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 Fusion Ortix 2020 Multiple Tester Review

Karhu is currently updating their entire line of running shoes with new uppers and complete reshaping of the shoes based on FIT ID measures from Fleet Feet stores. For the Fusion 2020, this meant slimming the heel, decreasing the volume of the toe box, and putting in the ortix insole in order to fit the "typical foot" based on these 100,000+ foot scans. We also reviewed the new Ikoni Ortix this year (and loved it), and that review has a lot of details regarding Karhu as a company as well as the research they have been conducting in conjunction with Fleet Feet. You can check out that review HERE. Does the Fusion 2020 live up to the hype that the Ikoni delivered? Read on to see what we thought.

Specifications (per Fleet Feet)
Weight: 10.9oz (men's), 9.3oz (women's)
Drop: 10 mm
Classification: Neutral Daily Trainer


Matt: The Karhu Fusion Ortix features one of the most comfortable uppers I have ever experienced.  The sole is firm and stable thanks to the fulcrum technology that will work very well for those that land midfoot forward.  Surprisingly light feeling for an almost 11 oz shoe.  A unique ride that will make you curious about the Finnish shoe brand.

David: The Karhu Fusion Ortix is a neutral trainer for the roads and track that features a fulcrum system unlike many shoes out there. The fulcrum and firm midsole contribute to a shoe that likes to run on the faster end with a high level of stability (for a neutral shoe). The upper is very breathable and the shoe fits well across all contours of the foot.

Nathan: The Karhu Fusion Ortix 2020 is the newest line of Karhu running shoes that utilized FIT ID foot scans to help create its shape, which makes for an extremely comfortable and well fitting shoe. A top tier (best I've felt all year) upper adds to this comfort with a ride that is faster than its weight thanks to the fulcrum technology. When it comes to neutral shoes, you won't find anything that beats this shoe in stability, which makes it a wonderful choice on long runs and when your legs are feeling beat.


Matt: The Karhu Fusion Ortix fits very true to size for me in my normal size 10.  The Ortix has one of the most comfortable uppers I have experienced.  Upon initial try on, the toe box was a little snug.  After a few miles, it loosened just a hair and fit very smoothly across the entire foot.  I have worn this shoe sockless from 15 miles onward exclusively and the inner aspect of the upper is extremely smooth.  The heel and midfoot are a little wider while the forefoot hits a little snug, but the mesh stretches well with time.  The heel cup is fairly rigid but is more external.  The heel collar is cushioned and protects the calcaneus well.  The heel is held very well by the heel cup without causing excessive pressure.  The midfoot locks down the foot well and I have had not slippage issues.

David: The upper of the Karhu fits very well to the foot without being excessively tight in any areas. When tied snug, the fulcrum might encroach on the midfoot slightly (either on medial or lateral side). The heel cup is rigid and won't allow for any slippage. I like how it is rigid through pretty much the whole calcaneus and then softens up about 1.5 cm from the top of the collar of the shoe. The heel cup is also nice because it is not overly narrow and does not "squeeze" the calcaneus, rather just hold it.

Nathan: This shoe fit very true to size for me. Since its previous editions, it has slimmed down a bit with a slightly lower volume forefoot. However, it still has ample space for me and the engineered mesh upper accommodates well. The heel is snug and secure, but has enough padding to protect the calcaneus and achilles from irritation. The midfoot is the second of Karhu shoes to include the Ortix insole, which is shaped by FIT ID scans at Fleet Feet and has a slight curvature to it to match the arch of the foot. Like the Ikoni was for me, it worked great with my foot and was very comfortable without impeding on my arch at all. The midfoot is also very secure because of the seamless mesh upper, the M-Lock lacing cords, and some of the overlays on the midfoot. For me, this was a great fitting shoe and even better than the Ikoni in terms of fit.


