This book presents the entire process that led to the development of a finite element model of the L4-L5-S1 segment, experimentally validated for biomechanical study of the human lumbar spine. In order to perform a more accurate validation of the model, in vitro experiments were carried out on segments L4-L4 and L5-S1 with gradual reduction method, a procedure that had only been applied to segment L4-L5, where significant differences were found in the range of motion (ROM) between segment L5-S1 and segment L4-L5. With experimental results, the finite elements model was calibrated adding step by step each one of the structures of the segment. Finally, a statistical analysis of the fibrous annulus was carried out in order to study the influence on the ROM of elastic constants of the fibers, the wedge angle and the ring area.