It looks like an ordinary black, comfy chair, perfect for a relaxing hour or two in front of the television.

But things are rarely as they seem at the CeBIT, the world's top high-tech fair, and this humble-looking chair is part-nurse, part-fitness coach designed for the increasing hoards of elderly in Germany and across the developed world.

Highly intelligent sensors in the chair register the owner's weight, blood pressure, heart rate and posture and builds a database of vital signs over a period of time.

And if the chair notices a few extra pounds, it flips into fitness coach mode and suggests a series of exercises.

The comfy arm-rests convert into a fully functioning rowing machine and the user "rows" down a river displayed on a big screen.

"Even in this mode, the sensors record all the vital signs and the health assistant notes if the person is not performing the exercises properly," said Sven Feilner from the Fraunhofer Institute as he put himself through his paces.

Users can link up with other users and "race" against each other or simply follow a tailor-made fitness programme, monitored closely by the chair.

The chair exercises the mind as well, with a "Simon Says"-type memory game in which the user shifts his or her weight in the chair in response to a random pattern.

"We're trying to make people more active, especially given our ageing population," explained project leader Matthias Struck.

Struck's team is working on bringing the prototype model unveiled at the CeBIT onto the market in "one, maybe two years," he said. The chairs could be particularly useful in old peoples' homes, he suggested.

One thing that might make you sit up straight: the price.

"Already without the technology, these chairs aren't particularly cheap," said Struck. "I would estimate 2,000-3,000 euros ($2,600-3,900) and then you're probably looking at the same again for all the sensors."

The CeBIT, self-styled Davos of the high-tech world in the northern German city of Hanover, runs until March 9 and has attracted some 4,100 exhibitors from around 70 countries.