Psoas Major Muscle: Function, Innervation, and Clinical Relevance

Function

1. Hip Flexion (Primary Action)

The psoas major is the primary flexor of the hip joint. It is most powerful at lower angles of hip flexion and becomes less effective near full flexion. It flexes the thigh at the hip joint.

2. Weak Lateral Rotation of the Hip

Lateral rotation occurs when the hip is slightly flexed. The psoas major contributes weakly to this movement.

Innervation

Ventral rami (anterior divisions) of spinal nerves L1–L3.

Attachments

• Origin: Lumbar vertebrae (bodies, transverse processes, and intervertebral discs of T12–L5)
• Insertion: Lesser trochanter of the femur

Clinical Relevance

The psoas major can cause or contribute to low back pain due to its attachment to the lumbar spine and its role in hip flexion and spinal stabilization.