The muscles in the back are primarily responsible for holding the human spine upright. In the normal, upright position, the spine can take a lot of strain. However, if it is subjected to pressure when curved (due to weak supporting muscles or poor posture) the intervertebral disks and vertebrae are abused. Acute pain and even degenerative chronic damage can result! The supporting role of the muscles in the back is therefore extremely important. Unfortunately conditions in day to day office work, with long periods spent sitting at a desk, lead to an unhealthy permanent tension of the back muscles which, in time, can develop into muscular hardening. But other muscle groups are affected as well: lifting the shoulders by as little as a few centimetres when typing on a keyboard, for example, is sufficient to produce tenseness in the shoulder/neck region.

The result of tenseness in the back is that, in the long tem, the muscles are unable to support the spine: the spine is no longer held upright, the sitter develops a rounded back with all the negative side effects - such as back ache, tenseness in the shoulders and neck and general disease, plus lack of concentration and digestive complaints.

We can, however, prevent muscular tenseness by developing a healthy posture, stretching and doing exercises. An essential element is the so-called dynamic posture: Dynamic posture means permenantly moving from the central active to a forward-leaning or reclining position. A good office chair promotes dynamic posture. The "homo sedens" in the office needs a dynamic posture to avoid muscular cramps and tension that would otherwise build up as a result of a purely static posture. It is therefore wise to aim for a healthy balance between tensing and relaxing the different muscle groups. To do this the chair must permit all seated positions, i.e. it must be able to support and follow all the movements of the user.

You can find further information on this under Ergonomics, Sitting Correctly and Techniques.