I’ve searched through the archives because I thought there were threads on this, but no dice.
What would be required to adapt a piece of aerobic-exercise equipment such that the energy could be captured and used as house current? Would this result in any economically significant energy capture if you could force a 4-person household to crank for an hour per person each day?
This recent thread discusses power output.
I don’t know if there’s an off-the-shelf solution for this. I’d guess you can connect a bike to an automotive alternator and use it to keep a UPS (Uninterrupted Power Source) battery topped up. A UPS already contains a battery and DC->AC inverter.
No. As a species, our might lies in our minds, not our muscles. You know those little bike lights powered by a generator that rubs against the tire? They are pretty feeble - the point being, we’re only good for <100W, and there just doesn’t seem like a lot you can do with that. Even say you could run a battery charger for an hour, you are only putting out <0.1 kWhr. At current electricity rates, this is pennies. 
To reduce dependence on the grid, I would think that reducing energy consumption and/or solar collection are more viable. Also, this is the reason that biking is so economical - not because humans are so efficient, but because you don’t have to lug around 3000 lbs of metal everywhere you go.
These guys did it but there are no specs on wattage output. Here’s another where among other things they suggest it would take up to 70 people pedaling to run an air conditioner.
When asked why they needed an air conditioner, they replied “Do you know how hot we get pedalling that hard?”
I just figured it was better to capture the energy than let it go. Every time I put in an hour on one of these bikes, I can’t help thinking how much energy I’m wasting. I can do something like 120 watts, probably more when I’m in shape. I hear that top cyclists can do something like 900 watts in peak bursts.
Lesse,
Armstrong, Basso, Ulriich, other major Tour riders can likely do around 500+ watts over thirty minutes at sea-level, around 260-280 watts over five to six hours, and sprinters like Petacchi can put out a stunning 1,500-2,000 watts in all out sprints for around ten to fifteen seconds. All of these are done at peak fitness with considerale efforts. Quite a few people, with training, would be able to do 200 consistently and comfortably for three or more hours.
So you could capture that energy, but as noted it’s about enough to run a light bulb. If you took all the effort to build a bicycle generator and instead put it towards going out and buying three compact flourescent light bulbs, you’d be better of, electrically speaking.
Of course, if you don’t have electricity, and need to power something, bicycles can make sense. That’s what these guys do : http://www.pedalpower.org/maya.html
Oops, this is a better link for cool bicycle machines: http://www.mayapedal.com