Psoas syndrome is an uncommon and often misdiagnosed, condition that can appear as refractory lower back pain (pain that stays even after treatment) accompanied by other symptoms. The condition occurs when the psoas muscle—the long muscle (up to 16 inches) in your back—is injured. The psoas muscle is located in the lower lumbar region of the spine and extends through the pelvis to the femur. This muscle works by flexing the hip joint and lifting the upper leg towards the body. A common example of the movement created from this muscle is walking.