UltrAspire Zygos Review

I didn’t know what to expect when I first fished the Ultraspire Zygos out from behind a plant on Eric’s front porch — but after a month of regular use, I have grown fond of the pack’s technical innovation and solid performance. This pack and I have gone everywhere together, from the northern Sierra Nevada, to the mountains of central Oregon, to the woods of New England and we’ve become inseparable on these day-long adventures.


Features Pockets

The pocket layout of the Zygos is relatively standard, except for the inclusion of the Isomeric Magnon pocket (which, to be honest, I didn’t have very much use for). It includes

  • Two drawstring mesh pockets on chest straps, good for bottles, cell phone, or food
  • Isomeric Magnon pocket on right-hand chest strap, specially designed to hold electrolyte tablets
  • Small zippered pocket on left-hand chest strap
  • Two large zippered pockets on either hip, good for headlamp, first aid kit, and food
  • Large open-top pocket on back, good for jacket or map main compartment
  • Small zippered pocket inside main compartment
  • Hydration bladder compartment




Gear Loops

The Zygos’ gear loops are some of the most elegant I’ve seen on a pack. While explicitly designed for hiking poles or an ice axe, I found these loops to be extremely versatile, with a little creativity. For example, I was easily able to attach pair of Yaktrax to the outside for my “cold weather scramble” configuration, detailed below.

At the top of the pack, a hook on an adjustable-length elastic can wrap around or through gear and secure it without any extra hardware. A second attachment to the fabric loop at the bottom of the pack, an adjustable fabric loop provides added stability.

gear loop, top

gear loop, bottom



Chest Straps

New to the Zygos, and latest Alpha packs is the “Max O2 Sternum Adjustment”, so named for its ability to expand and contract with the wearer’s chest while breathing. Instead of the weak elastic cloth found in previous Ultraspire models and competing packs, the Zygos uses strong elastic cord to hold the pack to your chest.


I must confess when I first looked at the pack, I was skeptical that this system would be comfortable: Would the elastics be too tight against my chest? Would I develop any kind of friction burn underneath the elastics or their clasps? Would they be easy to put on and take off when tired?

After using the pack for a month, I am a firm believer in this design. The pack hugs my body tighter than most, affording a very stable feel, even with heavy loads. The elasticity of the straps feels perfectly adjusted to the dynamicity of my lungs, allowing me to take deep breaths without any discomfort.

Additionally, the hooked design allows, with some practice, the pack to be put on and taken off while wearing thick gloves.


It is worth mentioning that the elastics on the Max O2 system need to be readjusted when the load on your back changes (food and water are consumed, jackets are put away) and the elastics can sometimes come out of the right-hand adjustment clasp if they are not secured properly. But both of these inconveniences become less apparent the longer I used the pack and learned its quirks.


Hydration System

The Zygos comes with a Ultraspire-branded Hydropack 2L bladder, large diameter hose, and “Infusion Valve”, optimized for high flow rate drinking.

The bladder slides into a completely isolated pocket in the back of the pack. While it is possible to slip a smaller 1.5L bladder into this pocket, a 3L bladder will not fit.


Attached to the bladder by a Plug-N-Play port, the large diameter hose (dubbed “The Bore”) delivers more water to the “Infusion Valve” which opens with a pull for a no-bite no-suck high- flow-rate drinking experience.


While I appreciate the innovation in the hydration system, I found it was overshadowed by a problem: It is so hard to get the valve open! I have to open it with hands because it feels like I’ll damage my teeth by opening it with them. The force it took to open the valve did not decrease significantly over a month of use. This is a prerelease model so I hope that Hydropack and Ultraspire can fix this problem before the pack hits the market!

Valve is hard to open!
Valve is hard to open!


The Zygos holds 14L of gear, on the high end of traditional running pack volumes but smaller than a traditional overnight pack. To give you an idea of what you can do with 14L, I will show the pack loaded down for two different types of gear-intensive outings: a cold weather scramble in the mountains and a warm weather overnight trip.

Cold Weather Scramble





  • pair of arm warmers
  • pair of warm gloves
  • buff
  • heavy jacket
  • running tights
  • pair of Yactrax Run
  • 5 energy bars and nut butter
  • chapstick
  • map
  • headlamp
  • pocket knife
  • first aid kit
  • 2L of water

Warm Weather Overnight

If you don’t mind people making fun of you for looking like a SCUBA diver, you can loosen the compression straps on the back all the way and slide in a small sleeping bag! Whether or not this use was intended by the pack’s designers, it works perfectly as long as your sleeping bag has a small diameter!




  • ultralight sleeping bag (Sea to Summit Micro ||)
  • ultralight ground pad (Klymit Inertia X Frame)
  • 5 energy bars and nut butter
  • bag of trail mix
  • battery pack (mophie outdoor edition)
  • lightweight jacket
  • extra tech tee
  • headlamp
  • pocket knife
  • first aid kit
  • 2L water



Most of the Zygos is made out of a very light, glossy, crinkly nylon, which is perfectly waterproof. If not for the zippers, you could probably use the pack as a drybag!

The shoulder straps on the front of the pack feature a mesh with larger openings than the previous generation of ultraspire packs. While I’m glad Ultraspire is raising the bar here, I didn’t notice much difference day-to-day.

Overall, the build quality of the pack is spectacular. After a month of rough use, it looks just like it did the day I got it.



  • Available March 2015
  • MSRP $160
  • Available colors: “Steep Green” (small / medium, pictured in this review), “Precipitous Blue” (large / extra large)
  • Maximum hydration bladder size: 2L (can also hold two bottles in front)
  • Weight without hydration system: 280 g (small / medium) 290 g (large / extra large)
  • Total volume: 14L
  • External dimensions: 44 x 37 x 4 cm (small / medium) 48 x 41 x 4 cm (large / extra large)



  • Hydration system optimized for high flow rate drinking
  • Large 2L hydration bladder
  • Exceptionally stable and comfortable fit
  • Versatile gear loops, designed for an ice axe or hiking poles
  • Isometric Magnon pocket, especially designed for holding electrolyte capsules
  • Just enough capacity for demanding day trips and ultralight overnight trips
  • Ability to hold a ultralight sleeping bag (though maybe not by design)
  • Waterproof material

Screen Shot 2014-11-24 at 2.58.40 PM


  • My hydration valve was extremely difficult to open!
  • The Max O2 Sternum Adjustment can be finicky to adjust



With its innovative sternum adjustment, this pack allows for fastpacking-sized loads to sit comfortably on a true running pack.

If you have ever felt like your running pack is a little cramped for the kinds of adventures you enjoy, give the Zygos a chance. It may not be the most fashion-forward pack on the market, but it is tops the charts in technicality and quality design.



This review was written for URP by Lucas, a 22 year old ultrarunner, software engineering student, and part-time orange farmer. After 3 years of regularly competing in ultras, culminating in a victory at the 2013 San Francisco 100 miler, he has taken some time off from racing to focus on exploring the Sierra Nevada mountains and developing his hobby project, Scree Maps (http://twitter.com/screemaps.)

Lucas recently reviewed the Ultimate Direction Fastpack 30 for us here. All words are his own, and neither he nor URP were compensated or encouraged for a positive review. After reading Lucas’ opinion, I think I probably should have kept this one for myself.


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