What is spinal extension? In the world of biomechanics, spinal extension is returning your spine upright after being in spinal flexion (rounded forward). That means back to your “natural curves.” However, in Pilates, we speak of extension as a backbend. For the sake of this article, let’s use this Pilates interpretation. Flexion is where inward curves round outward, and all curves match in degree of curvature relative to the natural outward curve of the thoracic (mid) spine. Therefore, extension is the direct opposite: where outward curves curve inward matching the degree of curvature relative to the natural inward curve of the lumbar spine (lower back).