Gravity exerts a force towards the center of larger objects. The gravity of the very large object sun keeps the earth in its orbit. The earth provides the center of a similar orbital relation for the moon and thousands of satellites. Our habit of walking on a flat earth named the direction towards the center of gravity 'down', and away from the center in any other direction 'up'.
As Buckminster Fuller would say, 'up' and 'down' don't exist in universe. While we still use these words, we should remember that they mean towards and away from the center of gravity. The words up and down attain special meaning anyway in the AT world.
Most people use two legs to move their body around, a difficult task in comparison to a quadruped, which can use four legs to balance its body weight. Kangaroos use their strong tails as third leg, and this three points of contact create a stable balance, not unlike a tripod. We find another triangular structure in each feet, allowing us to balance even on one leg. Our body structure coevolved under conditions of gravity, and copes well with it unless we interfere.
The comfortable chairs, in which we search relieve from the constraints of gravity, help creating and maintaining pattern of use that interfere with the natural ability of our nervous system to organise our movement efficiently and effortless. If we learn to work with gravity instead of against it, we give our nervous system the freedom to explore back that we had as children.
Nobody teaches a child how to walk. A child exercises its muscles and experiments how to use them. Crawling on all four demonstrates the relative stability of quadruped's movement and precedes any walking attempt. Only equipped with an adaptable nervous system the child learns to apply the same intent used for sitting, balance up and away from gravity, to the difficult act of walking. Gravity alone teaches our children to coordinate their nervous system so that their intent expresses itself by balanced movement.