| So what is the function of the Spine? |
SPS Clinics Ltd |
Fig 1: The Bicycle.
![Bicycle]()
A good comparison of the spine is to a bicycle i.e. sturdy framework around which the mechanics of movement take place.
Or: A bicycle chain of many inter linking sections. Kept well oiled and maintained the links run smoothly...left to rust the mechanics of movement can't take place efficiently. Or one loose link will have a reciprocal effect on its neighbor and set off a domino response and subsequent dysfunction.
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Index
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| The Skeletal System |
SPS Clinics Ltd |
The Spine
is the central core of our skeletal system. The main function of the spine like the bicycle is to provide support to your body and anchorage points to your other bones to allow you to move freely. It also has the important job of providing protection to the spinal cord which is the major nervous pathway in your body.
The spine is made up of vertebrae which can be sub categorised into three main areas:
- ) Cervical (7) - The neck
- ) Thoracic (12) - The middle back
- ) Lumbar (5) - The lower back
Each vertebrae as it connects to the next makes up a series of articulations or joints. Besides the obvious central articulations, the vertebrae have a series of joints to their sides known as facet joints.
Fig 2: Vertebrae
Side View
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View From Above
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Fig 3: The Spine
Stability is achieved with the help of three natural curves made up by the vertebrae which keep your body balanced. The curves are known as Primary and Secondary curves.
The Thoracic or middle spine demonstrates a Primary curve which was developed while we were still babies in our mothers wombs.
Secondary curves are developed as we grow and our needs to function and survive progress in early childhood.
The first Secondary curve to develop is in the Cervical spine or neck when we first begin to raise our heads to look around...probably a reflex in search of food. The second Secondary curve is at the Lumbar spine or lower back which is achieved similarly in our attempts to first get up and crawl.
Fig. 4: Physiological efficient posture
Eventually as we rise into a standing position we acquire the normal human upright posture. In proper alignment these curves allow us to achieve a normal Centre of Gravity thus making us the most able of animals in terms of all round function. So to dispel that age-old myth...we are designed to stand upright.
The correct alignment of the curves is needed to allow good function but also to prevent injury occurring. Keeping your lower back aligned is very important as the lumbar spine tends to bear the brunt of your weight plus it moves the most in functional activities. Indeed the lumbar spine has been described as the "Workhorse" of the spine.
To assist the spine to function properly and to provide strength, stability and flexibility, other structures are necessary:
- Discs are soft cushions that separate the individual vertebrae and absorb shock as you move.
The discs much likened to a soft centered sweet. The inner spongy centre is known as the Nucleus and the tougher outer shell the Annulus which contain many pain sensitive fibres.
The movement of fluid within the nucleus allows your vertebrae to rock back and forwards on your discs, giving you the necessary flexibility to move and bend.
Interestingly depending on the amount we move and with what functions we perform the discs alter in their shape, size and flexibility throughout the day.
At night while we're sleeping the discs "reconstitute" and the nucleus fills with fluid which increases the pressure on the outer annulus. This is possibly why we feel stiff and are prone to injuries on sudden movements in the morning.
As the day goes on the fluid is pushed out of the nucleus as we move and through the effects of our body weight and gravity. Depending on your view point this can make you more flexible generally or less flexible. It's true therefore that you start the day off taller and gradually get shorter by the end of the day.
- The Spinal Cord and Nerves are the structures responsible for transmitting chemical and electrical signals from your brain to every structure in the body and back to the brain from those structures (including pain signals) and so the importance of the nervous system is obvious. Although very resilient in its own make up the nervous system is incredibly vulnerable to injury or dysfunction and is often compromised in back injury. Hence the trapped nerve situation in the neck and shoulder and sciatic pain in the lower limb. Sciatica therefore is not a condition but a symptom and as such can be treated directly:
 
- Ligaments are an essential part of the joint complex. They are much like rubber bands because they can resist forces but buckle when subjected to compression.
Ligaments have several basic functions some of which seem in opposition to others.
- They must allow for correct alignment and spacing between vertebrae.
- They must provide protection to the spinal cord during normal daily activities by restricting joint movements within defined limits.
- Finally they must protect the spinal cord in traumatic situations where high velocity forces are experienced.
There are seven spinal ligaments that provide these all important services to your spinal segments.
It is interesting to note that a great deal of the trauma sustained in RTA's as in whiplash etc. is to the ligaments.
Spinal Ligaments - Side View
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Spinal Ligaments - Internal Side View
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- Muscles of the spine are needed to assist in maintaining the position of the three natural curves by holding the vertebrae in proper alignment. Although incredibly strong the back muscles are not designed for lifting. We rely on the musculature of the abdominals, hip and thighs as well as other groups to be strong and flexible to provide the power and work of moving and lifting to take the strain off of the precious back.
