What's the difference between a rucksack and a running vest? One plausible answer might be that the former is something you carry, while the latter is a thing you wear. In recent times, some packs ostensibly more for walking have begun blurring the distinction, with a running vest-style closer fit. I'm all for it. Broad body-hugging straps tend to be comfy, spreading the load to offer a well-balanced feel. In place of thick padding, which can be hot and obtrusive, these vest-style harnesses are thin, form-fitting, and highly perforated for maximum airflow. The addition of stretchy shoulder and hip pockets that you can access on the go for drinks, snacks, devices and nicknacks is useful for hillwalkers as well as runners. 

Rab's Aeon collection is a great example of the genre, with several models ranging from more walk-oriented to more running-like.

At the 'technical' end of things, the Aeon Ultra is my pick of the bunch. Coming in 20L and 28L versions, it's designed for 'fast hiking' (which a sceptic like me would probably just call walking with a sense of purpose), but ought to prove equally handy for a range of other uses, from scrambling to straight-up running. I'd recommend it for less performance-oriented walks too, since there is no particular speed you have to be going at in order to appreciate a pack with a close, well-ventilated fit, and a high degree of weather resistance.     

Capacity

As I'm on the larger side (bigger folks' spare clothes take up more room), I went for the 28L. Rab suggest this might do for ultralight bivvies and mountain marathons, but while I'm as keen as the next person on stuff-reduction, I'm not sure my packing is quite that disciplined. The suggestion of hut nights seems more realistic. For me the 28L capacity is in the ballpark for a summer hill day, with room to spare for a bulky SLR; and if you're under-capacity, the easily-adjusted side compression cords work really well to reduce the load.     

Weight

At just 640g (Rab quote a conservative 700g), the Aeon Ultra 28 may not be a featherweight but it does seem pretty darn light for a model with a comfy harness, taped seams, and lots of stretchy pockets. Compared to the often rather weighty walking day packs out there it definitely feels more like it's in running territory. 

Fit and comfort

A running vest-style harness is only going to offer maximum advantage if it fits you well, so perhaps more so than with a conventional design it'd be worth trying the Aeon Ultra on in a shop before buying. This is a unisex pack, with a fixed back length, and I imagine that for most women - and shorter men - the smaller capacity model would be the more successful.

While the Aeon Ultra 20's 43cm back length will best suit less-tall users, the Aeon Ultra 28 takes this up to 48cm. At 183cm tall, with a fairly long body, this greater length suits me well, allowing the hip belt to actually sit on my hips and do the job of load transfer and stabilisation it was meant to. I've rarely found a smaller day pack that's long enough to really work properly for me, and I'm not exactly a giant. Thanks go to Rab here for making a wee pack that suits the slightly-bigger-than-average wearer. 

A close, comfy fit is the defining feel of this bag. The broad, sculpted shoulder straps and hip belt hug the form and spread the weight, giving a well-balanced feel and reducing bounce to a minimum, even at pace. Running vest-style double height-adjustable chest straps help bring the fit in close, and since they're stretchy they don't restrict you getting a full lungfull when working hard. These fasten with a slot-in clip rather than a conventional buckle. Some users report that the clips can pop out in use (see the forum thread below) but I can't say it's something I've yet noticed myself. It may be a personal fit thing.

Your typical walking pack has a depth of foam cushioning over the shoulders, which can be both sticky and bulky; lacking this, the Aeon Ultra's soft, unpadded shoulder straps afford totally free arm movement - great for scrambling - while their open mesh is as breathable as it gets. The moulded foam back panel is pretty airy too, and overall this is a really cool and unsweaty pack that has proved totally ideal for warm weather use. 

Fabric and taped seams

Made from 50% recycled material, the nylon fabric is light but feels easily tough enough for the job. It's highly weather resistant, and with fully taped seams - a rarity on rucksacks, but always seems like it'd be a good idea - the Aeon Ultra is great on a rainy day. While I do still resort to stowing stuff in dry bags, this is more for organisation than because they're needed for keeping things dry.

Features

While this rolltop pack lacks a lid pocket, you're not exactly short of other options for all the bits and bobs that we tend to like to keep handy.

On the front, a big stretchy sleeve gives you somewhere accessible for a shell, or your map. Further roomy stretch pockets on the sides are good in theory, but actually I have found them of limited use. This is thanks to their double entry design, with a lower as well as an upper opening; this is supposed to make it easier to reach items without removing your pack (maybe if you're double-jointed) but carries the unintended risk of losing things like water bottles and gloves out of the bottom hole. They're fine for folding poles, and you can also attach poles runner-style between the shoulder and hip belt using neat stretchy loops - a good option on fiddly ground where you may want to deploy poles one minute and stow them away the next to free your hands, without having to actually stop and remove your pack. 

Another very worthwhile crossover from the running world is the stretchy chest pocket. In fact I'm so accustomed to using these that I miss them on any design that has failed to take this obvious advance. Whether it's a soft flask, a compass, your phone, or a handful of jelly babies, the ability to grab things on the move without having to root around inside your pack is just as much an advantage to walkers who have no interest in timing their pace as it is to runners racing the clock. If you're also a stretchy pocket fan, the Aeon Ultra will not let you down. On one side is a zipped expanding pocket shaped to fit a 500ml soft bottle, and designed to stop it slipping down as it empties, while on the other shoulder you get a zipped phone pocket. Overlaying each of these is an additional little stretchy sleeve that'll hold a few sweets.

More zipped stretch pockets down on the hip fins give you obvious places to keep a wooly hat, thin gloves, or a few bars and snacks. Should you want it, the pack will also hold a water bag in a space under the back panel. Inside is a small valuables pocket, with key clip.  

Summary

It does not matter what speed you're aiming to travel: Running-inspired pack designs are rightly becoming more popular for a range of mountain pursuits, and if you're a walker, a scrambler, or an occasional runner looking to benefit from this sort of thing then the Aeon Ultra must be one of the best choices out there. Close-fitting, lightweight, cool and breathable, and sporting a plethora of on-the-go stretch pockets, it offers a number of key advantages over your average pedestrian walker's pack, and would be an excellent alternative to something like the Montane Trailblazer. And let's not forget the weather resistant fabric and build, a big plus for UK use. While it's looking a little pricey for a smallish day pack, the addition of taped seams alone arguably justifies the cost. All told, the Aeon Ultra is a really accomplished example of its type.

Rab Aeon Ultra Pack from Rab on Vimeo.