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Karhu Fusion 2021 Review

David: The Karhu Fusion comes with a completely redesigned midsole and geometry and continues upon the great uppers that Karhu continues to come out with. The Karhu Fusion features the classic fulcrum that helps with guidance of the foot while still keeping a lot of the elements of the shoe neutral. The result is a rolling sensation throughout the shoe with a really stable platform.

Matt: The Karhu Fusion 2021 kicks off a new running concept from Karhu: Every Runner's Right. This is based around the understand that every runner has unique feet and biomechanics. This also comes from the Finnish belief in equal right and access to nature and the beautiful countryside. That every person has a right to be active and enjoy the outdoors. This has culminated in Karhu's continuation of using large scale foot scan data from every day runners to find a way to design a shoe to fit everyone's feet. The result is a fantastic fit, with a fantastic upper. The sole is more cushioned too, moving in the cushioning direction, yet reducing the weight from last year's model (REVIEW). The Fusion 2021 is far improved from the previous model and creates a much smoother ride. The continued use of a fulcrum is interesting and I think it is worth diving into a biomechanical discussion on what it may do.

Specifications (per Running Warehouse)
Weight: 10.5 oz / 300 g (men's size 10) 8.8 oz / 250 g  (women's size 8)
Stack Height: 24 mm / 18 mm
Drop: 6 mm
Classification: Neutral Daily Trainer


Matt: The Karhu Fusion 2021 is a complete update featuring a new midsole geometry, midsole height, midsole foam and upper. The ride is far softer with a smoother rocker overall. Now with a wonderful beveled heel, increased midsole foam under the forefoot and a shortened fulcrum, the ride is more forgiving. The upper features an incredibly soft liner and more anatomic fit that should work for a large population of runners given Karhu's use of thousands of scans to determine a best fit. With a fulcrum that creates great guidance, this shoe should work for those wanting a cushioned, smooth and unique ride.

The Karhu Fusion 2021 is a great update to the 2020 model with a completely redone midsole and geometry. The midsole durometer is softer and more forgiving throughout and the geometry has a much greater rocker to it throughout. The heel is beveled pretty well and transitions quickly through the fulcrum in the rearfoot/midfoot. The shoe feels like a very stable and quick transitioning neutral shoe (though Karhu follows more of a "guidance" framework, not the normal neutral vs. stability)


Matt: The Karhu Fusion 2021 fits me mostly true to size, maybe a hair short with socks, in my normal men's size 10. The upper is seamless and incredibly comfortable. I have worn this shoe sockless up to 10 miles without issue. The fit overall is a slightly snug and a little lower volume for a daily trainer (there is a high volume version called Hi-Vo). There is enough room for toe splay in the forefoot, it just fits a little closer to the foot. The heel and midfoot fit a bit more snug. I did not have to lace lock the shoe, even with running sockless. However, I did have to be careful with lacing the shoes as there was a large amount of pressure over the dorsum of my midfoot initially. There is a decent amount of cushion in the upper, both throughout and in the heel collar. There is a heel counter, but I have not noticed it due to the protection from the upper cushioning. Overall this is a slightly snugger feeling but plush upper that should fit a variety of foot types, although those who need a bit more room should check out the Hi-Vo version.

The Karhu Fusion 2021 has a performance like fit for a daily trainer. The shoe is pretty snug throughout with good lockdown through the midfoot and heel. The toe box is wide enough for toe splay and has enough volume for swelling. The mesh is very breathable but not so thin that it leaves your feet cold. This has a nice daily training feel to it, meaning there is a lot of effort into making the upper cushioned and balanced throughout with a good hold. The shoe does fit pretty snug through the midfoot and the dorsum of the foot though, and this may be a little much for some. Luckily they do have a Hi-Vo option for those that need a little bit more volume in the upper. Overall I like the upper a lot, but I do feel there can be some more volume through the midfoot to decrease some pressure over the dorsum of the foot in the base model.

