No two fads are growing faster than getting fit and going green. Is it possible that by achieving the former, one could also accomplish the latter? Harnessing human movement has long been a holy grail of renewable energy, but real-life implementations have been relegated to advertising stunts and commercially impractical gadgets. But ReRev.com, a startup company from St. Petersburg, Florida, thinks its technology can let us improve our own health, and that of our planet, by working up a sweat.