First, the anatomy - segmental instability occurs because a motion segment (vertebra - disc - vertebra)
allows a component motion of sheer which is not anatomic. In other words the vertebra above should never sheer against the vertebra below. This abnormal motion would occur when a disc has lost its volume and therefore its height. we see this as a narrowing of the disc space on x-ray. Once sheer is allowed, it is safe to assume that the static stability of the spine is lost. In this image you can see the loss of disc height.
Muscularly, there is an opportunity to stabilize the spine dynamically using the muscles that surround the core. In front and on the sides are the Rectus Abdominus, the Internal and External Oblique muscles and the Transverse Abdominus as well. In addition there is the strong lateral stabilizer - the Quadratus Llumborum and the posterior structures including the deep paraspinal muscles Multifidus and Rotatores and others. Further, there is the large back muscle Latisimus Dorsi that inserts into the thoraco-lumbar dorsal fascia and the Gluteal muscles that also do so and provide additional tension to the structures. Finally, there is the diaphragm above and the pelvic floor below that both play a significant role in providing true core stability.
Key core exercises tackle all these muscles. Front, back, sides, top and bottom. The secret is that it is not possible to compress water, and so by having a dynamic compression of the contents of the core, stability is created whereby the segmental instability described above is restricted from movement by the compression of the abdominal contents. Make sense?
There are other considerations such as muscle tightness and joint restriction above and blow the spine that contributes to the load at the unstable segment, perhaps perpetuating the instability and the consequence of that instability
on the pain one experiences. For example, the Hamstrings insert on the Ischeal Tuberosity and it is easy to imagine how the tight hamstrings would limit the ability of the pelvis to tilt forward during forward bending, which means that as one bends forward, the unstable segment in the spine would carry the load INSTEAD of the hamstrings. And because it is moving under load more than it should, and because the motion takes it past normal ranges due to sheering, an unstable segment is more likely to become even more unstable causing yet more pain and dysfunction as time passes.
What this means is that core stability needs to be a priority, but normalizing hip and thoracic spine mobility also need to be addressed for the core exercises to matter and for their effect to last.
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