Synergy of human spine in neutral posture

Disorders of the human spine account for a major part of disabilities in the workforce and have a significant socioeconomic impact. The present study makes an attempt to develop a new hybrid kinematic approach for analysis of the human spine in the neutral posture. The model combines two essential parts of the human spine: the passive osteoligamentious spine and spinal muscles with both parts interacting according to a proposed control algorithm.

Since the model focuses primarily on neutral posture with a limited range of motion, stiffness of the spinal motion segments is considered to be linear, and beams are used to simulate their elastic behavior. The trunk, reinforced by the rib cage and significantly more rigid than lumbar spine, is simulated by increasing the stiffness of the thoracic disks. In the equilibrium spinal analysis virtual springs are used to control the spinal configuration and to provide for interaction between the passive spine and the muscles. In the analyses, muscles are classified in two groups, global running from the pelvis to the rib cage and local running from the pelvis to the lumbar vertebrae. A simple formula for muscle stiffness is used to assign stiffness to each muscle based on its force and length determined from the equilibrium analysis. Constraints are replaced by the truss elements originating and inserting into the same points as muscles with corresponding stiffness properties and tension forces. This state of spine is subsequently analyzed in a linear perturbation step for a magnitude of an eigenvalue which corresponds to the load added onto the arms.

The proposed approach examines effect of changes in spinal configuration controlled by mechanisms such as T1 positioning on mobilization of the passive spinal resistance. The role of the pelvic rotation and muscle forces are then further investigated during the time dependent viscoelastic changes (creep) of the disks. Response of the spine under the postural load is evaluated also in cases with a simple and a detailed muscular architecture.

Findings indicate that the spinal load-bearing potential is enhanced by the mechanisms based on synergetic principles. The compressive resistance of the spinal structure considered as a vertical column is exploited by maintaining the higher buckling modes. Muscle forces necessary to maintain higher modal shapes when acting at characteristic buckling points are only minimal. One of the functions of multiplicity of the lumbar muscular architecture in the neutral posture is to prevent spine from transition into hypermobily when the spine deforms in the higher buckling modes. (Abstract shortened by UMI.)