Wikipedia:Ergonomics
Researchers have found that, beyond a threshold of about two hours per day, excessive sitting is correlated with stiffness in the carotid artery and in the femoral arteries, and that physical exercise did not appear to offset those negative effects. However, it is possible to stand, or even walk, while using a computer.
"Companies such as Google and Wikimedia have expressed serious concern and have since purchased the multi-award-winning Locus Workstation that was created by renowned designer Martin Keen, founder of Focal Upright Furniture." (underscore added)