Matt: The Karhu Fusion Ortix has a firm ride from heel to toe.  The ride from the midfoot forward is very smooth and fulcrum helps roll you forward.  Combined with the fulcrum and slightly wider forefoot last, the ride is extremely stable at the midfoot and forefoot.  The posterior flare at the heel causes jarring landings.  There is also a 10mm drop, which is fairly mild landing farther forward, but is noticeable landing in the posterior section.  The toe off is extremely smooth as the forefoot has wonderful flexibility, especially when being transitioned forward by the fulcrum.  The fulcrum provides a gentle rocking forward sensation that should be good for those who are looking to train their talucrural joint (ankle joint) to move well. 

David: The ride of the Karhu Fusion Ortix is definitely on the firm end. The smooth transition of forces that the fulcrum provides is most appreciated when running at moderate paces, making this a great progression run shoe.  The shoe was almost overly firm and stable for me to run through large road miles on... but ended up feeling amazing through grass and dirt.  Ironically, I felt like this road shoe was the best shoe in my current rotation for unstable non trail surfaces. The shoe also features a 10 mm drop ratio and favors to more mid/hind foot strikers. As a forefoot striker, this may be why I only enjoyed the fulcrum at faster paces.

Nathan: Given the 3/4 length fulcrum, which is made of a more dense foam, the ride is more on the firm and stiff side. Firm in that it doesn't give during landing, and stiff in that there isn't much flexibility during heel stike to toe off. This, combined with the forward propulsion that the fulcrum provides also makes this shoe run quicker than its weight and it feels responsive while doing it. There is a posterior heel flare that does create a slightly harsh landing for those who heel strike, but the transition from the heel to the forefoot is smooth and snappy. The slight heel bevel also does help a bit. I found the ride got better later in miles, as the fulcrum assisted in the heel toe transition as I fatigued. The 3/4 length fulcrum made toe off particularly smooth for me, as the end of the fulcrum is near the metatarsal heads, helping get up on the toes for push off. Unlike the Ikoni, there was not as drastic of demand placed on the calf and there was no acclimation period for me.

David's Outsole @ 40 miles


Matt: For a neutral shoe, the Karhu Fusion Ortix provides a very stable ride.  This is mostly due to the fulcrum, which stiffens the ride in the frontal plane and helps guide the foot forward rather than side to side.  There is no post or medial support, but the fulcrum does provide additional stability in the midfoot.  Those who need mild medial or lateral support will enjoy this ride as provides stability but not in a biased manner.

David: I was trying to convince myself that this was a neutral shoe for the first few runs I ran in the Karhu Fusion Ortix. The shoe is incredibly stable with no posting. I believe this in part to the firm midsole and and the fulcrum that almost stirrups the midfoot on the medial and lateral sides of the midfoot. As noted earlier, I trust this shoe with no hesitation in grass and dirt. This is much more than just a road or track shoe.

Nathan: This is an incredibly stable neutral shoe without trying to venture into the realm of "controlling pronation". The firm ride, full contact outsole, secure upper, and multidirectional component of the fulcrum make this shoe very forgiving without imposing its will on the foot. It is a rare shoe that could be applicable for both people who consider themselves "overpronators" or supinators. For those who typically do well with a posted shoe but would like to transition to a more neutral shoe, this could be a great option for you.


Matt: The Karhu Fusion Ortix is a certainly a daily trainer.  At almost 11 oz, the shoe should feel heavy, but rides like a much lighter shoe.  I personally found this shoe best for moderate to uptempo runs.  It does well hitting 6 to 7 min per mile pace, but the weight holds it back from going any faster.  The excessive posterior flare also makes the ride very clunky for those who find their rhythm landing farther back.  Those who land further forward (check out Dr. Salas) will not have an issue with this.

David: The Karhu Fusion Ortix is a daily trainer that enjoys picking up the pace. With that said, I was able to get it down to about 5:20 pace on progression runs with it still feeling pretty snappy. Though the shoe remained responsive, the weight just makes it hard to go much faster in workouts. Overall though, the shoe can hold it's own in both training/progression runs and tempo workouts.

