Psoas major varies a great deal in size and shape. It originates in the last thoracic and all lumbar vertebrae and inserts on the lesser trochanter of the femur. It traps the coxal bone and sacrum between its attachments.
It has two sections. One originates on vertebral bodies or T12-L4, and the other originates on the transverse processes of L1-L5. Nerve roots from the lumbar vertebrae pass between these sections. They blend together and then combine with the iliacus before inserting on the femur.
Lateral views in the standing position are not often illustrated. It highlights the sections
It also shows how psoas major bends around the pubic bone before inserting on the femur. Some studies show that it lifts off the head of the femur at about 14 degrees.
Variations in psoas major are reported as rare in literature, Dissections mention psoas quartus and psoas tertius, which are separate muscles originating on the transverse processes of lower vertebrae and the quadratus lumborum. These anomalies insert at the lesser trochanter.
Origin – Psoas major usually originates from two sections. The anterior bellies originate from the bodies and intervertebral discs of T12-L4. The posterior portion originates from the transverse processes of L1-L5.
Insertion – Both sections of psoas major blend with the fibers of iliacus, and cross the pubic bone. This combined muscle inserts on the lesser trochanter of the femur.
Function – Psoas major is primarily seen as a hip flexor but also flexes the trunk laterally.
Its other functions can get complicated and are argued extensively among researchers, physiologists, and clinical practitioners. For example, it is primarily seen as contributing extending the lumbar spine and compressing the lower lumbar vertebrae, however, when the lumbar is in a reverse curve, the upper sections can, paradoxically, flex the lumbar spine.
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