If you have taken a Pilates mat class, experience the one-on-one attention of a private Pilates session, or practiced along side a DVD, you have probably heard the term Pilates "V" feet, or Pilates first position (based on the first position in ballet, but not quite as exaggerated). That little Pilates first position isn’t for aesthetic reasons like back at Miss Loula’s Ballet class. Nope it serves a purpose.
The purpose is to get your psoas working. The psoas is a muscle that does hip flexion, meaning to draw the thigh up toward the front of your torso. The psoas is closer to the mid line of your body than your big bulky quads, of which the rectus femoris also works to flex the hip. They are both considered hip flexors, but the psoas has the advantage of being located closer to your spine than your other hip flexors, and therefore it is more efficient. Plus in Pilates you are always trying to get the deep muscles to work. You want to get everything fired up closer to core.
Also of importance it the fact that the psoas creates slight external rotation of the hip (the thighs rolling away from one another), as well as pulls the legs closer to one another, known in kinesiological terms as adduction. To make proper "V" feet you only open your feet about 45 degrees, not as wide as your feet will go. The rotation of feet happens not at the ankles or the knees but at the hips. You want to also press you heels slightly together to fire up your psoas even more and get your inner thighs to work a bit too.
Hope this helps with your 100's.
Oh! You can also make "V" feet with flexed toes. To see what that looks like
The flexed foot position is good to use if you tend to cramp in your calves or if you feel tight in your calves. Keep those heels together though!!!