Before the dawn of inventions, most of the work people did was powered by sheer human strength. "Historically, if you go way back, everything was human-powered," said Mark Archibald, chair of the Human-Powered Vehicle Committee of the American Society of Mechanical Engineers. But then the motor came along and did much of the heavy lifting for us.
Today many people are turning back to human-power as an energy source. It's free, it's clean and it's good for the heart.
The following is a look at some of the ways energy can be harvested from people -- for both transport and for electrical devices.
ABOVE: Scottish cyclist Graeme Obree designed this face-down, headfirst bike, which lets him pump his legs horizontally instead of up and down. The design improves the aerodynamics by reducing drag. In September 2013, Obree tested the bike in Battle Mountain, Nev., and reached 56.62 mph, breaking the record for prone cycling's previous speed of 54.9 mph.