A scientist named Keegan got five people to lie on their side, curl up into a ball and then unfold in increments while he X-rayed their lumbar spine the lower five vertebrae that carry the bulk of the body s weight and measured the distance between their vertebrae. The more strain you have on the vertebrae, Keegan reasoned, the more the vertebrae compress the discs in the back. Keegan found that the stresses on the spine were most evenly distributed when the legs were at 135 degrees from the torso. If you just watch how people sit naturally at a meeting or at the movies, within 10 minutes most people are sitting back in their chair. The only way you can often do that is by scooting out from the back of the chair.
Experts disagree on which way is best.
To this end, Scandinavian furniture designers came up with the Balans chair, which keeps the spine upright but has the legs pointing downwards and the shins resting against a padded surface. However, some experts say there can be problems with sitting at a forward tilt. The nerves and blood vessels that go down the legs can be compressed, leading to swollen ankles, and it can also be uncomfortable to rest the body weight on the shins for long periods. But, by and large, studies have shown that it doesn t reduce back problems for people and there are other problems associated with using it. The other way to open out the abdominal angle is to keep the seat pan flat but have the back of the chair recline.