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Adidas Adizero Prime X 2 Strung: Stable (!?) 50mm (!!) Training
By Matthew Klein

The Prime X series debuted in the words of Adidas as an "Illegal" racing and training shoe. With a 50mm stack height and multiple stiffening agents, the company has been open about intentionally and blatantly violating World Athletics rules to push boundaries. The first two versions were similar, outside of upper differences as relatively light shoes for how much midsole they had. They had incredibly bouncy midsoles that felt like higher and less stable versions of the Adios Pro 2. Unstable is an understatement. Unless you had highly stable and efficient mechanics, they were difficult to run in, at least for those with any form of stability needs. In fact, they were borderline dangerous during sharp turns. The trade off is that they were still incredible performance shoes that many people used to race because they were still light enough to move fairly fast. The new Prime X 2 Strung has completely changed that, not only because it looks different but it functions completely differently. A far more stable shoe that will be usable by more people for more types of training, but does lose the speedy edge the original had. Whether this is a good or a bad thing will depend on you.

Adidas Adizero Prime X Strung 2
Price: $300 at Running Warehouse
Weight: 10.4 oz, 295 g (men's size 8.5), 9.9 oz, 281g (women's size 8)                      Team Measured Weights: 10.7 oz (men's size 9), 11.5 oz (men's size 10)
Stack Height: 50 mm / 43.5 mm
Drop: 6.5 mm
Classification: Super Trainer





RUNNING SHOE SUMMARY

The Adidas Adizero Prime X 2 Strung is a super stack height maximal training shoe for daily training, long runs and uptempo efforts. A brand new, slightly more flexible Strung upper finally creates a sock-like fit that is true to size. A brand new midsole design makes the shoe far more stable, with two carbon plates instead of rods and three layers of Lightstrike Pro (with an interesting, mysterious, super soft middle layer). A dramatic increase in weight is the trade-off for these clear improvements, moving the Prime X series away from being a super racer to a true super trainer. Those wanting a solid long-run shoe that does an excellent job performing at easy and marathon-pace runs and has more stack height underfoot than any other shoe may be interested in this shoe. 

SIMILAR SHOES: New Balance FuelCell SC Elite v3
PAST SHOE: Adidas Prime X Strung

VIDEO



Check out Matt's video review from our latest podcast. You can also listen on the run here.

FIT


The Adidas Adizero Prime X 2 Strung fits me true to size in my normal Men's US size 10. The upper is a lighter, more flexible strung material than the previous versions with a knit tongue integrated into it. The volume is low and fits sock-like, especially in the forefoot. I initially thought these were short putting them on, but then walking and running felt true to size and the upper disappeared on my feet. The forefoot is normal width-to-slightly wide thanks to mild stretch. This is not a shoe for wide feet as anyone with wider or above normal volume will struggle to get this shoe on; the midfoot fits snug. The tongue is part of the upper, so I had no issues with slippage. It did roll in on itself every time I put it on, so I had to make sure it sat correctly before running. The heel is flexible, normal width with no heel counter. I did experience heel slippage due to the lack of major reinforcements. Fortunately, there are additional lace holes. Once I lace locked the upper, I had no further slippage, the sock-like upper fit my foot super well and I forgot about it. There is some mild additional structure in the midfoot and surprisingly I did not have major translation side to side with turns. The Prime X 2 Strung absolutely must be worn with socks despite my comments on the "sock-like' nature of the upper. The strung is quite scratchy against bare skin and I only made it a few steps before realizing it would not work sockless. So as long as you use socks, you will have a lower volume upper that disappears on the run as long as you lace lock the heel.  




