I heard about a concept like those for a gym powered by their stationary bikes, but the math just doesn’t work. A pro rider can put out about 2000 W for a short sprint, but about 300 W for an extended time. That 300 W works out to about 5¢ worth of electricity per hour of working out, or about $20 if a person averages that for an hour per day for a year. That $20/year has to be able to cover the cost of the added generator system in the bike, whatever battery/inverter system that’s storing the energy, and/or the interlock system to put that power into one’s electrical service.
It’s great if you want to power the electronics of the stationary bike, maybe with a USB port to power one’s phone/music player. Trying to offset electrical costs with human/animal labour is not going to be net positive except in very niche situations.