An article published in the journal Nature this week describes how bungee cords can decrease the force of a backpack’s load by 86 percent.

Here’s how it works: When you run or walk, you bob up and down, and if you’re wearing a backpack it bobs with you. During the bob, the pack pushes down on you with varying force.

For example, if you’re running with a 50-pound pack, the force of the pack on your body ranges from 150 pounds to nothing at all, says the study's lead author, Dr. Larry Rome, a biology professor at the University of Pennsylvania and Whitman Investigator at the Marine Biological Laboratory in Woods Hole, MA. Even though the force averages out to 50 pounds, Rome says the high impact moments, called “peak forces,” can do damage to your ankles and knees.

The secret to Rome’s new backpack design is minimizing the up and down movement of the pack, or the “vertical displacement.” Vertical displacement is related to acceleration of a person's hips: in physics-speak, acceleration is the second derivative of vertical displacement. Rome explains: “The bottom line is that if you have a big displacement there’s a good chance you have a big acceleration.” Acceleration matters because the more acceleration the greater the force on the carrier (recall Newton’s Second Law of Motion, Force = Mass * Acceleration).

In other words, less bobbing means that the pack’s force stays consistently around the actual weight of the pack, eliminating peak forces that can damage the body. The catch, of course, is you have to be moving. (“The backpack can’t take away weight,” says Rome. “If I could do that then I’d be really famous!”)

A few years ago, Rome invented a backpack that utilizes the same bobbing motion to generate electricity. (Read more about the electricity pack in the journal Science.) And he thinks he may have some interested buyers—like the military. The electricity pack would allow Marines to charge batteries for their equipment on the run, literally. The National Institutes of Health is interested in electrifying packs to keep vaccines cool, Rome says.

The bungee packs might be useful for soldiers, who often break their bones jumping out of trucks with heavy backpacks, Rome says. “Not only do you have to stop the mass of your own body,” he explains, “but also stop the mass of the backpack and that causes very high forces—enough to break ankles.”  

-Flora Lichtman

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