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.
Back Muscle Functions
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).