If you're in the market for a reasonably light (8.4oz/240g) road shoe that's somewhat firm and fits a healthy human foot, then the Duo is a good option. The Duo is not without its flaws, the main one being a tendency to pick up small stones under the sole. The other issue is that the sole seems to wear faster than you might expect. This is not as bad as early versions of The One, where the sole used to abrade like it was made of milk chocolate, but it's a little more than I would like. I'm only at about 120 miles, and there's already slight signs of wear on the outsole. On a more positive note however, the Duo is shaped like a healthy human foot, so it won't induce bunions, nor require you to have bunions to fit correctly. The Duo even has a little bit more height in the toe box than many Altra shoes tend to have. It's a little firmer than I would like for this weight, but that does confer some degree of protection without much cushioning, a plus point if you like a firmer running shoe. Like all Altra shoes, it's close to zero drop, with nothing to interfere with your natural biomechanics. I'm rating the Duo as "Recommended." (I use The Science of Running Shoes as the basis of how I test running shoes and what you should look for in a running shoe.)

Below are some pictures of the Duo after it's picked up some small stones. The first 2 pictures are from a run along and asphalt Greenway, that has just a few patches of loose stones. The later pictures with the darker sole are after a 43-mile run along a gravel trail (the New River State Park in Virginia.) You can see the Duo was crammed full of small stones, so you're probably wondering why I wore the Duo on a gravel trail. Let me talk you through my reasoning. I didn't want to wear a thin road shoe, like my favorite Altra Solstice as it wouldn't provide enough of protection from the stones, but I didn't want to wear a full trail shoe like the Altra Timp as I didn't need that much protection, nor did I want that much weight. The Duo is firm enough to provide the protection I needed, and the raised patches provide good grip on the patches of mud and other slick surfaces. The Duo did accumulate stones very rapidly, but not enough to significantly increase the weight of the shoe or compromise the cushioning. (The stones added about 0.4oz in case you're wondering, like I was.) While I found I can cut open the toe box of a shoe and run on gravel surfaces without getting any stones into the holes, I do prefer leaving the toe box intact if I have the option. That really limits me to an Altra, or a tiny subset of other shoes that are designed to fit a healthy human foot. And I found the Duo to be quite comfortable for a 40+ mile run.

1 Characteristics

Cushioning . The Duo has reasonable, but not great cushioning for its weight. It feels a little firm rather than hard, but my perception while running is that it's a little firmer than the numbers would suggest. This is probably because the Duo has a number of raised rings or patches of cushioning, creating an eggshell effect. Because the pressure is concentrated on these raised areas, they tend to compress a little more than the rest of the midsole. This creates a slightly softer, and more cushioned sole than you'd get from a more traditional, flat sole. I never felt that there was any unevenness from these raised areas, and the camp and you owe is a smooth and neutral underfoot. I would prefer that issue waiting this much provided more cushioning, or weigh far less, but the problem is far from egregious. However, Altra's claim of "maximum cushion and minimal weight" is somewhat laughable. It's a very long way from either maximum cushioning or minimal weight, and Altra's claim that "the cushion-to-weight ratio on this shoe is hard to beat" seems disingenuous at best. Every Altra road shoe I've measured has a better cushion-to-weight ratio! (The good news is that at least they're aware of this ratio.)
Drop. The Duo is effectively zero drop (not all Altra shoes are zero drop, regardless of what they claim.)
Structure. The Duo uses just one density of foam and doesn't attempt to manipulate or interfere with your natural biomechanics.
Flexibility. While the Duo is moderately flexible and quite springy, though I doubt if there's any performance benefit to be had.
Outsole. The outsole is the Duo's weak point. The raised areas are naturally prone to rapid wear, and the gaps between the raised areas are the worst stone magnet I've ever come across. And once they are full of tiny stones, the effective cushioning of the shoe is dramatically reduced. Even running on what I think of as a fairly clean asphalt surface, the camp and you owe manages to find a few stones to pick up. In the photos above, you can see the harder rubber outsole as its colored yellow, but even after just 100 miles or so, the wear was noticeable, and I'm typically pretty gentle on my shoes.
Shape. Altra shoes are some of the few that attempt to actually fit the human foot. The Duo does a great job of fitting the outline of the human foot, and like some other Altra shoes there's a little headroom for your toes. In fact, there's enough headroom that I didn't feel the urge to cut open the toe box.

