Gym Eco Mode
Actually, some of the machines in my old gym used to have some sort of Eco Mode where some of the energy you fed into the equipment went to reducing the amount of energy the treadmill itself drew from the grid (a bit).
Oh yes, those machines are so excruciatingly poorly designed that even when their whole purpose is to extract 100-200w, from the user, the machines themselves draw a great deal more from the grid. I couldn't find the spec on line but I did find a piece about a similar thing in this link (Daily Mail Warning)
http://www.dailymail.co.uk/news/article-2430771/Worlds-self-powering-gym-uses-energy-WORKOUT-lights-dont-break.html
For those who don't want to contribute towards Lord Rothermere's clickbait empire, the key sentence is this one:
"The new treadmills will also use 30 per cent less electricity and generate enough energy to power their own information screens"... but only if the user output is more than 100w.
And so they bloody should given that an ipad can run its screen, do some useful stuff and still charge it's battery from 12w