Zygos 4.0 Hydration Pack

(32 customer reviews)


Bryce Thatcher’s pack of choice, the Zygos 4.0 hydration pack is very versatile. This pack is lightweight and high capacity, a comfortable wrap around style with full compression allow for the lowest profile empty or fully loaded. The Zygos 4.0 comes with a 2L reservoir and a magnetic clip to keep your hands free. New expandable mesh on the back panel gives 4 liters of extra capacity as well as storage in the front with extra pockets. Whether you are fastpacking, mountain biking, in a fast race, ultra race, or summit assault this hydration backpack is the solution for any athlete. Available in three sizes.


Race Line: Built for the competitive athlete. Our Race Line focuses on Ultra & Trail running but can be used in any running scenario. Our Race Line products are sized to fit the athletic body, feature UltrAcool technology for maxiumum breathability on all race vests, made with very lightweight material for comfort during long distances, and created for pure performance.

  • Recommended Use:
    Versatile Pack utilized in any run scenario, training or racing.


    • Lightweight, soft, breathable, but strong, large-holed mesh pulls moisture away from the body to keep skin dry and cool.
    • Silicone and PU coated feather-weight nylon fabrics resist water, abrasion and tears.
    • Max O2 Sternum™ for unrestricted breathing while maintaining a stable pack. Uses shock cord with good stretch for unrestricted breathing but also good rebound on the stretch for a stable ride. Patent Pending.
    • Plush feeling micro-fiber polyester edges, soft and non-abrasive against skin and clothing.
    • Sweat proof webbing—won’t absorb water or get saturated and heavy.
    • Safety stitched bar tack on all stress points.
    • Quiet, bounce free, easy to see and use, zipper pulls.
    • Zig zagged stitching over elastic for ultra-durability, increased stretch and rebound.
    • Simple, fast, easy in and out bladder attachment.
    • Magnon pocket for sweat-free pills, with automatic magnetic closure for rapid access to pills and other essentials. Patent pending.

Fit:  Unisex
Size(s): Small (30″-36″), Medium (32″-38″) or Large (34″-42″)
Color:  Pitch Black, Emerald Blue*
Hydration (included):  2L Reservoir with mag-clip to secure hose.
Bottle Compatible (not included):  UltraFlask, Formula 250, Human 20 oz 2.0, or Collapsible Bottle
Capacity: 854 c.i. (14L)

Additional information

Weight N/A
Dimensions N/A



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Fluid Capacity

2 L (70 fl. oz) Reservoir

Total Volume

14 Liters

Weight (Includes Reservoir)

16.0 oz (454g)


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32 reviews for Zygos 4.0 Hydration Pack

  1. Jake Grossbauer

    Took the Zygos 4.0 on a test run and after first thought i am happy with purchase.
    Bought it for future all day or even overnight fastpack runs hoping the storage would be enough.
    For grins i tested the underarm pockets which i was able to easily fit multiple gels, waffles, and other nutrions in. The added front pockets allow for easy access to phone pr map while still having bottles in each as well.
    The fit is true to size amd the honeycomb wicked perfectly as it was warm bit the pack did not absorb any.

    The underside zipper pocket on pack was easily accessible and perfect storage for when it warmed up to put buff and gloves in, while never breaking stride

  2. Matt Ozanic (verified owner)

    This vest is AMAZING!! I’ve had many vests over the years but this one beats them all, hands down. I love the placement of pockets, materials used and the overall thought of design that will be long lasting and comfortable at the same time. It is the fine-wine of hydration packs. This pack is going to be great at keeping my fluids cool on my training runs in western Colorado and it will carry all of my needed gear on High Lonesome 100 and The Bear this year. Thank you for all that you put into this pack. It is absolutely perfect!!

