Our review and lab test 

The Ultraboost 21 is a neutral socklike trainer with roots in high-tech shoe design. To me though it felt heavy, outdated, and a bit sloppy. 

I think Adidas needs to get back to the drawing table and focus on performance over looks to stay relevant in this category. 

Adidas Ultraboost 21 pieces of the shoe

This shoe is recommended for runners who like that sock-like feel and usually train on flat, straight courses.

Don’t buy this shoe if you: 

Too stretchy, too loose

Adidas needs to stiffen this shoe up significantly. The flex that allows it to form around your foot so well, also lacks the proper lockdown runners need. 

They need to add some welded overlay as I felt like I was going to roll out of this shoe around tight corners. You’ll see in the video of me wiggling my toes just how flexible this material is.

Adidas Ultraboost 21: one of the heaviest road shoes! 

Boy oh boy, do they feel heavy. 

I wish the Ultraboost 21 were lighter because I did like the feel of them, the peppy ride, and the form-fitting upper, but I really can’t get past the weight. 

Weight of Adidas Ultraboost 21

At 12.7 ounces, these may be one of the heaviest road shoes currently available. Now if your goal is to do weighted training runs so come race day your light shoes feel like nothing but air, maybe these are what you need. 

Proper sock-like fit

The upper is very accommodating for a wide variety of foot shapes and widths. 

Ultraboost 21 cut in half

Here’s why: 

  • The upper on Ultraboost 21 is made from insanely stretchy Primeknit material that’s only 1.9mm thick and is designed to be more like a sock than a traditional upper.
  • It stretches to form around your foot. I would not say this is a high-volume shoe, but it easily molds to fit your foot in a very comfortable fashion. 
  • The lace is nice and stretchy though with 0.156 stretch ratio, keeping the top of the shoe comfortable as your foot flexes through your stride. 

Ultraboost 21 upper and lacing system

There is one challenge, however: if your socks are not super tight or are too thick, it's very easy for them to bunch up as you don the shoe. 

Fantastic peppy ride

The ride is sweet under foot and the toe-off is fantastic thanks to the LEP Torsion System which the Ultraboost 21 gains as an update. 

It adds rigidity throughout the midfoot and into the front of the shoe which feels nice and springy at the very end of the stride. 

Breathability at its best

The Primeknit is nice and breathable as well, passing our light test with a measurement of a whopping 28.2 LUX, the highest reading we’ve had so far, meaning it stays cool as you push yourself. 

Transparency test on Adidas Ultraboost 21

Heel stays put in Ultraboost 21

There’s also no real heel counter in this shoe, and what does exist is a dated design of a rigid, thick (4mm) external plastic cup that adds lateral stability only. I can proudly say that my heel did not budge, unlike the front of my foot.

The heel itself is very flexible measuring in at only 19.8N in the center, our softest shoe yet. However, thanks to the tight upper and the plastic heel cup, there’s no heel slip to report.

Ultraboost 21 heel design

The heel is topped off with an elf-like tab to help pull the shoe on, I’d prefer a loop but at least there’s something built into the design. 

External lacing system: outdated and too heavy 

I will admit, although I had issues with the overly-flexible upper around the toebox, the lockdown over the mid-foot works. However, it’s accomplished with a sewn-on plastic lacing structure. 

External lacing system on Adidas Ultraboost 21

I hate this design feature, it adds weight, it’s ugly, and it's outdated. 

Weight of the plastic lacing system on Ultraboost 21

I cut it off just to see how much unnecessary weight it adds to the shoe, and 22g (both sides combined) is just too much when a few welded overlays and eyelets would have sufficed. 

Even the laces are heavy

Adidas finishes off the lacing structure with a quality lace, but it’s heavy too. At 38.5’’, it's a pretty short lace but weighs in at 4g which is how much the lace on the Brooks Ricochet 3 weighed but is almost 10’’ longer. 

Ultraboost 21 is grippy and durable

The continental outsole rubber is soft enough (72.8hc on the durometer) to offer a nice grip across a variety of hard surfaces I tested this shoe on like cement and asphalt. 

Outsole design on Adidas Ultraboost 21

It gives the shoe some color and although it’s not technically full-coverage, there’s the very limited exposed midsole, adding some welcomed durability. I don’t see any problems with this shoe lasting 300-500 miles. 

Heel: too steep AND too heavy

A lot of the weight comes from the “junk in the trunk”. These have a big heel, and a lot of heel stack. 

Heel stack and heel drop on Adidas Ultraboost 21

We measured 32.8mm in the heel, giving these a 12mm drop, and although Adidas claims they are a 10mm drop shoe, they feel steep to me and like they have a considerable amount of heft in the rear of the shoe. 

The heel drags, and other than making them easier to slip off, I’m not sure they need this much foam on the back end. 

Conclusion

I find myself wondering if the Ultraboost 21 is really a running shoe. Has Adidas built a fashion shoe with running DNA that’s long since been outdone and one-upped by almost all the other brands out there? Is this just a slightly-more technical version of a Yeezy

To me, the Ultraboost 21 is too heavy, and the upper is too flexible to really be ultra-useful as a real running shoe. Maybe it will work on slow, long runs, but ultimately I will keep my fingers crossed for the 22 and a return to triumph! 

