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Adidas Adizero Prime X Review
By Chief Editor / Founder Matt Klein and Senior Contributor David Salas


With the introduction of so many maximal stack height racing shoes, World Athletics issued a height limit to racing shoes for the road at 40 mm. Despite the limit for road racing, Adidas released the Adidas Adizero Prime X, coming in with a 50mm heel stack height as a concept shoe. One athlete has already been disqualified for using it after winning the Vienna marathon (Derara Hurisa in 2:09:22). With such an insane stack height, carbon infused rods, carbon blades, one would think this would be the ultimate performance enhancing shoe. However, it ends up being a unique training/long run shoe, although perhaps "experience" is a better description than "shoe."


Adidas Adizero Prime X
Price: $249.95 at Running Warehouse

Weight: Not Provided  (men's size 9) Not Provided (women's size 8)
Measured Weight Men's US size 9.5: 9.6 oz / 272 g
Stack Height: 50 mm / 40 mm
Drop: 10 mm
Classification: Ultra Max Stack Height Training Shoe


RUNNING SHOE SUMMARY

Matt: The Adidas Adizero Prime X is a long distance training/uptempo shoe that provides a smooth ride, a super high stack height and two layers of carbon infused rods and carbon blades. With a 50 mm of Lighstrike Pro in the heel, this is the largest stack height of any known running shoe on the market. A wider but super thin upper provides plenty of room in the toe box while still providing a secure heel. The fit is actually slightly big, so many people may want to consider going down a half size. The ride is the smoothest of any of the super shoes on the market. Despite the numerous stiffening agents in the midsole, the triple layers of Lightstrike Pro and the significant rocker provides a very natural feeling ride at recovery, easy and uptempo paces. Despite the DNA, the Prime X is too large to be used as a racing shoe for most below marathon distances and instead serves as a unique training option/companion to other super shoes.

David: The Adidas Adizero Prime X is a performance trainer/racer for long road distances. The stack height is set at 50mm through the heel and uses Lightstrike Pro as the midsole. The result is a very high platform with a really soft and bouncy midsole that also utilizes two layers of energy rods. The ride of this shoe is really fun however the high stack does give some stability concerns for some. The outsole is very sticky and uses Continental outsole throughout which really helps with stabilizing the incredibly high platform. This shoe shoe does carry some weight being around the 9 ounce mark compared to other performance shoes, but we did see it go 2:09 in the marathon. I think for most it won't be a racer but the option is there. For me this is more of a long run/long workout shoe when you know you are going to be on road for the large majority of the run. 


FIT 

Matt: The Adidas Adizero Prime X fits me a half size large as a US men's size 9.5 fits me true to size rather than my normal men's US size 10. The fit is snug in the heel and opens up in the midfoot and forefoot. The toebox is wider for an uptempo shoe and I had plenty of room for my toes. The midfoot is more normal to slightly snug and can be be easily tightened or loosened with the laces. The heel is more snug and like the Adios Pro 2, I sometimes had trouble getting the shoes on my feet. There is no heel counter in this shoe, however the heel is very secure thanks to reinforcements on the upper. Adidas uses a "Slinglaunch" heel design, whereby additional reinforcements coming from the midfoot wrap around the top of the heel to lock in the foot. I found this works very well in the half size down, but am concerned that a true to size may feel a little loose like my experience with the Adios Pro 2. The tongue is very thin and the laces can cut into the top of the ankle if tied too tight. The upper overall is very light and see through. Despite the lightness, there are still several reinforcements built in that keep the fit fairly secure. The reinforcements do have stitching on the inner side of the upper and for that reason I suggest wearing socks with this shoe. Overall, the upper fits a little long, but has plenty of security in the upper despite being on the flexible side.

David: I am a half size down from my normal 9.5 men's and the shoe actually fits really well. The shoe does seem to be about a half size big giving me a great fit throughout a half size down. The heel and midfoot are normal width to slightly narrow with a wider toe box and forefoot. The upper is a very thin and synthetic Celermesh 2.0. The upper material itself is a little plasticky but it is very thin, very light, and still reinforced pretty well with some minor reinforcement using suede like fabric throughout the medial and lateral aspect of the shoe. The tongue is very thin and this upper mimics pretty similar to the Adios Pro 2 in that is very lightweight but still wants to lock down snug for a performance fit. There is no heel counter but there is a piece of fabric used to help keep some structure through the heel along the midline of the calcaneus region. Overall I am really pleased with the lockdown throughout but you do need to size down a half size. I could easily see this being a bit roomy in my normal size with both length and width. 




