Functionally the iliacus and the psoas are both hip flexors, hence many anatomy books call these two muscles the iliopsoas. When you view these muscles in a detailed anatomy book you will see that the iliacus portion terminates at the anterior crest of the iliac bone. While the psoas origin is the body of T12 and the first 4 lumbar vertebrae, its insertion is the less trochanter of femur in the groin between the adductor. Thus its contraction creates hip flexion, external rotation and the natural lumbar curve and when excessively tight a lordosis. The latter condiiton is rare. Many teachers overly correct the lumbar curve and flatten it when they have students pull the coccyx down in standing poses, thus lessening the tone of the psoas.
To test for strength of the psoas lay down and lift one leg at a time 24" with 12" of abduction (out to the side) and externally rotate the femur until a valley can be felt at the upper thigh separating the gracilis tendon adjacent to the genitals and the quadriceps. One should be able to hold the position for 12 steady breaths to have adequate strength. Poses which require a strong psoas include adho mukha svanasana - down facing dog, navasana - boat; and urdhva prasarita padottanasana - upward extended legs. If the iliopsoas is weak these are difficult. A sign the muscle is strong is that the natural lumbar curve shape can be maintained in these poses.
A stretch test is to lay on the belly and pull the heel to the buttocks. This will stretch the quadriceps and if they have normal flexibility then the psoas will react as it is located posterior in the groin relative to the quadriceps. If the quadriceps are tight they will feel the stretch in mid thigh. If they are released and the psoas is tight, then the stretch will be felt as a pull in the lumbar vertebrae or upper groin. Poses that stretch the muscle are also quadricep stretches -- runner, natarajasana - dancer; ustrasana - camel.
The boat requires strength of the abdominals and the psoas in harmony. The rectus abdominis does trunk flexion (rounding the spine) and the psoas does hip and lumbar flexion (arching the low back and lifting the legs). If they are equally toned the pose looks great. It is not excessive for the body. All poses chosen for my book are ones i consider natural range of motion regardless of one's age, sex, or health. When done well one can see optimal skeletal muscle tone. When not done well, body reading can reveal which specific muscles are weak and/or tight.
One helpful reference is [u]The Psoas Book[/u], Liz Koch, Guinea Pig Productions, Felton, Ca. www.guineapigpub.com My favorite anatomy atlas reference is [u]Anatomy - A regional Atlas[/u], Carmine Clemente, 4th edition, Williams & Wilkins. Also see my website www.yogatherapycenter.org for a more thorough recommended reading list
