Backpack Evolution: How Modern Ultralights Tackle Heavier Loads with Real-World Comfort

Published February 25, 2026 06:59PM

The landscape of ultralight backpacking packs has undergone a significant transformation, moving beyond the minimalist, frameless designs favored by some purists, like this editor’s well-worn Mountain Laurel Designs pack. Contemporary ultralight offerings, exemplified by models such as the Durston Kakwa, are now integrating advanced features like lightweight aluminum framestays and cutting-edge fabrics. These innovations empower backpackers to carry heavier resupplies and manage substantial water loads with significantly less discomfort and strain.

For several years, the Outside Lab at CU Denver has been at the forefront of objectively measuring pack performance. Through rigorous testing involving sensor-equipped test dummies, the lab meticulously analyzes how much weight a pack can effectively distribute, adhering to the fundamental backpacking principle of the 80/20 rule. This rule dictates that 80 percent of a pack’s load should be borne by the hipbelt, with only 20 percent resting on the shoulders, a configuration designed to optimize comfort and minimize fatigue on long treks. However, the quantitative data generated by sensors, while invaluable, cannot fully capture the subjective human experience of carrying a load. Do these precise load distribution numbers truly translate to a more comfortable hike? Since mannequins, however sophisticated, cannot articulate their sensations, Backpacker Editor-in-Chief Adam Roy joined Outside Lab Test Editor Adam Trenkamp for a direct, side-by-side comparative test of two highly regarded packs, bridging the gap between laboratory precision and real-world trail feel.

The Human Element: Bridging Lab Data with Trail Experience

“Hey everybody, I’m Adam Trenkamp, lab test editor here at the Outside Lab at CU Denver, and today I have with me Adam Roy, editor-in-chief at Backpacker,” Trenkamp began, initiating the testing session. Roy responded, “Pretty good today, Adam. How about you?” Trenkamp expressed his satisfaction, “I’m good, thanks for coming down to the lab and joining me for a little test operation we have here.”

Roy then sought clarification on the day’s objectives: “So run me through what we’re doing today. What are we going to be testing?”

Trenkamp elaborated on the lab’s ongoing research into pack performance. “Yeah, so as you’re familiar, loading your backpack for a trip can get a little heavy at times. So recently in the lab, we did an effective load test where we took a mannequin, put some force sensors on that, fit the packs to it and loaded them until it was basically showing an uncomfortable percentage of load on the shoulders versus on the hips. What we don’t know, because our mannequins can’t talk, surprise, is that we don’t know how that actually feels. So we know that the pack can physically hold the load on the hips at these heavy weights, but how’s that feel when you’re actually out on the trail, actually moving around and trying to shift that much weight? That’s where you come in.”

Roy embraced his role, stating, “So I’m the human test dummy for the test today.”

Trenkamp affirmed Roy’s suitability for the task, highlighting his extensive experience. “You’re uniquely qualified for how much time you spend out on the trail, how many packs you’ve worn, how much weight you’ve carried in them and not carried in them and really learning how to efficiently pack. So what I would like to see is if we put a normal comfortable load in a pack and we go walk around, see how it feels, and then we’re going to extra load that pack and we’re going to really push the limits of what we found in the lab and then see how that same pack actually feels. Is it comfortable? We know it can carry that load on the hips, but is it comfortable to carry that much load? And the important part here is we’re actually going to do this with ultralight packs. So some of the things we found in testing is that ultralight packs have gotten way better at carrying an excessive load.”

Roy underscored the critical importance of understanding a pack’s behavior beyond its ideal load capacity. “It’s worth mentioning it’s important to know how a pack feels loaded at its max capacity in addition to its comfort capacity, because you may not always have the option of keeping a pack loaded to its ideal weight limit. Sometimes you’re carrying extra food, sometimes you’re carrying extra water on a long carry, sometimes your partner shows up and tells you to carry the tent, so you gotta load it up a little more. So it’s good to have a reference point to how a pack feels when it’s beyond its ideal capacity.”

Trenkamp concluded this segment, “Yeah, 100%. So let’s load some packs and get started.”

The Contenders: Durston Kakwa 55 and Outdoor Vitals CS40

Roy inquired, “So Adam, tell me what packs we’re going to be testing today. Take me through it.”

Trenkamp introduced the two packs under scrutiny. “Good question. So we’re going to start with the Durston Kakwa 55, and then we are also going to test the Outdoor Vitals CS40. So the CS40 definitely falls more in that ultralight category that we’re used to in terms of how much weight it can carry, how much load. The Durston, however, really, really competed on that upper load limit with more traditional packs with a frame. So we were pushing that 60-pound load limit with this pack.”

Roy then turned his attention to the packing process, asking, “We’ve got a bunch of stuff to put in these packs to get them up to weight. You’ve got a mix of weights, you’ve got some gear, I see sleeping pads, sleeping bags. Tell me about what you picked to load up these packs and why that would make for a realistic test.”

Trenkamp explained the methodology behind the load selection. “Yeah, so in our lab test, we did a few pads on the bottom of each bag to kind of support the weight and be a little more realistic to the load, but after that we started just putting weights in so we could really get that weight pushing down. When you’re out on the trail, you’re not just putting heavy weights in your pack. It’s going to start to bulge and really push out, so we want to make sure we have a realistic test with us moving around with that same weight in the pack, but that it’s going to fill the pack out as well so it kind of gets us a little off balance and lets us feel how they’re really going to be if you load them up this much out on the trail.”

Roy then outlined the two-tiered testing approach, incorporating both lab-derived comfort limits and the packs’ maximum carrying capacities. “So from what I understand, we’re going to load each of these packs to two different weights. We’ve tested these in the lab with sensors on mannequins and figured out at what weight each of them puts 80% of the weight on the hips and 20% on the shoulders. We’re going to start with that. Then we’re going to load it up a little closer to its max carrying capacity, get it to the point where it’s putting 40% of the weight on the shoulders and 60% on the hips. To most people, we’d assume that’s going to feel a little less comfortable, a little more strain on your shoulders. So let’s get going.”

Round One: The 80/20 Comfort Zone

Trenkamp began the loading process, announcing, “So we are a little over 25 pounds for the Outdoor Vitals. 25 pounds is what we found for its 80/20 rule. So I think 25 pounds 11 ounces is going to be okay. We’ll start there. You know what? This one weighs less. I’m going to take this one for myself. Adam, you get the heavier pack.”

Roy playfully responded, “Thanks for that, Adam.”

Trenkamp then proceeded to load the Durston Kakwa. “So now we’re going to move on to the Durston Kakwa. The Durston in testing actually performed a little more like a traditional pack in terms of how it carried. The Durston got up to 35 pounds before we surpassed the 80/20 rule. So 20% on the shoulders still at 35 pounds. Again, we’re going to start with our sleeping bag. This one’s a little beefier, too, so beefier bag for the beefier pack. There we go, 35 pounds 8 ounces. Our 35-pound mark was about our 80/20 rule in the lab test on this pack, so we’re good to go. We’ll venture outside so it feels like we actually have backpacks on. Along the way, I would love if you could tell me from your expertise how the pack feels with this much weight, this kind of volume in it that’s a little more normal. Does it feel comfortable? Does it feel like it’s starting to overstep its bounds in any way? Do you feel it shift? Any notes that you find.”

As they began their initial walk, Roy provided immediate feedback on the Durston Kakwa. “So off the bat, this pack is carrying really well for something this light. You know, almost all of the weight is on my hips, feels fairly rigid. I mean, this is really, really supporting itself well. You know, I think for the spirit of this test, we have to get off-road a little bit. This looks like rough terrain.”

Trenkamp, experiencing the Outdoor Vitals CS40, echoed similar positive sentiments. “Alright, I’m ready. Let’s find some steps, some hills, some grass. It’s great to hear that about the Durston. I feel almost the exact same thing with this Outdoor Vitals pack. Now I’m carrying a little bit less weight than you, but we’re looking at the same sort of 80/20 rule and I feel like almost all of the weight is on my hips. There’s just a little on my shoulders to help balance that load so it doesn’t really rock back and forth too much with my steps.”

Roy then delved into the design principles contributing to efficient ultralight pack performance. “So Adam, tell me a little bit about what makes a good ultralight pack in terms of efficiency and how it carries.”

Trenkamp identified key technological advancements. “Yeah, I think one of the big things is these newer ultralight packs have started to introduce frame stays that really help the back stay rigid, so a lot of the load doesn’t push into your back. The idea in an ultralight pack, you know, is to carry a lot less, to carry really only what you need and be lightweight. So something that can do that efficiently through the shoulders and even a lightweight hip belt, I think that’s what’s most important.”

Roy added historical context and the impact of material innovation. “For a long time, the vast majority of ultralight packs were frameless. I know that the ones I have at home are mostly frameless. So some of these companies like Durston have moved to novel materials like hollow aluminum stays to carry loads more efficiently while still keeping that weight down.”

