Why don't we recycle energy from human exercise?

The majority of all young people I know routinely do some form of vigorous exercise, whether at a gym or on the street (biking, riding, etc.). While it may be tough to convert the energy from biking / running outdoors, it surprises me that the calories burned in indoor exercise are not stored and used for powering electrical devices. Worse still, these machines (e.g., treadmills) actually consume energy!

According to wikipedia (http://en.wikipedia.org/wiki/Watt ), a person climbing a flight of stairs burns energy at the rate of 200 W, while energy-conserving incandescent bulbs require 8W of energy. Thus, one person's one day's worth of light exercise could power one bulb for the whole day (assuming person burns energy at 200W for 1 hour! I bet it will not be too tough to devise some contraption to convert calories burned in lifting weights to energy as well.

Why aren't vendor coming out with products like these?

Details:
1. @amiram_a: we need not feed energy back into a grid.. think simple: the energy may be used for powering a bulb in the same room using just one wire and capacitors to store energy. Your dollar figures are also misleading: we replace the motor in a current treadmill with a generator.. why is that going to add a significant figure to its *existing* price?

You are misinformed, it is existing and currently in use at California fitness center in Hong Kong. Please click the link below for full details,

http://www.voanews.com/english/archive/2...

However, outdoor activities was not yet done, though, there were some attempts at charging batteries and cell phone while biking, in Japan, it is still not bringing positive result. Though they were able to harness wind power in residential houses and the design are quite beautiful. See link below for full details

http://www.accessmylibrary.com/coms2/sum...


Alvin

assume we build a treadmill that produce electricity and feeds it back to the grid. Now, such a thread-mill will cost 1000$ at least and another 200$ per year for maintenance (very optimistic figures) and after 10 years will be discarded, and it will produce about 0.15 kW of energy, and even that not all the time, say only 50% of the day.
that is more than 2$ per kWh, but power stations produce electricity for 0.04$ per kWh.
Thats why not.

You might enjoy checking out Humboldt State University's CCAT (California Center for Appropriate Technology) program. Their area of study is sustainable tehcnology in all shapes and forms. Through them, many of the events and festivals held on campus are powered by stationary bikes that supply energy to a generator- totally human powered!
Here is a great link that tells about how they do it, and how you could do it too!
www.humboldt.edu/~ccat/pedalpower/

The science and calculations here way out of my league.
However, I wonder considering this age of the "obesity crisis" of kids and adults, about the benefits of linking exercise equipment to cell phone batteries, TV and video equipment. This would require the participants to power up the cells to use the TV or video game etc. This could have some mktg appeal to provide motivation where lack of self discipline, and parental monitoring is prevalent.
Fast food vendors could have such equipment installed near the entrance, or within their kids play areas. The appeal could be to have visible, audible gages of calories burned/stored etc.