The psoas (so-as) runs from the thigh bone through the length of the belly and is the major flexor of the hip. It’s the psoas that lifts the thigh as you walk. It also acts in conjunction with the spinal muscles to support the lumbar spine. The psoas is a paired muscle, originating on the lowest thoracic vertebra and each of the five lumbar vertebrae of the lower back, and extending down through the pelvis to attach on the inside of the upper femur. It crosses three major joints—the hip socket, the joint between the lumbar spine and the sacrum (L5-S1), and the sacroiliac joint (SI joint between the sacrum and the pelvis). So it’s easy to see that if the psoas is not healthy and strong, there are major repercussions throughout the body. That back pain and twitchiness you feel? There's a good chance it is actually your psoas.
In aerial, we need our psoas to be strong and not holding excess tone. When stretching the psoas, breath is important because the muscle is intimately connected with the diaphragm through fascia and connective tissue.