The Latissimus Dorsi muscles (also known as the Lats) are the largest muscles of the back.
Being large, fan-shaped muscles, they are able to provide force in a wide range of body positions, e.g. leaning back to straight vertical and all points in between.
The Lats are attached to the upper end of the humerus with fibers running down in a fan down the vertebral column and pelvic girdle.
The Trapezius (trap) muscle is a long, trapezoid-shaped muscle that runs down the upper section of the spinal cord, originating at the base of the skull and attaching down in the middle to lower back.
The angles of the Trapezius fibers provide pull in three different directions: up, down and in towards the centerline of the body.
The function of the Latissimus Dorsi is to pull the arm down towards the pelvis. When the arm is fixed (e.g. during a chin-up), the lats serve to bring the body up towards the arm. It is the same basic movement but with the directions reversed. The Lats also function to stabilize the torso during many movements, including the flat bench press.
The functions of the Trapezius muscle include scapular elevation (shrugging up), scapular adduction (drawing the shoulder blades together) and scapular depression (pulling the shoulder blades down).
The Erector Spinae is a group of muscles that support the spinal column. They include the Longissimus, the Spinalis and Iliocostalis. The muscles of the Erector Spinae attach to the vertebrae, the ribs and the pelvis. The functions of the Erector Spinae group are to extend the spine as well as provide support for it.
The Teres Major muscle originates on the outer (lateral) edge of the scapula and attaches to the humerus.
The Teres Major muscles work with the Rotator Cuff muscles to stabilize the shoulder joint and works with the Latissimus Dorsi muscles to pull the humerus back.
The Rhomboids (Major and Minor) originate on the spinal column and attach to the middle (medial) surface of the scapula.
The Rhomboid muscles get their name from their shape: rhomboid. The Major and Minor designations refer to their relative size to each other.
The function of the Teres Major is to move the humerus posteriorly, meaning that it brings the arm towards the back.
The Rhomboids function to bring the scapula in towards the spinal column, essentially squeezing the shoulder blades together when the Rhomboids of both sides are used at the same time.