Parametric Control of the Stiffness of Lumbar Segments

To clarify relations between kinematics and mechanics of lumbar spinal segments (L3/L4, L4/L5) versus thoracic T6/T7. Measurements of rotational angle-torque characteristics and migrations of the instantaneous helical axis (IHA) in cyclic segment motions; correlation of IHA-position and differential segment stiffness by applying theorem of Steiner. In neutral axial position of the thoracic segment (alpha = 0), IHA was found within central areas of the intervertebral disc. No change of axial IHA-positions by flexion/extension. In axial rotation IHA-migrations along wide arches from one joint to the other and S-shaped characteristics: strong dependence of IHA-migration and the differential stiffness on the degree of flexion/extension. In flexion/extension, J-shape of the characteristic and simultaneous anterior/posterior IHA-migration. Correlation of initial differential segment stiffness and IHA-position via theorem of Steiner. The following conclusions are obtained: 1. Non-linearity of the characteristics is hardly produced by the soft tissues but mainly by IHA-migration and IHA-tilting. 2. The segment stiffness in axial rotation is parametrically controlled via the degree of additional flexion/extension. 3. The autochthon muscles are able to control the segment stiffness in axial rotation via shifting the resulting force line in anterior/posterior direction without change of its amount about the factor 3-5.