The Evolution and Function of Human Lumbar Lordosis Variability

Abstract

Curvature of the human lower spine, known as lumbar lordosis, was a key adaptation for bipedalism that evolved to balance the upright trunk, but measures of lordosis in modern humans and estimates from the fossil record indicate substantial variability within and among hominin groups. This thesis explores the biomechanical consequences of lordosis variability in modern humans in three studies to understand the evolutionary pressures that have shaped the human lumbar spine. The first study used magnetic resonance imaging and a postural analysis to test three biomechanical models of lordosis variability. The first model tests the correlation between lordosis and trunk-balancing hip moments; the second tests a beam bending model of lordosis variability; and the third tests an interaction and tradeoff between trunk strength and lumbar flexibility. Results show that hip moments are not associated with lordosis, and the beam model explains 25% of lordosis variation. Lordosis is best explained by the strength-flexibility model, which shows that lumbar flexibility modulates the effects of trunk strength such that stronger backs correlate with greater lordosis and stronger abdominals correlate with reduced lordosis. At the same time, low trunk flexibility is associated with reduced lordosis regardless of trunk strength variations. The second study examines how lordosis affects the lumbar spine during static axial loading. A weight vest experiment measured changes in lordosis under load. The first hypothesis tests whether lordosis decreases under load to provide stability; the second tests time-dependent variations in lumbar deformations during loading; and the third tests the correlation between lordosis and the spine’s resistance to bending. Results show that average lordosis decreases with load, and that straighter spines show less variability in lordosis change during loading. Within a loading trial, straighter spines also tend to straighten while curved spines tend to become more curved. In addition, lordosis negatively correlates with both bending stiffness and elastic modulus, suggesting that straighter lumbar spines provide greater stability. The third study used accelerometers mounted on the back to examine how lordosis affects lumbar shock attenuation during barefoot walking and running. The first hypothesis tests the effect of lordosis on shock attenuation; the second tests a viscoelastic model of dynamic lumbar motion; and the third tests how intervertebral disc height affects attenuation. Results show that lordosis has no effect on attenuation during walking but a strong effect during running. In addition, greater attenuation is associated with greater lordosis angular displacements and slower angular displacement velocity. Thicker intervertebral discs are also associated with increased attenuation, but lordosis is a stronger predictor of attenuation when controlling for both discs and lumbar posture. These findings suggest that lordosis increases lumbar shock attenuation during running when dynamic impact forces are highest. The results of this thesis provide context for interpreting lordosis in fossil hominins and shed light on the etiology of modern lumbar spinal pathology. The first study suggests that trunk geometry passively influences spinal shape. This result may explain why the larger upper bodies of straight-backed Neanderthals, who had deeper rib cages and longer pubic bones, may have experienced higher bending moments acting to reduce lordosis. The second and third studies imply a functional tradeoff underlying evolutionary differences in lumbar curvature. Lordosis is associated with increased lower back mobility and greater lumbar shock attenuation capacity, but curved lumbar spines are also associated with less resistance to bending deformations, less stability, and increased risk of injury due to higher intervertebral shearing forces. This tradeoff has important implications for understanding the selection pressures driving adaptive changes in lumbar lordosis through hominin evolution and perhaps may explain the recent rise in lower back pain prevalence.