PERFORMANCE 

Matt: The Adidas Adizero Prime X has a very soft and surprisingly smooth ride. There is a carbon plate in the rearfoot, 5 carbon infused rods in the midfoot to forefoot and 3 carbon fiber energy blades in the forefoot below the rods. There is a 50 mm heel stack height with a 40 mm forefoot stack height, making this the tallest shoe I have ever run in. Although I expected an extremely stiff ride, I experienced one of the smoothest rides of any carbon fiber super shoe I have tried thus far. The ride is very rockered, with a smooth midfoot and forefoot transition. The toespring and softer ride are stabilized well by the rods and blades, creating a very natural feeling toe off. The heel is less stable, although on flat ground transitions decently. The ride is softer, but is not mushy. The foam feels more bouncy and everything helps tip you forward. There is a 10mm heel drop that I did not notice, likely thanks to the toe spring and extreme posterior lateral heel bevel. The ride is best for training miles and for uptempo efforts. I found this shoe does very well with longer uptempo runs where the pace is being pushed a little. At anything faster, the size of the shoe and 9.6 oz weight is immediately noticeable and gets in the way. I had a difficult time running faster in this shoe, but really enjoyed the protection and smooth ride for normal and easy runs. Further evidence that this is a training shoe is how durable the outsole is. The Continental rubber outsole is extremely thick and I have not made even a dent in them after 25 miles. This is in deep contrast to the Adios Pro 2, which I began to tear through within a few miles. For that reason, the Adizero Prime X may work best as a training companion to racing shoes like the Adios Pro 2 to eat up easy/normal/long miles. However, this training shoe should be kept strictly to roads as the exposed Lightstrike Pro can be easily ripped through and the ride is NOT stable for anything but level road.

David: Similar to the Adios Pro 2 the Prime X is definitely geared more towards performance like efforts. The ride of the shoe is very soft, bouncy, and responsive. The shoe likes to go fast on the road. The 50mm stack height is very high and does take some getting used to. For me, I can go off road with it for a few steps but I'm definitely sketched out a tad. The usage is definitely for uptempo uses on the road. The Lightstrike Pro foam is very soft and deforms quite a bit when loaded. At slower paces the shoe can feel a tad "floaty" but when the pace begins to pick up the transition onto the forefoot happens quicker and you feel much more stable. The traction underfoot is really great. It saves the stability of the midsole, because even with how sticky the outsole is the shoe still feels a little suspect when running slower or off of road conditions. I am a fan of the Adidas Adizero Prime X but it is not for those that are already a tad wobbly when running. Turning is a little difficult in the shoe but ironically when the shoe is loaded with more force at higher speeds the turning seems to be cleaned up quite a bit. There is plenty of protection and I find that I do have less soreness following efforts like I would with other "super shoe" configurations. I'm not sure I'd race in this, but it is definitely something I may look at for long runs or workouts when I know they are on road. I do get a small amount of ankle soreness after wearing these for long periods of time due to stability demands but overall I am ok enough to just disregard it and keep moving forward. Despite the outsole this certainly is a road shoe. If you have pretty neutral mechanics and really like soft and bouncy rides this could be something to look into. Is it worth the price? I'm still trying to figure that out because the shoe is certainly responsive. It's really responsive. It's a matter of the stability demands and mechanics of the runner that is in it. Its a really niched shoe but something I could still see some people reaching for on race day depending on the person if they don't mind the little bit of added weight. For me I also like the forefoot transition. When loaded it is the most stable piece of the shoe and provides quite a bit of pop when you load it hard and quickly. The Prime X is a shoe that might be amazing for some and a nightmare for others. 




