Although a hard, statically contracted tummy plays a critical role in a retaining wall when the spine bends, it also does important things just as you sit there. A strong co-contraction with the back muscles buoys the spine up and stops the segments telescoping down into the pelvis and compressing the discs.When the abdominal retaining wall is weak, it cannot generate enough upthrust on the lumbar segments to offset the downward forces.

The low intra-abdominal pressure cannot lift the spine and interrupt the bearing down pressures on the base. As the girth expands with sitting and the belly falls forward over the belt like a bag of chaff, the abdominal contents spill forward, dragging the spine downwards. As the multi-segmented column ploughs down ever more firmly onto the sacrum, it increases the pressure on the lower discs.

By moving forward onto the front of the seat incidentally, the abdominal wall works more dynamically and gives better support. You can see this yourself. If you sit slumped on a sofa your tummy balloons forward, flaccid and inert. If you sit up free of back support you feel the internal corset of your tummy reefing in the retaining wall and making a firm, flexible cylinder. 

It also makes the top of your spine better balanced and free-wheeling to work more efficiently over its base.The best sitting arrangement at a desk is a kneeling chair.Its seat is inclined forward a few degrees which promotes optimal hollowing of the lumbar spine instead of a slumped ‘c’. 

Some weight taken through the knees and lower legs on the upholstered cushion minimizes that taken through the lower back. But even though it is ingenious, this chair should be used with caution if you have knee or ankle problems, or if your low back already has a too much lumbar arch. It is worth noting that carrying loads balanced on the head-another time-honoured custom of less developed societies-invokes a superb dynamic response from the tummy muscles. 

Automatically they pull in and brace, converting the lower abdomen into a taut, supportive cylinder. The raised pressure within the abdominal cavity creates extra lift for the spine and safeguards the lower segments from excessive compression. We might do well to copy this way of carrying.