Lindsay Grymes Movement on Instagram: "Lumbar extension vs thoracic extension. They are not the same thing. What you’re seeing here is a thoracic extension, not a hinge dumped into the low back. Most people think extension means “arch your back.” In reality, that’s often lumbar compression doing the work while the thoracic spine stays stiff, quiet, and underused. The thoracic spine is designed to extend, rotate, and support posture. When it doesn’t move, the body steals motion from the lumbar sp