STABILITY 

Matt: The Adidas Adizero Prime X is a neutral training/uptempo shoe. The carbon infused rods, carbon blades, well placed toespring and wider forefoot make for a stable ride up front. However, the heel has a narrower last and despite the large lateral heel bevel and rearfoot plate is unstable. Despite being a heel striker, I found myself naturally trying to move my foot strike forward given that it was way more stable up front. The heel instability is particularly noticeable on uneven road and downhills. For that reason, I would only suggest using this shoe on flat, even road. Rocks, dirt and uneven terrain may be dangerous given how tall the stack height is (without adequate sole width or flaring in the heel). Additionally, this unique shoe is only going to work well for those that have stable mechanics in the rearfoot. Those that need mild stability in the forefoot will be able to get away with running in this shoe. However, overall the softer foam, which is not mushy, and tall stack height make this shoe best for those without stability needs.

David: Ok... this is a shoe that is dynamically stable when loaded. What do I mean by that? The harder and faster you run on this the smoother it will be. The shoe does have a dual layer of rods/plates that extend through the forefoot and that is noticeable when you load the shoe. The midsole height and softness itself make for a very high and unstable platform for those that don't have neutral mechanics. The design of the shoe is meant to move forward through and if you are loading the shoe quickly in a linear fashion the stability isn't too bad. The outsole also provides really good traction throughout and saves the stability for more people in my opinion. The last through the midfoot is rather narrow for a shoe of this kind of stack height as well which makes it feel a little teeter totter like but everything changes once you are in motion. I'm not going to say this is a stable shoe. It isn't. But when loaded appropriately it isn't horrible. I think this is really going to depend on the runner and usage of the shoe and platform to determine stability proper. For me as long as I'm on road and the pace is a little uptempo I do enjoy this shoe. 



THOUGHTS AS A DPT / FOOTWEAR SCIENCE

Many people have asked whether the Adidas Adizero Prime X truly provides an unfair advantage. Despite the violation of the stack height limits, I think the answer is no. When it comes to footwear design, there are many factors that go into making a shoe fast, not to mention the individual characteristics and responsiveness of the individual wearing them.

While foams are getting lighter, shoe weight still plays a large factor in speed, with every 100g decrease improving running economy by 1% (Franz et al., 2012). While the lightest shoe is not going to be the fastest shoe, as evident by the same research just referenced determining that a light cushioned shoe is more economical than running barefoot or in a minimalist shoe, a lighter cushioned shoe is going to be best. The challenge with the Prime X is that it is not light. At 9.6 oz / 272g for a men's US 9.5, that weight is more associated with that of a trainer or lightweight trainer than a racing shoe. It certainly feels lighter thanks to the rebound from the foam and efficient ride, but trying to run fast in this shoe is difficult with that much relative weight on your foot.

The other issue is that just because you have a ton of cushioning does not mean you have more protection. There is increasing evidence that running in maximal or high stack shoes does not necessarily provide more impact protection. Part of this is due to the lack of ground feel, which impairs proprioception (body sense) and the bodies ability to sense impact and adequately react to it (Kulmala et al. 2018). For that same reason and potentially others, people tend to land harder in maximal cushioned shoes, which keeps the impact forces similar or even increases them (Chan et al., 2018; Pollard et al., 2018). There is also evidence that another part of this is due to the instability or softness of so much foam, which can cause the body to compensate by stiffening the lower extremity (Kulmala et al., 2018). Landing on legs that are too stiff means less shock absorption by the muscles and more by passive structures like joints, ligaments, etc.

There are additional elements that have been suggested but not confirmed to improve performance in shoe types like these. The taller midsole means that lever arm of the lower extremity increases, potentially allowing the running to generate more torque with each stride. That however requires the runner to be able to generate that much force in the first place. The rockered sole is another component, whereby there may be some improvements in efficiency, although the amount this provides is questionable. The durometer of the foam and rod stiffness are additional factors that do have some research behind them and may improve running efficiency, 
although how much specifically with Lightstrike Pro and two layers of rods is unknown (most research focuses on a single carbon fiber plate). Additionally, how this level of stack height, the instability in the rearfoot, the higher weight and all the other factors respond together is another question.

