Normal Neutral Mechanics

With the lumbar spine in the neutral position, lumbar vertebrae and the sacrum will move in opposite directions. For example:

• When lumbar flexion occurs (forward bending), the sacrum nutates posteriorly (the sacral base tilts backward) as L5 flexes.
• When lumbar extension occurs (backward bending), the sacrum nutates anteriorly (the sacral base tilts forward) as L5 extends.

The axis around which this anterior-posterior nutation occurs has been described differently by various investigators but appears to be related to the upper and lower limbs of the sacroiliac joint and their junction somewhere around S2. to visualize sacral flexion/extension from an oblique view.

• If the sacrum rotates to the right and side bends to the left, the lumbar vertebrae will rotate to the left and side bend to the right (Click here to view an interactive animation of neutral lumbosacral mechanics from a
• If the sacrum rotates to the left and side bends to the right, the lumbar vertebrae will rotate to the right and side bend to the left (Click here to view an interactive animation of neutral lumbosacral mechanics from a

Click here to join Dr. Philip E. Greenman, D.O., F.A.A.O., as he describes normal neutral mechanics.

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Normal Non-Neutral Mechanics

If side bending of the lumbar spine is introduced when the lumbosacral spine is in either flexion or extension, non-neutral coupling occurs. The sacrum responds to non-neutral lumbar spine coupling by moving around an oblique axis, resulting in sacral torsion. By convention, the runs from the upper extremity of the left sacroiliac joint to the lower end of the right sacroiliac joint, and the runs from the upper end of the right sacroiliac joint to the lower extremity of the left sacroiliac joint. Although the exact biomechanics of the torsional movements of the sacrum are unknown, the hypothetical left and right oblique axes are useful for descriptive purposes.

Sacral torsion occurs in an anterior direction when the lumbosacral juction is extended and side bending is introduced. When the lumbar spine moves into extension, the sacrum responds with symmetric (bilateral) flexion (anterior nutation). If the extended lumbar spine then side bends to the left with coupled rotation to the right, the sacrum will rotate to the right on the right oblique axis (Click here to view an interactive animation of non-neutral lumbosacral mechanics as the lumbar spine extends and then side bends left with coupled rotation right from a --front view --back view --top view

Similarly, when the lumbar spine moves into extension followed by right side bending with coupled rotation to the left, the sacrum will rotate to the left on the left oblique axis (Click here to view an interactive animation of non-neutral lumbosacral mechanics as the lumbar spine extends and then side bends right with coupled rotation left from a --front view --back view --top view

Sacral torsion occurs in a posterior direction when the lumbosacral junction is flexed and side bending is introduced. When the lumbar spine moves into flexion, the sacrum responds with symmetric (bilateral) extension (posterior nutation). If the flexed lumbar spine then side bends to the left with coupled rotation to the left, the sacrum will rotate to the right on the left oblique axis (Click here to view an interactive animation of non-neutral lumbosacral mechanics with the lumbar spine flexed, side bent left, coupled rotation left from a --front view --back view --top view

Similarly, with lumbar flexion followed by right side bending with coupled rotation to the right, the sacrum will rotate to the left on the right oblique axis (Click here to view an interactive animation of non-neutral lumbosacral mechanics with the lumbar spine flexed, side bent right, coupled rotation right from a --front view --back view --top view