Although caution must be exercised in comparing the
elasticity of the ligamentous LS to that of an elastic metal
strip, there are nevertheless some interesting similarities.
Like the string fastened to the top of the strip, the PM is
able to increase lordosis of the LS, without affecting vertical position, by moving upper and lower regions in opposite directions
Furthermore, the course of the
PM with respect to the LS corresponds strikingly well with the specific direction of string pull in the metal strip
experiment (Fig. 9).
These similarities suggest that the PM may have an effect on the LS that is comparable to the effect achieved by
the specifically directed string on the metal strip. According to the metal strip experiments, the PM may theoretically act in two different ways: by actively pulling the upright LS into more lordosis, or by passively stabilizing
any given upright LS lordotic position by tightening its individual fascicles.
Bogduk et al. [3] pointed out that pulling the LS into
more lordosis would require maximum PM effort, as its
fascicles have short moment arms (pass at short distances
from the motion centres) and hence will tend to exert severe compression forces on the lumbar motion segments.
Consequently, this does not seem a purposeful function.