PERFORMANCE

The Adidas Adizero Prime X 2 Strung is a super maximal daily training, long run and uptempo shoe. The first thing I noticed was the dramatic increase in weight. This makes the shoe feel less bouncy than the previous versions and far more grounded (as much as a 50mm stack height shoe can feel grounded). My size 10 pair comes in at 11.5 oz, up from my Prime X Strung which weighs 9.8 oz. There is still plenty of bounce to this shoe with this much stack height of Lightstrike Pro, it just feels less crazy than prior versions. This does change as you put more force into the ground, which is why uptempo paces feel great. There is a 6.5mm drop thanks to an increase in stack height in the forefoot. It feels exactly like this, although the massive rockers at the heel and forefoot took away my ability to notice it feeling a little lower. The heel transition is extremely smooth with some lateral bias (see stability section) similar to the Adios Pro 2. The midfoot transitions well into a stiff but easy transition forefoot. The roll was not as aggressive as I expected and worked really well at both easy and uptempo paces. With two carbon plate and 43.5 mm of stack height, the forefoot has zero flex. However, the rocker rolls you forward nicely and those with any toe mobility issues will do well here (because the shoe will not flex).

Due to the higher weight and more sturdy sole, the Prime X 2 Strung is a solid super trainer. I have used it for several easy runs and a long run. I found it does well at moderate paces best but thanks to the weight felt natural at easier efforts. Even recovery runs felt decent. When I picked up the pace, I had varied responses. Uptempo and marathon-paced efforts feel awesome in this shoe. Once you get into a longer effort rhythm, the shoe moves with you and your pace will surprise you. However, trying to push the shoe any faster than that was a struggle. The weight holds it back from doing anything into the faster tempo or interval range.

For me, this shoe is best for daily training, long runs, marathon pace long runs, uptempo runs and road marathons/ultramarathons. It is not meant, or at least does not function well, at faster paces, be it from half marathon pace and downward. I would not race a marathon in this shoe and would pick the Adios Pro 3 over it. However, I would easily pick the Prime X 2 Strung for training hands down over that or any Adidas daily trainer at this time.

Durability and surface-wise the Prime X 2 Strung does well on road only. The tall stack height and smooth outsole grips well on the road but no other surface. This absolutely shoe not be taken on unstable trails. I accidentally went on smooth trail and was fine but would stick to road. Durability has been excellent as I have 40 miles on my pair with no dent or wear on the outsole. If you keep to the road, I would expect this shoe to last you a long time, as hopefully a $300 shoe should. I will get >100 miles on my pair and report back on this. 





STABILITY

The Adidas Adizero Prime X 2 Strung is supposed to be a neutral shoe, but due to several elements is actually at least a mild stability shoe with lateral bias. The heel design is the most obvious point with a posterior lateral bevel that is so large it creates a medial wedge. There are also small sidewalls on the medial and lateral sides but the lateral bias, similar to the Adios Pro 2, is apparent if you are a really posterior landing heel striker. The sole of the shoe is wide. Even the midfoot, which narrows slightly compared to the wider heel, is certainly wider than the last version. There is also an extra strip on the medial side of the outsole that does raise the medial heel and medial posterior midfoot higher than the lateral side. This creates subtle but slightly medial resist throughout this transition. It also offsets the small gap between the outsole of part of the medial midfoot, which is still filled in (compared to the older versions where there was no medial aspect to the midfoot).

The majority of the forefoot is the most centered part of this shoe. It is super wide with a large amount of midsole flaring on both the medial and lateral side. The forefoot rocker is biased slightly lateral, adding some lateral guidance at the forefoot for those who need it. For these reasons, the Prime X 2 Strung may feel like a stable/mild stability shoe for those with medial stability needs especially in the heel and forefoot and may feel unstable for those with lateral stability needs



Thoughts as a DPT: Surprising Stability/Bias/Wedge in a Maximal Stack Height Shoe
By Matthew Klein

The Prime X and Prime X Strung were interesting but terrifying shoes. The narrowness of their soles compared to their stack heights made it difficult for anyone but those with stiff neutral mechanics to run in. There were even reports of people suffering terrible ankle injuries due to fast turns or rolling their ankle (with resulting ligament tears and fractures). This is the inherent challenge of super max stack height shoes. If you want to keep weight down, you either have to reduce stability by keeping the sole narrower while raising the stack height. Or you have to use a super low-density and light foam. Even then, the amount of material will increase weight. 