Nathan: Why is this shoe as quick as it is? It is a bit heavier and feels like a robust shoe, but I loved tempo days in these things (5:30 - 6:20 min/mile). I spoke to this above, but the fulcrum and more firm ride take the weight specs and throw them out the window. On the other end, I actually found it difficult to drop my pace way down for recovery days. I ended up pulling this one out for moderate paces to tempo runs.


Matt: To echo Dr. Salas's comments, I was surprised how durable this shoe is.  I have over 100 miles on the Ortix and have very little on the outsole.  There is a little wear on the posterior lateral heel, which is normal for me, but not when it takes me 80-100 miles to get there.  I have used this shoe on road, trail, track and can't seem to make a dent in the sole.  The fulcrum and cushioning feel almost exactly the same if not a little more forgiving than when I first tried this shoe on.  Thus I expect this shoe to last longer than the industry standard of 300-500 miles for many.

David: I thought this shoe's outsole was going to break down much quicker than it has. It's surprisingly durable and the firm midsole shows no signs of breaking down in the foam. There is a little wear in the forefoot (to be fair, I run on more than just road) and I see this shoe lasting somewhere in the high 300 low 400 mileage range.

Nathan: Like the Ikoni was for me, this is one of the most durable shoes I've had. The outsole has no signs of wearing down, the fulcrum midsole hasn't changed in the slightest, and the upper looks great. I think this shoe is going to run me to a high number of miles (like the Ikoni has)...more like 600-800 range.


The concept of a fulcrum in a shoe is interesting and I can understand it is supposed to emulate the ankle rocker portion of the foot.  Given that Karhu is trying to emphasize certain parts of the biomechanics of the foot, I am confused to see such a massive posterior heel flare.  As I have discussed before, flaring the outsole past where the foot sits is a great way to provide natural resistance to motion.  This works well to guide the foot forward and restrict frontal plane motion when placed on the medial or lateral side of the shoe.  However, when there is posterior heel flare, little good can come out of it for anyone who lands at the rearfoot.  A posterior heel flare may cause an early initial contact of the foot before the body is ready.   During the last part of terminal swing (right before the foot hits the ground) all the muscles of the lower extremity are activating to brace for impact.  When that impact occurs early, the muscles aren't really ready and they may take time or may not adapt to this as that extra posterior extension is outside of the body's normal proprioception.  Having a posterior heel flare also creates a longer lever arm, meaning that the anterior tibial muscles must work harder since the front of the foot will travel in a greater arc to reach foot flat position.  In English, this means that a posterior heel flare causes an early landing and overworks the anterior tibial muscles.  Which can put people at risk for irritation of the anterior tibial muscles or some variant of medial tibial stress syndrome in those that are not properly conditioned or are sensitive to this.  So I applaud Karhu for the fantastic upper and unique biomechanical design, but please change that heel.


I spend a lot of time walking through the biomechanics of the fulcrum in my Ikoni review, so if you want more on that, check out my "Thoughts as a DPT" section there. The one major difference here with the fulcrum is that the Fusion has a fulcrum that spans to 3/4 length of the shoe. As I mentioned above, this places the "hinge/end" point of the fulcrum near the metatarsalphylangeal joints (MTP joint or the ball of the foot). For some of the runners I've worked with who are dealing with MTP joint pain or even Morton's neuromas, the placement of this fulcrum has really helped. No, I don't have the research to back this up, but having the hinge point here does change the demand of motion through the MTP joints (in theory decreasing it) because you are rolling on the toes via the end of the fulcrum and not the MTP joints. Having less strain through these joints has helped some of my patients have less pain even with walking.