  3. Sean Haverty

    I got the Zygos 4.0 after Long time use of UD vests. First impressions right out of the box was Quality. The mesh and fabrics used are seriously the finest I have seen.
    Skip to use: the Zygos 4.0 come last with a insulated bladder pocket. This is huge for me since I seem to get the most refreshment from cold water, and this allows it to stay cold for a long time. Special bonus; the bladder closure is attached to the bladder itself! More than once did I forget or lose the closeure on my UD bladder rendering it useless.
    The fit: at first it seemed a little strange that the vest didn’t fit snug on my shoulders, yet hung above them a bit. However after miles of use now I found that this is a blessing. It’s like wearing nothing at all!
    The pole straps are different in that they are located at the bottom back of the pack. I thought I would have a problem easily accessing these but I was pleaseanlty supprised. I can take them out and put them back in with ease, while continuing to run.
    Then there is the magnetic drinking tube clasp. So convienient and quick. My only problem was that I felt I would lose the receiving magnet that attaches to the vest on a clip. I solved this by attaching the clip to the top vest closure bungee. It doesn’t come off of there unless you want it to.
    Next is storage space. There are plenty of pockets and places to put anything you need to carry, without a problem.
    I really am impressed with this vest. If you are thinking about it, pull the trigger and buy it. You won’t be disappointed.

  4. Sean Haverty

    After being a UD vest user, I ordered the Zygos 4.0, mainly because of the support that UltrAspire gives to everyday mortal runners, unlike UD.
    First impressions: out of the box the first thing I noticed was the quality. The mesh and fabrics used are such high quality.
    Let’s get to the use: The bladder is easy to fill, and it has an attached closure! I can’t tell you how many times I have lost or forgot the closure on the UD vests, rendering it completely useless.
    The bladder pocket is insulated which I love as I feel more refreshed with cold water and this keeps the water cool for quite some time.
    The fit of the vest for me was at first unusual. Perhaps it’s because I have a big frame, but the shoulders seemed to rise above a bit. After many miles now, it’s actually a godsend. It’s like I’m not wearing anything at all.
    The pole straps are located in the bottom back. I thought I would have trouble reaching and using these but so was pleasantly supprised. I can take my poles out and put them back with ease even while running.
    The storage space is massive, the pockets are easy to reach.
    Did I mention the magnetic drinking tube holder? This is really awesome and fast. At first Inwas afraid I would lose the attachment that goes on the vest with a clip, but I remedied this by attaching it to the top vest bungee closure. It doesn’t come off of there unless you want it to.
    I love the bite nozzle on the bladder. It delivers water with no effort.
    If you are undecided on this product, go ahead and pull the plug. You won’t be dissatisfied.

  5. Sascha Jurak

    I used it on my last marathontrail and it is the best of the best.
    Everything is very good reachable and the pockets have the perfect size. The only thing i don´t like to much is the position of the poles. I have to put the Zygos of my back to put the poles in and out. And sometimes when i run downhill i hit the poles with my elbows. Mybe there is a possibility to clip and add on in front to sstre the poles there?

  6. Joshua Hamilton

    First off, I am an Ultraspire Immortal Ambassador so I currently only use Ultraspire packs. In the past I have primarily used Salomon gear and I’ve always been a big fan of their stuff for all mountain activities. This pack is one of the highest quality pieces of gear that I have ever used….period. Phenomenal fit and finish with excellent features that all seem to work seamlessly together. The materials are lightweight and super high quality. Ultraspire has definitely outdone themselves with this one. I absolutely love it.

  7. Jeff Fullmer

    Like many others, I was a diehard UD fan. I was shocked to take this out on my first run and realize I could breathe in it! With previous vests from other companies, I constantly have to adjust the chest straps to breathe more on the uphills and tighten it up on the downhills. I have yet to adjust the chest straps mid-run with the Zygos 4 as it breathes with you! I LOVE the lower pouch on the back of this pack. It has my tp and first aid kit easy to access but out of the way. Most runs I don’t use them but when I do its nice to not even have to take my pack off to access this small tucked away lower compartment. I LOVE the waterproof pockets like the magnetic pill pouch, cell phone, and interior pack pockets. The all-day comfort is huge for me as well. I don’t ever feel like this digs into my shoulders. I can carry A LOT of gear for all-day mountain adventures (and enough emergency supplies to make it through a night if I had to) or just throw my cell phone in it for a quick morning run. The versatility is astounding. Open access to the water blatter compartment and insulation are a big bonus as well. The quality is outstanding and meticulous; from the bungees to the stitching, straps, and pockets. I foresee this being a 10 year+ pack and will probably leave it in my will for my favorite son. HA! 🙂

  8. Aaron Smith (verified owner)

    Zygos 4.0: awesome pack! I took it on two 50k’s and it stored everything I needed, I probably took a little more than I needed. The races were supported with aid stations at decent intervals so I decided not to take the included 2L bladder but I did take two 500ml Softflask bottles (sold separately) with me each time.

