The major muscles of your back are responsible for almost all pulling movements you make, including pulling yourself up to a chin-up bar, pulling a box down from a shelf or just drawing a cup of tea across the table toward you. Some back muscles also work to stabilize your spine. A firm grasp of the major back muscles allows you to better understand your own body mechanics and how to get the most benefit out of every back exercise.
These three columns of muscle that run to either side of your spine help stabilize your spine against the forces of your rectus abdominus and obliques. The erector spinae are also responsible for extension and lateral flexion of your spine. In other words they arch your back -- the opposite of doing a crunch -- and flex your spine to either side.
This group of four small muscles' importance belies its size. Together the diminutive supraspinatus, infraspinatus, teres minor and subscapularis are responsible for holding your humerus or upper arm bone in the shallow shoulder socket. These muscles bear the greatest strain during movements that require internal and external rotation of the shoulder. To get a feel for these motions, hold one arm at your side with your elbow bent at 90 degrees. Keep your elbow tucked close to your body as you rotate your hand toward your body's midline -- this is internal rotation. Rotate your hand the other way, away from your midline, and you're doing external rotation.
Latissimus Dorsi And Teres Major
A well-developed latissimus dorsi lends the pleasant V-shape associated with a strong back. This muscle activates strongly during any activities that involve pulling down against resistance, whether your elbow is traveling from front to back of your body or toward your midline. Examples include rowing, pull-ups or climbing a rope. The teres major, sometimes known as "Lat's little helper," assist with most of the motions your latissimus dorsi perform, including shoulder extension and adduction.
Shoulder Girdle: Trapezius, Levator Scapulae, Rhomboids
Both of your shoulder blades or scapulae "float," connected to your body by soft tissue alone instead of bony joints. Your trapezius muscles form a rough diamond shape on your back and are divided into upper, middle and lower fibers. These fibers respectively lift and upward-rotate your scapulae, upward-rotate and adduct your scapulae and depress your scapulae. Your rhomboids also stabilize the scapulae during every arm movement, drawing your shoulder blades up, in, or rotating them down. The levator scapulae connects to the processes of your cervical vertebrae and is, as you might expect, responsible for elevating your scapulae.