Discussion-worthy 

Yes, it’s true, Ultraboost is a great midsole, and 7 years ago when it first arrived on the scene it was groundbreaking. But, it's heavy. And most other brands have caught up in the last ¾ of a decade and have a product that’s similar in comfort, shock absorption, and responsiveness without the heft. 

Adidas is not the only game in town with high-tech midsoles, hell, the midsole on Saucony’s Endorphin Speed and Endorphin Pro even looks like Ultraboost, but they found a way to do it better and lighter. 

Complete lab-specs overview 

Adidas Ultraboost 21
Weight - Left 355g
Weight - Right 361g
Weight - Insole 15g
Weight - Lace 4g
Length - Overall 303mm
Length - Insole 275mm
Width Midsole - Forefoot  117.2mm
Width Midsole - Heel 92.8mm
Width Midsole - Middle  65.3mm
Width Upper - Forefoot 100.8mm
Width Upper - Heel 91.7mm
Width Upper - Middle 80.2mm
Stack - Forefoot with insole 20.7mm
Stack - Heel with insole 32.8mm
Stack - Forefoot without insole 16.1mm
Stack - Heel without insole 28.3mm
Insole Thickness 4.2mm
Drop 12.1mm
Outsole thickness (Forefoot) 2.6mm
Outsole thickness (Heel) 3.6mm
Lugs Depth NA
Laces (without stretch) 38.5inches
Laces (with stretch) 44.5inches
Lace Stretch Ratio 0.156
Laces - Thickness (Height) 0.8mm
Laces - Width 5.5mm
Room temperature 56.3F
Durometer Outsole Forefoot (Room temperature) 72.2HC
Durometer Outsole Heel (Room temperature) 73.3HC
Outsole % of change from Heel to Forefoot (Room temperature) 1.62%
Durometer Midsole Forefoot (Room temperature) 26.7HA
Durometer Midsole Forefoot 2nd layer (Room temperature) N/A
Durometer Midsole Heel (Room temperature) 19.2HA
Durometer Insole (Room temperature) 28.2HA
Flexibility of the shoe  27.5N
Durometer Outsole Forefoot (Freezer 1 hour) 76.8HC
Durometer Outsole Heel (Freezer 1 hour) 78.8HC
Durometer Midsole Forefoot (Freezer 1 hour) 30.7HA
Durometer Midsole Forefoot 2nd layer (Freezer 1 hour) N/A
Durometer Midsole Heel (Freezer 1 hour) 20.7HA
Durometer Insole (Freezer 1 hour) 30.7HA
Flexibility of the shoe (Freezer 1 hour) 38.0
Durometer Outsole Forefoot (% change with temperature) 6.47
Durometer Outsole Heel (% change with temperature) 7.50
Durometer Midsole Forefoot (% change with temperature) 15
Durometer Midsole Heel (% change with temperature) 7.83
Durometer Insole (% change with temperature) 8.88
Flexibility of the shoe (% change with temperature) 22.54
Thickness - Heel Counter/Insert 4mm
Thickness - Ankle Collar (Front) 3.7mm
Thickness - Ankle Collar (Middle) 9.8mm
Thickness - Ankle Collar (Back) 7.4mm
Thickness - Tongue 1.9mm
Flexibility of the heel counter  19.8N
Light test (transparence) 28.8LUX
Lace slip test with the knot 42.0N
Longitudinal flexibility (0-5) 2
Torsional flexibility (0-5) 4
Heel counter material plastic
Tongue: gusset type One-piece upper without a tonge
Tongue gusset material Knit upper
Laces: profile  Flat
Laces: material braided
Laces: extra hole  no
Laces: are they long enough to use the extra hole  N/A
Heel: pull tab  yes
Insole: removable yes
Control devices:
Multi-density midsole
Rigid heel counter
Elevated medial insole under arch
Supportive tensioned medial upper
Medial flare
Thermoplastic medial post

no
yes
no
yes
yes
no
Longitudinal flexibility (on a 1-6 scale) 2
Torsional flexibility (on a 1-6 scale) 4
How minimalist the shoe is in % 20

Note: all the tests were done on a men's shoe US size 9. 

Expert reviews:

Video reviews

How Ultraboost 21 compares

This shoe: 79
All shoes average: 82
57 93
This shoe: $180
All shoes average: $120
$40 $300
This shoe: 12oz
All shoes average: 9.5oz
3.5oz 15.4oz
Author
Jens Jakob Andersen
Jens Jakob Andersen

Jens Jakob is a fan of short distances with a 5K PR at 15:58 minutes. Based on 35 million race results, he's among the fastest 0.2% runners. Jens Jakob previously owned a running store, when he was also a competitive runner. His work is regularly featured in The New York Times, Washington Post, BBC and the likes as well as peer-reviewed journals. Finally, he has been a guest on +30 podcasts on running.

jens@runrepeat.com