The quadratus lumborum lifts the ilium superiorly and anteriorly, flexes the spine laterally, and, through coupled motion, rotates the spine ipsilaterally into counternutation.
Note: In the video, the iliolumbar ligaments are show to stretch only because of the large range of motion needed for demonstration purposes. In reality, these ligaments stretch minimally, if at all. They tie L4 & L5 to the ilium, which pulls these two vertebrae counter to the sacrum and the rest of the lumbar spine.
The quadratus lumborum consists of 3 laminae:
- 12th rib inferiorly to iliac crest
- 12th rib & transverse processes of L1-4 inferiorly and laterally to iliolumbar ligament & iliac crest
- Transverse processes of L1-4 laterally and superiorly to 12th rib – this lamina can only be viewed from the front but is often very small and sometimes missing entirely.
- Lamina 1: Because the 12 rib is anterior to the iliac crest, the 12th rib is pulled posteriorly and inferiorly and the ilium is pulled anteriorly and superiorly, pulling L5 and, possibly L4, with it.
- Lamina 2: The 12th rib and L1-4 are pulled posteriorly (flattening the lumbar curve) and the iliac crest is pulled anteriorly and superiorly.
- Lamina 3: Its anterior-posterior pull appears to be difficult to visualize but, as a lateral flexor, it may cooperate with the psoas in providing a coupling motion that would flatten the spine on the side of contraction. (see Coupled Motion)
- Lumbar spine extension p328 p527
- Stabilizes the lumbar spine  p574 p331
- “participates with the deep erector spinae and psoas major muscles (both counternutators) in preparing the lumbar spine for force transference… in the horizontal plane”  p83
- Acts as a vertical stabilizer to the diaphragm in forced inspiration  p100 p375 p33
- Adams et al. states that inspiration is its principle function.
- Inspiration is a function of counternutation  .
- Major function is lateral flexion p100 p527.
- Raises the ilium (hip hiking)  p328
- Observation reveals that it promotes ipsilateral lumbar rotation through spinal coupling.
- Ipsilateral flexion (counternutation).
- Raises the ilium while rotating it anteriorly 
- Lowers the rib cage while rotating it posteriorly, straightening the lower thoracic spine. 
Some textbooks mistakenly say the quadratus lumborum provides lumbar extension (increased lordosis) by bringing both ilia anteriorly. This does not take into account the fact that the ilia and sacrum move in opposite directions relative to each other. When viewed in the context of nutation/counternutation, with anterior movement of the ilia, the natural proprioceptive movement of the sacrum is to go posteriorly, toward reducing the lordosis. However, looks can be deceiving: the ilia can carry the entire pelvis forward relative to the gravitational line and, as such, the sacrum does move anteriorly, even though it moves posteriorly relative to the ilia. The lumbar spine moves with the sacrum and, accordingly, will move posteriorly, reducing the lordosis. Junghanns p148 noted that “the greater the inclination of the pelvis, the more the lumbar curve reverses…”, which is consistent with the principles of nutation/counternutation. An example of this concept is seen in pregnancy in which exaggerated anterior pelvic rotation is seen together with a flattened lumbar spine. This movement pattern demonstrates the inherent balance of nutation & counternutation in weight distribution by keeping the body’s mass centered and serves as a key point in understanding spinal motion in the context of nutation and counternutation.
The quadratus lumborum has been shown to be overactive in patients with back pain  p97, which would be expected in a nutation lesion, since it promotes counternutation. Accordingly, it has been shown to increase activity with increased axial loading on the spine , which also induces nutation.
The quadratus lumborum is difficult to investigate with EMG due to depth and thickness of fascia surrounding this muscle. However, using fine wire EMG,  found that the quadratus lumborum increased activity with increased axial loading on the spine. Axial loading induces nutation in the sacroiliac joint. In a normal SI joint, there would be no overactive counternutation response but, in a SIJ nutation lesion, the quadratus lumborum and other counternutation muscles would increase activity relative to the degree of stress inducing nutation.
In another EMG study, Andersson  found that the quadratus lumborum was activated at the same time as the deep erector spinae (a counternutator), whereas the superficial erector spinae (a nutator) had little activity during those movements. This study indirectly indicates that the quadratus lumborum induces counternutation because it is similar in function to the deep erector spinae.
These statements go along with the concept that, as a counternutation muscle, the quadratus lumborum would be in a state of contraction with a nutation lesion.
The anterior band of the iliolumbar ligament may be derived from the quadratus lumborum muscle p105. Kuchera p683 referred to Luk in stating that the iliolumbar ligaments, which limit nutation, are formed from fibers of the quadratus lumborum muscle, and should not be considered ligaments until after the age of 25. He also noted that calcification of a ligament can be evidence of adaptation to chronic stress and suggests that pain at its attachments may be a sign of postural decompensation due to gravitational stress. His studies suggested that the quadratus lumborum would provide a counternutation effect to the gravitational forces inducing nutation, thus calcification would be considered a result of a prolonged counternutation response to a nutation lesion. This would suggest that the high proportion of iliolumbar ligament calcification in our society (which is considered to be a normal occurrence) is an indication of the prevalence of the nutation syndrome.
Although in the references that I have studied, no mention is made of the quadratus lumborum causing lumbar rotation, we can infer this motion by combining several studies on coupling.
The major function of the quadratus lumborum on the spine is lateral flexion. p100 p527. Additionally, it raises the ilium  p328. When starting from an erect posture, lateral flexion, initiating in the lumbar spine, creates a coupling effect to cause the lumbar bodies from L1-3 to rotate into the concavity of the curve, and the pelvis to rotate contralaterally   [19, 20]. Thus, in a lateral bend to the right, the pelvis rotates to the left and, although the pelvis carries the lumbar spine with it towards the left (the primary rotation), relative to the pelvis, L1 to L3, and sometimes L4, will rotate to the right. However, the lower quadratus lumborum fibers, attaching L5 and L4 to the innominate, are ligamentous in the adult . As such, L4 and L5 rotate with the innominate due to the relatively non-elastic bond. For example, as the innominates rotate to the left, L4 and L5 would rotate to the left while L1 to L3 rotate to the right  . Other muscles may also cause these counter-movements of the upper and lower lumbar vertebrae, including the psoas  p98, multifidus, and lumbar parts of the longissimus and iliocostalis p42. Importantly, this counter-rotation at L3-4 and L5-S1 may significantly influence disc degeneration at those areas, especially when combined with posterior compression, as seen in the nutation lesion. For more on coupling, see Coupled Motion (make link).
Treatment is aimed at relaxing the quadratus lumborum rather than strengthening it p30.
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