The 5 Main Muscles of Movement.
Anatomy in detail:
The left and right trapezius muscles form the most superficial muscle layer from mid-back to the base of the skull.
They are 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"
The left and right trapezius muscles meet midline, attaching to the cervical and thoracic vertebrae (bones of the spine) via the nuchal & supraspinous ligaments.
These midline attachments mean the trapezii are the main muscles that are responsible for state of alignment of the upper body. The sensory information provided by the trapezius muscles allows us to feel the relative positioning of thoracic and cervical spine so we become more aware of our state of alignment and are thus able to move improve our posture and work towards a balanced body.
Current descriptions split each trapezius into 3 functional sections, based on the direction of the muscle fibres.
a.k.a. superior (i.e. higher than the other sections) trapezius.
a.k.a. descending (i.e. the muscle fibres descend) trapezius.
The upper trapezius attaches to:
a.k.a. transverse (i.e. the muscle fibres run approx. horizontally) trapezius.
The middle trapezius attaches to:
a.k.a. inferior (i.e. lower than the other sections) trapezius.
a.k.a. ascending (i.e. the muscle fibres ascend) trapezius.
Movement of the upper body should begin from the lower trapezius.
With Base-Line support in place, think of activating both your lower trapezii starting from midback, extending upwards and outwards towards your shoulder blades.
The lower trapezius attaches to:
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.