structure and function
Classical anatomy textbooks such as testut describe the psoas major muscle originating from the top of the lateral aspect and base of the transverse processes of the first four lumbar vertebrae and the twelfth thoracic vertebra and interposed intervertebral discs. insertion into the discs and transverse processes is through short tendinous tabs, which insert into the vertebral body with a medially concave arcuate tendon to allow the passage of the lumbar vessels and the communicating branches of the orthosympathetic nerves.
From the lumbar spine the numerous muscle bundles travel forward, downward and laterally to form a single, cylindrical, fusiform muscular body, thickest at the sacroiliac joint. the muscle crosses the lumbar region and the iliac fossa, whence it emerges through a notch in the superior margin of the iliopubic ramus between the anterior inferior iliac spine and the iliopectineal eminence (laguna musculorum). at this level, the posterior cords adhere firmly to the pelvic brim, and the muscle fibers change direction, bringing it posteriorly and laterally to insert through a thick tendon on the posterior aspect of the small trochanter of the femur.
The iliacus muscle has a fan-shaped triangular shape that occupies the internal iliac fossa. originates from the upper two-thirds of the iliac fossa, from the medial border of the iliac crest, from the iliac-lumbar ligament, where it fuses with the fascicles of the quadratus lumborum muscle, from the base of the sacrum, anteriorly from the iliac spines superior and inferior anterior, and finally in the anterior capsule of the hip joint. the muscle bundles move to the lateral side of the tendon of the psoas major muscle on which they insert. few lateral and inferior muscle bundles insert directly on the femur. the psoas minor muscle is a small, inconsistent muscle bundle that often becomes fibrous tissue, it is present in about 60-65% of the population .. it originates from the body of the last thoracic and first lumbar vertebrae and the interposed disc and is located anterior and medial to the psoas major muscle. It is shaped like a thin, flattened ribbon and fits over the iliopectineal eminence and iliac fascia.
psoas bursa. The psoas muscle is separated from the bony margin of the ilium and the fibrous capsule of the hip joint by a sizable sac of serous fluid (the iliopectineal bursa) that sometimes it communicates with the synovial space of the hip joint. a smaller bursa lies between the tendon of the muscle and the anterior surface of the lesser trochanter of the femur.
The iliopsoas muscle has important fascial relationships in its upper or lumbar abdominal portion, in its medial portion in the iliac fossa near the inguinal ligament, and in its femoral portion in the lower limb.
lumbo-pelvic portion. the psoas major muscle sits posterior to the diaphragm, psoas minor muscle, kidney and renal vessels, ureter, gonadal vessels, ascending colon on the right, and descending colon on the left. it is placed anterior to the intertransversal muscles of the transverse process of the vertebrae on which it inserts, with the quadratus lumborum muscle, from which it separates from the deep layer of the thoracolumbar fascia,, and from the anterior branches of the lumbar nerves. its right medial margin corresponds to the inferior vena cava and the descending part of the duodenum, the left medial margin is related to the aorta, the ascending part of the duodenum and the duodenojejunal angle. inferiorly it is traversed by the ureter and the common iliac vessels and runs parallel to the common iliac vessels. the psoas muscle is crossed by the branches of the lumbar plexus, which emerge from its surface: anteriorly the lateral femoral cutaneous nerve and the genitofemoral nerve; medially the lumbosacral trunk and the obturator nerve; laterally the iliohypogastric nerve, the ilioinguinal nerve and the femoral nerve. it should be kept in mind that the lumbar plexus and peripheral branches have great anatomic variability. the iliacus muscle is positioned posterior to the cecum on the right and the sigmoid colon on the left; its posterior surface lies in the iliac fossa and is related to the fibrous capsule of the sacroiliac joint, the iliolumbar ligaments, and the deep branches of the circumflex iliac vessels. the iliolumbar ligaments are an integral part of the thoracolumbar fascia following the posterior aspect of the psoas major muscle which is covered by the deep layer of the thoracolumbar fascia and continues superiorly with the endothoracic fascia.
inguinal portion. the iliopsoas muscle passes under the inguinal ligament, occupying the entire space between the ligament, the iliopectineal band, and the anterior border of the ilium: a space called the lacuna musculorum. the iliopectineal band separates it from the femoral artery and vein. the femoral nerve is on the anteromedial side.
femoral part. the lateral surface of the iliopsoas muscle forms the lateral part of scarpa’s triangle. its posterior side rests on the fibrous capsule of the hip joint. its medial margin corresponds to the lateral border of the pectoralis muscle with which it forms a groove for the passage of the femoral artery. its lateral margin is related to the sartorial muscle and the rectus femoris muscle.
The function of the psoas muscle is to connect the upper part of the body with the lower part of the body, the exterior with the interior, the appendicular with the axial skeleton and the anterior with the posterior, with its fascial relationship. Combined with the iliopsoas muscle, the psoas is a major contributor to hip joint flexion. Unilateral psoas contraction also helps with lateral movements, and bilateral contraction can help elevate the trunk from the supine position. The psoas muscle also works in conjunction with the hip flexors to lift the upper leg toward the body when the body is static or pull the body toward the leg when the leg is in a fixed position.
The biomechanical and postural function of the psoas complex muscle is to flex the hip, adduct the femur, and externally rotate the hip. in the upright position, the muscle takes a fixed point on the femur and acts on the pelvis and lumbar spine. its action is to flex the spine, bend it laterally ipsilaterally, and rotate it to the opposite side. the proper function of the psoas major muscle is to stabilize the lumbar spine in the sitting position and to flex the femur in the supine or standing position. the psoas major acts as a stabilizer of the femoral head in the hip socket in the first 15 degrees of movement, maintains the erector action from 15° to 45°, and is an effective flexor of the femur from 45° to 60°.  it appears that the inferior cords flex the lower lumbar spine and the upper cords to extend the upper lumbar vertebrae: the flexion-extension movement is small, while the compression and shear forces are large, collapsing the spine in a sigmoid fashion and forcing it in. lordosis while severely tightening the lumbosacral segment. an anchoring retinaculum at the level of the lumbosacral junction holds the psoas muscle in place along the lumbar spine, making its action on increasing lumbar lordosis independent of lumbar curvature. during gait movements, the ipsilateral psoas muscle is activated during early hip flexion and during the latter part of the swing phase.
the psoas major is in relationship with the medial arcuate ligament and is continuous with the thoracic diaphragm and the endothoracic fascia: the psoas major muscle seems capable of influencing the balance of the dynamics of the respiratory function and the functional relationship between the diaphragm and pelvic floor. the fascia of the muscle appears to show continuity with the fascia of the pillar of the diaphragm, blending with the anterior longitudinal ligament; while the inferomedial fascia of the psoas muscle thickens and becomes continuous with the deep fascia of the pelvic floor: this forms a link with the conjoint tendon, the transversus abdominis muscle, and the internal oblique muscle. From this point of view, the psoas muscle can influence the functions of the pelvic floor, the balance between the movements of the pelvic floor with the rhythm of the thoracic diaphragm; It may be involved in the venous and lymphatic drainage of the abdomen, pelvis, and lower limbs.