Psoas Major

Original Editor - Oyemi Sillo

Top Contributors - Vidya Acharya, Samrah khan, Lucinda hampton, Oyemi Sillo, Kim Jackson, George Prudden, Samson Chengetanai, Joao Costa, Maram Salem, Kardelen Aktas and Rachael Lowe;

Description

The psoas muscle is a low back 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.

1. It also acts to both laterally flex the lumbar spine as well as stabilize and flex the thigh.
2. It is essential for correct standing or sitting lumbar posture, stability of the hip joint, and during walking and running.

Anatomy

Origin

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.

Insertion

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.       

Nerve Supply

Branches from the ventral rami of lumbar spinal nerves (L1, L2, and L3) before they join to form the lumbar plexus.

Blood Supply

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. 
Action and Function

The psoas major muscle

1. Stabilizes the lumbar spine during the sitting position
2. Flexion of the thigh in a supine position or standing.
3. Acts as a stabilizer of the femoral head in the hip acetabulum in the first 15 degrees of movement.
4. 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.
5. Combined with the iliopsoas muscle, the psoas is a major contributor of flexion of the hip joint.
6. Unilateral contraction of the psoas also helps with lateral motions and bilateral contraction can help elevate the trunk from the supine position.
7. 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.

Psoas Syndrome and Iliopsoas Tendinitis:

Psoas syndrome and iliopsoas tendinitis are less common musculoskeletal conditions therefore, they often go misdiagnosed. Clinically it presents symptoms like low back pain, pain in groin and pelvis, radiating pain towards knee; which halts the primary function of psoas muscle that is hip flexion (hence difficult walking) and to sustain fully erect posture. The condition is mostly found in acute/overuse injuries, more commonly sports specific like runners, athletic activities, plyometric and among ballet dancers. It is known as internal coxa saltans.

Prolong sitting and work which requires long sitting periods, tend to put psoas in shortened and tight position leading to psoas syndrome. Because they are major flexors, weak psoas muscles can cause many of the surrounding muscles to compensate and become overused. That is why a tight or overstretched psoas muscle could cause low back and pelvic pain. Iliopsoas tendinitis may also cause snapping hip syndrome which is characterised by pain and snap (click) in groin.

Moreover, restriction of diaphragm can also occur due to psoas dysfunction and psoas spasm can potentially cause more disability of back muscles than other pathologic condition. A tight psoas muscle can create a thrusting forward of the ribcage.  This causes shallow, chest breathing, which limits the amount of oxygen taken in and encourages over usage of your neck muscles.                                                                  

'Psoas, “the muscle of soul”                                                     

The unique location of psoas makes it the deepest core muscle of human body and the only muscle that connects trunk to lower limb. Therefore, it plays a key role in stability and balance of human body. The psoas muscles support internal organs and work like hydraulic pump allowing blood and lymph to be pushed in and out of the cells. It creates a muscular shelf on which the kidneys and adrenals rest. The movement of diaphragm, during breathing gently massages the psoas muscles and neighbouring organs, thereby stimulating blood circulation. But, when the psoas muscles become imbalanced, kidneys and adrenal glands also get affected, resulting into physical and emotional exhaustion.

So, psoas may seem like small muscle, but its pivotal role in walking, postural alignment, balance, flexibility, joint mobility and organ functioning, makes it, clinically important, during assessment and screening.

Related links

Psoas minor