Since the majority of back pain emanates
from problems related to the intervertebral disc,
a discussion that is centered around the inter-relationships
of anatomy, physiology, and mechanics is most pertinent.
The effect of forces acting upon the spine which generate
pain can be understood best by considering what physically
occurs when forces are applied.
When
a person is standing erect, the entire weight of the upper
body above the pelvis is supported by the lumbar spine.
Look at a skeleton, when the ribs end, the lumbar
spine is the only structure left to support the
weight. That means better than half the weight of the human
body is resting upon the vertebral column in the upright
position. This force is directed from above on every vertebral
disc, compressing it. The bones are rigid, but
the consistency of the discs more approach that of a liquid.
The bones don't physiologically compress, but the discs
do.
The intervertebral disc's management of compressive forces
can be demonstrated by figuratively placing a liquid filled
balloon between two vertebral bones (Figure 22). It is known
in the laws of hydraulics that the pressures acting on a
liquid are equal on any surface upon which the liquid acts.
With both the anterior and posterior surface areas nearly
the same, the pressure acting upon them is roughly equal
when the material within the disc is largely a liquid.
Now,
consider what is happening when the body bends directly
forward in flexion with weight on the vertebral column.
The posterior aspects of the vertebral bodies separate and
the space between them widens, increasing the surface area
upon which the pressure acts, and causing the liquid center
to bulge posteriorly. So, when the enormous pressures, as
delineated above, are applied to the disc in anterior flexion
they are felt by the posterior aspects of the annulus fibrosus
and the capsuleas in Figure 24.
When in flexion, the anterior component of the dish- or
bowl-shaped (See Figure 23) disc containing surfaces of
the vertebral bodies close while the posterior aspects of
the bowl-shaped surfaces of the vertebral bodies open. In
the anterior, the pressure is contained by two bony walls
that have come together.In the posterior,
the pressure is received by relatively weaker, stretched,
ligamentous structures, causing the contents of the nucleus
pulposus to protrude towards the back (posteriorly and peripherally
as in Figure 24). Like clapping jell-o with half-open cupped
hands, the anterior aspects of the vertebral bodies are
closed so the nucleus pulposus is forced posteriorly by
the pressures exerted when these similarly bowl-shaped surfaces
close. The "jello" has no alternative than to
be squeezed out through the widest opening--the posterior.
Under the tremendous forces generated during a flexion event
(as in lifting a heavy weight), the nucleus pulposus, its
hardened center, and the more gelatinous components of the
annulus fibrosus are all caused to forcefully move and expand
posteriorly. The central disc material is caused to move
away from the anterior compression force generated by the
weight of the superior vertebral body pressing down on the
bony surface of the inferior vertebral body. The vertebral
bodies above and below the disc have concave surfaces which
direct the central disc material posteriorly. This central
disc material (especially when it is not liquid as in an
older person) often must move under the influence of these
dish-shaped pressures and has no where else to go except
posteriorly during WEIGHT-BEARING FLEXION especially during
traumatic or forceful events. It cannot move superiorly
or anteriorly because it meets the bony surfaces of the
vertebral bodies. The anterior component is closed by virtue
of the flexion, and there is only one place for the hard
central disc material and the liquid component to travel--peripherally
and posteriorly, that is, away from the anterior compression
induced by the WEIGHT-BEARING FLEXION. The damage to the
disc occurs when these forces are so great as to exceed
the strength of the materials meant to contain them.
When
a person is standing erect, the entire weight of the upper
body above the pelvis is supported by the lumbar spine.
Look at a skeleton, when the ribs end, the lumbar
spine is the only structure left to support the
weight. That means better than half the weight of the human
body is resting upon the vertebral column in the upright
position. This force is directed from above on every vertebral
disc, compressing it. The bones are rigid, but
the consistency of the discs more approach that of a liquid.
The bones don't physiologically compress, but the discs
do. 
Now,
consider what is happening when the body bends directly
forward in flexion with weight on the vertebral column.
The posterior aspects of the vertebral bodies separate and
the space between them widens, increasing the surface area
upon which the pressure acts, and causing the liquid center
to bulge posteriorly. So, when the enormous pressures, as
delineated above, are applied to the disc in anterior flexion
they are felt by the posterior aspects of the annulus fibrosus
and the capsule as in Figure 24.
Under the tremendous forces generated during a flexion event
(as in lifting a heavy weight), the nucleus pulposus, its
hardened center, and the more gelatinous components of the
annulus fibrosus are all caused to forcefully move and expand
posteriorly. The central disc material is caused to move
away from the anterior compression force generated by the
weight of the superior vertebral body pressing down on the
bony surface of the inferior vertebral body. The vertebral
bodies above and below the disc have concave surfaces which
direct the central disc material posteriorly. This central
disc material (especially when it is not liquid as in an
older person) often must move under the influence of these
dish-shaped pressures and has no where else to go except
posteriorly during WEIGHT-BEARING FLEXION especially during
traumatic or forceful events. It cannot move superiorly
or anteriorly because it meets the bony surfaces of the
vertebral bodies. The anterior component is closed by virtue
of the flexion, and there is only one place for the hard
central disc material and the liquid component to travel--peripherally
and posteriorly, that is, away from the anterior compression
induced by the WEIGHT-BEARING FLEXION. The damage to the
disc occurs when these forces are so great as to exceed
the strength of the materials meant to contain them.