Anatomically, the rear torso (back) consists of several layers of muscle, stacked like a sandwich.
Functionally, and for bodybuilding purposes, the back is best considered in three sections, resembling triangular segments of a quilted blanket.
The upper back is made up of large triangular-shaped muscle called the trapezius. It orginates along the upper spine from the skull down to the last rib (that is, all the cervical and thoracic vertebrae). The upper fibers of the trapezius (in the neck) attach to the outer tip of the shoulder on the clavicle, acromion, and scapula. The middle and lower fibers of the trapezius (in the upper back) attach to the scapula (shoulder blade). The upper traps elevate the scapula to shrug the shoulder abduction. The middle traps retract the scapula, pulling the shoulders backward; the lower traps depress the scapula downward.
Underneath the trapezius are three muscles that anchor the scapula to the spine: the levator scapulae, rhomboid major, and rhomboid minor. The levator scapulae muscles assist the upper traps to elevate the scapula. The rhomboid muscles work with the middle traps to retract the scapula. These scapular retractor muscles lie under the trapezius and add muscular thickness to the upper back.
The middle back consists of the latissimus dorsi, a large fan-shaped muscle that arises from the lower half of the spinal column and the rear ridge of the pelvic bone (posterior iliac crest). From its large origin, the latissimus converges into a bandlike tendon that attaches to the upper humerus (next to the tendon of the pectoralis major). When the latissimus dorsi contracts, movement takes place at the shoulder joint.
The latissimus dorsi pulls the upper arm downward and backward (shoulder extension); hence this muscle is targeted by pulldowns, pull-ups, and rows. The latissimus also pulls the arm in against the side of the body (adduction). The lower back is made up of the erector spinae (or sacrospinalis) muscles that run alongside the entire length of the spinal column. In the lumbar region, the erector spinae split into three columns: the iliocottalis, longissimus, and spinalis. These muscles are the pillars of strength in the lower back that stabilize the spine and extend the torso, arching the spine backward.
The trapezius and latissimus dorsi are concerned primarily with movements of the shoulder and arm. It is the sacrospinalis muscles that cause movements of the spine and torso. Exercises that target the back muscles include shrugs, pulldowns, pullups, rows, and lumbar extensions. The deadlift is a compound, multijoint exercise that utilizes all of the back muscles.
Source: wikipedia.org | book – Bodybuilding Anatomy by Nick Evans