The psoas, straightening the lumbar spine (with the exception of the lumbosacral area), combines with the iliacus, anteriorly rotating the pelvis, to induce counternutation.
Origin: Superior 2/3 of the iliac fossa, inner lip of the iliac crest, and small region of the sacrum across the sacroiliac joint
Insertion: Lesser trochanter
Function: Anteriorly rotates the pelvis
Psoas Minor: Counternutation
Origin: Bodies of T12 to L1 and the discs between them
Insertion: Fascia over the psoas major and iliacus
Function: Flexes the trunk, decreasing the lumbar lordosis 
Psoas Major: Counternutation
Origin: T12 to L5 transverse processes & discs
Insertion: Lesser trochanter
Functions: p232 p97-98 [4-7]
- On the hip, its principle action is flexion of the femur on the hip joint and advancing the leg while walking. It also laterally rotates and abducts the femur.
- On the spine, its dominant function is lateral flexion, as well as axial compression 
- With the thighs flexed on the pelvis, it increases the lumbar lordosis.
- During gait, it decreases the lumbar lordosis 
There is considerable controversy as to whether the psoas increases or decreases the lumbar lordosis. Below are some examples of both ideas, with my conclusions at the end.
Sullivan  took tracings of x-rays in the neutral position and in flexion and extension and compared the lines of pull of the psoas muscle from all three positions. He found that, “Depending on the starting position of the spine, therefore, the vertebral portion of the psoas major muscle can act as a flexor, a stabilizer, or an extensor of the lumbar spine.” This statement appears to be consistent with Lovett who, in a study on scoliosis , found that the starting position of the movement (posture) determined the accompanying coupling motion in the lumbar spine. For example, he found that the lumbar spine rotates toward the side of lateral bending (bodies toward the concavity) when the person is initially standing or in extension but towards the opposite side (towards the convexity) when the person is initially in flexion.
In the neutral anatomic position, the lines of force of the muscle bundles from the L5 vertebrae lie slightly anterior to the horizontal axis of rotation, causing a slight flexion force at L5 p98 . At the L4 to L5 disc, the force is neutral with respect to flexion and extension. From L4 to L1, the lines of force go progressively posteriorly until they are slightly posterior to the vertebral axis of rotation, causing a mild extension force from L1 to L3. The net effect is a slight increase in lordosis.
Calais-Germain p62 stated that the psoas is generally considered to increase lordosis. “However, electromyographic recordings taken from moving subjects suggest a paradoxical action. The psoas, in combination with the posterior transverso-spinalis muscles, forms a system of four muscle bundles arranged around the lumbar spine. By contracting together, these four bundles can act to erect (straighten) the lumbar spine, rather than increasing lordosis.”
Bachrach  suggested that both increased and decreased lordosis can happen as the psoas tightens. In most cases, “The lumbar lordosis is increased (or, less frequently, the trunk is flexed on the hips, the lordosis decreased or reversed)…Acute, prolonged, or repetitive flexion stress of the lumbar spine may result in spasm of the psoas major uni- or bilaterally. The trunk is forward flexed on the pelvis. The lumbar spine is flattened or reversed.”
Kappler  stated “Chronic contracture of the psoas muscles results in loss of the normal lumbar antero-posterior curve, with flattening or even reversal of the curve.” He continued, “As the condition becomes chronic or recurrent, the instability of the lumbosacral area increases as a result of chronic stress, and the pain may lateralize. The patient assumes a typical posture at this point, with flexion of the lumbar spine and pelvic side shift or lateral deviation of the pelvis. This position is sometimes referred to as sciatic scoliosis…”
Oatis p683 mentioned that “tightness of the psoas major is often manifested by increased lumbar extension, that is, by an excessive lumbar lordosis…If, however, the subject lacks hyperextension flexibility in the spine, tightness of the iliacus or psoas major can produce a forward lean and a flattened lumbar spine in upright posture.”
In addition, Levangie p374 stated that “Given the attachment of the psoas major muscle to the anterior vertebrae and the iliacus muscle to the iliac fossa, activity of the passive tension in these muscles would anteriorly tilt the pelvis (iliacus muscle) and, apparently, pull the lumbar vertebrae anteriorly into flexion (psoas major muscle). In closed-chain function (head vertical), however, these muscles seem to create a paradoxical lumbar lordosis (lumbar extension) that results from the body’s attempt to keep the head over the sacrum with anterior pelvic tilt and lower lumbar flexion.”
