In The first part of this series we laid out some unique qualities of the psoas muscle. Now we will look a little closer at each one. The psoas muscle is the bridge between the upper and lower body. There are only two muscles that connect the legs to the spine- the psoas and the piriformis. Healthy tone and function of these muscles is a prerequisite for the proper placement of the pelvis.

The piriformis attaches to a bony prominence high on the outside of the thigh bone, and then crosses the back of the pelvis to insert on the inside front portion of the sacrum. While the psoas has a greater role as a postural muscle, it can't function effectively without having a happy relationship with the piriformis. Dysfunction in one invariably involves the other.

A lot of what we are dealing with as modern man relates back to our shift from quadruped to biped. Everything changed when we stood up on two legs. On all fours the weight of the body falls through the arms and shoulders as much as the legs and the pelvis isn't asked to support the skeleton above it. The psoas doesn't play that important a role in a four-legged creature unless you are a butcher and prize it by its other name: the tenderloin. In the quadruped the psoas never actually makes contact with the pelvis crossing from the spine to the leg.

In standing the psoas is pulled across the front rim of the pelvis as it rotates from horizontal to vertical. This changes its relationship to the body dynamically. When we stood up the psoas became a main part of the support system for the upright skeleton. This happens because of the tension created when the psoas crosses the rim of the pelvis on the way towards vertical.

The psoas begins at the base of the rib cage attaching on the outer edge of the 12th thoracic vertebrae (the thoracic spine is between the lower back and the neck and includes all of the vertebrae where ribs attach to the spine) and four of the lumbar (lower back) vertebrae on its way down to cross the rim of the pelvis and head backwards and down to connect to the back half of the inner thigh.

The tension created when the psoas moves down and back after crossing the front of the pelvis supports, among other things, the proper curve of the lumbar spine as the downward action of the psoas pulls spine into its proper curve. Another equally important action is a reciprocal engagement of another set of muscles, the erector spinea or spinal erectors. When the psoas lengthens down, its forward pull on the lumbar spine allows for the erector spinea muscles to find tone lengthening up in opposition. The engagement of these muscles brings support all the way up the spine.

This action of the psoas, which creates a pulley like force that we will discuss further in another article, is a main determinant of the position of the pelvis. It would be impossible to find this action without the independence of the leg and the pelvis. Their separation is what allows the psoas to generate so much power in support of the spine. The piriformis, among its other jobs, helps to stabilize the pelvis in harmony with the psoas through its connection to the inside of the sacrum. Proper balance and function of these two muscles determine a great deal of our balance and coordination in standing and their importance as a bridge between the upper and lower half of the body cannot be underestimated.