All of the above is not to say the Adidas Adizero Prime X isn't a great shoe. It is super interesting and has been a blast to run in. The double layer of rods (carbon infused rods and blades), the rockered sole and extremely bouncy level of Lightstrike Pro make it really fun to run in. However, I would argue that it is not a super racing shoe but a super training shoe. Many people are going to love using a shoe like this for a marathon (or road ultramarathon) while others may want something lighter for racing. That is where individual variation in response is going to come in. However, the key here is that more is not necessarily better. With more cushioning comes more need for stability, whether from the shoe or the person. With more rods/plates comes the need for a more rockered shoe or more force generation from the runner. That is why World Athletics has several rules beyond the 40mm stack height limit, but  I still think this is not a cheater shoe despite the marketing from Adidas. It is a clear example of again how more is not necessarily better and the optimal racing shoe is going to have to find a unique balance of lighter weight, midsole cushioning/responsiveness, geometry, plate stiffness and more.


References


Chan, Z. Y., Au, I. P., Lau, F. O., Ching, E. C., Zhang, J. H., & Cheung, R. T. (2018). Does maximalist footwear lower impact loading during level ground and downhill running? 
European Journal of Sport Science, 18(8), 1083-1089.


Franz, J. R., Wierzbinski, C. M., & Kram, R. (2012). Metabolic cost of running barefoot versus shod: is lighter better. 
Med Sci Sports Exerc44(8), 1519-1525.

Kulmala, J. P., Kosonen, J., Nurminen, J., & Avela, J. (2018). Running in highly cushioned shoes increases leg stiffness and amplifies impact loading. 
Scientific Reports, 8(1), 17496.

Pollard, C. D., Ter Har, J. A., Hannigan, J. J., & Norcross, M. F. (2018). Influence of maximal running shoes on biomechanics before and after a 5K run. 
Orthopaedic Journal of Sports Medicine, 6(6), 2325967118775720.



RECOMMENDATIONS

Matt: I do understand this is a concept shoe that is available in limited quantities. However, if there is going to be a version 2, I have a few suggestions. There is first the sizing issue. I honestly got lucky that these fit as this 9.5 was actually supposed to be a surprise for our social media manager (who is men's size 9). My curiosity got the better of me and I tried them on before sending them to him and realized they fit (so obviously I had to get some miles in them for review before sending them out). However, My concern is that these would fit too big and have some security issues if I went true to size. While I like the wider fit in the toes, I think the length needs to be adjusted to fit more true to size, particularly for $250. The other major issue is heel stability. Despite the lateral heel bevel, the 50mm of foam in the relatively narrow heel creates a high level of instability. As much as I enjoyed these, I came away from most runs fairly sore due to that (also due to the softness as discussed in the Thoughts as a DPT). Those with any rearfoot instabilities are not going to do well in this shoe. This could be adjusted by increasing the sole flare at the heel. The extreme lateral bevel will still adjust for this on landing at the posterior side. While there is a rearfoot plate, I do not feel it is doing enough. So only those with very neutral mechanics at the rearfoot will do well here.

David: The Prime X is wild. There are some pretty common themes I think that can be looked at a little closer though. The first is the sizing issue. This shoe clearly is a half size big so order your shoe a half size down. The other is the rear foot mechanics. For me the forefoot is actually quite good but the shoe is very floaty and a tad sketchy up until that point. I know there is a rear foot plate there but that heel doesn't feel super trustworthy at slower paces. When the pace picks up and the transition is quicker it isn't too bad, but the rearfoot stability could be cleaned up a little more to help with the towering stack height of the Prime X.   

WHO THE ADIDAS PRIME X IS FOR 


Matt: The Adidas Adizero Prime X is an extremely high stack height training/uptempo shoe for those who want as much cushioning as possible under their feet. The double layer of rods and blades stabilize the forefoot well and combined with the well down toe spring provide one of the smoothest toe offs of any super shoe I have tried. The upper is minimal and secure, although those interested may need to try the Prime X on first to determine what size they need. The heel is unstable, so those with neutral mechanics in the rearfoot will do better. Overall the ride is heavily cushioned but not mushy, with the ability to push a little bit. The weight and size however make this far more of a training and uptempo shoe that works best for easy miles, longer uptempo efforts and extremely long races. Those wanting an elite (and legal), fast and aggressive racing shoe will want to look elsewhere (like the Adios Pro 2) but those wanting a ton of cushioning and a very unique running experience should check this shoe out.