Nathan: I was able to test both the normal fit Karhu Fusion 2021 as well as the HiVo edition. "HiVo" stands for "high volume" and is Karhu's take on creating a wide version of a shoe. What this does well is that it is not simply creating a wider upper or last, but changing the toe box construction to provide more 3D space. The fit difference between the "normal" and "HiVo" was substantial, where the "normal" gives a quite locked down and performance like fit (in fact, to locked down that my toes went numb on some runs), whereas the HiVo provided nice toe box space that made it feel like a daily trainer. What the HiVo version did well is that there was very substantial room in the toe box and midfoot, but the lockdown to keep the foot stable on the platform was just as good as the "normal". Anytime moving forward, I'm leaning to the HiVo option for fit from my Karhu shoes, particularly because I want them to function either as a workhorse trainer or daily miles shoe, not necessarily for tempo days and workouts. For many of my patient populations, the HiVo option will be a much easier recommendation due to the accommodating fit (for various foot types and foot deformities). 


Matt: The ride of the Karhu Fusion has been completely overhauled from the previous version. The biggest changes come from a very well beveled heel, a shortened fulcrum, increased forefoot cushioning and a lower drop. The ride overall is on the softer side and the Karhu Fusion 2021 is definitely a training shoe. The new beveled heel provides much smoother heel transitions and the foam is much softer than previous. The fulcrum sits between the rearfoot and midfoot and is noticeable. It is a bit stiff, but transitions you quickly into a softer forefoot. This softness is relative as it is certainly softer than the previous version, but compared to other shoes I would put it at the mildly soft range (thanks to firmness from the fulcrum). The fulcrum is shorter than previous versions (shortest in the new Karhu line), however is still noticeable. It feels like an additional firm spot and does provide a very quick pivot right where it is. The transition to the forefoot is soft, but a bit stiff. The new increased midsole height in the forefoot is nice, but is best for easy mileage. It isn't smooth enough to pick up the pace and the Fusion 2021 is best for easy long runs and mileage. The new 6mm heel toe drop is not super noticeable. This is likely thanks to the really nice heel bevel and fulcrum taking care of the transition. So those used to 8-10mm drops will easily transition here. I have well over 50 miles in my current pair and 40 miles in another pair. Both have next to no wear on the outsole and the midsole foam/fulcrum are still good as new. I expect a very high number of miles out of this shoe, but that should come as no surprise given this is a premium training shoe.

The Fusion 2021 has a really quick transitioning ride throughout. The heel is beveled pretty well and the shoe snaps you forward through the midfoot really quickly onto the toe off. The forefoot does not have as much toe spring as say the bevel in the heel and so there is still some "push off" that you have to create to propel yourself. At first I did get some knee pain since my foot was pitched forward quicker than I was used to but that cleaned up after the second run. The foam itself is softer than the previous fusion model, but still does ride a little on the firm side. The Fulcrum is noticeable through the midline of the foot and helps stabilize the ride a lot. I think this also contributes to that really fast heel to midfoot transition that is experienced in this shoe. Overall the probably could use some more forefoot flexibility to offset the rigidity in the midfoot and really quick transition that comes from the heel into the midfoot. The shoe rides like a really stable neutral shoe, which is the aim of the shoe with the fulcrum. 


Matt: The Karhu Fusion 2021 is technically a neutral shoe, but per Karhu is designed to provide stability without a post. This comes from the fulcrum, which is shortened fulcrum in this version. The Fulcrum spans the width of the midsole in the rearfoot/midfoot. There is a center base and four prongs on each side. The medial prongs reach a bit farther anterior and posterior than the lateral ones, which may provide a little extra resistance to medial motion. This does create a very stable midfoot without any kind of post. The additional transition function of the fulcrum also does a great job of keeping the foot moving forward, rather than laterally. This primarily occurs in the heel and midfoot, with the forefoot doing its own thing. The softer forefoot is a little less stable due to the softer foam. However, for those that need mild to moderate stability in the rearfoot/midfoot, this may be a great "alternative" option if you do not like posts. 

The fulcrum in the Fusion 2021 does wonders for stability in this shoe. The shoe legitimately feels "guided" from the heel through the midfoot transition. The fulcrum also creates some rigidity through the midsole and midfoot regions of the shoe. This allows the shoe to keep motion linear and prevent deformation of the shoe from unstable surfaces or forces. The upper security is good throughout and I have no worries through turns or in unstable conditions. The midsole does ride a little bit more on the firm side which adds to stability. I think the forefoot could use some refinement to make the shoe more fluid and transition easier through toe off to further help stability since it feels a little bit like a bump in the forefoot which takes away from stability. 


