek τραπέζιον (trapézion), literally "a little table", a diminutive of τράπεζα (trápeza), "a table", itself from τετράς (tetrás), "four" + πέζα (péza), "a foot; end, border, edge"
Trapezius keeping it simple
Current descriptions split the trapezius into 3 functional sections, based on the direction of the muscle fibres.
a.k.a. superior (i.e. higher than the other sections) or descending (i.e. the muscle fibres descend) trapezius.
a.k.a. transverse (i.e. the muscle fibres run approximately horizontally) trapezius.
a.k.a. inferior (i.e. lower than the other sections) or ascending (i.e. the muscle fibres ascend) trapezius.
Between the 6th cervical and 3rd thoracic vertebrae (the base of the nuchal ligament and start of the supraspinous ligament) the trapezius muscles are connected to the midline by a broad semi-elliptical aponeurosis (thin sheet of strong connective tissue), forming a tendinous ellipse between the shoulder blades.
Photo human cadaver tendinous ellipse.
The trapezius muscles should be free the extend in all directions.
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