Psoas major muscle
The psoas major muscle is a muscle of the abdomen that connects the spinal column in the lower back to the femur (the bone of the upper leg). When contracted, it flexes and laterally rotates the hip. The psoas major is one of the primary movers in walking and running in people with good posture. It is one of the major components of what is known as the 'core' muscles.
The psoas major originates at many points in the low back: the transverse processes of the first through fifth lumbar vertebrae, the bodies of the twelfth thoracic vertebra and the first through fifth lumbar vertebrae and the intervertebral discs between them.
The psoas major is innervated by the anterior primary rami of L1 and L2.