The psoas major is a muscle found on the front of the hip that is responsible for flexing the hip, or bending the leg forward relative to the trunk, and laterally rotating the hip, or turning the leg to the outside. Psoas is often used as shorthand for the iliopsoas group, which is made up of the psoas major and iliacus. In some cases the psoas features a division known as the psoas minor. Collectively these muscles are commonly referred to as the hip flexors, although the term may also include other muscles that flex the hip, such as the tensor fasciae latae and the rectus femoris. In a large percentage of the population this muscle can become very short, tight, and overactive, often the result of sitting for long periods.
Originating on the spine, the psoas major is a bi-layered muscle. It possesses both deep and superficial fibers, with the deep layer found close to the spine and the superficial layer above it, nearer to the skin. The fibers of the deep layer arise via tendons attached to the transverse processes, or sideways bony projections, of all five lumbar vertebrae, L1-L5. Above that, the superficial fibers arise from the bodies of the bottommost thoracic vertebra, T12, and the top four lumbar vertebrae, L1-L4, as well as from the intervertebral discs.
Descending along either side of the spine, the psoas major crosses the crest of the iliac bone, the large wing-shaped bone of the hip. Here it unites with the iliacus muscle and together they are known as the iliopsoas. They are distinguished as a single muscular unit by the fact that both are encased in the iliac fascia, a sheath of fibrous tissue that compartmentalizes the muscle. Crossing the pubic bone, the iliopsoas continues downward to insert via a tendon deep in the thigh on the lesser trochanter, a small bony protuberance on the upper inside edge of the femur bone.
As the psoas major crosses just anterior to the hip joint, it along with the iliacus is the primary flexor of the hip, whether in bending the trunk forward at the hips or in lifting the legs relative to the trunk. This action can be seen in performing a military-style sit-up as well as in hanging suspended and lifting both legs in front of the body. Another action of the psoas is external or lateral rotation, in which the thigh is turned outward at the hip as seen in a ballet dancer with toes pointed out. The iliopsoas is particularly powerful when the actions of hip flexion and external rotation are performed simultaneously. An example of this is sprinting, as during a running stride the knee tends to be brought in front of the body and slightly to the outside rather than directly in front of the hip bone.