Origin
Insertion
Innervation
Vascular Supply
The psoas major muscle is a long fusiform muscle originating from the transverse processes and the lateral surfaces of the first four lumbar vertebrae. It involves the transverse processes of the last thoracic vertebra, vertebral body and intervertebral discs. The muscle fibers are directed downwards, parallel to the lumbar vertebrae.
The fibers of the psoas major muscle and the Iliac muscle join together in the iliac fossa, passing under the inguinal ligament and inserting their robust tendon on the small trochanter of the femur. Generally, the right muscle is greater in size than the left muscle.
The psoas major muscle is the primary connector between the trunk and lower limbs and it is involved in various activities like running, sitting and walking. It also helps in stabilizing the spine and maintaining an upright posture.
The psoas major muscle is divided by some studies into upper and lower fibers.
The upper fibers tend to pass posterior to the mediolateral axis of the sagittal plane motion of the spine and cause extension, whereas the lower fibers tend to pass anterior to the mediolateral axis of the sagittal plane motion of the spine and cause flexion.
Keep in mind that the degree of lumbar lordosis would affect the line of pull of this muscle relative to the joints it crosses. (See the Psoas Paradox.)
The psoas major muscle fibers are mainly represented by 60% of anaerobic or IIA fibers and about 40% of aerobic or type I fibers. At its origin at the lumbar level, there are present more red fibers, while at the hip joint, there are more white fibers. This means that at the lumbar level its function is more static as a posture stabilizer of the lumbar spine, sacroiliac and hip joints (postural function), while at the hip level it has a more dynamic function as a hip flexor.
The psoas major muscle has been found to have fascial attachments into the diaphragm, sacroiliac joint, and the pelvic floor. Therefore, the psoas major muscle could help to stabilize all these structures.
The Core Structure & Function
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The Spine & Neck – Back Muscles Lectures – Part 2
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The Core Structure & Function Lectures – Part 2
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The Core Structure & Function Lectures – Part 1
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