- Iliopsoas (Psoas major + Iliacus)
- Action: Primary hip flexor.
- Clinical Pearl: Irritation causes a positive Psoas Sign. Key differential includes appendicitis, psoas abscess (often from TB/Pott disease or S. aureus).
- Quadratus Lumborum (QL)
- Action: Lateral flexion (side-bending) of the trunk.
- Erector Spinae Group (“I Love Spine”)
- Action: Primary extensor of the back.
- Clinical Pearl: Most common muscle group injured in nonspecific low back pain (lumbar strain).
