Usually over shadowed by the more famous, psoas major, the QL is a hard working core stabilizer, and has a strong influence on respiration and even digestion.
While the QL is often associated with the lower back, in reality it is a very deep abdominal muscle. Nestled in the depths of the core, it is in fact the deepest muscle of the abdomen. It originates at the posterior iliac crest and stretches upwards toward the rib cage. Finally, it inserts onto five different points: the transverse processes of the 1st, 2nd, 3rd, and 4th lumbar vertabrae as well as the 12th rib. The QL is also the only other muscle that runs along side the psoas completely through the diaphragm (connecting the upper body to the lower body).
When we consider all of its attachments it is obvious that when it contracts, the QL will draw the pelvis and ribcage towards one another, effectively shortening one side of the torso. Therefor, it is primarily responsible for lateral flexion (side to side bending). It also assists with spinal extension (back bending). But next consider the surrounding tissues. The QL not only penetrates the diaphragm and attaches to the rib cage, it is also surrounded by the organs of digestion. If it is tight and stiff, the QL will prevent the rib cage from fully expanding to take a deep breath, and it will pull unfavorably on the diaphragm and surrounding connective tissue. A tight QL would also limit mobility and blood flow to surrounding digestive organs. A healthy QL must therefor be vital for optimal respiration (breathing!) and good digestion.
To stretch & activate the QL, practice these side bending poses: