You may not be particularly excited by counting grams, but any backpacker can benefit from stripping out unnecessary weight; and this starts with the rucksack you carry everything else in. Rab's Muon could really help here. Built light, without compromising too far on toughness, comfort or features, this little trekking pack would be ideal for one or two nights camping or bivvying in the hills, or longer backpacking adventures with a bit of a minimalist approach to packing.

There's a lot to like about the Muon, and what we think really makes it stand out is the shoulder harness, which has clearly been influenced - for the better - by running vest design.

The Muon range

There are four models: the Muon 50L and Muon ND50L (a women's version with a slightly narrower fit), and the Muon 40L and Muon ND40L (smaller capacity packs, both minus a lid - £200). If you pack very light and are only out for a night or two the 40 might do, but for me 50 litres is a more useful and versatile size for backpacking trips of various lengths since it allows for that bit more food and gear, and you don't have to completely fill it every time. I also like a lid pocket. With this in mind, we picked the 50L to review.

In use

A number of brands now make walking and climbing packs that borrow some of the characteristics of a running vest, particularly the close minimally-padded fit around the chest, and the addition of shoulder pockets for easy access on the go to snacks, drinks and phones. Rab's Latok mountain pack is a great example, and one that we got on really well with in a recent review:

The Muon shares not only some of the aesthetics of the Latok, but elements of this running style too. As a trekking pack designed to carry significant loads there is of course more padding for comfort, and a light wire frame that gives the necessary support, so it's not the bare-minimum back system of the Latok. Nevertheless, the close yet airy fit around the chest is clearly running vest-inspired; and it works fantastically even on this much bigger pack. So too the shoulder pockets, which can hold a soft flask, hat/gloves, snacks and phone, helping to keep you smooth and efficient on the trail because you don't have to stop to root around every time something small is needed.

Weighing about 1kg for the men's 50L pack (marginally less for the women's version, and 895g for the Muon 40L) this is clearly a lightweight by the standards of backpacking packs, so while it may not quite compete with ultra-minimalist niche specialists in head-to-head weight terms, it's comparable to the lighter options from mainstream brands such as Deuter and Osprey. For this little weight you get a lot of pack with the Muon, which makes few compromises in terms of comfort, support, or useful features. Of overnight rucksacks I've reviewed in the last few years the nearest equivalent in feel would be the Deuter Aircontact Ultra 50:

While it's slightly lighter (and every 200g helps) the Muon is as versatile as the Aircontact Ultra, particularly well suited to a speedy and lightly-laden approach, but also sufficiently user-friendly for your mainstream hiker. If you're bivvying or camping in the UK hills, on a hut tour in the Alps, or planning a multi-day backpacking trip somewhere warm like the Pyrenees in summer, then the Muon would be basically perfect. Were I lucky this year to have time for a big-ish trip like the Cape Wrath Trail, this is the pack I'd pick. Perhaps it's not quite robust or techy enough for regular winter mountain use, and it's not got the capacity for really serious wilderness journeys going many days between resupply points, but other than that the world is pretty much your oyster with a Muon.

Fabric and build

Most of it is made from a high tenacity 100D Nylon, with added oomph from a Spectra ("15 times stronger than steel") ripstop grid. I'd certainly say the Muon seems tough and well-made by the standards of a very light pack, and it should last well. But I'm not sure some of the other fabrics and components are as durable as the Spectra-reinforced bits, since I put a micro-ding in the base of the pack after only a couple of days' use. At 1kg, completely bombproof would be a lot to expect.

Fit and comfort

My sole real criticism is that it comes in only one back size. It's a fixed back system with no adjustability, too, so either it'll fit you or it won't. The back on the men's Muons is 48cm long (43cm for the Muon NDs), and at 183cm tall with a fairly long back I am towards the upper end of wearability. The hip belt sits slightly higher than I find ideal - a matter of a couple of centimetres - though still does a decent job of transferring some of the load off the shoulders, and keeping everything stable. If you're much taller than me, or a lot shorter, then try it to ensure it'll work for you.

