Allow me to introduce you to your Psoas. It’s a deep hip flexor and external rotator. Its origin is the lumbar spine and its insertion is the lesser trocanter of the femur.
The story of the psoas often ends there, but it’s really so much more interesting than that.
The psoas actually has facial attachments directly into the diaphragm (the main muscle of respiration), the inside of the pelvic bowl (via the illiacus muscle) and onto the disks of the lumbar spine.
So a tight, weak or overly developed psoas muscle is not just a pain in the hip, it can effect breathing patterns and also pull lumbar disks out of place.
A great way to isolate the psoas in movement is to lie supine on the floor, externally rotate the leg while lifting it at the hip. To give the psoas a great stretch, access it through low lunge with one knee on the floor, the other foot on the floor, pelvis set low and posteriorly tilted as much as possible. Raise the same arm up as the leg in the rear and lean over towards the opposite side till a stretch sensation is felt in the deep front of the hip. Remember the psoas has a direct connection to the diaphragm so focused breathing will definitely help access the stretch.