Humans don't produce a lot of usable energy that way. Let's say you burn 300 Cal riding that stationary bike for 30 minutes. That's barely 0.3 kWh. Most of that energy you burned is just heat. The rest is minuscule. After subtracting inefficiencies of the alternator, and transformers, and all the rest of the hardware required to convert human power into electrical power... you're not left with much. It would take far, far longer than the lifespan of the exercise equipment to pay for the operation.

And, most modern cardio equipment consumes more energy as electricity than the user of the equipment is producing.

There is a gym chain in the Chicago named Eco Gym[0] that uses solar and human power to power their gyms.

[0] https://www.ecogymworldwide.com/

At that point it is more a PR play than really reusing the energy.

Having not read anything about it. Why is it just PR? What energy does a gym need? Solar and Homan power should be more then enough to power a few LED lights and a fridge or something.

Because we are talking about a negligible amount of energy.

It would take more energy to build, install and maintain the equipment that would be able to do that.

I've always wondered why we can't do more with the kinetic energy of the ocean. I know there are many projects and tidal energy companies. But considering the scale of the ocean, waves, and so on, I've always been interested to know why we haven't cracked it at a large scale.

The huge amount of energy in ocean waves and currents, compared to air (water is about 800 times as heavy as air) also is a challenge. It destroys anything that isn’t built extremely sturdy.

And that’s just water. Sea water is salty. It destroys steel and electronics even if it isn’t hitting it.

Ah yes makes sense, it seems solar, then wind, would be easier to harness. I just look at the ocean and see all that potential in the waves...!