The mysterious psoas muscle is famous as a hip flexor, but it does a whole lot more! Running from the mid back to the inner thigh, it's connected to the diaphragm, plays a role in breathing, and is stretched and toned by our breathing. The psoas also plays a role in external rotation of the leg and acts as its own antagonist—both extending and flexing the lower spine. Kat explains what these roles of the psoas mean for us in practice.