The Psoas Muscle: What Is It and How Can It Better Support You? The psoas (pronounced “SO-as”) is a deep core muscle that plays a vital role in connecting your upper and lower body. When people refer to “the psoas,” they usually mean the psoas major—a long, powerful muscle that runs from the sides of the lower thoracic and lumbar vertebrae (T12 to L5) through … Continue reading The Psoas Muscle: Anatomy, Function, and How to Keep It Healthy