The two major muscle groups of the back and shoulder of the cat are the trapezius group and the deltoid group.  The clavotrapezius arises from the back of the skull and middorsal line of the neck and inserts on the clavicle.  Its action is to extend the humerus in a forward direction.  The acromiotrapezius (which arises on the middorsal line of the neck and thorax and inserts on the metacromion and spine of the scapula) adducts the scapula.  The spinotrapezius arises from the spinous processes of thoracic vertebrae and inserts on the fascia covering the scapular muscles.  Its action is to adduct the scapula and pull it posteriorly.  The major muscle group of the shoulder is the deltoid group.  The clavodeltoid originates from the clavicle and inserts on the ulna.  This muscle flexes the forearm.  The acromiodeltoid lies posterior to the clavodeltoid.  It arises from the acromion process of the scapula and inserts on the spinodeltoid muscle.  The spinodeltoid arises from the scapula spine and inserts on the proximal portion of the humerus.  The acromiodeltoid and spinodeltoid work synergistically to flex the humerus and rotate it outward. There are two back muscles associated with these two groups of muscles; they are the levator scapulae ventralis and the latissimus dorsi.  The levator scapulae ventralis is a band-like muscle lying between the clavotrapezius and the acromiotrapezius.  It originates from the skull and atlas to insert on the scapula.  When the levator scapulae ventralis contracts it pulls the scapula forward.  The latissimus dorsi is a large triangular muscle posterior to the trapezius group.   It arises from the midline in the thoraco-lumbar region and inserts on the proximal end of the humerus.  Contraction of this muscle results in the humerus being moved posteriorly and dorsally.