ANATOMY:
The iliopsoas muscle is found in the abdominal cavity and in the anterior part of the thigh.
It is one of the most powerful muscles of the body,
the main flexor of the thigh and the most powerful of the flexor muscles of the hip.
It is constituted by two parts: a long medial part,
the psoas,
and a wide lateral part,
the iliac.
The psoas or psoas major portion originates in the T12 vertebrae and the first five lumbar vertebrae (T12-L5),
as well as at the base of the corresponding transverse processes,
and descends towards the iliac fossa where it joins with the iliac (Fig.1).
It increases slightly in cross-sectional diameter distally,
and normally is bilaterally symmetric.
The iliac portion is inserted above into the inner lip of the iliac crest, the superior two thirds of the iliac fossa, sacrum and ventral aspect of the iliolumbar and sacroiliac ligaments.
Both muscular bodies joined together to pass under the crural arch in the external zone, to reach the lesser trochanter of the femur.
If we study the psoas iliac muscle in the sagittal plane (Fig.2) we see that it courses anteriorly till the superior pubic branch where it curves posteriorly to reach the lesser trochanter. This angle will depend on the position of the pelvis measuring approximately 45-60º in the supine position and will increase as the hip extends (Fig.3).
MUSCLE FUNCTION:
The iliopsoas functions dynamically as a primary hip flexor,
with a significant role in advancing the lower limb when walking,
running or kicking.
Additionally it has major static functions,
in balancing and stabilizing the body when standing,
sitting or lifting.
The action of the muscle will depend on which of the insertion points remains fixed during contraction.
When we keep the spine and pelvis fixed and the muscle pulls its distal insertion,
the anterior hip flexion occurs.
If we keep the pelvis and hip fixed and it pulls from the proximal insertions of the spine,
trunk flexion occurs.
Unilateral contraction of the psoas also participates in lateral flexion of the lumbar spine.
Since it is inserted in the inner face of the femur it has the function of external rotation of the hip.
On the other hand due to its orientation in the sagittal plane it plays an important role in the static of the spine causing lumbar hyperlordosis.