.
Upper. The upper is unpadded except for a tiny bit around the ankle opening. The upper is inflexible, which I prefer over stretchy uppers. For some reason, Altra claim the Duo is for "Hot Weather Endurance Racing," and while it's reasonably breathable, there's nothing particularly noteworthy.
Tongue. The Duo has a traditional tongue that is has a tab to attach it to the laces. There's a bit of padding down the middle of the tongue, which might help a little bit with the pressure on the top of the foot.
Lacing. The thin flat laces never came untied, something that surprised me a little given the problems I've had with the Altra Solstice, an otherwise superior shoe.
Heel Counter. The Duo has no noticeable heel counter.

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2 Update After 210 Miles

I've come to quite enjoy running in the Duo, far more than I expected, and it's one of my preferred shoes for speed work.

As is usual with running shoes, the Duo has softened a little over the miles as the foam becomes more compliant. I rather like this, but it's still a reasonably firm shoe. There's a hint of non-linearity in the cushioning I've noticed; the initial contact is relatively soft, and then as you land the cushioning firms up. Part of this is due to the "egg crate effect" of the sole and part due to the rather soft insole, but it seems to have become more noticeable over time.
As you'd expect, the outsole has worn quite a bit as the contact patch is much smaller than you'd get on a shoe with a flat sole. I'm generally quite gentle on my shoes, and I've got quite a bit of wear on the outsole after just 210 miles.
The more rapid wear on the outsole exacerbates the compression of the midsole foam, making the forefoot uneven and twisting the foot towards the midline slightly. At this mileage, the effect is not a problem, and I'd estimate the area under the ball of the foot is only 2-3mm lower. This is partly because the insole has only compressed a tiny fraction. Most insoles are as durable as milk chocolate on a hot day, so this is a noteworthy difference.
The gaps in the outsole continue to be a magnet for small stones, and prying them out is quite tedious, requiring a tool such as a flat head screwdriver.
You'll notice on the pictures that there's quite a bit of wear on the outside rear of the heel, which occurs on only a subset of shoes I run in. I don't generally heel-strike, and my current guess is that the heel is scraping slightly before landing. The Duo doesn't grip my heel in the way most shoes do, so it's possible it's dropping away from my foot slightly.
The Duo has fairly good grip on wet surfaces, especially if there's a little mud on the ground. This is much better than I've found in a flat soled shoe like the Altra Solstice.

3 A Comparison with other Recommended Shoes

If you're looking for "the best of the best" running shoe, here are my top picks. Of course, the answer will depend a little on what you're looking for, so I have recommendations for various categories.

Best All Round Shoe. The Altra Escalante is my current all-round favorite. It has plenty of cushioning for its weight, it has a very springy midsole, it lasts well, and it has a shape that's closer to the shape of a human foot than most shoes. It's a great shoe for any runner, including those Starting to run. It's also a fairly easy shoe to find due to its popularity.
Best Maximalist Shoe: If you want something massively cushioned, then I'd recommend the Saucony Kinvara 8. It's remarkably light and remarkably cushioned, beating Hoka at their own game.
Best Optimal Shoe: For those looking to trade cushioning for speed, the Nike Zoom Streak LT 3 was my top pick, but the latest version (LT 4) falls short of it's predecessor. There are lots of great optimal running shoes, which provide just enough cushioning with light weight and minimal frills, but all have their weak spots. Probably the best option at the moment is the Altra Vanish-R, which offers great cushioning for just over 5oz/150g.
Fastest Shoe: If you really want speed, then check out the Nike Vaporfly 4%. It's light, massively cushioned, and has a carbon fiber plate. Nothing comes close, not even the now defunct New Balance RC5000‏‎ or New Balance RC5000v2‏‎. There are a number of caveats; it's really expensive, it's really hard to find, and there's a significant injury risk.
Best Minimalist Shoe: Merrell Trail Glove. I recommend the trail glove for road running in spite of the 'trail' moniker. It's not a fast shoe by any means, but it's comfortable and will last well.
All Terrain Shoe. For trail running, I've become a fan of Altra, and I think their best shoe is the the Altra Timp, though the Altra Lone Peak is really close.
Honorable Mention: It's not really a running shoe, but the Vivobarefoot Ra is comfortable, minimalist and can more or less pass as a dress shoe. I've worn mine to weddings with a suit and they've not looked out of place. You can run in the Ra, but the leather means it doesn't breathe well.