When I saw the changes to the Prime X 2 Strung, I was excited but also scared based on my experiences with the previous versions. As amazing as they were for some people, I found myself more sore after using them due to how hard my muscles had to work to stabilize myself on the platforms. When I kept a faster pace they were incredibly, but slowing down at all was problematic.

The changes to the Prime X 2 do make it more stable, but only for certain people. There are so many elements in the heel especially that create a wedge. A wedge is different from a post in that it actually sets the foot in a different position. A post is merely a firmer part of the midsole that is supposed to resist motion as the foot moves through that section and may potentially move more through the softer (relative) section (path of least resistance). A wedge will keep the foot in an inverted or everted position. Some people do need this and it dramatically increases stability in one direction without the hard pressure posted shoes have. This can work well for those with positional issues or needing a bit more aggressive stability. However, it can be problematic for those who are sensitive to going in the opposite direction. The medial side of the outsole is also built up and it is clear the shoe tilts lateral. Those with medial stability needs are going to describe this shoe as stable, enhanced by the taller stack height the shoe and cushioning that will cause them to move more in the direction the shoe is pushing them. Those with lateral stability needs are going to describe this shoe as unstable for the same reasons.




RECOMMENDATIONS

The Prime X 2 Strung is a major step forward for those who wanted a more stable ride but a major step backward for those who liked amazing height-to-weight ratio of the previous versions. Version 2 is completely different from the prior versions, enough that I would almost recommend giving this shoe a different name like the Prime X Trainer. It is an excellent daily trainer, long run and uptempo shoe but not something I would ever consider racing. It is purely meant for training purposes, rather than its predecessors that while illegal for major competition, were still racers at heart. My first mild suggestion would be to secure the heel a little more. I would go back to version 1 and 2 that both had a little more structure in the heel while keeping a heel counter.

My major suggestion is to expand out whatever that middle layer of Lightstrike Pro foam is. This version lost some of the bounce from the previous versions. I would love to see this newer foam as the three primary layers of the midsole. This clearly lower density and maybe more resilient foam could be the key to dropping the weight while increasing this bounce. This may make the shoe more unstable, but the wider sole and elements added will still keep it stable, at least for those with medial stability needs. There is a whole other discussion on the fact this shoe MAY need to be more stable neutral rather than medially biased, but I don't want them to change it because selfishly it is working so well for my mechanics. So in the interest of the public, I might suggest centering the forefoot rocker a little to be more stable neutral, but do like how the heel and midfoot are set up.

WHO THIS SHOE IS FOR

The Adidas Adizero Prime X 2 Strung is for those who want a super maximal daily training shoe with a sock-like upper, a ton of Lightstrike Pro underfoot, double carbon plates for stiffness and a mildly lateral biased ride for daily training, long runs and uptempo workouts. The Prime X 2 is now far more safe than the previous versions, but loses its edge as a racing shoe. The weight increase is noticeable although the improvement in stability and upper sizing are good. The cost at $300 is a little less justified now given the similarity between the performance of this shoe and something like the New Balance Fuelcell SC Elite v1, which came in at $180, had a similar weight and functioned similarly as a daily super trainer.

My gut says that Adidas may be on the verge of really taking Lightstrike Pro to the next level, but hesitated to use more of its evolution in this model. I am excited to see where they take this foam as changes are clearly happening. I think if they use more of this and give this shoe the same bounce as the originals while keeping the weight down, that price tag may be more justified.

I am happy that the clear stability lessons have been learned from the previous as this shoe works so well for someone like me. However, as much as I hesitate to say this, it may still need to be more centered for those with lateral stability needs. You cannot please everyone, so to have such a tall shoe that will work for those with neutral and medial stability needs as a daily trainer is great, albeit pricey. Just know it isn't the same shoe and it is certainly not the same racer.


