Everyone tucks their pelvis. This is my mantra and I am sticking with it. When the pelvis is tucked under it throws the whole skeleton into disarray but it especially affects two muscles, the psoas major and the rhomboids, which have an interesting relationship in the body. The psoas straps the pelvis to the legs and spine at the front of the body while the rhomboids strap the shoulder girdle to the spine at the back of the body.
The rhomboids are a muscle that gives people fits in that they are often lacking the tone necessary for upper body stability. Other than one bony connection at the front of the chest, the shoulder girdle is only connected to the spine by the rhomboids, and only the rhomboids, so their tone determines the resting position of the shoulder blades which are often sliding off the back, often pulled that way by a tight pectoralis minor.
The psoas major is a muscle that acts like a pulley to provide support and elevation to the spine. When the psoas is aligned correctly the spine is able to stack successfully on top of the pelvis. A well aligned psoas means that the curve of the lower back is in its optimal position. The emergence of the psoas as we came to stand established the lumbar curve and helps to maintain it with good posture.
The rhomboids need that curve of the lower back if they want to find a proper resting place. When the pelvis tucks under we lose or change the lumbar curve which has the effect of stretching the rhomboids and pulling the shoulder blades towards the front plane of the body.
This isn’t hard to feel. If you stand up and tuck your pelvis you should be able feel the shoulder blades being pulled out to the side. When you rotate your pelvis to a neutral position that allows for the proper curve of the spine you should feel your shoulder blades move subtly towards each other.
All the rhomboid exercises in the world will not help to change the nature of these muscles until we improve our posture in order to give them the best chance to do their jobs correctly. The rhomboids and the psoas major, as well as every other muscle in the body, need a well aligned pelvis in order support the upright body.