By Elizabeth A. Murray, Mount St. Joseph University
Back pain affects an estimated 80% of people at some time in their lives. Acute back pain can become chronic, especially in the low back. The back muscles are just as complicated and complex as the spine. There are a great number of muscles in the back—so many, in fact, that we consider them in layers, mostly related to their functions.
Muscle Layering
Here’s a fictional story to understand muscle layering: After watching a competition on television, a group of teenagers decided to take on the sport of knife throwing. The teens weren’t very proficient with their knives, and one went to retrieve his blade from the target tree, not realizing another was yet to throw.
The unsuspecting kid sustained a penetrating wound that was medial to his right scapula, situated roughly between the transverse and spinous processes of the right side of his spine, and slightly superior to the spine of the scapula. Before the knife was fortunately stopped by his spine, what muscles did it penetrate? Although the wound was less than an inch or so deep, it could penetrate at least eight different muscles—so one can see what we mean by intricate layering!
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Appendicular Muscles of the Back
The first group of muscles we’ll discuss are not really back muscles at all—they are muscles of the upper limb that happen to ‘live’ on the back. Muscles cross a joint to cause movement at that joint, and with all muscles of the limbs, the origin is always proximal and the insertion is always distal.
So, our first of so-called back muscles are called the appendicular muscles of the back because they take origin off the axial skeleton—either the spine or skull—for the anchorage they need to gain their leverage. And they move some aspect of the upper appendage—so their insertion is on either the scapula, clavicle, or directly on the humerus.
There are generally considered to be five of these superficial or appendicular back muscles: the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae.
The Trapezius
The trapezius—sometimes called the trap—has a long origin from the base of the skull, all the way down through the thoracic vertebrae, taking origin along the midline of the spine, from the spinous processes of cervical and thoracic vertebrae. It inserts on the spine of the scapula, and around to the lateral clavicle—together the scapula and clavicle are called the shoulder girdle or pectoral girdle. The trapezius is a good example of a muscle with a complex shape—somewhat like a trapezoid—and as a consequence, it has complex and even contradictory actions.
The upper region of the trapezius elevates the shoulder girdle. The lower fibers are used to depress the shoulder girdle—as when we’re sitting on the floor and use our hand to help push ourself up. The middle fibers retract the scapula, pulling them together, an action that some references call adducting the scapula.
But because the trapezius attaches to the posterior skull, it can also extend the head and neck. The trapezius is supplied by the same nerve that serves the sternocleidomastoid muscle—the spinal accessory nerve, cranial nerve XI. What puts accessory in its name is that some branches from the cervical plexus assist, or ‘accessorize’, this cranial nerve.
The Latissimus Dorsi
The latissimus dorsi is another muscle on the back that isn’t a muscle of the back—it’s a muscle that has its belly on the dorsal side of the trunk, originating from a layer of broad, flat connective tissue called the thoracolumbar fascia. The latissimus dorsi muscles—sometimes called the lats—insert on the anterior and proximal part of the humerus.
Since the latissimus dorsi is posteriorly located, it has several jobs. It extends the shoulder, and since it crosses the axilla—or armpit—it also adducts the humerus toward the body. Finally, because it attaches anteriorly, it can turn the humerus medially—as in rolling it inward, toward the body.
The Rhomboids and Levator Scapulae
The other three superficial or appendicular muscles of the back are the rhomboid major, rhomboid minor and levator scapulae.
The levator scapulae elevates the scapula by originating on the transverse processes of some cervical vertebrae and inserting on the superior aspect of the scapula. It works with the upper fibers of the trapezius to shrug the shoulders.
Both the rhomboid major and rhomboid minor retract the scapulae, in conjunction with the middle fibers of the trapezius. The rhomboids do this by generally originating on the spinous processes of thoracic vertebrae and inserting on the medial border of the scapula. Their slightly oblique orientation also lets them elevate the scapula and rotate it so that the shoulder joint faces inferiorly.
The other thing we are reminded of by the rhomboids is our rule of adjectives—if we have a major, we’ll have a minor. The rhomboid major is larger and more inferior—attaching inferior to the spine of the scapula, while the rhomboid minor is smaller, but more superior—usually at and slightly above the spine of the scapula. Some cadavers show little differentiation between the two, while others have a clear division between rhomboid major and minor.
Common Questions about Superficial or Appendicular Back Muscles
There are generally considered to be five of superficial or appendicular back muscles: the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae.
The trapezius performs complex and even contradictory actions. The upper region of the trapezius elevates the shoulder girdle. The lower fibers are used to depress the shoulder girdle—as when we’re sitting on the floor and use our hand to help push ourself up. The middle fibers retract the scapula, pulling them together, an action that some references call adducting the scapula. Also, because the trapezius attaches to the posterior skull, it can also extend the head and neck.
Since the latissimus dorsi is posteriorly located, it has several jobs. It extends the shoulder, and since it crosses the axilla—or armpit—it also adducts the humerus toward the body. Finally, because it attaches anteriorly, it can turn the humerus medially—as in rolling it inward, toward the body.