Lewitt p262 stated that “Psoas spasm is usually associated with spasm of the thoracolumbar erector spinae and the quadratus lumborum, and relaxation of one muscle induces relaxation of the other.” In the sections on the two muscles, it will be presented that both the quadratus lumborum and thoracolumbar erector spinae are shown to induce counternutation. Through Lewit’s association, the psoas would also induce counternutation, and decrease the lumbar lordosis.
Liebenson p26 discussed reciprocal inhibition, using Janda’s example of the psoas and gluteus maximus having an agonist-antagonist relationship. This opposing functional correlation indicates that, since the gluteus maximus is a pelvic extensor (a function of nutation), the psoas can be considered to promote lumbar flexion (a function of counternutation).
Kuchera refers to Jungmann p482  in stating that “The ilio-psoas is in the forefront of the structures which would oppose the thrust of gravity on the spine and pelvis.” As such, the psoas would induce counternutation since gravity induces nutation.
Perhaps the key to understanding how the psoas works is observation of its function during movement. For this, we must consider coupled motion of the spine, which can be simply described as the motion that accompanies other motions. For example, lateral flexion is accompanied by rotation. Here again, there is inconsistency and controversy in how the vertebral movements are coupled. However, when we closely consider how the lumbopelvic mechanism moves in the context of nutation and counternutation, these problems are explained. The following examples show that a complex set of factors can influence spinal coupling.
As noted in the section on the quadratus lumborum, lateral flexion creates a coupling effect to cause the lumbar bodies to rotate into the concavity of the curve, decreasing the lumbar lordosis on that side, as the pelvis rotates contralaterally  [8, 19, 20]. Thus, in a lateral bend to the right, the pelvis rotates to the left. Although the pelvis carries the spine with it towards the left (the primary rotation), L1 to L3, and sometimes L4, will rotate to the right, relative to the pelvis, along with the sacrum. Because the lower lumbar vertebrae are tied to the ilium by the iliolumbar ligaments, they will rotate counter to the rest of the lumbar spine and sacrum. This pattern describes an innate counternutation pattern where the rotation of the innominates to the left (anterior movement) is balanced by rotation of the spine, including the sacrum, to the right (posterior movement).
It is agreed that the psoas contributes significant compressive forces on the lumbar spine to provide stabilization. Bogduk stated that the psoas does not produce much action in the lumbar spine, presumably because there is no good use of the psoas inducing lordosis, since it can cause anterior shearing moments that can damage the lumbar spine. However, Santaguida et al. stated that the “dominant function at all levels is lateral bending. The ability to create axial rotation is greater than generating flexion-extension.”
In agreement with Lovett, Gracovetsky stated: “As the left leg advances and the right leg is in extension, contraction of the lateral flexors forces the spine to flex to the left, as viewed from the back. The left facets engage and the spine flexes as it bends to the left thereby reducing lordosis. The coupled motion of the spine induces a clockwise torque, as viewed from above as well as the reduction in lordosis…The left shoulder moves backward, as the spine winds up and flexes to the left.” He continued “When the spine derotates, it is necessary to increase the lordosis (on the left), maintain axial compression, and reduce shear. This can be done by a downward and forward pull on the convexity of the spine (on the right side). The right psoas is the only muscle that can perform this task. The psoas-induced and controlled axial counterclockwise torque generates a lateral bend to the right. The spine begins to straighten up and the (left) lordosis is restored.” At the same time, the right psoas is flattening the spine on the right (counternutation shifts to the right). As noted above, Santaguida stated that the dominant function (of the psoas) at all levels is lateral bending.” This concept is reinforced by the psoas’ ability to create axial rotation is greater than its ability to generate flexion/extension.
Gracovetsky also stated that “It is of extreme importance to control the lordosis. Psoas is a prime candidate for this. It has the proper line of action, very close to the center of reaction, which allows it to change the lordosis without disrupting the balancing of the moments at the intervertebral joints. This hypothesis is mathematically consistent with the hypothesis of stress minimization and equalization, and the available experimental evidence on psoas.”
In light of these statements and, combined with the coupling patterns described above, it would seem reasonable to assume that, in most cases, especially in chronic contracture, the action of the psoas is to decrease the lumbar lordosis, inducing counternutation.
For more, please see the section on Coupled Motion.
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