David: The Adidas Adizero Prime X is a double maximum stack height shoe that leans more in the performance trainer/ long distance racing category. For those with really neutral mechanics and not too much stability demands the shoe is very responsive through the forefoot and provides a lot of bounce when loaded. The heel isn't very stable and those who have stability needs through the rearfoot are warned. The stack is very high and proprioception and ground feel is certainly effected. With that said the shoe is very protective, very responsive, and a good shoe to pick the pace up on the road. It is a little heavy to race in for me, but could be a long run/long workout companion that I reach for when I know I'm on the roads. This is a very interesting shoe and for the right person this could be the shoe they have been waiting for. For others a nightmare to be avoided. It's polarizing but really shines in that responsive forefoot category on the roads. 



GRADING 

Matt
Fit: (Plenty of room in forefoot with secure heel and midfoot despite no heel counter. Sizing is slightly off, so may need to go half size down)
Performance: 
A- (An excellent training shoe, but not a racing shoe for most. Very smooth and cushioned ride, although a little unstable in the heel. Best for training miles and uptempo miles. Too large for faster runs for most)
Stability: B-/C+ (Surprisingly stable forefoot. However heel is narrow and unstable. Best for those with neutral mechanics)
DPT/Footwear Science: B (Surprisingly stable forefoot and the smoothest ride of any super shoe I have tried. However, more shoe is not necessarily better. Thus marketing this as an elite racing shoe is off the mark, given that it works better for extremely long efforts and easy runs due to the weight.)
Personal: B+ (Although the heel is not stable enough for me, this is an extremely unique shoe that I continued to reach for during shorter runs and uptempo efforts)
Overall: B/B+ (A unique running experience that works best for those wanting a minimal upper, tons of cushioning and a smooth ride best for training and uptempo work)

David
Fit: (This is because of the sizing issue. A if true to size. Solid heel width and midfoot lockdown for being a half size down as well as a roomy forefoot for swelling accommodation. Performance fit mixed with long distance considerations.)
Performance: 
A- (This shoe is really responsive, but niched. It has to be on roads for me. It's just too unstable on anything else. But long distance uptempo efforts this shoe does shine pretty brightly. The stability issues does make me apprehensive and I hold back in certain situations though.)
Stability: B- (For the stack height the forefoot stability is quite good and the stability cleans up with pace. But... that heel and midfoot can be quite sketchy until you adapt to it. It's not for everyone but when loaded quickly with force some of it cleans up.)
DPT/Footwear Science: B (I like that Adidas took a swing at something beyond 40mm. I think in some ways they hit a home run and others they missed the mark. That forefoot is good. Its really responsive and surprisingly stable. The rearfoot takes away from that experience though. With some refinement this may be a shoe in the future seen on a lot of people but for now it isn't for the faint of heart. Neutral mechanics and long uptempo efforts.)
Personal: B+ (The Prime X is a lot of fun. It is a very responsive shoe with a reasonable weight for most uses. It might be a tad too much shoe for racing... I'm still trying to figure that out but the responsiveness makes for a really fun ride. When I know I'm doing long road efforts with some uptempo work this is something I consider reaching for.)
Overall: (A highly unique shoe that requires some pretty sound mechanics. The shoe feels best on road at uptempo paces. The forefoot is VERY responsive and surprisingly stable but the rearfoot and midfoot can be a quite sketchy. If you can get to that forefoot cleanly this a very fun and fast shoe up front.)


VIDEO REVIEW OF THE ADIDAS PRIME X



Chief Editor Matt Klein takes his thoughts to YouTube for a video review of the Adidas Prime X.

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TESTER PROFILES:


Matthew Klein, PT DPT OCS FAAOMPT
Doctor of Physical Therapy
Board Certified Orthopedic Clinical Specialist
Fellow of the American Academy of Orthopedic Manual Physical Therapists

Dr. Matthew Klein is a 150 lb male with notable PRs of 14:45 for 5k and 2:32:44 for the full marathon.  He typically runs 70-100 miles per week and trains at a variety of paces from 8min per mile recovery runs to 4:40 per mile 1k repeats.  He prefers firmer and responsive shoes with snug heels and medium to wide toe boxes.  The stability guy of the group, he also prefers a little stability in his footwear. However, as a researcher, clinician and running shoe aficionado, he will run in anything. 

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 purchased from eBay. 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 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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