It ain't heavy: the effort of hefting a pack can be reduced with bungee cords. Click here to watch a video of the bouncing pack in action.PunchstockBouncing backpacks can make hiking easier, researchers have shown.
Lawrence Rome and his colleagues at the University of Pennsylvania in Philadelphia have designed a backpack in which the load hangs suspended by bungee cords from a frame strapped to the walker's back1.
They find that, with this bouncing pack, a load weighing 27 kilograms 'feels' more than 5 kilograms lighter: the walker uses only as much energy as they would for a normal rigid pack weighing 21.7 kilograms.
The reason, Rome and colleagues report in Nature, is that the bungee pack bounces up and down on the frame exactly out of step with the vertical movements of the walker's body. So these movements cancel out and less energy is wasted shifting the load up and down.
This vertical movement serves no useful purpose, but is the result of our ordinary gait: as we pivot on one foot in taking a step forward, our hips move up and down. In effect this means that a walker with a loaded backpack is repeatedly lifting it up. The Pennsylvania team's design aims at reducing these motions, so that the load stays at a more or less equal distance from the ground even as our bodies go on bobbing along.
Power pack
Last year, Rome and his co-workers designed a suspended backpack in which the vertical motion of the load relative to the frame was used to generate energy by turning an electricity-generating wheel2. In that case the moveable pack required more energy to carry than a fixed one.
The Pennsylvania team has now lowered the overall energy cost of carrying the moving pack below that of a fixed pack. Unfortunately this isn't possible while generating energy at the same time. "You can't get around the fact that if you do additional mechanical work to turn a generator then it will cost you more energy than it would if you didn't turn the generator," Rome says.
The design seems so simple that it is surprising no one has thought of it before. But getting it to work involved some careful tinkering, according to Rome. "Using the right diameter and length bungee cord was crucial," he says.
The researchers point out that the same principle of carrying a load on springy supports has been long exploited by merchants and porters in Asia, who carry loads on bamboo poles. In China such porters are often known as 'pole men'.
Pain in the back
Rome and colleagues say that, as well as making life easier for the tired hiker, the springy packs could have some significant advantages for public health. They say that children carrying heavy backpacks to school often suffer from back pain.
The design also makes it easier to run with a backpack, which, with a fixed pack, is usually uncomfortable and can increase the vertical forces on the back by up to a factor of three. This could help emergency personnel to carry equipment quickly at sites of accidents or disasters.
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The Pennsylvania team intends to commercialize their pack, through the same company they formed to develop and market the energy-generating backpack, Lightning Packs LLC.
They suspect that one hurdle will be the weight of the pack, which is slightly more than for one with a fixed frame. Rome points out that the ergonomic benefits greatly outweigh the excess heft — but it might prove difficult to persuade customers of that. "People seemed to be more concerned with weight than biomechanics," he says.
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