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Muscle between the lower spine and the shoulder blade

The trapezius is a large paired trapezoid shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It moves the scapula and supports the arm. The trapezius has three functional parts: an upper (descending) part which supports the weight of the arm; a middle region (transverse), which retracts the scapula; and a lower (ascending) part which medially rotates and depresses the scapula.

The trapezius muscle resembles a trapezium, also known as a trapezoid, or diamond shaped quadrilateral. The word "spinotrapezius" refers to the human trapezius, although it is not commonly used in modern texts. In other mammals, it refers to a portion of the analogous muscle.

The superior or upper (or descending) fibers of the trapezius originate from the spinous process of C7, the external occipital protuberance, the medial third of the superior nuchal line of the occipital bone (both in the back of the head), and the ligamentum nuchae. From this origin they proceed downward and laterally to be inserted into the posterior border of the lateral third of the clavicle. The middle fibers, or transverse of the trapezius arise from the spinous process of the seventh cervical (both in the back of the neck), and the spinous processes of the first, second, and third thoracic vertebrae. They are inserted into the medial margin of the acromion, and into the superior lip of the posterior border of the spine of the scapula. The inferior or lower (or ascending) fibers of the trapezius arise from the spinous processes of the remaining thoracic vertebrae (T4–T12). From this origin they proceed upward and laterally to converge near the scapula and end in an aponeurosis, which glides over the smooth triangular surface on the medial end of the spine, to be inserted into a tubercle at the apex of this smooth triangular surface. At its occipital origin, the trapezius is connected to the bone by a thin fibrous lamina, firmly adherent to the skin. The superficial and deep epimysia are continuous with an investing deep fascia that encircles the neck and also contains both sternocleidomastoid muscles. At the middle, the muscle is connected to the spinous processes by a broad semi elliptical aponeurosis, which reaches from the sixth cervical to the third thoracic vertebræ and forms, with that of the opposite muscle, a tendinous ellipse. The rest of the muscle arises by numerous short tendinous fibers. It is possible to feel the muscles of the superior trapezius become active by holding a weight in one hand in front of the body and, with the other hand, touching the area between the shoulder and the neck.

Motor function is supplied by the accessory nerve. Sensation, including pain and the sense of joint position (proprioception), travel via the ventral rami of the third (C3) and fourth (C4) cervical spinal nerves. and in an abnormal mobility or function of the scapula (scapular dyskinesia). There are multiple causes of trapezius dysfunction.

Trapezius palsy, due to damage of the spinal accessory nerve, is characterized by difficulty with arm adduction and abduction, and associated with a drooping shoulder, and shoulder and neck pain. Intractable trapezius palsy can be surgically managed with an Eden–Lange procedure.

The trapezius muscle is one of the commonly affected muscles in facioscapulohumeral muscular dystrophy (FSHD). The lower and middle fibers are affected initially, and the upper fibers are commonly spared until late in the disease.

Although rare, underdevelopment or absence of the trapezius has been reported to correlate to neck pain and poor scapular control that are not responsive to physical therapy. Absence of the trapezius has been reported in association with Poland syndrome.

The upper portion of the trapezius can be developed by elevating the shoulders. Common exercises for this movement are any version of the clean, particularly the hang clean, and the shoulder shrug. The uppermost area can be trained through neck extension. Middle fibers are developed by pulling shoulder blades together. This adduction also uses the upper/lower fibers. The lower part can be developed by drawing the shoulder blades downward while keeping the arms almost straight and stiff. It is mainly used in throwing, with the deltoid muscle and rotator cuff.