When people sit down, they often do so very badly.

For example, the vast majority of people sit down with their buttocks too far from the back of the chair.

In this position, their lumbar spine is not supported, and people often lean backwards. This causes the lower back to curve away from the base of the chair (shown below); as a result, the joints in the spine are compressed, the intervertebral discs become squashed and the wrong muscles are engaged.

Posture - slouching

People who use a computer also tend to lean forwards (shown below). This time, the upper back curves away from the chair, and the shoulders curve in, and down, towards the chest. To maintain the line of sight, the angle between the neck, and the back of the head, is reduced. This compresses the neck.

Posture - slumping

In addition to this, the head (which on average weighs about 5 kg or 11 pounds) is positioned in front of the spine, as opposed to on top of it.

This increases the strain on the neck and shoulder muscles substantially. Fatigue and muscle tension will result.

To correct this, most people will tell you to sit upright, on your sitting bones (or ischia). This is done by sitting down and then rolling your weight forward onto your pelvis. It is easier to practice on a hard chair: when you roll forward, you should be able to feel your ischia in contact with the surface of the chair.

The sitting bones (ischia)

However, whilst this is definitely better than slouching, it is not the best way to sit (most people tend to over-arch their lower back by forcing their weight forward).

Instead, you should sit with your buttocks close to the back of the chair, then lean back into the chair, so that your lower back is supported (shown below). This may be difficult to achieve without the use of a small cushion, placed behind your lower back. In addition to this, keep your upper back and neck straight, with your head supported over your neck. This can be done by gently pulling your head back, whilst keeping it horizontal.

Sitting correct

Do not tip your head back; this will compress the neck, without correcting the curve in the upper back. Keep both feet planted on the ground (do not cross your legs or ankles) and, if sitting for a long period of time, make sure that you take regular breaks. Get up and move around.