    For my solo training runs, however, I have taken the bladder, both bottles, and fully-loaded the pack with additional weather-dependent layers and emergency gear. I love the many pockets and they store everything I need (keys, phone, ID, clothes, gloves, hat, food). Everything can be accessed very easily while moving so I can grab a snack from a pocket without slowing down (too much!).

    Quality seems to be solid, I have fallen a few times and it hasn’t torn or ripped. I enjoy the magnetic clip of the 2l bladder, I love the various features such as the bungee system that tightens the pack as needed or the clips/zippers of the pockets. Everything is very appropriately placed, you can tell that a runner/adventurer designed this with other runners/adventurers in mind.

    Great pack!

  9. Tim

    This is a very low profile race vest despite the storage capacity which is a good thing! This vest is ideal for self support long runs in the mountains or races in which a mandatory gear kit is required. My one purpose of buying this vest was to test that specifically. I have used the vest twice with the gear kit requirements for UTMB on an 5 hour and 8 hour mountain run. I have never used the bladder as I do not like using them and refer the flasks as the are more versatile/quicker at aid stations. The vest was excellent on both runs, no movement, everything was accessible when needed. I did have issues storing poles as they hit my elbows and arms when running. My preference would be for UltrAspire to look at the ability to store poles vertically vs horizontally. Other than that one suggestion the vest has everything one would need for kit specific races. One suggestion is put the side pockets that are on the Zygos 4.0 on the Momentum.

  10. Holly

    This is my new favorite all-purpose hiking and trail running pack. I can fit everything I need for many different kinds of adventures and can distribute my gear weight and bulk throughout the many pockets so that I don’t feel weighed down. I love that my chest does not feel constricted with this vest – breathing is easy with the flexible straps. It is easy to adjust while running as my pack looses (or gains) volume with gear. The back pocket fits my puffy jacket and rain shell nicely and it feels secure (not bouncy).

    My suggestions for improvement are: have more size choices (I need the small normally, but the medium when wearing a jacket or a padded sports bra), remove the foil liner for the bladder (it’s noisy, hot, and doesn’t seem to add value), and consider a more secure clip design for the bladder straw magnet (I need to consciously remember to pull down to release it from the magnet when I want to drink, even though my instinct is to pull up and towards my mouth).

  11. Danny Widerburg

    I’ve ran in quite a few vests over the years, I seem to have always come back to my UltrAspire Zygo’s. I had the original for several years, it never failed me. Recently I acquired a Zygo’s 4.0 and the best compliment I can give it,
    “I never notice that I’m even wearing it”. It carries what ever I need for the current adventure, its quality that you can see and feel and it fits correctly. Hands Down, the BEST Running Vest I have ever used!

  12. Ron Anderson

    If Tony Stark made hydration vest, this would be it. I love a good piece of kit and Ultraspire did not disappoint. First rate materials and craftsmanship with plenty of pockets/storage for your ultra mileage arsenal. Look forward to sharing more miles with this vest.

  13. Olan Johnson

    I have been running with the Zygos 4.0 for a few months now, but wanted to see how the pack performed during my first 100 miler before posting a review. The bottom line up front is that this is a must-have piece of gear if you are taking on the 50 mile or 100 mile+ distances.

    I love that the low-profile design allows the vest to wrap around you in such a way that it feels like a part of your body instead of another layer that you are wearing. As a result, the vest is super comfortable and guarantees multiple solid points of contact. This means that I was able to wear this vest – fully loaded – for 29.5 hours straight without experiencing any fatigue or chafing. For reference, my version of fully loaded means carrying a full 2L water reservoir, 1x 550 UltraFlask, phone, running fuel, blister kit, first aid kid, additional layers of clothing, and a headlamp. I’ve also been impressed with how secure yet adaptable the Max O2 sternum system is. With previous vests, I would constantly have to adjust and re-adjust the straps throughout my run. The Max O2 system eliminates that completely. I can honestly say that I have yet to re-adjust the shock cords on the go once I’ve set them to my liking.