The harness is brilliant, with comfy and supportive padding over the top of the shoulders where of course you get the most pressure, tapering to unpadded over the chest to allow a form-hugging fit. In place of the usual sternum clip there's a webbing of drawcords as per Rab running vests, which helps get everything close-fitting and bounce-free, tailoring it all to your body shape by moving the hooks up and down a series of loops. On a running vest these straps tend to be elastic, but given the extra weight in a 40 or 50 litre pack Rab have gone for non-stretchy cord. At first I wondered if this might be a bit unforgiving, and yes they do cut into the pecs if you don't set them at the right height; but properly adjusted I actually find this webbing really comfortable. Out in the wind, with cold hands, the hooks can prove a bit fiddly.

For warm weather comfort there's a lot of open mesh around the hips, shoulders and upper back. Padding in the mid-back is non-breathable, but in use there doesn't seem to be much contact here and as yet I've not got sweaty. I shall have to test it in hot weather and report back.

There's more lumbar padding than may be apparent in the pictures, and while it feels quite unyielding when you first don the pack, you soon get used to it and, like me, will probably appreciate the support and comfort it offers over a long day. Hip belts can be behemoths on trekking packs, so this one counts as minimal, which I think is spot on for a 50 litre rucksack. The belt adjusts smoothly, but there are no loops to secure the spare webbing tails, which get flappy in the wind.

Overall the Muon 50 is a comfortable, stable and well-balanced load carrier, with enough support for the sort of weight you can expect to fit into it; in testing I've been very happy with 8-10kg and I'm sure the back system could handle a bit more. While I doubt you'd want to run with 50 litres of gear I actually think you more or less could, courtesy of that body-hugging harness.

Features

As a top-loader with a lid the Muon 50 offers the best of both worlds, since the lid can be removed if you want to go lighter and more streamlined. I personally like a lid pocket on a pack, particularly bigger models in which small loose items are otherwise a pain to locate. The zipped top pocket has plenty of space for hat, thick gloves, map and sunglasses. You also get an under-lid pocket with a key clip, a nice touch being that this is in transparent mesh so you can see what you're looking for. Worth bearing in mind in the rain is that the top zip isn't water resistant. Another very mild disappointment is that the lid can't be easily converted into a bumbag. If you're on a summit detour and have stashed your main pack it would be nice to have an integrated mini-bag option; I do appreciate that comparatively few designs offer this.

In the interests of keeping things simple there's a single main compartment, and no side or bottom entry zips. Being fairly sceptical of side zips anyway I absolutely don't miss them, though perhaps you may. But the thing is, there's so much potential external storage on the Muon that anything you might want to keep quickly to hand can probably find a place outside. Rab have included three huge stretchy outside pockets, which between them can take any number of spare layers, water bottles, maps and such, and these render its stated 50 litre capacity fairly nominal since they can hold more still.

A great addition for convenience are the running-inspired stretchy zipped pockets on the shoulder straps; every bag should have them, but few still do. I've been using them for light gloves, jelly babies and a phone. Very handily they'll take a small soft flask, though 500ml is a squeeze and unfortunately they don't fit the bottles used in Rab's own Veil running vests. I wonder if these pockets could be made slightly larger in future, perhaps with one as an open sleeve for a 500ml bottle and the other with a zip? Naturally the pack also takes a water bladder if you prefer.

Outside storage doesn't end there. You also get daisychains for clipping stuff on, and elastic retainers at various points for stowing poles; it's possible to attach them between the shoulder and waist belt for quick access when you want to be hands-free on the trail (a scrambling section, perhaps) or keep them more out of the way (and less liable to spear you if you fall) against the pack itself. While you can attach axes it feels a bit ad-hoc, another reason why the Muon is probably best regarded as primarily a three-season option.

The thin webbing compression straps work well to squish things down if you're not packed to the brim, and double as places to hold things (camera tripod etc). An additional set of compression cords runs across the bottom of the pack, and these offer plenty of length should you want to carry a bulky rollmat cross-ways; they're nifty, but I'm not sure how secure they'd be for something heavier like a tent.

Ethics and environment

Rab's clothing and equipment is covered by the 'Rab Promise': if an item fails they will repair or replace it. Damage due to wear and tear or improper care can also be repaired for a fee.

Verdict

Among lightweight midsized backpacking packs, the Muon really stands out for its functional design and its close and comfortable fit. Bring on summer bivvy trips!