Karhu Fusion 4.0 Review: Bringing Modern and Classic Together
By Andrea Myers & David Salas
Karhu Fusion 4.0Price: $160 at Karhu
Weight: 10.7oz, 302g (men's size 9), 8.8oz, 250g (women's size 8)
Stack Height: 6mm
Drop: 28mm/22mm
Shoe Purpose: Neutral daily trainer
Pros: wider toe box, overall comfortable fit
Cons: high weight, particularly for a lower stack shoe
RUNNING SHOE SUMMARY
The Karhu Fusion 4.0 is a lower stack, neutral daily trainer with a subtly rockered ride, thanks to a quarter length version of Karhu's Fulcrum+ Propulsion unit. Version 4.0 features an updated AeroFoam midsole that is now nitrogen-infused, which provides a nice balance of cushion and responsiveness. The updated IdealKnit upper was designed to be more breathable, although I (Andrea) did not get to test that out in the Connecticut winter. The Fusion 4.0 is a nice option for those who are looking for a more classic, low stack design with a more modern feeling midsole.
Past Model: Karhu Fusion 3.5
FIT
(To learn how a shoe should fit, check out our full podcast on fit by Matt Klein.)
David: The Karhu Fusion 4.0 fits true to size in my Men's 9.5. The width through the heel and midfoot are normal with a slightly wide forefoot. The upper mesh material is decently comfortable and is a tad on the thicker end. The heel collar is very built up with padding and does have a pretty rigid heel counter. The instep is pretty forgiving and the shoe is pretty easy to get on and off. The midfoot has decent space and integrates pretty well with the tongue. The tongue is partially gusseted and on the thinner side. The forefoot does have enough for room for swelling accommodation and toe splay. All around this is a pretty balanced fit and upper not steering too far into the snug or loose category. The upper material has a little bit of stretch to it, but did well for me in all instances where I needed it to. I do think I would like a little more padding on the tongue, as I do get a bit of biting from the laces. Otherwise the dimensions work pretty well for my foot.
David's Typical Size: Men's US Size 9.5
Shoes that have fit David well: Nike Vomero 17, Saucony Ride 16, Puma Velocity Nitro 3
Shoes that have fit snug: HOKA Arahi 7
Shoes that have fit large: Salomon Aero Glide
Andrea: The Karhu Fusion 4.0 fits short, with Karhu officially recommending on their website to go 1/2 size up. I received a 10 instead of my normal 9.5 and it fit correctly in both length and width for me. The toe box provides enough width and volume for comfort without allowing foot translation in the shoe. The midfoot and rearfoot are normal width and Karhu's M-lock lacing makes it easy to lock my feet down without needing multiple adjustments during the run. The partially gusseted tongue is thin and lightly padded. It stays securely in place, but I did experience mild discomfort on the dorsum of my ankle due to a stiff seam between the main part of the tongue and the thinner edge material. This was fixed by wearing thicker socks on subsequent runs, but could cause an issue for those who run sockless. The heel counter is fairly rigid and wraps around both sides of the calcaneus. The heel collar has ample padding and I experienced no heel slippage or irritation in this shoe. The IdealKnit upper is soft and comfortable, but it is on the thick side, and would likely be quite warm in the summer.