    Another major reason I love the Zygos is all the massive amounts of storage space! The pockets on the front (2 on each side) are deep and spacious. No zippers up front makes for easy grab and go whether its fuel or flask bottles. Another favorite aspect of this vest are the side pockets because they span the entire side portion of the vest. I’ve stored everything from fuel to headlamps to cold weather layers in these pockets, something I could not do comfortably – or at all – with other brands. One storage space I had not used until my 100 miler was the pocket at bottom of the pack. The placement seemed strange to me and I wasn’t sure how I’d use it. But this is where I ended up keeping my gloves and I can’t tell you enough how awesome it was to quickly access and store them here at various points throughout the race. The only aspect of this vest I haven’t been able to test yet is trekking pole storage. But I’ll be sure to check back in once I’ve had an opportunity to utilize them!

  14. Megan

    For reference, I am primarily a trail runner and run long distances so this review is tailored toward that style of trail running. I can’t say enough about this pack. The little attention to detail all over the pack makes it such an exceptional piece of gear. Right out of the box the quality was seen. No loose threads, no clear weak points. If anything the points that would typically be the weakest are instead reinforced. You can TELL that an actual ultra runner went through this pack and thought of everything. Simple things like the way the bladder quickly attaches to the pack, its quick and easy, no velcro, no fuss. The chest cord really does allow for easy breathing without restriction. I was hesitant to believe this based on the description, but it really held true. This in itself could make the pack worth it, but there is SO MUCH MORE. The pockets are all very accessible, They stretch, zippers are easy. You can shove a LOT of gear in here and it doesn’t hinder the fit of the pack much. The back is not only insulated from you back, but it actually does allow for sweat to disperse rather than soak your pack. This is huge for keeping your water cool, and contents dry. The last thing I will mention is the pole attachment. This is what initially drew me to this pack. All of the other packs I have tried have been unsuccessful in this area. The UD pack anchors poles to the front of the pack. They are extremely hard to attach while running, they are awkward, and they hit your arms to boot. All the other packs i’d just stuff my poles in the back, which means my pack had to come on and off each time wasting so much time. This pack has a super simple lower back attachment. You can do it while running with a little practice. Overall i’m incredibly excited about this pack and plan on writing up another review after an even longer run over 6-7 hours. But so far having used it for a 3 hour trail run I am very happy with this purchase.

  15. Thomas Murray

    LOVE LOVE LOVE this vest! An older version of the Zygos was my first hydration vest and that’s when I fell in love with UltrAspire products. Everything on that vest was so intuitive – things were exactly where I needed them and easy to access. So when that vest began to suffer from a little too much use, I figured to go for the same model. The updates on this vest are incredible and it is amazingly comfortable.

    Space: this vest was great for a 100K race where I was not going to have any crew. Cold temps in the morning and after sunset but really warm daytime temps made it important for me to be able to layer. The vest had plenty of room for me to pack in my jacket, gloves, and hat plus have dry socks, shirt, and buff available. Sounds like a lot, the way this pack sits I didn’t feel like I had all of that on my back. Between the two UltraFlask 550s I could carry in the front pockets and the 550 race handheld, I always had plenty of fluid/nutrition handy between aid stations.

    Fit: I am not a tiny person…6′ 0″ and 220lbs, so often vests don’t fit quite right. This one is a dream. The elastic cord closures allow me to get the perfect fit while not restricting my breathing at all. No chafing, no rubbing, no bouncing. It’s like it’s not even there. The way the shoulder straps fall are genius! No gapping, which happens with a lot of other vests for me.

    Versatility: This vest has replaced my Camelback for day hikes too. I can load up with 2 liters in the reservoir, two 550s in the front plus a handheld and be set for hours and hours of hiking in the desert, which is awesome when you can’t count on water sources.

  16. John Danese

    The Zygos line has been my ultramarathon and general lightweight running pack for years. I still have my Zygos 1.0 and 2.0 and needed something with a bit more storage for my latest Nolan’s 14 attempt, but not as big as my Epic pack, which I use for the 20+ hour legs.
    The Zygos 4.0 was awesome. The waist and front packet storage is commodious and it’s still super light and comfortable. I have a thing about stuffing too much into pockets and get frustrated trying to find various food items I want. Having the 4 pockets on the front help me organize and know where my various items are, so i can grab them efficiently.
    I like the slightly lower hose clip location on the left chest, compared to the other models. As with the Zygos 2.0, the ability to stash and retrieve z-poles without taking the pack off is appreciated.
    And, as with all UltAspire packs, I have NEVER had a material failure or other quality issue. I’ve had multiple friends with other brand packs have zippers fail and that’s just not acceptable during an ultramarathon, high alpine mountain adventure, or just a long run.

