The paper presents development of multi-segment lumbar structure based on the human spine. The research is performed within the project based on development of socially acceptable robot named "SARA". Two approaches for spine realization of humanoids exist: multi-joint viscoelastic structures (5-10 joints) that have variable flexibility and structures that consist of one joint - torso/waist joint, which has low elasticity and high stiffness. We propose multi-joint flexible structure with stiff, low backlash and self-locking mechanisms that require small actuators. Based on kinematic-dynamic requirements dynamical model of robot is formed. Dynamical simulation is performed for several postures of the robot and driving torques of lumbar structure are determined. During development of the lumbar structure 16 variant solutions are considered. Developed lumbar structure consists of three equal segments, it has 6 DOFs (2 DOFs per segment) and allows movements of lateral flexion ±30° and torsion ±45°, as well as the combination of these two movements. In development phase the movements of flexion/extension are excluded, for the bending of the body forward to an angle of 45° is achieved by rotation in the hip joints. Proposed solution of the lumbar structure is characterized by self-locking of mechanisms (if for any reason actuators stop working, lumbar structure retains current posture), low backlash (high positioning accuracy and repeatability of movements), compactness, high carrying capacity and small dimensions. [Projekat Ministarstva nauke Republike Srbije, br. III44008 and by Provincial secretariat for science and technological development under contract 114-451-2116/2011]