The latissimus dorsi (latissimus meaning “broadest”) are the large fan-shaped muscles on the back, starting on the lower vertebral column, spreading across the width of the back, and attaching posterior to the arm. When looking at the size, the latissimus dorsi are certainly the body’s largest muscles. Their fibers originate along the vertebrae of the lower half of the spine, along the posterior ridge of the hips,and along the last three ribs.

They then twist together to attach to the humerus,forming the back of the armpit. The lats are extremely busy muscles because they play many roles. They extend, abduct, horizontally abduct, flex, and internally rotate the shoulder joint. In short, the lats bring the arms down and toward the body (adduction). They also have a synergistic role, because they assist in lateral flexion of the spine. Everything that the lats do, however, is assisted by the teres major, a small strip of muscle that originates on the scapulae and attaches to the humerus, just above where the lats do. Because the lats are the largest muscle group on the posterior upper body,strengthening them will make a big difference in both functional performance and the aesthetics of your back.

The lower back muscles consist of two sets of muscles referred to as the erector spinae, or spinal erectors. These muscles are recruited for most heavy lifting. Within these muscles are three additional sets that further consist of three muscles: the iliocostalis, the longissimus, and the spinalis. They span the entire length of the spinal cord, all the way up to the neck. These are the longest muscle groups in the human body, having the important job of stabilizing the spinal column. 

This muscle group is particularly important because it is required to carry and maintain all the weight of the body. Viewing your body from the side, if you were to stand so that you could draw a straight line from your ear to your shoulder, down to your hip and straight to your lateral ankle bone, you would be standing in a natural, appropriate posture—spinal extension. 

When performing spinal flexion (a bent forward posture), these muscles bring your body back to an upright position known as spinal extension.These muscles also counterbalance spinal flexion and extension by taking your spine into hyperextension. Spinal hyperextension is nothing more than arching your back or leaning backward, which is a normal daily occurrence with typically no negative consequences.In well-developed physiques, the spinal erectors appear as developed ridges that run along the spine all the way up the back. It is important to have well-developed spinal erectors because these muscles work hard to support the low back during heavy weightlifting exercises such as deadlifts and squats, and bent-over rows, which we explore in this chapter.

The quadratus lumborum (or QL) also assists the spinal erectors when the spine bends sideways (lateral flexion) or twists. Each set of these QL muscles lies under the spinal erectors and originates on the top of the pelvis and inserts on the lower lumbar vertebrae, attaching to the bottom rib. 

When the lower fibers of the erector spinae muscles are weak, the QL pick up the slack. As a result, the QL are often the source of low back pain (especially as a result of sitting for long periods of time at a computer or driving, or of overusing low back supports such as pillows or stability belts). 

Unfortunately, sitting for long periods can cause these muscles to be put into a state of constant contraction, resulting in muscle fatigue that can cause decreased blood flow to these muscles and adhesion in the muscle and fascia.

These undersized muscles in particular may seem unimportant, but they are mighty important for maintaining spinal integrity and stability. All the muscles that support the spine (the core muscles and the many larger, superficial muscles, including the latissimus dorsi and trapezius, for example), should be trained for strength as well as endurance because their job is not always necessarily to move the spine, but to shore up any spinal weaknesses and to maintain structural integrity.