Clinical Analysis of Running, Running Footwear, and Injury Prevention/Performance. The Doctor(s) of Running, using knowledge of human movement, clinical biomechanics and performance to bring you cutting edge reviews, science and knowledge.

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Karhu Synchron 2020 Review

     The Karhu Synchron 2020 is a maximal cushion and guidance trainer featuring the brand new midsole AERO FOAM NOVA. The shoe utilizes a full length fulcrum with Karhu's stingray design to guide an efficient stride pattern and smooth transition from heel to toe off. The main goal of this shoe: maximum cushion without compromising responsiveness or smooth transitioning.

Specifications (per Karhu)
Weight: 12.7 oz (size 10 mens)
Stack Height: 23mm/ 15mm
Drop: 8 mm
Classification: Maximum cushion/guidance daily trainer


     The AERO FOAM NOVA midsole is one of the premium foams out there creating a fluid and plush ride without jeapordizing responsiveness. One unique feature to the foam is that it is actually liquid based. The full length stingray composite fulcrum creates rigidity in all of the right ways and creates a fluid and fast transition throughout the shoe. This a plush, stable, and smooth shoe.


     Karhu's Ideal Knit upper does a very good job of hugging the foot from heel to toe and providing enough struture to trust the upper with turns. The Ortix design of the upper creates the "Karhu fit", using 3D foot scans of runner's feet and looking at all of the unique contours and builds of feet to make a nearly custom fit for all runners. The Synchron specifically has a normal to slightly wide forefoot, narrow midfoot, and a normal and cushioned fitting heel. The heel collar is very padded and has a moderately rigid counter built in that holds the heel nicely. I did find slightly excessive rubbing the midfoot of the shoe (did blister), but I like the upper and shoe enough I was willing to look the other way. Outside of the slightly too narrow midfoot, this is a very impressive upper.


     For a max "guidance" and stability shoe, the ride is one of the smoothest rides out there thanks to the midsole and fulcrum design. Fluid is the word I would use to describe the ride of the Synchron 2020. The liquid-based foam does a good job of absorbing the shock of landing, while the stingray fulcrum "guides" the foot linearly. The rigidity throughout the fulcrum almost creates a momentous plate like effect as well. I found that if you are able to generate a certain pace (fast or slow), the shoe did a good job of responding and creating a smooth ride at the pace. The shoe does have an 8 mm drop and should work pretty well for most foot strikes. I do think the heel could be beveled slightly more to smooth out the landing for heel strikers (I am more midfoot/forefoot, but occasionally fall to heels when tired).


     This shoe is crazy stable and I have nothing but good things to say about it in this category. The stingray fulcrum encapsulates the sides of the foot well and is hardly noticeable from a landing perspective (only uncomfortable once on a very hard landing in the midfoot/heel junction). The design reminds me a lot of one of the elite marathon racing flats's plates on the market right now, but instead of a forefoot emphasis, this trainer's emphasis is on the the entirety of the foot from heel to toe. The foam is dense enough that you get surprisingly decent ground feel as well. The upper holds the foot very well and I had no problems with translation of the foot in any direction. 


    The shoe runs MUCH faster than it's weight. Probably because of the liquid-based foam, the shoe is probably the heaviest trainer I have run in for a good while now (12.7 oz size 10) but does not run like it. I think in large part this is due to the fulcrum's rigidity. The shoe does a great job at daily paces and at uptempo paces (5:50 ish for me). The shoe can get down to faster paces (5:20's), but the weight starts to show it's colors at those kinds of paces. For daily and uptempo efforts, this shoe is great, but you may want to look at other shoes for faster workouts or really long efforts. 


     This shoe is very durable. The midsole and ride of the shoe show no signs of wear, while the fulcrum will most likely help maintain the ride for miles to come. The outsole does have some exposed foam through the midfoot, but shows hardly any wear after testing (could break down over time further down the road). The rubber portion of the outsole barely shows any scratches after testing. Durability shouldn't be a major concern with this shoe.


     Biomechanically, this shoe is sound. The liquid-based foam in the AERO FOAM NOVA does a great job of absorbing shock, while the fulcrum does a great job of pushing the runner forward without being overbearing in any way. By creating an environment within the shoe that stabilizes both the medial and lateral sides of the foot without posting or limiting motion, the runner and their individual strike pattern will influence how this fulcrum will be used. The more stability the runner needs, the more pressure and "guidance" they will get from that specific region of the fulcrum. The fulcrum is also full length and appears to look like a "stingray" or like the letter "H", creating side rails within the midsole and relief over the high pressure areas of the heel and forefoot. This guiding fulcrum as a full-length feature is highly innovative and exciting from Karhu. 


     The shoe is very stable and very smooth, but could come off a little clunky initially. This could be due to the weight (requiring minor adjustment from the runner), or the lack of heel bevel built into the shoe. I would like to see the heel beveled more for the heel striking runners and further improve the transition. I would also like to see the midfoot widened out ever so slightly to allow for a little more room without rubbing but still having security. 


     The Karhu Synchron 2020 is a plush and stable daily trainer that runs much faster than its weight. The shoe is best at daily to uptempo paces, but not the best at workout paces. The shoe is highly durable and a shoe someone could log many miles into without too much worry of deterioration of the shoe's midsole or outsole. This shoe is for someone who needs a plush trainer and may require a little more "guidance" or stability built into their shoe and would like a durable workhorse feel to it.


Fit/Upper        9.25/10 (Great fit overall, though excessively narrow midfoot that can rub)
Ride/Midsole  9.5/10 (Heel may be a little clunky for some without heel bevel)
Stability           10/10 (Great fulcrum to guide movement, great upper to hold foot well)
Speed               8/10 (FOR IT'S WEIGHT very fast. Great daily paces. Not for workouts)
Durability        9.25/10 (exposed midfoot foam could break down early. Great midsole) 

TOTAL (%):  92%


Dr. Salas is a 135 lb male with notable PRs of 3:54 1500m, 14:56 5k, 31:06 10k, 1:08 for half marathon. He typically runs 40 to 50 miles per week and trains from about 7:30 recovery runs to fast shorter efforts at 4:30 pace. He normally prefers neutral shoes with a firmer ride, but is completely open to other types of shoes.  He is a footwear enthusiast at heart and will always appreciate a high quality shoe when it comes around. For updates on training or testing, IG handle: @docsofrundavid

Thanks for reading!

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.

Doctor of Physical Therapy
Board Certified Orthopedic Clinical Specialist
Fellow of the American Academy of Orthopedic Manual Physical Therapists

Nathan Brown PT DPT MS
Doctor of Physical Therapy 
Masters in Anatomy and Clinical Health Science
Movement Performance Institute Certified in Advanced Functional Biomechanics 

David Salas PT DPT CSCS
Doctor of Physical Therapy
Certified Strength and Conditioning Specialist
***Disclaimer: These shoes were provided free of charge in exchange for a review.  We thank the  people at Karhu for sending us a pair.  This in no way affected the honesty of this review. We put at least 35-75 miles on trainers and 10-25 miles on racing flats prior to reviewing them. Currently I have 35 miles on my pair (David). My views are based on my 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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