  17. Billy Grimes

    I am so stoked to have found this pack. For long ultras and big days in the mountains where layers, storm protection and supplies to address the unknown are necessary, while being compact, low profile and carrying this stuff efficiently; I am convinced this is the best option on the market today. There are a couple issues I’ll address but nothing major. It is clear to me that this is one company that takes gear testing and leveraging their athletes to create solid products very seriously (e.g. this pack, the speedgoat hip pack) while other company’s with long lists of heavy hitting pros produce products with glaring shortcomings that should be caught in the prototyping process.

    First of all the overall “frame/system” is robust and solid to account for the potential load this pack can handle. I at first questioned the elastic closure but I am now a believer, when the pack is full you can really appreciate how it accommodates lung/ribcage expansion. I was initially a little concerned by the claimed size (14L?) thinking it would be too big and was thinking the Zygos 2.0 older model was what I might want (I was really looking for 10L range), but the additional volume is all I think attributed to the added kangaroo pocket so it is not truly larger overall that a 10L.
    The pockets are truly dialed! The top left holds my inReach mini and a small mp3 player (not wasting my phone batt for music). The top right pill pocket with magnetic closure is is an amazing idea and the whole right side has a sweat proof lining. The dedicated phone envelope pocket with top closure is great especially in a rain storm, though I have started mostly carrying my phone in the stretch pocket outside of it because the envelope is a little bit of effort if you are taking it out/putting it back a lot using it for navigation or taking lots of pictures while running through amazing landscapes! The back bottom pocket is awesome for small things you want with you but don’t necessarily need access to on the fly (First aid/TP-and bag to carry it out!) etc.. The side pockets are the best I have ever used (they are actually functional!). They bag out so loading them up does not press into your rib cage like other packs and I can actually open/close and pull stuff out of them on the run which I constantly struggled to do with other packs. Same with the pole carrying system. First I have ever used where I can easily pull out/store on the run. Other reviews mention the poles get in the way of arm swing. Maybe theirs don’t collapse? They don’t get in the way for me at all with my BD z-poles.

    The minor issues I have relate to the hydration system. First is the insulating material. It started ripping away at the top seam one of the first few times i dropped in the resevoir. Like most packs this is difficult to do when the pack itself is stuffed which contributed to the difficulty/ripping. I love the no closure envelope pocket but it does seem an inch too low and is hard regardless to get the reservoir and the top bar of it below top of the envelope. Which leads me to the hydrapack reservoir itself. I have to wonder if I am the only one who gets irritation from the bar (especially as the reservoir drains and the water itself is no longer pushing into the back. With a stuffed pack the bar starts to really irritate my shoulder blades. No fault of UltraAspire as no other company they can source has a great solution except Camelback which is what I will upgrade my reservoir system too. Camelback reservoirs have their own issues with the closure, difficulty to open, and room for error in sealing… BUT its the only system that is comfortable on the back so I deal. Still some low hanging fruit in reservoir design!

    But, all in all, THANK YOU UltrAspire! I am typically not compelled to leave reviews but you deserve Kudos on this!

    One last note on size. I got the S/M, I measure a ~39′”chest. When I got this I thought it was too small (felt like it was cutting into the sides of my ribs a bit. I ordered the larger size and it was noticeably too big mostly length of back and short closure across chest. I like a pack that rides higher and not into the small of my back so I stuck with the smaller one and it has been the right choice, I don’t notice my initial concerns when running.

  18. David (verified owner)

    I just recently used this pack on an 82 mile (3 day) weekend adventure, and it was nearly perfect. With plenty of pockets, it held all of my food and essentials for the trip . Personally , I would wear a more minimal vest on a race, since there are aid stations, but this was great for a self supported adventure, which is the reason for my purchase.

