Spinal lordosis optimizes the requirements for a stable erect posture

We developed a musculoskeletal model of the lumbar spine, that includes an asymmetric, dorsally shifted location of the spinal column in the body, realistic moment arms, and physiological cross-sectional areas (PCSA) of the muscles as well as realistic force-length and force-velocity relationships. The model was used to analyze the stability of an upright body posture. According to our results, lordosis reduces the local joint torques necessary for an equilibrium of the vertebral column during an erect posture. At the same time lordosis increases the demands on the global muscles to provide stability.We conclude that the development of a spinal lordosis is a compromise between the stability requirements of an erect posture and the necessity of torque equilibria at each spinal segment.Lordosis is the typical convex bending of the human lumbar spine, and is thought to be an adaptation to bipedalism [1-3]. The upright body posture distinguishes humans from most mammals. Despite lordosis and the substantial evolutionary modifications of the human lower spine and hip, the topography of back muscles in humans is remarkably similar to that found in other primates [3]. The development of a lumbar lordosis in humans is apparently not genetically determined. Children develop a lordosis as they adopt bipedal standing and walking. Even Japanese macaques gradually acquire a pronounced lordosis of the lumbar spine when they are trained to walk bipedally [1]. In women, lordosis proliferates substantially during pregnancy [4]. Thus, why do humans and some animals develop a lumbar lordosis while learning to walk bipedally? Why is this apparently a solution that is spontaneously arrived at by the motor system? When regarding the coronal plane, the spine is medial in the body, so the spinal-muscular system is symmetric (Figure 1A,B). Normally the spine does not develop a curvature in the coronal plane (known as scoliosis). On the other hand, the spine does have an eccentric, dorsal po