Psoas major

Psoas major is a polyarticular muscle, which runs inside body from lower back bone to its forward inward downward direction, front inner thigh bone, by crossing over pelvis. On its way, Psoas major combinds with Iliacus to form Iliopsoas.

Actions of Psoas major are flexion and external rotation of the hip joint.
Since it is an inner muscle, it is very hard to be aware its working. However, it starts functioning in the early stage of human life because when a baby starst to sit up or crawls it is already awakened.

Iliopsoas (Psoas major together with iliacus) is used constantly when we stand, walk or run in regular basis. But if it is not used for a long time, it will get shorten and contracted because it is a typical posture muscle and made with slow twitch fibers. That’s why those old people who have a sedentary life style have lower back pain.

With almost all asanas you can strengthen or stretch this muscle.

Examples
 Utthita Hasta PadangusthasanaD (strengthen by flexion and lateral rotation of the leg)
 Virabhadrasana 2 (strengthen by flexing the upper body, forward tilting the pelvis, straightening and supporting the lower back)
 Ustrasana (stretches by extension of the upper body and contracting hip muscles together with contraction of quadriceps)

Antagonists
 Gluteus maximus
 Hamstrings

Synergists
 Tensor fascia lata
 Satorius
 Rectus femoris
 Pectineus