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Basic Biomechanics

Basic Biomechanics

Understanding the way the spine is supported and stabilized is not an easy task.  For now we will just take a very simplified view, I believe this simple understanding will go a long way in understanding the need for certain rehabilitation and exercise and help enhance your compliance with your exercise program and treatment.

The spine is built as a stack of vertebral bones interspersed with discs which are made up of fibrous material which can stiffen with pressure.

Photo: public domain

The muscles work to support the spine in its various positions, whether it is standing, sitting, flexing, extending, twisting, etc.  As an overall functional unit the muscles act as global stabilizers of the spine, they function similar to ropes in a ships mast for example.

Photo: Stan Shebs CC BY-SA 3.0

Both of these masts are given extra stabilization by the wiring which attach the structure to its foundation in various directions.  Without the stabilizing wires the structure would easily be pushed or pulled to one side and collapse.  The muscles of the spine work in a similar fashion.  From a frontal view of the spine we can see how the lateral muscles act to stabilize the spine from lateral motion and from twisting motions.  These muscles have an intrinsic stiffness or tightness which imparts stability to the spinal column, if they are too tight they cause too much compression on the spine, discs and joints and do not allow a proper range of motion.  If they are not tight enough they do not provide enough protection against whatever load is put against the spine.  The normal stiffness of these muscles should always be present depending on activity.

From the side we can see the muscular support system between the extensors and the flexors.  Not only do we want an intrinsic level of stiffness in these muscles, we also want a balance system between groups of muscles which pull against each other, termed antagonistic muscle groups.

Quite commonly injury creates a pattern of movement which is painful.  The body, in its attempt to avoid pain, creates a new movement pattern which attempts to avoid pressure on the area of the painful injury.  Over time many things can happen which help to ingrain this new substandard movement pattern into our systems.  If the injury does not properly heal there may be residual permanent pain due to scar tissue formation and pain receptor hypersensitivity, that is the receptors in the injured region will become more and more sensitive to pain.  This means that a hypersensitive pain receptor will need less of an irritation to produce sensations of pain, this continues to encourage the brain to develop and maintain alternate movement patterns which do not cause pain initially, but put mechanical pressure and strain on tissues which were not designed for this wear and tear eventually causing secondary injury to these structures as well.

A large portion of the rehabilitation program is aimed towards recognizing these aberrant movement patterns and correcting them through a learning program with the correct movement patterns.  Care must be taken to properly progress through different difficulty grades involving both varying ratios of force and velocity or the patient may be quickly reinjured.

Intra-Abdominal Pressure:  We initially began our discussion with the concept of the core being a simple cylinder or barrel.  We would have the round circumferential wall of the cylinder which would be akin to our Transverse Abdominus, Internal and External Obliques, and their attachment points at both the front of the body (Rectus Abdominus, abdominal sheath) and at the back of the body (thoracolumbar fascia).  We would also define the top of the cylinder as the diaphragm muscle and the bottom as the pelvic floor muscles.  In synergy these muscles function to always have a degree of pre-tension or stiffness which causes increased pressure inside of the cylinder they form.  This cylinder surrounds our trunk and provides stabilization, especially as we load the spine as seen in flexion and/or lifting of a weight just to name a few.

Spinal Compression:  The spine and discs act as shock absorbers in the trunk.  They function this way due to the viscoelastic properties of both the discs and the endplates of the vertebrae themselves.  It has also been theorized that the valveless venous system which supplies blood to the vertebrae also acts as a passive hydraulic shock absorber.  A small amount of compression is a good thing, it provides stiffness to the spinal discs by slightly pressurizing them, providing a degree of stored energy which can help stiffen and stabilize against load.  Too much compression is unhealthy and can over time cause damage to spinal structures.  The first structures to suffer are the discs themselves which begin to tear and shrink over time.  This shrinking lowers the stability of the spine and puts the spinal facet joints into more of a load bearing position, also eventually causing dysfunction and eventual wear and tear.  The most obvious sequelae of this dysfunction is osteoarthritis which is the bodies attempt to stiffen the ligaments and structures around these joints.

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