Why this deep muscle may influence the lower back, pelvis, nerves, and the ability to rest in a deep squat. The psoas major is often described as a tight hip flexor that simply needs to be stretched. That explanation is easy to understand, but it is incomplete. The psoas is a deep muscle that connects the lumbar spine directly to the femur. It helps coordinate the lumbar spine, pelvis, and hip, while much of the lumbar plexus lies within the substance of the muscle. Because o