GRADING

Matt
Fit: A-/B+ (Truly socklike upper that fits true to size. Does need to be laced locked at the heel for security)
Performance: B+/
A- (Excellent shoe for daily training, long runs and uptempo/marathon-paced runs. Struggles at faster efforts but not sure this is meant for that)
Stability: B-/A- [Mild Medial Stability Shoe Unintentionally?] (Excellent shoe for those that need medial stability almost along the entire length of the shoe. Those with lateral stability needs may struggle at the heel [which is why the extreme in grades). Stable for this incredible stack height)
DPT/Footwear Science: A- (Excellent stabilization of this model compared to previous. Unsure if stability measures were intentional. Shoe redesign did come at a cost to the higher level performance of this shoe and relegates it to an expensive trainer)
Personal: A- (I really like this shoe. It doesn't feel super weird for training and is excellent for long runs and tempos. I personally wouldn't use a shoe this size for speed work and didn't use the last version for that. So this is a major improvement, just wish it was lighter)
Overall: A- 

SHOP | SUPPORT DOR

Adidas Adizero Prime X 2 Strung
Price: $300 at Running Warehouse

Shop Men | Shop Women


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FURTHER READING: RACING SHOES FOR 2023

Adidas Takumi Sen 9  - An upper update drops the weight down to a swift 6.4 oz for race day
Asics Metaspeed LD [Track Spike] - Flytefoam Turbo and a carbon plate power this super spike
Brooks Hyperion Elite 3 - We look at Brooks premier racing shoe
Hoka Cielo Road - Hoka's new 5k/10k PLATELESS racing flat
Hoka Rocket X 2  - Hoka finally has its first real super shoe with this PEBA-based racer
New Balance FuelCell SuperComp Elite v3 - A full redesign of their super shoe offers the best ride yet, but with one issue
New Balance SuperComp Trail - A cushioned, rockered ride for all-terrain
Nike Vaporfly Next% 3 - A better fit and new geometry make the Vaporfly 3 the best yet
On Cloudboom Echo 3 - On Running's first real super shoe hits the market
Puma Deviate Nitro Elite 2 - One of our favorite racers sees its upgrade, but is it better?
Puma Fast-FWD Nitro Elite - Puma's highly aggressive, uniquely design 5k/10k racer
Saucony Endorphin Elite  - A brand new super shoe rivaling the Alphafly in performance
Saucony Sinister - Sub 5ozs of speed
Skechers Speed Beast - A new long distancer trainer/racer from Skechers
Saucony Terminal VT [Track Spike] - Saucony's new middle distance super spike, featuring PWRRUN HG
Under Armour Flow Velociti Elite - UA releases their racer, featuring an ultra comfortable platform

Find all Shoe Reviews at Doctors of Running here.

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Editor's Note: As always, the views presented on this website belong to myself or the selected few who contribute to these posts. This website should not and does not serve as a replacement for seeking medical care. If you are currently injured or concerned about an injury, please see your local running physical therapist. If you are in the Los Angeles area, I am currently taking clients for running evaluations.

***Disclaimer: These shoes were provided free of charge in exchange for a review.  We thank the people at Adidas for sending us a pair.  This in no way affected the honesty of this review. We systematically put each type of shoe through certain runs prior to review. For trainers and performance trainers, we take them on daily runs, workouts, recovery runs and a long run prior to review (often accumulating anywhere from 20-50 miles in the process). For racing flats we ensure that we have completed intervals, a tempo or steady state run as well as a warm-up and cool down in each pair prior to review. This systematic process is to ensure that we have experience with each shoe in a large variety of conditions to provide expansive and thorough reviews for the public and for companies. Our views are based on our extensive history in the footwear industry and years testing and developing footwear. If you are a footwear rep looking for footwear reviews or consultations on development, we are currently looking to partner with companies to assist, discuss and promote footwear models. Partnership will not affect the honesty of our reviews.

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Contact us at doctorsofrunning@gmail.com

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Hoka Cielo Road

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