For a more detailed on running shoes see the Recommendations for Best Running Shoes. This table lists the key attributes of What to Look for in Running Shoes. For more detailed information, on the shoes see detailed shoe comparison.

Full Review	Brand	Name	Rating	Recommended price	Benefit	Weight (oz)	Penalty (sec/mile)	Forefoot Thickness	Heel Thickness	Loaded Drop mm	Cushioning	Flexibility
Saucony Type A Review	Saucony	A/A7	Recommended	$100	6.4	6.7	12.1	19	22	1	4.3	7
Saucony Type A6 Review	Saucony	A6	Highly Recommended	$100	8.2	6.1	9.5	17	21	4	5.0	7
Saucony Type A8 Review	Saucony	A8	Highly Recommended	$90	7.5	6.2	9.7	19	20	-1	4.7	7
Adidas Adios Boost 2 Review	Adidas	Adios	Worth considering	$140	4.7	8.6	13.4	17	27	11	4.0	6
Hoka Bondi 5 Review	Hoka	Bondi	Recommended	$150	6.1	11.6	18.1	38	42	5	7.1	2
Hoka Clayton 2 Review	Hoka	Clayton2	Highly Recommended	$150	9.1	8.3	12.9	23	28	1	7.5	5
Hoka Clifton 4 Review	Hoka	Clifton4	Worth considering	$130	7.7	9.3	14.5	30	35	10	7.2	3
ON Cloudflash Review	On Cloud	Cloudflash	Worth considering	$180	4.4	8.3	14.6	19	26	7	3.7	6
On Cloudracer Review	On Cloud	Cloudracer	Worth considering	$130	5.7	8.2	12.8	19	27	5	4.7	7
ON Cloud X Review	On Cloud	CloudX	Not recommended	$140	4.5	8.3	16.3	21	27	7	3.8	7
Mizuno Wave Cruise Review	Mizuno	Cruise	Worth considering	$120	6.6	5.9	12.5	17	20	7	3.9	6
Newton Distance IV Review	Newton	Distance	Worth considering	$155	7.5	9.1	14.2	26	31	3	6.8	5
Asics Gel DS Racer 10 Review	Asics	DS Racer	Worth considering	$110	8.2	7.0	10.9	21	26	6	5.8	5
Altra Duo Review	Altra	Duo	Recommended	$130	6.0	8.4	13.1	29	29	1	5.0	5
Mizuno Wave Ekiden 8 Review	Mizuno	Ekiden	Worth considering	$115	5.7	5.7	14.6	13	18	6	3.2	8
Saucony Endorphin 2 Review	Saucony	Endorphin 2	Worth considering	$115	8.0	5.1	9.6	15	13	-1	4.1	8
Adidas Energy Boost Review	Adidas	Energy	Worth considering	$160	7.2	10.0	15.6	20	30	7	7.2	5
Altra Escalante Review	Altra	Escalante	Best of the Best	$130	9.1	8.7	13.5	28	25	-1	7.9	6
Altra Escalante 1.5 Review	Altra	Escalante 1.5	Best of the Best	$130	8.2	9.0	14.0	26	27	-1	7.4	7
Saucony Fastwitch Review	Saucony	Fastwitch	Highly Recommended	$90	9.5	7.1	11.1	20	22	4	6.8	7
Topo Fli-Lyte 2 Review	Topo	Fli-Lyte2	Highly Recommended	$100	6.7	9.1	14.2	24	26	3	6.1	5
Reebok Floatride Run Review	Reebok	Floatride	Worth considering	$150	7.7	9.6	14.9	28	34	10	7.4	5
Saucony Freedom Review	Saucony	Freedom	Recommended	$160	5.4	10.7	16.6	25	29	3	5.8	6
Skechers GORun 4 Review	Skechers	GORun	Not recommended	$100	6.1	7.5	11.7	15	23	3	4.5	7
Skechers GOrun Ultra 2 Review	Skechers	GRU	Worth considering	$90	7.5	10.0	15.6	28	34	8	7.5	4