    But like Holly from a previous review, I had noticed the same small issues —
    – The foil liner for the bladder was noisy to me as well. I’m wondering if a foil “pouch” can be designed to go around the bladder if desired , but is also removable. Also, as Holly also mentioned, I removed the bladder hose magnet, and replaced it with a clip from another vest . I just don’t like the magnet on the hose. It would undo itself on technical areas of the trail , but it also felt too magnetic when I wanted to draw it to my mouth. Neither of these issues would cause me to choose anything else though , as I really did like the way it felt – with or without a shirt, and how much I was able to carry .
    I gave it four stars, because I didn’t have the option to give it 4.5 stars

  19. justin turnbow

    Perfect modified it so i can carry my trek poles up front but it is my all race vest.

  20. J Tufte (verified owner)

    zygos 4 is my third UltrAspire pack. Needed a larger one for a longer event and this pack fits great, feels great, no bounce, and holds what I need. when the spry isn’t big enough, the zygos is a GREAT option.

  21. Jonathan Miller (verified owner)

    Just received this vest and used it on a 5 hour run. I have a 37″ chest and ordered the Medium. Seemed snug at first but the fit was great on the run and didn’t notice any bounce or noise from the insulation liner. Love the large capacity as it comfortably held lots of cold weather gear along with all the food and water I needed. I use 500 ml soft flasks instead of a bladder and 2 fit well up front in the outside pockets (gave the included bladder to my wife for her vest). iPhone 7 in a case easily fit up front too. I thought the soft flask would push the hard phone case into my ribs, but I adjust the phone’s position a bit and had no issues. Quality seems excellent so far. I’ve owned Salomon and Nathan vests in the past but I think the Zygos 4 will be my favorite!

  22. Stephen Cooper

    Would love to buy this pack in a large size but impossible to find.Is the Zygos 4.0 being phased out or just in short supply?Based on what I see are its features and favorable experience with the larger Epic XT I’m guessing it’s a 5 star pack!

    • Jodi

      Hi Stephen,

      The Zygos 4.0 isn’t being phased out. We are simply out of stock. We do have a new shipment of Zygos 4.0 scheduled to arrive at our headquarters in January. So…it should be back in stock shortly. We will update the website as soon as the Zygos 4.0 arrive.
      Let’s get you one!!
      Please let me know if you have any questions.


  23. George

    As an Ultra Runner that works in a Running Store in SoCal, I get to try on a lot of Gear and the Zygo’s 4.0 is as close to perfection as I’ve ever found in a Hydration Vest! I’ve used it in 2 Trail Marathons, 3 50k’s and a 50 Miler, as well as 3 Mt San Jacinto summits and it does everything I need. I do big mountain races in Colorado, Utah and Tahoe, so I needed something that held all the gear I need for high elevation surprises and running through the night and this vest fits the bill with ease. Additionally, it’s super comfortable, has everything I need, in all the right places, including the zipper compartment on the bottom, that holds my collapsible cup. The fit feel and expert design is apparent in every feature. Thank You … UltraSpire!

  24. James Nalley, 2021 UltrAspire Team

    I recently purchased the Zygos 4.0 and decided to break it in on a 21-mile Grand Canyon run from South Kaibab down to Phantom Ranch and back up Bright Angel trail. After running with it on that day and on other long runs, I believe that this gear is really excellent for many reasons.

    First, I love is low-profile design in which the vest literally wraps around you and holds on. It feels like a koala bear is holding on and hugging you for the entire run. Despite the faster speed and rugged terrain of running the downhills, the vest never bounced. The vest was also fully loaded with 2 liters of water, numerous snacks and gels, and a headlamp, with my jacket folded into the expansive large pocket. I was actually surprised when I took off my running jacket and it neatly fit into the pocket, due to the stretchy mesh material. I also love the shock cords, which stayed in place the entire run and “breathed” with me as I exerted energy on the 9,000 feet of climbs. As far as trekking pole storage, the two loops in the back were great for holding my z-poles and they did not annoy me, despite being at my lower back.

    Well-designed overall!! Very please with this product! I look forward to running my next 100 miler in this vest!!

  25. Joseph Farrington

    The UltrAspire team outdid themselves with the Zygos 4.0. My go-to vest was the Ultimate Direction Mountain 4.0. The Zygos took everything I liked about that vest and made it all way better. Closed/zipper pockets on the straps, very large and easy to access pockets throughout. The back compartments are amazing! The magnetic hose clip is a god send! But wait, there’s more, the reservoir retention device is amazing and easy to do quick exchanges without messing with snaps!!! I’m never going back!!!!

