Sure, if you use energy to produce it new instead of repurposing (as said, rebuilding) and can’t amortize over a long timeframe. Reduce, Reuse, Recycle. If you take things destined for landfill and adapt them it’s fine, likely a net win, with a good fitness boost. It’s the design that would be important.

Also while the power output is not huge, human muscles are capable of some impressive torque, especially with geared mechanical advantage.

It is nothing compared to the energy used to produce the setup.

What is missing here is the fuel input - the food humans need to consume to power such a device. Everybody here knows how many calories riding a bicycle hard can burn and those calories need to come from somewhere. Growing plants to feed them to animals which are then slaughtered and fed to a human who is processing the meat into energy which is going into such a generator is really, really inefficient. Getting those calories from plants is better, but a solar panel with a battery is better. Simply taking the energy from a grid which is powered from renewables might be even better.