The psoas muscle is a long fusiform muscle that lies on either side of the vertebral column and the brim of the lesser pelvis. It is among the most significant muscles that overlie the vertebral column and has traditionally been described as having a deep and superficial segment. At its distal end, it combines with the iliacus muscle to form the iliopsoas muscle. The psoas muscle functions 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. Unilateral contraction of the psoas also helps with lateral motions and bilateral contraction can help elevate the trunk from the supine position.

Psoas syndrome is a rare condition that occurs when the psoas muscle is injured. It can appear as refractory lower back pain accompanied by other symptoms. The psoas muscle is located in the lower lumbar region of the spine and extends through the pelvis to the femur. It works by flexing the hip joint and lifting the upper leg towards the body. Psoas syndrome is best treated with physical exercises, which are often demonstrated by a doctor or physical therapist in the outpatient office and done at home. Additional treatments may include osteopathic manipulative treatment, ultrasound, and rarely, injections to muscle or associated tendon structures.

In summary, the psoas muscle is a long fusiform muscle that connects the upper body to the lower body and is a major contributor of flexion of the hip joint. Psoas syndrome is a rare condition that occurs when the psoas muscle is injured and can cause lower back pain. It is best treated with physical exercises.