What a clever idea: Hong Kong Gym Uses Exercises to Generate Electricity
I’m thinking you could make this fairly cost-effective, since as the article suggests, a generator is pretty basic technology, and is really just a motor running in reverse. That means that many of the parts required are already present in some types of exercise gear, like treadmills and exercise bikes. I can see it having some qualitative impact on the customer experience, depending on how you set things up–for example, I’ve always preferred powered treadmills to the kind you have to drive yourself, for example. But that might be a surmountable obstacle, and I’ll bet you could add generators to some gear, like weight lifting equipment, with little if any effect on the experience. Resistance previously provided by the weights would be provided by the generator, instead.
I wonder how much power could have been recaptured by now if all gyms did something like this? I doubt it would be much relative to world power demand, but it might well be enough to run the lights at the gyms.