When out for a stroll, a person's hips move up and down between five and seven centimeters during every step. Larry Rome of the University of Pennsylvania and his colleagues exploited this trait to design their new "suspended load" backpack. The bag is based around a frame, which is connected to a wearer's hips, and as the frame gets raised and lowered, the backpack's contents move up and down. This pogo stick-like motion generates mechanical energy, which in turn is converted into electricity by an attached motor. In laboratory tests, volunteers wearing backpacks that weighed between 20 to 38 kilograms generated about 7.4 watts of power, an amount that could simultaneously operate multiple small electronic devices. (Equipment bags of target users typically weigh more than 36 kilograms, the scientists say, with up to 25 percent of the load attributed to replacement batteries.) Because the amount of mechanical energy correlates to how much weight is moved, a heavier pack or a faster gait both translated into greater power generation.
The scientists note that the backpack's outer metal frame, which weighs under six kilograms, slightly altered how subjects walked, making their strides more efficient. "Metabolically speaking, we've found this to be much cheaper than we anticipated," Rome remarks. "The energy you exert could be offset by carrying an extra snack, which is nothing compared to the weight of extra batteries." The team hopes to refine the design, which is currently a prototype, and perhaps someday remove the need to lug replacement batteries on long treks.