Satisfied with the initial comfort assessment, Trenkamp posed the question, “Alright Adam, how’s this weight feel?”

Roy’s reply was enthusiastic: “I could carry this all day, man.”

Trenkamp then proposed escalating the challenge: “Alright, well, let’s put some more weight in these packs and do it again. I think that was a good test. You ready for some more weight?”

Roy readily agreed, “Yeah, let’s load these up, see how they do.”

Round Two: Pushing the Limits to the 60/40 Mark

Trenkamp explained the objective for the second phase of testing. “For each of these packs, what we’re going to do is push them now to their 60/40 rule that we’ve found in the lab. So about 40% of the weight on the shoulders, 60% on the hips.”

Roy anticipated the increased challenge: “And this is going to be a little more uncomfortable, or probably will be. We would expect something weighted that much to really put some strain on your shoulders over the long term.”

Trenkamp detailed the weight adjustments. “Yeah, and so we wanted to keep the range for both of these the same, but that means different weights. So on the Outdoor Vitals, we’re now going to bump this up to 40 pounds, whereas I had 25 in to start. For the Durston pack, we’re going to bump that all the way up to 55 pounds, and we started with 35. So that’s really showing how much weight it was able to carry in our test here in the lab, and I’m really curious to see your thoughts when we have that much load in the pack. 39 pounds 13 ounces, so we are about 3 ounces shy of 40 pounds. That’s fine. 55 pounds 9 ounces.”

Roy acknowledged the slight discrepancy, “Close enough.”

Trenkamp shared his initial sensations with the heavier load on the Outdoor Vitals. “Close enough, but I imagine I am going to be feeling a little more of that weight on my shoulders and not quite as much on my hips. I don’t have as much weight as you, but I can certainly feel a difference in this pack.”

Roy described the increased strain on the Durston Kakwa. “I’ve got the hip belt cinched up about as tight as I dare and I can already feel there’s a little more weight on my shoulders. Not unlike some packs I’ve carried in the past, but I can tell you off the bat this is going to be less comfortable.”

Trenkamp explored the impact of body posture on load management. “Now, one question to ask with that, do you feel like it makes you lean forward at all just because of that extra weight?”

Roy confirmed the tendency, stating, “It’s definitely making me want to lean forward a little bit just to keep my balance.”

Trenkamp explained the biomechanical advantage of this posture. “Yeah, a little bit of balance and when you do that, it actually helps take some of the load off your shoulders as well and starts pushing it a little more through your back. That’s one thing we don’t do in our lab test with the mannequin. So I’m curious how that affects the comfort of the pack, just that more human movement of leaning forward a bit versus our mannequin always staying upright and seeing at what point the hip belt slides. But let’s put it to the test.”

Roy expressed anticipation for navigating more challenging terrain. “Let’s get schlepping. Back to the great indoors. And outdoors. Yeah, I’m really interested in how these stairs up and down are going to feel with this. So with this extra weight, just how does it carry with these extra kind of bumpy steps making your body move back and forth.”

Roy provided a detailed assessment of the Durston Kakwa’s performance at the higher load. “One thing I will say is that this actually carries a lot closer to a traditional pack than most ultralight packs I’ve tried. Feels pretty stable. You know, I don’t feel like my load’s kind of rocking all over. Obviously no barreling because of the frame sheet. I wouldn’t call it comfortable, but it’s more comfortable than it might otherwise be. So the hip belt is actually rock solid on this, and I think part of that’s down to the padding on it. There’s not a ton of padding, but there’s kind of minimal padding compared to some ultralight packs where it’s straight webbing. That’s a lot more comfortable. Dummies can’t really test that because they don’t have the skin and fat and muscle that that wears on. I think this is pretty rigid. And while the padding on this is respectable for a pack of this weight class, I could definitely see if I were carrying this on a multi-day trip, I think I’d probably wake up in the morning and feel a little sore after carrying this load. And I will say having carried this weight in a frameless pack before, it’s night and day. I mean, carrying this kind of weight in a frameless pack, it feels like it’s mostly on your shoulders because the pack’s kind of bending a little more, it’s not maintaining that rigidity, and that really does, you know, it’s hard on your body.”

