The Physics Behind Lightning Packs’ Radical HoverGlide

One of my first gigs in journalism was as a tech reporter for Popular Mechanics, where I wrote with boundless enthusiasm about amazing new gadget prototypes that were always just around the corner. That formative experience is probably one of the main reasons I’m so hesitant to hype new technology these days—because I eventually realized that most of the prototypes I wrote about never made it to the consumer market.

There are exceptions, though—if you’re patient. In late September, I saw a Kickstarter campaign from a company called Lightning Packs for the HoverGlide, which it bills as the “world’s first floating backpack.” The premise is simple: a backpack connected to its frame by shock-absorbing bungee cords, which minimizes its up-and-down motion while you walk or run. (A gif is perhaps worth a thousand words here.) The initial research was published in Nature way back in 2006—and yes, I wrote about it for Pop Mech—and patented a year later. It promised a decline in “peak accelerative vertical force” of 82 percent while walking and 86 percent while running, and enabled subjects to carry a 60-pound pack for the same energy as it would normally take to carry a 48-pound pack.

As with a lot of the research into the science of backpacking, most of the initial interest in the new pack design came from the military. But they were mostly focused on the potential for harvesting electricity from the up-and-down motion of the pack, to reduce the dependence of troops in the field on cumbersome disposable batteries and oil for portable generators delivered at great risk and a cost of $ 60 per gallon. So that’s where the development team, led by University of Pennsylvania professor Larry Rome, focused their energy. And that’s where things stood until about 18 months ago, when Rome and his colleagues decided to revisit the consumer market for an ergonomic pack.

The result is a (still hypothetical) family of backpacks—estimated delivery date July 2019—ranging from a 28-liter commuter pack to a 55-liter hiking pack, which attach interchangeably to either a small or a large frame. The concept is obviously popular: they blew through their stated funding goal of $ 75,000 in a matter of hours, and are currently sitting at over $ 222,000, with a deadline of October 28. That’s still not a happy ending, of course: many are the Kickstarter projects that have foundered after raising piles of cash. But it’s an encouraging sign, so I got in touch with Rome by email to find out more. Here are some lightly edited highlights from our conversation:

OUTSIDE: Where did the idea for this type of pack come from?
ROME: I have been a professor of biology for the last 30 years, and I studied the design and function of fish muscle in swimming and calling fish. Sixteen years ago, I was in contact with the Office of Naval Research as they wanted to build a submersible which moved like a fish. One day in November 2002, they said Special Forces Operators in Afghanistan were carrying around 80-pound packs with an additional 20 pounds of batteries. They asked if there was a way to get energy from their movement and convert it to electricity so they didn’t have to carry so many batteries. In that phone call, I came up with the idea of getting it from the movement of the pack.

So that’s where the electric pack came from. When did you decide to take out the generator that converts up-and-down motion into electricity and just make it as comfortable and efficient a pack as possible?
All of the subjects in the initial study loved walking with and running with the [electricity-generating] backpack compared to a normal pack. We found that the accelerative force exerted on the subject was much less than with a normal pack. I hypothesized that if I removed the generator—more difficult than it sounds—the force would become even lower and metabolic cost for carrying the load would be less than with a normal backpack and make it much easier for soldiers, firemen, and hikers to carry loads.

It turns out that the electric pack reduces the vertical accelerative forces by about 65 percent compared to the HoverGlide which reduces it by 82 to 86 percent. So both the ergonomic and electric versions share the same Suspended-Load Technology (SLT), but the actual suspension systems differ.

What’s so special about how the packs carry loads?
These packs do two things which seem opposed. The technology adds about 2 pounds to the weight of the pack, but the contents in the pack exert much less force on you when you walk or run. It is the only device that I can think of which increases the weight but reduces the dynamic forces. The mantra in the military and outdoor world is to “lighten the load,” but this may be wrong. What you want to do is reduce the forces. It is hard for many people to wrap their heads around that concept.

Does the backpack have to be “tuned” to a particular person’s body weight or walking/running gait, or does it adjust automatically?
At normal walking speeds up to the fastest running speeds, HoverGlide does not have to be adjusted for a person’s gait. Since there is an elastic coupling in the pack, that has to be tensioned depending on how much weight is being carried—otherwise the load will ride very high or very low.

What are the most important benefits the pack provides by reducing impact forces? Is it reducing injury risk? Improving comfort? Reducing metabolic cost?
All of the benefits are interrelated, so it is hard to say which is most important. Since I have bad knees, the reduction of impact force is most important for me as it very likely reduces joint injury. Also when going downhill (or down stairs), muscles undergo so-called eccentric contractions—generating force while being stretched. This quite often leads to muscle soreness, particularly in the following days. We believe that this will be reduced with our packs.

The reduction in metabolic cost associated with HoverGlide might be especially important at higher altitudes that may limit maximum oxygen consumption. Interestingly, when people put on the pack in suspended mode and run around, they don't necessarily think it's so special—but when we lock it [i.e. fix the pack rigidly to the frame instead of suspending it from bungee cords] and convert into a normal pack, they feel the difference immediately and insist we suspend it again!

Are there any disadvantages to the pack, or situations where you’d recommend not wearing it?
We don't see that there are disadvantages to the pack, because no matter what, the pack can be locked at which point it becomes a normal backpack. This fall-back function—if all else fails, it reverts to a normal pack—is a key component in our military packs as well.

Having said this, we don't recommend that you use the pack where movement of the load could be a problem. For instance, if you lay flat on your stomach, the load would move up the frame slowly towards your head. So if you are looking over a cliff and not yet used to the pack, you would definitely want to lock it. You may also want to lock the pack if you are jumping off a large boulder so you can absorb the full weight by bending your knees.

What’s next after the Kickstarter campaign?
We are looking forward to finalizing the designs, and then to start machining the molds for the frames and manufacturing HoverGlides. We are anxious to get them into the hands of our backers. We believe they will greatly enjoy them, and will find new uses for them beyond our imagination.


My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.

Outside Magazine: Fitness

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