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.