Lordosis is historically defined as an abnormal inward curvature of the lumbar spine. However, the terms lordosis and lordotic are also used to refer to the normal inward curvature of the lumbar and cervical regions of the human spine. Similarly, kyphosis historically refers to abnormal convex curvature of the spine. The normal outward (convex) curvature in the thoracic and sacral regions is also termed kyphosis or kyphotic. The term comes from the Greek lordōsis, from lordos ("bent backward"). Lordosis in the human spine makes it easier for humans to bring the bulk of their mass over the pelvis. This allows for a much more efficient walking gait than that of other primates, whose inflexible spines cause them to resort to an inefficient forward leaning "bent-knee, bent-waist" gait. As such, lordosis in the human spine is considered one of the primary physiological adaptations of the human skeleton that allows for human gait to be as energetically efficient as it is. Lumbar hyperlordosis is excessive extension of the lumbar region, and is commonly called hollow back, sway back, or saddle back (after a similar condition that affects some horses). Lumbar kyphosis is an abnormally straight (or in severe cases flexed) lumbar region. Types Lumbar lordosis Normal lordotic curvatures, also known as secondary curvatures, result in a difference in the thickness between the front and back parts of the intervertebral disc. Lordosis may also increase at puberty, sometimes not becoming evident until the early or mid-20s. In radiology, a lordotic view is an X-ray taken of a patient leaning backward. Lumbar hyperlordosis Lumbar hyperlordosis is a condition that occurs when the lumbar region (lower back) experiences stress or extra weight and is arched to point of muscle pain or spasms. Lumbar hyperlordosis is a common postural position where the natural curve of the lumbar region of the back is slightly or dramatically accentuated. Commonly known as swayback, it is common in dancers. Imbalances in muscle strength and length are also a cause, such as weak hamstrings, or tight hip flexors (psoas). A major feature of lumbar hyperlordosis is a forward pelvic tilt, resulting in the pelvis resting on top of the thighs. Other health conditions and disorders can cause hyperlordosis. Achondroplasia (a disorder where bones grow abnormally which can result in short stature as in dwarfism), Spondylolisthesis (a condition in which vertebrae slip forward) and osteoporosis (the most common bone disease in which