In important group of muscles in maintaining the upright postural position are known as the "Anti-gravity Muscles". These muscles spread from the head right down to the feet and keep our Centre of Gravity (CoG) in the anatomical position in the vicinity of the body of the 2nd Sacral vertebrae. This however is variable depending on your body shape and the postions we adapt for function.
Fig. 4: The Muscular System
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Abdominal Muscles
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Superficial Muscles
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Deep Back Muscles
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Index
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| Biomechanics |
SPS Clinics Ltd |
The Line of Gravity is a vertical line that can be drawn through the centre of gravity. In good posture if such a thing exists the line of gravity passes through the mid cervical and the mid lumbar vertebrae and in front of the mid thoracic vertebrae.
The Base as applied to the rigid body is the area by which it is supported.
Equilibrium results when forces acting on the body are perfectly balanced. If forces acting on the body at rest tend to restore it to it's original starting position quickly after it has been displaced, the body is said to be in Stable Equilibrium. if the body is given an initial displacement, however small, and the reactive forces to correct it also increase the displacement even momentarily the body could be said to be in Unstable Equilibrium.
Stability is improved when the CoG is low and the base wide. The reverse obviously reduces stability.
Orders of Levers
Biomechanically the body employs a set up of levers to achieve and maintain our body postures. These are termed the Three Orders of Levers. In the body a lever is represented by bone which is capable of movement around a Fulcrum. Effort is achieved from muscle contraction at boney insertions and the Weight can either be at the CoG of the part moved or an external object to be moved.
Figs 5a/b/c: Orders of Levers
Posturally one can illustrate the 1st Order very nicely by the balancing of the head on neck/shoulders and in maintaining the pelvic tilt needed to maintain the lumbar curve.
Practically the 2nd Order of Lever can be illustrated by the use of a wheel barrow and the 3rd by the lifting of say a ladder with its base against the wall or even using a fishing rod when reeling in the big one.
Movements of the spine are three dimensional or around three planes.
The main pure movements are Flexion, Extension, Side flexion and Rotation.
Using these movements in isolation and as combined movements allows us to achieve our activities of daily living.
Good body mechanics keeps your spine well aligned and running smoothly just like the well oiled bicycle we started with.
By maintaining the 3 natural curves in alignment throughout the day you minimise the stress on your spine and you help to prevent back pain and injury.
By using poor biomechanics and moving in the "wrong way", injury is more likely. The same applies to positions sustained over long periods including sitting which can provide incredible stress forces to the lower back.
Therefore by recognising the potential hazards in terms of body position we can help to plan ahead and minimise stress to the spine.
E.g.s:
- Stoop Standing
- Carrying heavy bags
- High heels
- Slouching
- Lifting
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Index
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| Basic Spinal Biomechanics |
SPS Clinics Ltd |
Four Types of Forces
Each spinal segment is subject to a variety of loads and forces. These include loads from the weight of body segments as well as from the weights being handled. The pattern of injury to the spine is a function of the type and size of forces present at time of injury. It is therefore important to understand these forces and the types of tissue deformations and failures that result.
There are four types of forces acting on spinal structures:
- compression;
- tension;
- shear; and
- torsional.
Compression
A downward force on the vertebrae compresses the discs and causes them to bulge or shorten and widen. This is called a compressive force. As a compressive force increases so does the disc pressure until damage occurs to the disc structures. The lifting of excessively heavy loads may cause injury due to compressive forces.
Side View
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Front View
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Key Pont: The pattern of injury to the spine is a function of the type and size of forces present at time of injury. Lifting excessively heavy loads may result in damage to the disc structures.
Tension
Tensions pull apart the structures being loaded. In the spine it is the ligaments that are usually under tension, causing lengthening and narrowing. Overstretching of the ligaments may result in tears to parts of or all of a ligament. Low back pain is usually experienced with these types of sprain injuries.
Side View
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Front View
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Key Pont: Overstretching may result in tearing of ligaments and consequently low back pain.
Shear
Shear forces involve the application of a load parallel to the vertebral surface. On forward bending of the spine there is a tendency for a vertebra to slide forwards on the one below it. Shear forces are most commonly experienced in the lumbo-sacral region and are an important mechanism in disc prolapse or herniation.
Shear Forces Acting on a Vertebrae
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Key Point: Shear forces, as a consequence of forward bending of the spine, may result in disc prolapse or herniation.
Torsional
Twisting movements of the spine cause soft tissue strain through the generation of large muscle forces and loads on the intervertebral discs. These types of movements cause the soft tissues to be exposed to a combination of compression, shear and tension forces.
Torsional Forces Acting on a Vertebrae
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Key Point: Twisting movements of the spine may cause soft tissue damage
During manual handling a combination of compression, torsional, tension and shear forces occur throughout the activity.
© 1998 The SafetyLine Institute.
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