In the summer of 2005, Jamie Pigman was deployed as a Navy medic to the Korengal Valley in Afghanistan with a scout sniper platoon. His three-man team ventured out on missions ranging from a few hours to a few weeks, with minimal resupply. “The mission always dictates the equipment needed and hence the weight carried,” he says. With a baseline weight of 30 to 40 pounds for body armor, weapons, ammunition, and survival gear, plus supplies for extended missions, he and his team were often carrying to 70 to 100 pounds each.

The gear was loaded into a modified ALICE pack—that’s All-purpose Lightweight Individual Carrying Equipment, a vintage external-frame carrying system adopted by the military in the 1970s and still favored by some units for its ruggedness and versatility. But Pigman, an experienced hiker and outdoorsman who had joined the Boy Scouts at age 9 and hiked 120 miles across the New Mexico desert, noticed something odd: “No one I worked with used a hip strap,” he recalls. “You really can’t use the hip strap with all your kit on.”

That observation was the spark behind a study published last month in the journal Human Factors, on the energy savings from using a hip strap while carrying a load. After being shot in the left knee during a firefight, Pigman decided to pursue exercise science and biomechanics as a post-military career. His masters research, with Peter Hosick at Montclair State University, focused on better ways of carrying the loads he’d humped in the Korengal Valley—a topic that turns out to be surprisingly understudied.

The hip strap study was fairly straightforward. Pigman had 15 men carry a 53-pound ALICE pack on a treadmill for 10 minutes at a self-selected pace, with and without the hip strap done up. Sure enough, oxygen consumption and subjective perceived effort were both slightly lower by the end of the walk with the hip strap. While the differences after 10 minutes were subtle, they add up quickly when you’re walking for hours or days. “If the reduced energy expenditure is great enough,” Hosick says, “it could allow people to walk longer, or faster, or even reduce how much food they have to pack.”

What caught my attention most, though, was all the gaps and uncertainties in the existing literature on backpacks. To support the seemingly obvious claim that it takes more energy to carry a heavier pack, Pigman and Hosick cited a relatively recent paper from 2014. When I looked up that paper, its first sentence was: “Although humans clearly expend more energy to walk with an extra load, it is unclear what biomechanical mechanisms contribute to that increase.”

What caught my attention most, though, was all the gaps and uncertainties in the existing literature on backpacks.

Really? I found it hard to believe that we don’t know why carrying a heavy backpack is harder than a light one. So I called Max Donelan, the head of Simon Fraser University’s Locomotion Laboratory in Vancouver.

The problem, Donelan explained, is that the act of walking is way more complicated than most of us realize: “Because it’s subconscious, we think walking is easy.” In Isaac Asimov-era science fiction, he pointed out, everyone assumed that in the future we would have athletic but dumb robots. Instead, we have Deep Blue and Watson, but walking robots can’t yet out-toddle a 2-year-old.

The key point about walking, from a physics perspective, is that you don’t perform any overall mechanical work if you’re traveling at a steady speed over level ground. You have the same amount of kinetic and potential energy at the start of a hike as you do at the end—which means that, despite how you may feel, you haven’t done any net work, since work is the change in energy. That still holds true if you strap on a pack, Donelan points out: “In theory, you can make the extra weight ‘disappear,’ because you don’t need to do any net mechanical work on it.”

In practice, of course, walking—with or without a pack—does take effort. With each step, your center of mass rises and falls in an arc, like an upside-down pendulum. Back in 2002, Donelan and his colleagues showed that much of the energy you burn while walking comes from the transition from one pendulum swing to the next: negative work by the knee joint to brake as your foot lands, then positive work by the ankle to push off again.

So when you strap a pack on, the actual cost of supporting the pack against gravity isn’t a big deal. Instead, it’s decelerating and then reaccelerating it with each step that costs energy—which is why some researchers are pursuing ideas like suspended packs that counteract some of this up-and-down motion. That can offer big savings on level ground; once you start climbing a hill, though, then you can’t avoid spending energy to hoist the pack against gravity.

Of course, energy cost isn’t the only consideration in backpack design. Pigman, who is now working on a Ph.D. in biomechanics at the University of Delaware, has further data coming out soon on force transmission to various parts of the body. Hip straps are supposed to redistribute force from the shoulder muscles to the much larger muscles around the hips, which should improve comfort and fatigue resistance, even if you burn the same amount of energy overall. They also reduce the risk of “rucksack palsy” resulting from compressed nerves under the shoulder strap.

But even those factors aren’t as straightforward as I might have thought. Digging into some of the military literature on load carrying, I found that there’s a massive body of research that, as a longtime backpacker, I’d never encountered, answering questions I’d never even thought to ask.

Where is the most efficient place to position a load? Believe it or not, it’s on your head. Women in Kenya and other East African countries, researchers reported in the 1980s, carry up to 20 percent of their body weight either balanced on their heads or supported by a strap around their foreheads (like porters in Nepal sometimes use) without expending any extra energy. The problem, as a review in Military Medicine points out, is that it takes too long to learn how to do this.

There’s a lot of interesting stuff here that I hope to dig into in future columns. For now, suffice to say that you should make sure your pack has a properly fitting hip strap, and buckle it up.

“I know the use of a hip strap is quite universal in the recreational world,” Pigman says. “But in the military, it’s not used, and I feel pack designs for combat use should focus on creating a better pack-to-body interface for our fighters.”

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