Because of the direct attachment to the vertebrae, both passive and active strain from the musculature influence the spinal kinematics in normal or destabilized motion segments. Kinetic behavior of the spine refers to studies of the spine motions. Kinetic behavior & stability not only of the spinal motions is studied extensively but also the neutral region is studied. A transition zone for example between spine movements such as flexion & extension occurs is called the neutral region.
Segmental instability in the lumbar spine is associated with abnormal intervertebral motion. Most of biomechanical studies have studied the common stabilizing structures i.e., intervertebral disc, facet joints, and ligaments and have not simultaneously considered the effects of active musculature on spinal kinematics. Recent researches reveal that:
1. Axial translation increases in response to injuries to the disc.
2. Sagittal rotation and shear translation changes in response graded injuries to the facet joint.
3. Significantly increased coupled motion and rotation in neutral region in injury of the facet + transverse processes
Injuries to the common stabilizing structures increase the range of motion. But increased muscular activity stabilizes the injured motion segment by smoothing the erratic rotation pattern of motion, particularly in the neutral region. Surgeons & rehabilitation specialists dealing with facetectomy must take a special note of it.
Kaigle AM et al; Spine. 1995 Feb 15;20(4):421-30.