Asics GT 2000 Review	Asics	GT 2000	Not recommended	$120	4.8	11.2	17.4	28	35	5	5.4	2
New Balance Hanzo S Review	New Balance	Hanzo	Worth considering	$110	7.6	6.9	10.7	21	19	2	5.2	5
Hoka Hupana Review	Hoka	Hupana	Recommended	$115	6.1	8.9	13.9	31	36	7	5.4	4
Asics Gel Hyper Speed 7 Review	Asics	Hyper Speed	Highly Recommended	$75	10.9	6.3	9.8	22	26	5	6.8	6
Altra Instinct 4.0 Review	Altra	Instinct 4.0	Worth considering	$120	6.0	9.8	15.3	29	25	-1	5.9	5
Asics Tarther Kainos 3 Review	Asics	Kainos	Worth considering	$130	10.0	6.9	10.7	17	27	9	6.8	6
Altra King MT 1.5 Review	Altra	King MT	Recommended	$140	7.5	10.7	16.7	19	21	3	8.0	5
Saucony Kinvara 8 Review	Saucony	Kinvara 8	Best of the Best	$110	9.3	8.6	13.4	26	31	3	8.0	5
Altra Lone Peak 3.5 Review	Altra	Lone Peak	Highly Recommended	$120	4.2	11.4	17.7	24	25	4	4.8	5
Nike Zoom Streak LT 3 Review	Nike	LT3	Best of the Best	$80	9.8	5.4	8.4	16	21	5	5.3	7
Nike Zoom Streak LT 4 Review	Nike	LT4	Recommended	$90	8.5	5.5	8.5	15	21	3	4.6	6
Nike LunarSpider R5 Review	Nike	LunarSpider	Recommended	$125	6.9	6.7	10.4	17	21	3	4.6	6
Hoka Mafate Speed Review	Hoka	Mafate	Best of the Best	$170	7.6	11.9	18.5	39	40	4	9.0	1
Pearl Izumi EM Road N0 v2 Review	Pearl	N0	Highly Recommended	$100	7.9	6.5	10.1	14	20	4	5.2	8
Saucony Nomad Review	Saucony	Nomad	Worth considering	$110	4.3	10.5	17.2	25	27	2	4.5	4
Hoka Odyssey Review	Hoka	Odyssey	Highly Recommended	$130	8.5	9.4	14.6	37	45	5	8.0	3
Altra Olympus 2.5 Review	Altra	Olympus	Highly Recommended	$150	4.6	11.8	18.4	35	34	0	5.4	6
Altra One 3.0 Review	Altra	One	Recommended	$100	6.1	8.8	13.7	23	23	0	5.4	6
Altra Paradigm 4.0 Review	Altra	Paradigm	Highly Recommended	$150	5.3	11.0	17.1	28	30	5	5.8	8
Asics Piranha SP 5 Review	Asics	Piranha	Recommended	$100	10.1	4.2	6.5	11	15	3	4.2	9
Brooks PureFlow 5 Review	Brooks	PureFlow	Worth considering	$110	6.0	9.7	15.1	26	29	5	5.8	8
Salming Race Review	Salming	Race	Worth considering	$130	6.9	6.5	10.1	16	19	4	4.5	6
Altra Escalante Racer Review	Altra	Racer	Highly Recommended	$140	6.6	7.6	11.8	24	26	6	5.0	8
New Balance RC1600 v2 Review	New Balance	RC1600	Highly Recommended	$110	8.8	5.6	8.7	15	21	5	4.9	8
New Balance RC5000v2 Review	New Balance	RC5000v2	Best of the Best	$125	14.2	4.0	6.2	13	21	6	5.7	7
Skechers GoRun Ride 3 Review	Skechers	Ride	Worth considering	$85	5.9	8.5	13.2	18	28	6	5.0	8
Nike RN Distance 2 Review	Nike	RNDist2	Worth considering	$120	8.0	9.2	14.3	25	28	4	7.4	7
Inov-8 RoadXTreme 220 Review	Inov-8	RXT-220	Worth considering	$120	5.2	8.0	18.2	14	17	3	4.2	8
Salomon Sense Ride Review	Salomon	Sense	Worth considering	$120	8.5	10.0	15.6	26	29	6	8.5	0
Altra Solstice Review	Altra	Solstice	Highly Recommended	$90	7.1	8.1	12.5	23	25	0	5.7	5
