A complex web of small, subtle muscles and large, powerful muscles make up the muscles of your back. These muscles are responsible for controlling and gently moving one of the most precious parts of your skeleton -- your spine. The muscles of your back are also responsible for impressive feats of strength, like a chin-up.
The deeper muscles of your back tend to be smaller but are very important for the health of your spine and also help you maintain good posture. The superficial muscles of the back are larger and are meant to move the body by producing a lot of force.
Deep Muscles of the Back
The deep muscles of the back that connect the spine and skull are the splenius capitis and splenius cervicis. The iliocastalis runs along the ribs and spine, helping you rotate your trunk and neck. The longissimus and multifidus are long muscles that run down the sides of your spine and help extend your spine and neck.
The spinalis runs down the middle of your spine and also helps extend your head and neck. The semispinalis is a slightly shorter muscle, running down the middle section of your spine on the sides. It helps extend and rotate your spine.
The erector spinae lies on top of the deeper muscles that run down the sides of your spine. It's a big muscle and is even divided into three separate parts: the long head, lateral head and medial head. The erector spinae spans most of your spine, starting from the bottom of your skull and going all the way down into your pelvis. Each head acts on the three major parts of the spine: the lumbar, or lower; thoracic, or middle; and cervical, or upper.
The larger muscles of your back need to be powerful because they move your shoulder blades and your shoulder joint. They allow you to do things like chin-ups and rows. The rhomboid major and minor lie between your shoulder blades and help pinch them together. The levator scapulae run from your shoulder blade to your neck and assist in shrugging your shoulders towards your ears.
The trapezius is a complex muscle because it covers so much distance and performs three different actions. It spans from the base of your skull, out to the very end of your collar bone, and down to the middle of your back. The upper part also helps you shrug your shoulders towards your ears. The middle part pulls your shoulder blades together. The lower part actually does the opposite of what the upper part does: it lowers your shoulders away from your ears.
The latissimus dorsi is the largest and most powerful of your back muscles. It starts all the way down in your lower back, climbs up to the middle of your back, and stretches out into your shoulder. If your latissimus dorsi is well-developed, it sticks out to the sides of your body and can make you look like you have wings! This muscle brings your arms down and closer to your body. It's very active in exercises like the chin-up, where you are pulling an object towards your body.