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Looking at the diagrams to the left, you will notice that the
vertebra extended is more anterior at the inferior aspect than
it was originally.
You might think one vertebra can be pulled into extension like
this, but it does not happen this way because neither the
vertebra above nor below the one being extended is stable with
regard to the muscle pull.
When the
muscles pull they also move the vertebrae above and below
as shown here:
neutral
contracting
The diagram to the
above right illustrates an anterior vertebra compared
to the ones above and
below with the extensor muscles contracting to bring
the spinal column into extension.
The vertebrae above and below are also brought into extension
forcing the middle vertebra, which was anterior to begin with,
further anterior.
As
you can see, because of the anatomical realities, the
physiological result is not quite what you expect on a quick
and simple look. There are no muscles aligned to pull
vertebrae in a posterior direction.
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