The psoas lies in the abdominal cavity behind the internal organs and cannot be seen. It is important to understand that the psoas attaches to the lumbar vertebrae. Because of this, when you are lying down and you raise your legs, the psoas not only pulls on the femur to lift the legs, but it also pulls on the lower vertebrae. When this occurs spinal hyperextension can result if the abdominal muscles are not strongly contracted to counteract this effect.
Origin
The psoas originates on the sides of the last thoracic and all the lumbar vertebrae.
Insertion
At the lower end the psoas attaches to the lesser trochanter of the femur.
Function
Adducts the leg and tilts the trunk laterally and anteriorly.
