During spinal extension, what happens to the superior vertebra?

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During extension the moving piece is the superior vertebra relative to the one below, which tilts backward and slides posteriorly. This posterior glide brings the facet joints closer together on the posterior aspect and compresses the neural canal and foramina. As a result, the intervertebral foramen narrows (often around a 20% reduction reported in some segments) and the central canal also becomes smaller due to posterior structures encroaching, including buckling of the ligamentum flavum. This is why extension narrows the spaces through which the nerve roots and spinal cord pass.

Flexion, in contrast, would open these spaces by tilting and gliding anteriorly, which is the opposite pattern. A pure stationary description ignores the clear postero-posterior movement, and lateral gliding would indicate side bending components rather than pure extension.