Original Editor - Oyemi Sillo
Description[edit | edit source]
The psoas muscle is a paraspinal muscle located deep in the body, very close to the spine and the brim of the lesser pelvis. At its distal end, it combines with the iliacus muscle to form the iliopsoas muscle. This depth, combined with the fact that the psoas originates from the sides of the five lumbar vertebrae, means it plays an important role in back health.
- It also acts to both laterally flex the lumbar spine as well as stabilize and flex the thigh. 
- It is essential for correct standing or sitting lumbar posture, stability of the hip joint, and during walking and running.
Anatomy[edit | edit source]
The psoas muscle is anatomically considered to have superficial and deep parts owing to the presence of branches of the lumbar plexus running through it. Superficial part - overlies the lumbar plexus and takes origin from the sides of the T12 and L1 to L4 vertebrae including the intervening intervertebral discs. The deep part which lies mainly deep to the branches of the lumbar plexus takes origin from the transverse processes of lumbar vertebrae L1 to L5.
Image: Psoas major muscle (highlighted in green) - anterior view 
The fibres of the muscle converge from its wide origin as they descend on the posterior abdominal wall, They cross the pelvic inlet/brim to form a long tendon, which is joined within the pelvic region by multitudinous fibres from the iliacus muscle, finally inserting into the lesser trochanter of the femur.
Below the insertional tendinous unit is the iliopsoas bursa, which separates the tendon from the bone surface and the proximal portion of the femur.
- At the lumbar level (origin) there are more anaerobic fibres ie postural, static function
- At the hip joint there are more aerobic fibres ie dynamic function.
Branches from the ventral rami of lumbar spinal nerves (L1, L2, and L3) before they join to form the lumbar plexus. The lumbar plexus is embedded within the Psoas major muscle and its branches emerge from it.
Image 4: Lumbar plexus in abdominal cavity. The lumbar plexus is embedded within the Psoas major muscle and its branches emerge from it.
The muscle receives blood from the four lumbar arteries from the aorta, from small branches of the renal arteries, from small muscular branches of the common iliac artery, and from the deep circumflex iliac artery.
The psoas muscle is enclosed by the psoas fascia (it is this that retains the pus in a psoas abscess). The psoas fascia invests the surface of the muscle, attached to the vertebral bodies, fibrous arches, and the transverse processes, and extends along the pelvic brim attached to the iliopubic eminence at the margins of the muscle. The lateral edge blends with the anterior layer of the lumbar fascia (over quadratus lumborum).
Action and Function[edit | edit source]
The psoas major muscle functions include:
- To connect the upper body to the lower body, the outside to the inside, the appendicular to the axial skeleton, and the front to the back, with its fascial relationship.
- Combined with the iliopsoas muscle, the psoas is a major contributor of flexion of the hip joint (in a supine position or standing).
- Stabilizes the lumbar spine during the sitting position (see also Exercises for Lumbar Instability)
- Acts as a stabilizer of the femoral head in the hip acetabulum in the first 15 degrees of movement.
- Unilateral contraction of the psoas also helps with lateral motions and bilateral contraction can help elevate the trunk from the supine position.
- The psoas muscle also works in conjunction with the hip flexors to elevate the upper leg towards the body when the body is static or pull the body towards the leg when the leg is in a fixed position.
The psoas muscle is also one of the core muscles.
Physiotherapy Relevance[edit | edit source]
If the psoas is weak, it will likely be harder to flex the hip joint, negatively affecting the ability to climb stairs, walk uphill, get up from supine or preparing to stand up from a sitting position.
Resources[edit | edit source]
A good 2 and 1/2 minute video on the anatomy of the psoas major. 
[edit | edit source]
References[edit | edit source]
- Radiopedoa Psoas Major Available: https://radiopaedia.org/articles/psoas-major-muscle-1?lang=us(accessed 16.1.2022)
- Very well health Psoas Muscle and Your Low Back Health Available:https://www.verywellhealth.com/psoas-muscle-and-your-low-back-health-297061 (accessed 16.1.2022)
- Bordoni B, Varacallo M. Anatomy, Bony Pelvis and Lower Limb, Iliopsoas Muscle. StatPearls [Internet]. 2021 Jul 21. Available:https://www.ncbi.nlm.nih.gov/books/NBK531508/ (accessed 16.1.2022)
- Psoas major muscle (highlighted in green) - anterior view image - © Kenhub https://www.kenhub.com/en/library/anatomy/psoas-major-muscle
- Psoas major muscle video - © Kenhub https://www.kenhub.com/en/library/anatomy/psoas-major-muscle