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adidas Adizero Prime X Review
By Chief Editor / Founder Matt Klein


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."




Specifications for the adidas Prime X (per 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

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. 


FIT 

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. 




PERFORMANCE 

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.




STABILITY 

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.



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

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.

WHO THIS SHOE IS FOR 


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. 



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)

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.

Please feel free to reach out, comment and ask questions!
Contact us at doctorsofrunning@gmail.com

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