Physical Therapists Using Clinical Analysis To Discuss The Art And Science Behind Running and The Stuff We Put On Our Feet

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Karhu Synchron 2020 Multiple Tester 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


David: 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.

Nathan: The Synchron 2020 has one of my favorite step in feels and is made with extremely durable materials, so you will certainly get the miles you want out of this shoe. The Synchron 2020 boasts their new AERO FOAM NOVA and a unique stability mechanism that is not harsh on the foot and would be suitable for many types of runners, even those who typically run neutral.


David: 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.

Nathan: In my opinion, Karhu does sizing right, and there are a few special features to the Synchron 2020 that add to the well-fitting nature of this shoe. First, they develop their sizing based off of 3D scans of over 100,000 people around the country (in Fleet Feet stores), so they are taking into account a large population as they develop their sizing. Second, they just rolled out their "HiVo" option (which is their term for wide). This, in my opinion, is also wise. Instead of just making a wider last, they create shoes that have overall higher volume that can accommodate for those who have a high arch and overall larger volume foot, not just taking into account the width. Finally, the Synchron has an oblique lacing that spans to the ball of the foot, which allows for small adjustments in toe box width. So for those who have more narrow feet, you can really synch it up there.

All that said, I had the non-HiVo option and it fit true to size for me. Compared to older versions, the toe box has much more ample room, and the midfoot locks down really well with the M-Lock lacing. The "Ideal Knit" upper is extremely comfortable and is my favorite upper to date -- for comfort. Overall the upper feels like a Cadillac -- you can see and feel the quality. The one issue I have with this year's Karhu line for fit is the material they use to pad the heel counter. It is a very smooth/silky material, and I find that my heel likes to slides around quite a bit in there if I don't really lock it down. Given it's smooth nature I've never had any irritation, but it's just a bit more tricky for me to get my heel locked in.


David: 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).

Nathan: First impression of this shoe is that it's heavy. Their new AERO FOAM NOVA is a liquid based foam and the weight does reflect that. That said, the shoe doesn't ride as heavy as it's listed. Overall, I'd say that the ride of this shoe feels "flat" and there isn't a notable bounce or pop to the shoe (but that doesn't mean it ran poorly...stick with me). In comparison to the Ikoni, which has a half length fulcrum, this shoe has a full length fulcrum. For me, the Ikoni feels propulsive because you are able to drive off of the fulcrum during the transition to the forefoot. However, given that the fulcrum is full length in this case, there is less of a propulsion sensation, and you just feel a smooth and steady transition from heel to toe. In addition, the "flat" sensation led to this feeling almost like a 0 drop shoe for me (listed at 8mm). It was difficult to transition to getting up on the toes quickly. The fulcrum (photo below) gives gentle guidance to the medial and lateral foot, possibly mitigating some unnecessary movement laterally that decreases efficiency and keep the foot progressing forward. I use the word gentle because I never once felt the fulcrum pushing into the foot, and if you asked the guys on the team, I am the most sensitive when it comes to shoes with stability built in. So this shoe is going to give a smooth, consistent, and cushioned ride that feels more efficient than you'd think given the lack of responsive feel. Lastly, this truly is my new favorite walking shoe, so for those looking for a shoe that you want to use for walking and some running, this is a great option.


David: 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.

Nathan: Overall this is a very stable shoe. The full length fulcrum can help some (with limitations I'll explain soon), there is a rigid heel counter, cushioned foam that gets rigidity from the fulcrum, and a full contact outsole with some lateral flaring. The limitation of the fulcrum in this shoe compared to Karhu's other models is that it is "floating" on top fo the foam and doesn't have full depth of the shoe. I have pictures below to hopefully help with the visual. The Ikoni and Fusion use a more dense foam that spans from the top of the bottom of the midsole to create the fulcrum, where the Synchron utilizes a composite fulcrum that does not have contact with the top and bottom of the midsole. This means that the foam above and below the composite fulcrum can compress however it wants, limiting the full effect of the guidance laterally and medially. So although this is considered their "highest stability" shoe, I would say that the Fusion carries more stability in the medial/lateral direction for those who need it most. However, Synchron is is the most rigid from heel to toe and provides the most forefoot stability of the three.

Synchron Composite Fulcrum in Pink

Ikoni Foam Fulcrum in Yellow/Dark Gray


David: 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.

Nathan: Yes, I agree that this shoe runs faster than its weight and feels smooth. However, weight does play a big role, and this shoe was exclusive for daily runs for me as I felt the weight during swing phase while trying to pick up the pace. This Cadillac is perfect for daily miles, long runs, and recovery runs. 


David: 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.

Nathan: Durability is my highlight for this shoe. Like I said, for someone who is looking for a shoe that can do casual running and double as their walking shoe for outings with the family, this is great. The foam is going to last, the outsole is doing great, the fulcrum gives integrity to the midsole. All around great durability.


     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. 


David: 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.

Nathan: I believe every shoe in a company's rotation is meant to serve a certain role, and I think the Synchron is doing a great job on the daily miles role. To make it a bit better, I would switch up the material through the heel counter that is a bit too smooth to get a lock down. I spoke about the "flat" nature of how this shoe runs. Possibly increasing the drop to 10mm or adding a bit more of a forefoot rocker could help with propulsive feel of the shoe.


David: 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.

Nathan: The Karhu Synchron 2020 is a uniquely stabilized shoe that can provide gentle stability to neutral and stability runners alike. This shoe is perfect for someone looking for a long-lasting shoe that can handle casual running, daily miles, recovery runs, and then double as their walking shoe. If top notch comfort and durability are what someone is looking for, this is a great option.


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) 

Fit/Upper        9.5/10 (wonderful fit and good HiVo option available, -0.5 for slippery heel)
Ride/Midsole  7.5/10 (-2.5 for flat ride and difficulty getting up onto toes)
Stability           9/10 (some slippage from heel, "floating" fulcrum gives less overall stability)
Speed               7/10 (fast for its weight and fine for daily miles, doesn't have the pop for speed work and slow transition up onto the toes)
Durability        10/10 (this thing is a beast for durability)

TOTAL (%):  D: 92%, N 86%

Interested in purchasing the Karhu Synchron? Visit our partner Fleet Feet with the link here. It doesn't impact your purchase, but it helps our brand grow! Thanks to Fleet Feet for their support.


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

Dr. Brown is a 155 lb male with notable PRs of 18:18 5K, 39:25 10K, 1:29:01 half marathon, and 3:54 marathon. He typically runs between 20-40 miles per week at a variety of paces from 7:30-8:30 min/mile for recovery runs to 6-6:45 min/mile for tempo runs. He typically prefers shoes that provide some cushioning underfoot but still maintain a more firm and responsive feel. Current goals for 2020 are to break the 1.5 hour half marathon and 3:30 marathon. IG handle: @nate.docsofrunning

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) and 36 miles (Nathan) on mine. 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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