Bloomberg BusinessWeek:

“Short of sitting on a spike, you can’t do much worse than a standard office chair,” says Galen Cranz, a professor at the University of California at Berkeley. She explains that the spine wasn’t meant to stay for long periods in a seated position. Generally speaking, the slight S shape of the spine serves us well. “If you think about a heavy weight on a C or S, which is going to collapse more easily? The C,” she says. But when you sit, the lower lumbar curve collapses, turning the spine’s natural S-shape into a C, hampering the abdominal and back musculature that support the body. The body is left to slouch, and the lateral and oblique muscles grow weak and unable to support it.

One way to keep back muscles strong is Perching – a hybrid between standing and sitting:

The best sitting alternative is perching—a half-standing position at barstool height that keeps weight on the legs and leaves the S-curve intact. Chair alternatives include the Swopper, a hybrid stool seat and the funky, high HAG Capisco chair. Standing desks and chaise longues are good options. Ball chairs, which bounce your spine into a C-shape, are not. The biggest obstacle to healthy sitting may be ourselves. Says Jackie Maze, the vice-president for marketing at Keilhauer: “Most customers still want chairs that look like chairs.”