Here is something that will put a spring in your step: a backpack that bounces up and down on bungee cords instead of pounding the shoulders and back. The bag's designer envisions his creation as just what the doctor ordered to relieve the spines of schoolchildren as well as military and emergency personnel--all who risk injury from carrying heavy loads.

When people walk their hips move up and down by as much as seven centimeters, which normally causes a backpack to bob up and down, too. That is bad news for the joints and back, because on its downswing the pack exerts added force on the wearer. A 50-pound load, for example, can slam down with 80 pounds of force when a person is walking and up to 150 pounds when running, says physiologist Lawrence Rome of the University of Pennsylvania.

The new backpack, crafted by Rome, is designed to keep a load level with the ground during motion and, therefore, counteract the jarring force of walking or running with a heavy pack. In Rome's device, the load is suspended by a system of pulleys and bungee cords, allowing it to slide up and down on a mounting of metal rods. As a person's hips rise, the mounting rises as well, but the bungee cords let the load dip down, limiting its movement. [Click here for video of Rome walking with the backpack]

The backpack tamps down vibrations for the same reason a slinky stays level if you hold it upside down and shake it rapidly--it vibrates too slowly to catch up with your motions, says biomechanics researcher Rodger Kram of the University of Colorado. Kram has found the same mechanism at work in bamboo poles hung with heavy objects and slung over the shoulder by people in Asia.

With Rome's device, "the cleverness is in the simplicity," Kram says. "There's not a lot to it." Others have tried to build similar backpacks, but they were more complex and didn't work as well, he says.

The pack reduces the maximum force on the wearer by about 80 percent and dampens up and down motions by more than half while walking, Rome and his colleagues reported in a paper published online by Nature. "You're aware of some movement," he says, "but it's actually more comfortable than the different type of movement you get in a rigid backpack." [Click here to see the backpack in both bungee mode and rigid mode]

The team also compared the energy expended when the bungee power was turned on with when it was off. Walking with the suspended pack is equivalent to lightening the load by 12 pounds, they found.

"Energetics isn't really a big issue for most people--it's comfort," says Kram, who adds that he expects the pack to appeal to those who run with heavy loads, including emergency personnel and possibly children.

Eager to buy one? You may have to wait a few more years. Rome started a company, Lightning Packs, LLC, to commercialize the pack along with a similar one he created last year (that generates electricity from its motion), but he says he needs more funding to get the business off the ground.