  26. Michael O. West

    I’ve been waiting for a pack from Ultraspire that I could show some of my running friends and out do anything they have.
    Well I’m able to do that now.
    This pack does it all and more!
    So many expandable pockets and zippered ones as well that expand.
    This pack will win best in show.
    Thank you Ultraspire for listening to us ultra runners and being one yourselves .
    Michael West AKA Mr. Shawnee

  27. Anya

    What a gem!! I’ve been in the alpha for years (which I also love) but needed a little extra packing room for a 50 miler coming up. So far I am loving the Zygos. It has so many nifty pockets but doesn’t feel bulky at all. I’m about 5’2″ and 112 lb for reference and the Small fits really well. One of the things I love about UltrAspire is that there are sizes for us little peeps! no bounce, no chaffing and all the hydration. 😀

  28. Jesse

    Best pack I’ve ever used! I’ve been trail running for 20 years and have used several packs. Zygos 4.0 has amazing carry capability with significant front storage, for fast and easy accessibility. I find that most packs struggle with sweat wear and tear on zippers. So far this pack is holding up.

  29. Jeff Boyer (verified owner)

    Just got my vest and have had it on two runs so far – 6 miler and 12 miler – loving the fit so far and the capacity is more than enough to handle my miles right now! Rides soft and smooth on my shoulders and the adjustability in the front allows for normal breathing for me. Ready to tackle some bigger mileages now that I can be prepared to hydrate and carry enough gear for a few hours! Thanks, Jeff

  30. Paolo

    I needed a running vest for daily trainings, including long ones, as I didn’t want to use my main race backpack. In the past I had almost pulled the trigger on the Zygos 2.0 but 10 liters wasn’t enough. When I saw Zygos 4.0 in pitch black I just couldn’t resist and got it straight away!

    This vest is outstanding:
    – It is incredibly comfortable and breathable
    – the 3D mesh back panel wicks sweat away very well
    – All pockets are easy to access with the vest on
    – Even the poles can attached and removed quickly
    – The stretch mesh makes it easy to throw in and get out clothing and gear
    – The reservoir compartment is insulated and keeps the water fresh for quite some time
    – The pitch black color is simply awesome – it’s worth stating it twice!

    I have been using it for about four weeks now and clocked almost 500 Km running in the mountains. I couldn’t be happier with it.

    The only thing that is missing for me so far is a couple of loops on the sides: they would be great for attaching carabiners and secure gloves and a cap on the go without having to put them into the pockets each time. I can still do that by clipping the carabiners over the side pockets though but they might make holes into the inner mesh over time.

    I am so happy with it that I even considered using it for my next 200 miler in the Alps but the size won’t cut it: it could work if temperatures don’t drop too much but it’s too hard store full cold weather kit comfortably, even if all my layers are in the ultralight category, the Zygos 4.0 is not large enough to carry them, access them easily, and stay comfortable.

    If there was a larger 17-21 liter Zygos, I wouldn’t think twice about it. In fact, it would actually be a nice addition and fill the gap between the current 14L Zygos 4.0 and the Epic XT 2.0 which I also own.

    I have been mountain running for 10 year now and over the years I have tested or used many different running vests and packs: in my opinion the attention to detail and design of the Zygos 4.0 is second to none and better than most. I even got a second one to use just for races. If you are in the market for a high quality, well-designed running vest, look no further. Ultraspire customer service is great too.

    I am not an Ultraspire athlete and purchased both vests with my own funds.

  31. Lori Smith

    I bought the Zygos because I needed a pack I could use for long 12-14 hour unsupported training runs. I had a difficult time finding a pack that would fit my small upper stature, and the size small fits me perfect. I can tighten it up to fit me without having any bounce. This pack has lots of easy access pockets for on the run snacks and the capacity for a bladder and 2 flasks. I now wear it on my longer races, such as 50 miler and plan to use at the Bear 100 this month. I love it for all distances!

  32. Pavel Ivanov

    I love this pack! I’ve used it as is with 2L of water in the bladder and gels in the front pockets on shorter and cooler runs, and also with additional two water bottles in front pockets and gels in side pockets on longer and hotter runs. In both cases the pack sits comfortably without causing any issues or discomfort. And extra storage on the back can fit lots of additional stuff, so I can easily run with 4 litres of water, jacket and everything else that I need with me.

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