A healthy human back should look most like the second picture. When we talk about spinal curvature, we talk relative to the baseline of a small lordotic curve in the lumbar spine and a small khyphotic curve in the thoracic spine, followed again by mild lordosis in the cervical spine. These men show high levels of khyphosis, which is often caused, in modern times at least, by a combination of poor posture and excessively weak back muscles.