David: Today I want to take a look at the fulcrum technology and how it impacts the midsole and the ride of the shoe. Does the fulcrum work? Does it enhance your ride and responsiveness? Well the answer is yes and no. The fulcrum essentially creates a rigid platform that is integrated in the middle of the foam/midsole of the shoe with strategic points throughout it that either has more flexbility or is nonexistent (like a strategic hole/crash pad or different geometry). The most obvious integration would be the Synchron 2020 model that has a Stingray shape fulcrum literally shaped as a "H". The heel and the forefoot don't have the rigid platform in the midline of the heel or forefoot, but it is present in the midfoot and along the medial and lateral aspects of the shoe through the full length. The Fusion is not quite as long in length, but plays by the same rules. The rigidity is in the rearfoot through parts of the midfoot and makes for a VERY fast transition from heel to midfoot. The effects are a little different from a traditional plate design and the feeling is much more of a "guidance" sensation. This is why Karhu does not classify any specific models as a neutral or stability shoe, but by the levels of guidance a specific model provides. Is this a good thing? It very much can be. A lot of companies are taking similar approaches by incorporating guide rails, dual foam densities, and other components to make a guidance type platform. The big challenge is integrating it well with midsole and the geometry of the shoe as a whole so the ride is still natural and pleasant. When done right this design is quite intriguing and fun.

Matt: I do like the updates to the Fusion 2021. However, I have some thoughts on the fulcrum. We frequently talk about rockers of the foot and ankle, which help aid in efficiency and forward motion naturally. They are, in a sense, the body's own fulcrums. The fulcrum utilized in the Fusion 2021 acts to replace/emphasize the ankle rocker. The ankle rocker is really the ankle joint (Talocrural joint). As you transition through midstance, the ankle joint moves naturally into more dorsiflexion (bend) IF you have enough dorsiflexion. Normal motion at this joint is supposed to be 20 degrees, but it is rare that people have that much motion given that most shoes, particularly dress shoes, have a very high heel drop. This keeps people in a plantarflexed (ankle/foot pointed) position, which can lead to calf stiffness, joint stiffness, etc. This isn't necessarily bad, it just means you need to be careful transitioning to shoes with a lower drop if you have limited motion there. The fulcrum in the Karhu 2021 combined with the beveled heel (helps with the heel rocker), will help those interested in lower drop shoes try something even if they have limited ankle range of motion. HOWEVER, for those who have plenty of motion here, the fulcrum may serve to pivot you too far forward. Those with too much dorsiflexion often have knees that translate too far forward. This places additional stress on the quadriceps tendon, patella and patellar ligament. Both David and I had some anterior knee pain initially running in this shoe and my suspicion is that because we both have decent motion at the ankle, this mechanism may be inducing a knee forward to toes running movement impairment. I have written on this movement impairment HERE and this is a common issue leading to quadriceps overuse and knee symptoms. So those with this movement issue already should be cautious of this shoe. If you want to run in it, ensure you have a high level of hip extensor strength and coordination, as this will prevent the knee from going to far forward over your toes during midstance. However those with limited ankle motion will likely do better. Remember shoes are tools and some will work better for others.




Matt: The Karhu Fusion 2021 is a great update. The cushioning and rearfoot are far better than the 2020 version. However I have a few recommendations. With the softer and more cushioned forefoot, it is a bit unstable and stiff transitioning off the fulcrum. My suggestion is to increase the forefoot flexibility or increase the toe spring to help promote forward motion over the forefoot. While the upper is extremely comfortable, I think the laces on top of midfoot need to be reworked to not put so much pressure on the dorsum of the foot. The slightly snugger fit already does a great job of holding the foot, so some of the overlays may be better placed around the heel.

I enjoyed the Karhu Fusion 2021 much more than I did the Fusion 2020. The main recommendations I would make for the 2021 model would be to make the forefoot more flexible to offset how fast and rigid the transition is from the heel through the midfoot. My other suggestion would be to make the upper a little bit more accommodating with volume through the midfoot so there isn't so much pressure on the dorsum of the foot.