Trenkamp shared his experience with the Outdoor Vitals CS40 at the increased weight. “This Outdoor Vitals CS40 pack, I can certainly feel the extra load and I notice it on my shoulders. At our first round of test, I didn’t really feel it. I could tell it was there, but it was more of a balancing act than it was actually load. And now I’m feeling that load start to push. It’s still really comfortable, feels like a stable pack. But again, to echo what you said, I think after multiple days or even really one good full day, I would really start to feel this and kind of have some sore shoulders at the end of the day and maybe starting to regret my choice of how much weight I brought or maybe which pack I chose. So it can certainly handle the load, but it’s not quite as comfortable.”

Roy quantified the load distribution, explaining, “So let’s do a little bit of math. At this weight, our lab has found that 60% of the weight should be on my hips, 40% should be on my shoulders. At 55 pounds, that would mean I’m carrying 22 pounds on my shoulders right now.”

Trenkamp then pondered the real-world implications of dynamic movement on pack stability. “And what we really want to see is that sort of movement here because that’s now what we’re getting. Does that weight start to shift in these packs and does the hip belt actually fail before what we find in the lab because in the lab it’s such a static test where now we’re kind of kinetic and on the move. So does that motion moving around cause the hip belt to slip or cause that weight to kind of push even more than what we actually felt like, you know, with every step do we add a pound or two to the load? It’s good to hear that maybe not with this pack.”

Roy noted hip belt stability but acknowledged emerging hip strain. “The hip belt is staying pretty solid here. I will say that I’m starting to feel a little bit of strain on the hips. I think over the course of a couple of days carrying this pack I’d end up pretty tender here. Let’s head up the steps and kind of debrief on what we felt about these packs. What do you think now that you actually put these on and walked around yourself? Because mannequins, unfortunately let’s call him Steve, he can’t tell me how it feels. I’m going solely by what the lab equipment says.”

The Verdict: Lab Precision Meets Human Nuance

Roy summarized his key takeaways, emphasizing the accuracy of the lab testing while highlighting the subtle, yet significant, human factors. “Yeah, so my big takeaway here, Adam, was that your testing in the lab is actually pretty accurate. When we loaded this pack up according to the 80/20 rule, it was very comfortable. When we pushed it to 60/40, that’s 40% of the weight on the shoulders, 60% on the hips, not so comfortable. I started to feel that strain. So what I think this lab test didn’t necessarily reflect were the details. Things like the Durston has a slightly thicker fabric than a lot of ultralight packs, slightly stiffer fabric. I think that helped cut down on barreling at the bottom when we loaded it up. Also your excellent packing job helped with that. Likewise, it has a little bit of padding on the hip belt which some really minimalist packs don’t have, and that made it more comfortable to carry even at a higher load. So I would say the place that the lab test falls a little short is in figuring out how those details interact with a real human body and that’s where the field testing comes in.”

Trenkamp concurred, reinforcing the synergy between lab and field testing. “Yeah, that’s a great, great point. So we always like to say that the field testing and the lab testing combined to give you a more complete review of the product itself. And the lab is really to help us find some key data points that might be hard to pinpoint in the field or at least consistently find in the field based on conditions or weather or even really what you’re going to carry for that particular trip. If you show up at the trailhead and someone asks you to carry all of the food for six days because they can’t, now you know your pack can take that extra weight. So on the Outdoor Vitals, I had a similar sort of feel. While we walked around, I felt like the load was pretty accurate about what was on my shoulders in terms of percentage of the total weight. As we moved around, it maybe slipped a little bit, but I could have probably pulled the hip belt just a little tighter there. What I noticed that you didn’t feel, though, is as we bounced around at the heavier weight, the pack started to kind of barrel on the bottom. And during my lab testing, that wasn’t something I noticed as much until I got to really extreme weights and it was really pushing down. But moving around outside, having the pack on myself, it kind of bounces that weight a bit and starts to settle everything down. It didn’t add any more weight to my shoulders, but I could feel that hit my lower back in different points just below the hip belt. And that was kind of a note for me to take away and maybe something we should start to look for in our lab test here is really how does the material itself hold up, not just how much load does the pack hold.”

Roy reiterated the importance of material properties. “Right. And again, that’s the details. I mean, when you feel this pack, you can feel that this is slightly less of a stiff fabric as opposed to the Durston, and that does have an effect once you start taking it out, especially over the long term.”

Trenkamp, with a hint of competitive spirit, concluded, “So with that said, I think what we learned here today is that I nailed it in the lab, and that means I may take the title of Adam number one now. Are we switching number one, number two?”

Roy playfully countered, “Let’s not get ahead of ourselves here.”

Trenkamp maintained his stance, “I think I won.”