A Brief Tour of Spinal Anatomy

[Picture of the spine] The spinal column is made up 33 vertebrae separated by 23 intervertebral discs. If you are doing the math you'll need to know that the sacrum and coccyx are fused and there are no discs in between the fused vertebrae. There is also no disc between the first and second cervical vertebrae; hence, only 23 discs. Also, there is some variation in the population with regards to the actual number of vertebrae so the above is simply a general guideline.

The spinal column has three distinct curves - the cervical, thoracic and lumbar curves. We are born with a C-shaped curve and the spinal curves develop in response to different gravitational stresses that affect the body as we grow. The first curve to develop is the cervical curve and it develops in response to the baby learning to lift it's head, sit and then crawl. The lumbar curve develops as the baby learns to stand and then walk near the end of the first year of life.

he thoracic curve is already there as part of the C-shaped curve we start out with. It is very important to let these curves develop naturally so that the baby will have a healthy spine. Encouraging a child to walk or stand too early in life (as seen in a "Jolly Jumper" apparatus) may put unnatural forces through an undeveloped spine.

The three curves of your spine are important because they actually allow the spine to support more weight than if it were straight. This is because the curves increase resistance to axial compression - that is, a head-to-toe squishing of the spine. The physics of the spine state that resistance of a curved column is directly proportional to the square of the number of curves plus one. In math terms that can be written as: (3)2 + 1 = 10. That means that 10 times more weight can be supported by a curved spine than if it were straight. That is why proper lifting is so important. If you bend at the waist to pick up a box you straighten out your lumbar spine. By losing a curve you cannot support as much weight and are therefore at risk for an injury. By bending at the knees and squatting down you are able to maintain your spinal curves and protect your back.

While we're on the topic of injury, let's talk a little about disc herniations. People sometimes complain that they have a 'slipped disc' in their back. In reality, a disc can't slip out of place. But discs can be injured. An intervertebral disc is built like a jelly donut, it has a soft, jelly-like centre called a nucleus pulposus and a tough outer casing called the annulus fibrosis. The outer annulus is made of ligament and serves to join the vertebrae to one another while the jelly-like nucleus provides some cushioning for the spine. Sometimes the outer annulus can tear and this allows the inner jelly to leak out. If the leak presses on the spinal cord or on the nerve it can cause pain into the leg (in the case of a lumbar disc herniation) or into the arm (in the case of a cervical disc herniation). For a graphic of a lumbar disc herniation click here. If you would like to see and interesting picture of the spinal cord in relation to the vertebrae click here.

Different positions can place different stresses on the spine. The following chart outlines how posture can affect the low back.

Approximate Load on L3 Disc in a Person Weighing 70kg

Position Force
Standing at ease 500 N
Lying on your back 250 N
Sitting, no back support 700 N
Sitting with lumbar support 400 N
Coughing 600 N
Forward Bend, 20 600 N
Forward Bend, 40 1000 N
Forward Bend, 20 with 20 kg 1200 N
Forward Bend, 20 rotated 20, 10 kg 2100 N
Sit-Ups 1200 N
Holding 5 kg, arms extended 1900 N

As you can see, proper posture can greatly decrease the forces that are applied to your low back. Here are a few things you can do to protect your back:

  • Lift objects by bending at the kness, not at the waist. This will reduce the stress on your low back.

  • If you are carrying an object, hold it close to your body, not with your arms extended

  • Never cough while bending forward as this really puts you in a position to hurt your back.


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