Contribution of pelvic rotation to lumbar posteroanterior movement

Variability in lumbar PA stiffness has been found to relate to many factors. Sagittal pelvic rotation has been suggested as one determinant of lumbar PA stiffness. Previous studies have shown that decreased pelvic rotation is associated with increased lumbar PA stiffness. However, it is not known whether variations in pelvic rotation cause changes in PA stiffness. This study aimed to investigate the role of pelvic rotation in determining lumbar PA stiffness, and to investigate whether this role varies with vertebral level of the applied load. A mechanical device was used to apply PA forces to the skin overlying the spinous processes of L₂-L₅ with the pelvis constrained and unconstrained in 37 subjects without low back pain. Significantly higher PA stiffness (P < 0.05) was found when the pelvis was constrained. The degree of increase in PA stiffness depended upon the vertebral level being loaded, with loads at L₅ producing the greatest increase in stiffness (24%) and loads at L₂ producing a non-significant increase (6%). The findings indicate that sagittal pelvic rotation plays a significant part in the lumbar PA stiffness at L₅ but has a lesser influence at more cephalad vertebral levels.