Topo ST-2 Review	Topo	ST-2	Highly Recommended	$90	8.2	7.3	11.4	20	18	0	6.0	7
Hoka Stinson Lite Review	Hoka	Stinson	Highly Recommended	$160	7.3	11.6	18.1	35	40	6	8.5	0
Altra Superior	Altra	Superior	Highly Recommended	$110	5.6	9.9	15.5	21	25	-1	5.5	4
Adidas Takumi Sen 3 Review	Adidas	Takumi Sen 3	Highly Recommended	$160	7.7	6.6	10.2	17	21	4	5.1	5
Topo Terraventure	Topo	Terraventure	Review Pending	$110	2.6	10.8	27.5	24	24	0	2.8	10
Altra Timp Review	Altra	Timp	Best of the Best	$130	3.7	11.6	18.1	30	31	-1	4.3	6
Altra Torin 2.0 Review	Altra	Torin	Worth considering	$125	3.4	9.6	20.7	30	25	1	3.3	9
Altra Torin 3.5 Review	Altra	Torin 3.5	Worth considering	$125	5.8	9.3	14.5	27	28	0	5.4	8
Hoka Tracer Review	Hoka	Tracer	Recommended	$130	7.2	7.4	11.5	21	24	2	5.3	5
Merrell Trail Glove 4 Review	Merrell	Trail Glove 4	Best of the Best	$100	3.4	8.4	23.8	13	13	0	2.8	10
Topo Tribute Review	Topo	Tribute	Recommended	$100	5.9	7.3	11.4	20	18	-1	4.3	6
Mizuno Wave Universe 5 Review	Mizuno	Universe	Highly Recommended	$125	10.7	3.1	10.6	9	12	1	3.3	9
Altra Vanish-R Review	Altra	Vanish-R	Recommended	$100	16.1	5.3	8.2	19	21	4	8.5	6
Merrell Vapor Glove 3 Review	Merrell	Vapor Glove 3	Highly Recommended	$85	2.1	6.1	27.6	6	5	0	1.3	10
Nike Vaporfly 4% Review	Nike	Vaporfly	Best of the Best	$250	10.4	7.2	11.2	25	37	8	7.5	2
New Balance Vazee Pace Review	New Balance	Vazee Pace	Worth considering	$110	6.0	8.6	13.4	18	24	6	5.2	5
Asics TartherZeal 3 Review	Asics	Zeal	Worth considering	$140	10.9	6.3	9.8	17	27	9	6.8	6
Saucony Zealot 3 Review	Saucony	Zealot3	Recommended	$130	8.9	9.5	14.8	29	32	4	8.5	6
Nike Zoom Fly Review	Nike	Zoom Fly	Worth considering	$150	8.3	8.7	13.5	29	36	8	7.2	3

It's not a running shoe, but I love the Hoka Tor Ultra hiking boot and I've tested the Altra Lone Peak Boot, the Hoka Tor Speed 2, and the Inov-8 Roclite 325 hiking boots
Older shoe reviews: Saucony Hattori Review, Mizuno Cursoris Review, Skechers GO Bionic 2 Review, Hoka Clifton Review, Saucony Virrata 2 Review, Brooks PureCadence 3 Review, Brooks PureConnect 3 Review, Brooks PureFlow 3 Review, Skechers GO Bionic 2 Review, New Balance 980 Review, Brooks Transcend 2 Review, Hoka Huaka Review, Patagonia EVERlong Review, Asics 33-DFA Review, Hoka Conquest Review, Saucony Cortana Review, Puma Faas 100 R Review, Saucony Fastwitch Review, Nike Free Review, Asics Gel Lyte 33 Review, Skechers GOmeb Speed Review, Skechers GOrun Ultra Road Review, Nike LunaRacer Review, Altra Paradigm Review.

Altra Duo Review

1 Characteristics

2 Update After 210 Miles

3 A Comparison with other Recommended Shoes

Fellrnr.com, Running tips