There are three primary muscles groups in the back – the trapezius muscles in the upper back, the latissimus dorsi muscles in the mid-back, and the erector spinae muscles in the lower back.
There are several other smaller muscles in the back, all of which will be developed by performing exercises that work the three main back muscles.
The trapezius (traps) muscle is made-up of three separate regions. The upper portion of the trapezius is mainly responsible for shrugging your shoulders. The middle and lower regions are used during rowing type movements. Well developed traps help to counterbalance the chest and front (anterior) deltoids. This helps to improve posture and reduce the risk of injury.
The latissimus dorsi (also known as the “lats”), lie across the middle section of the back on each side. This muscle group when well developed gives the upper body that V shape. The lats originates from attachments along the spine and inserts with a large tendon to the humerus (upper arm bone) on each side of the body.
The erector spinae muscles of the lower back are like two thick columns of muscle on either side of your spinal column. The spinal erectors run from the base of your pelvis to almost the full length of your spine. However, they are most easily visible from the middle to the lower back.