system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't fitness centers harness energy that its patrons create when exercising?

We go to the gym and same story, usually packed during peak hours and everyone is on a weight machine or some form of cycle. Is the amount of energy expended not quantifiable or can it not be captured? Or would the short answer just be that its cost prohibitive?
gpt
An excellent idea! The bad news is humans are too puny. From the Wikipedia page on human power *... adults of good average fitness average between 50 and 150 watts for an hour of vigorous exercise.*

So you on a treadmill could barely power a lightbulb or two. Sounds crazy, but there it is.

Some cardio machines have displays that are powered by the users, but that's a tiny amount of energy...