From Cornell University Ergonomics:
The Perils of Sitting Sitting for more than 1 hour has been shown to induce biochemical changes in lipase activity (an enzyme involved in fat metabolism) and in glucose metabolism that leads to the deposit of fats in adipose tissue rather than these being metabolized by muscle, and extensive sitting also relates to heart disease risks, so people are advocating standing to work because this use more muscle activity (burns about 20% more calories). These changes happen in both fit people who regularly aerobically work out and also unfit and obese people, so regular exercise doesn’t address this.
The Perils of Standing But, standing to work has long known to be problematic, it is more tiring, it dramatically increases the risks of carotid atherosclerosis (ninefold) because of the additional load on the circulatory system, and it also increases the risks of varicose veins, so standing all day is unhealthy. The performance of many fine motor skills also is less good when people stand rather than sit.
Sit-Stand Workstations We have tested computer use when sitting and standing in different ways. The problem with standing is that when you raise desk height for keyboard/mouse use you need to also raise screen height above the desk or you get neck flexion. Also, for standing computer work the computer fixes the person’s posture there is greater wrist extension and pretty soon people end up leaning which also compromises their wrist posture, thereby increasing the risks of a musculoskeletal disorder like carpal tunnel syndrome.
In our field studies of sit-stand workstations we have found little evidence of widespread benefits and users only stand for very short-periods (15 minutes or less total per day). Other studies have found that the use of sit-stand stations rapidly declines so that after 1 month a majority of people are sitting all the time.
Others have proposed a treadmill workstation or a bicycle workstation. Both of these have been tested and shown to decrease computer work performance (typing and mousing slows down and significantly more mistakes are made).
Sit-stand workstations are expensive and generally ineffective in addressing the issues to hand.
The bottom line: Sit to do computer work. Sit using a height-adjustable, downward titling keyboard tray for the best work posture, then every 20 minutes stand for 2 minutes AND MOVE. The absolute time isn’t critical but about every 20-30 minutes take a posture break and move for a couple of minutes. Simply standing is insufficient. Movement is important to get blood circulation through the muscles. Research shows that you don’t need to do vigorous exercise (e.g. jumping jacks) to get the benefits, just walking around is sufficient. So build in a pattern of creating greater movement variety in the workplace (e.g. walk to a printer, water fountain, stand for a meeting, take the stairs, walk around the floor, park a bit further away from the building each day).
So the key is to build movement variety into the normal workday.