- Nathan Brown PT, DPT, MS


Matt:  My biggest recommendation is to get rid of the massive posterior heel flare and replace it with an appropriately and biomechanically sound beveled heel under the foot (not behind it).  This contributes to an excessively firm ride (I like firm, but not like this) that could be so much better. I have high hopes for the fulcrum system, but that heel design needs to change.

David: I can't believe I am about to say this... but one recommendation I would have for this shoe would be to make the midsole a little more forgiving on roads.  If this shoe was designed for all terrains, I believe it would be ok, but for being a road shoe, I feel like I have to push the pace in these shoes to make the firm midsole comfortable. The fulcrum system is great and handles well through all terrains and quicker road miles (so no recommendations there).

Nathan: Two recommendations I've thought through for this shoe, although only one may be necessary to achieve the goal. One would be to soften the foam in the heel. The ride is overall just a bit too firm and heel strike is not as smooth as it could be (and I like more firm riding shoes). The other would be to rid this shoe of the posterior heel flare and match the heel for this shoe to the Ikoni Ortix. Heel strike on the Ikoni felt much better than the Fusion 2020, and I could see these changes making this shoe even one step better.


Matt: The Karhu Fusion Ortix is for the medium to narrow footed runner that lands farther forward looking for a stable ride with possibly one of the most comfortable uppers.  The firmer ride lends itself well to moderate to uptempo paces and the fulcrum creates a very unique ride that may propel some runners forward.  For those looking to try something unique and stable, try the Fusion Ortix out.

David: The Karhu Fusion Ortix is for a runner that want's to get a lot out of their shoe for moderate pace miles on the road and doesn't mind a little added stability on a neutral shoe. The shoe is firm, and so for those who like shoes on the more rigid end with a snappy feel when the pace picks up a little, this is for you! The shoe has a relatively high drop (10 mm) and so people who respond well to higher drop shoes may also want to check the Fusion Ortix out.

Nathan: For someone looking for a very durable, stable, and comfortable shoe that will do well for moderate to fast paces (and lots of miles), the Karhu Fusion 2020 is a great option to check out. It has the best upper I've worn all year and has a snappy ride that runs faster than its weight would suggest. This could be a great marathon shoe option given its ability to pick up the pace a bit as well as giving support as fatigue sets in.

Fit                     8.5/10 (-1.5 points for snug forefoot and wider heel.  Extremely comfortable upper)
Ride                  6/10 (-4 for clunky ride due to posterior heel flare)
Stability            9.5/10 (High stability from fulcrum despite neutral ride)
Speed                6/10 (-4 Heavy weight and lack of smooth ride prevent higher speeds)
Durability         9.5/10 (-.5 for normal heel wear  Highly resilient sole and cushioning)

Fit                     9/10 (can feel the fulcrum encroach on the midfoot a little when tied snug)
Ride                  8/10 (too rigid for easy days, has to be quicker paces to get fulcrum engaged)
Stability            10/10 (fulcrum almost acts as an unintentional post, as stable as neutrals come)
Speed                9/10 (great at moderate paces, a little too heavy for faster workouts)
Durability         8.5/10 (Relatively durable for road miles, some wear in outsole forefoot and midfoot)

Fit                     10/10 (best upper I've worn this year, ortix midsole felt great, held foot well with no slippage)
Ride                  7.5/10 (-2.5 harsh heel strike/slightly firm, high marks for snappy ride)
Stability            10/10 (3/4 length fulcrum does the job)
Speed                8/10 (-2 unable to hit top speeds or slow down enough for recovery days)
Durability         10/10 (no signs of wear, particularly in the midsole)

Total Score: 86.3% (M: 7.9/10  D: 8.9/10 N: 9.1/10 )

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.

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

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 *** for sending us a pair.  This in no way affected the honesty of this review. We put at least 35-50 miles on trainers and 10-25 miles on racing flats prior to reviewing them. Currently we have 106 miles (Matt), 38 miles (Nathan) and 42 miles (David) on our pairs. Our 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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