Matt: The Karhu Fusion 2021 is for those looking for a unique, mildly soft cushioned but protective ride. While this is a neutral shoe, there is some additional stability the the rear to midfoot that should have provide an interesting transition forward. This may be especially helpful for those wanting a lower drop but cushioned shoe if they have limited ankle motion. The upper is also incredibly comfortable, but a bit snug through the midfoot. So those wanting a shoe to wear sockless will find a very good fit. Best for long and easy miles, the Karhu Fusion 2021 is a big leap forward and I am excited to see the direction Karhu continues to move in.

The Karhu fusion is a shoe for those looking for a shoe that deviates from the normal "neutral" versus "stability" concept and focuses more on "guidance". The shoe rides like a really stable neutral shoe with a really quick transition from heel through the midfoot. The shoe also has a fulcrum that creates rigidity through that region that helps with that transition and stability. So for those that like a slightly firmer riding shoe for daily mileage that has some elements of guidance and stability this would be a shoe worth looking into.


Fit: B+ (Very comfortable seamless upper that can be worn sockless. A bit lower volume, especially in the midfoot. Fits almost slightly short when wearing socks, but a Hi-Vo option is available for those who need more room.)                     
Performance: B (Best for easy and long miles. Softer forefoot provides more protection while a beveled heel provides a nice landing. The fulcrum creates an interesting transition, but the midfoot is a bit stiff. The forefoot transition is also a bit stiff and limits the shoe from going faster)
Stability: B+ (Good stability without a post in the heel and midfoot. Forefoot a bit soft and a little unstable. ) 
DPT/Footwear Science: A- (Much better use of fulcrum, heel bevel and cushioning. The fusion is much smoother than it was in the past. However the transition between the mid and forefoot is a bit off. May be problematic for those with extra ankle motion, but a tool for those with limited motion) 
Personal:  B- (A consistent and durable shoe. Bonus points for sockless running, but the ride is a bit uninspiring in today's market. A bit too stiff transitioning from mid to forefoot. Really nice as a walking shoe/recovery shoe though) 
Overall: B (A slightly soft, slightly lower volume, stable neutral shoe with a unique ride. Best for long and easy mileage providing a protective, facilitated ride. A bit stiff through the midfoot, but those with limited ankle motion wanting to try a protective, slightly lower drop shoe)              

Fit: B (Overall the upper is comfortable and pretty accommodating, but the shoe has too much pressure over the dorsum of the foot for me, and I think I may have done better with their Hi-Vo option.)                    
(Best for daily training paces and mileage, the shoe has a quick transition and can pick up the pace a little bit but feels the most nice and smooth and controlled efforts.) 
A- (Incredibly stable from the heel through the midfoot, but the forefoot has a bump like feel to it that takes a little bit away) 
DPT/Footwear Science:  
A- (Huge improvement from previous model. Integration of the bevel and fulcrum is done pretty well, though the forefoot could be refined a little more with flexibility or toe spring) 
(Overall the shoe is pretty pleasant. A firm riding stable neutral shoe. I am not sure I like the ride of it enough to run in it over some other neutral shoes that I enjoy, but it certainly is not a bad option.) 
Overall:  (A firm riding stable neutral shoe that does lack some responsiveness but ultimately provides a pleasant experience to the runner.)           

Interested in purchasing Karhu’s Fusion 2021? Available at Fleet Feet here. Using this link helps support our work at Doctors of Running! Thank you so much.


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Thanks for reading!


Dr. Matthew Klein is a 140 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.  He is particular to less cushioned shoes and close to the ground shoes, but can handle a little cushion when he gets beat up. IG handle @kleinrunsdpt

Dr. David Salas is a 135 lb male with notable PRs ranging from 3:54 in the 1500m to 1:08:36 for half marathon. He typically runs 60 to 70 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

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

Bach Pham MS
Marketing and Social Media Manager
Master of Arts in Cultural Anthropology

***Disclaimer: These shoes were provided free of charge in exchange for a review.  We thank the  people at Karhu USA 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 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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