Andrea's Typical Size: Women's US Size 9.5
Shoes that have fit Andrea
well: New Balance Beacon v3, Brooks Hyperion Max, Topo Cyclone 2, Nike
Vaporfly 3, Altra Via Olympus 2
Shoes that have fit snug:
Saucony Kinvara 14 (length and toe box width), Altra FWD Experience
(length and width), Hoka Cielo Road (toe box width), Saucony Endorphin
Speed and Pro 1-3 (length)
Shoes that have fit large: Adidas Boston 12 (length), Adidas Adios 8 (length)
Doctors of Running Checklist
Is This a Good Shoe for Walking: Yes
Is This a Good Shoe for Standing: Yes
Is the Forefoot Flexible: Somewhat
How Flexible is the Shoe: Rearfoot stiff, Forefoot somewhat
Is This a Good Heel Bevel: Yes
Recommended for Haglunds: No
Recommended for Sockless: No
Durability Expectation: Average
PERFORMANCE
David: The Karhu Fusion is a daily running shoe with a relatively low to moderate stack height by todays standard. The shoe does maintain a relatively firm ride throughout, but does have enough cushioning for daily mileage and long runs. This does use the fulcrum technology seen in many Karhu models with this one being 1/4 length of the shoe. The ride of the shoe is relatively fluid and gives you a gentle rocker with a somewhat flexible forefoot. The fulcrum and heel bevel give you a sensation that you are gently getting pitched forward. This combined with the flexible forefoot give you a relatively natural feel underfoot. The weight of the shoe feels consistent with most daily training shoes. I would not classify it as light or heavy. The overall construction feels pretty built up and this could easily operate as a workhorse shoe. The outsole construction does fit more of a road profile, but due to the firmness and decent stability I was able to take this off road as well. The transitions do feel pretty natural underfoot for me and I could just relax into my runs. This does feel best at daily mileage paces and I would not necessarily reach for this on workout day. Those wanting a neutral workhorse shoe with a small amount of guidance and a flexible forefoot can definitely look into this one.
Andrea: The Karhu Fusion 4.0 feels like a modern version of a classic lower drop daily trainer. The nitrogen-infused AeroFoam has some nice pop to it and the mildly rockered ride provides a hint of forward guidance in an otherwise very neutral shoe. Between the comfortable fit and neutral ride, I basically forgot the shoes were on my feet during my test runs. The only thing holding this shoe back from being a do-it-all performance trainer is its weight. With a 28mm/22mm stack height, the Fusion 4.0 is heavy at 10.7oz for a men's 10 and 8.8oz for a women's 8. I think some of that weight is in the upper, so perhaps the next version could see this shoe get even lighter.
The shoe feels like its stated 6mm drop and I found it well suited to midfoot landings. The 1/4 length Fulcrum means that the heel bevel and forefoot rocker are small compared to the Ikoni, but even with the muted heel bevel, I had no issues with the rearfoot getting in the way. The later forefoot rocker provides a mild forward rolling sensation, but is by no means aggressive or intrusive. The forefoot rocker is further muted by some flexibility in the forefoot, which really gives the shoe a neutral feel from initial contact to push off for me. I primarily used the shoe for easy runs, but I was able to do strides in it without feeling like I was fighting the shoe. Outsole traction is average due to the partial rubber coverage. The outsole has rubber coverage in high wear spots, including the lateral midfoot where I land, but the lack of rubber on the medial midfoot may result in early wear for some. I would expect average durability from the outsole in this shoe.
STABILITY
David: The Karhu Fusion 4.0 is a neutral shoe but does have some elements that help with giving it a stable platform. The stack height is not very high and the base of the shoe underfoot is decently wide through the forefoot. This gives you a good amount of ground feel and cross sectional area for this platform as you are pushing off. The midfoot is undercut a tad, but it is consistent with other neutral shoes on the market. The heel counter and fit does give a good hold and a secure feeling to the platform. I geometry and fulcrum does seem to give you sense of guidance as well. This is certainly still a neutral shoe, but does give you some gentle guidance and stability needs.
Andrea: The Karhu Fusion 4.0 is marketed as Karhu's neutral shoe, and I would agree that is exactly what it is. There are certainly no traditional stability features in the shoe. The relatively narrow rearfoot and midfoot open up into a slightly wider base in the forefoot, but not enough to be considered a stable neutral feature. The 1/4 length fulcrum (heel bevel and forefoot rocker) do not provide significant enough forward guidance to fall into the stable neutral category, either. There is mild sole flaring in the rearfoot, but not enough to make up for the potential lack of stability in the narrow midfoot. There is a medial sidewall in the midfoot that I did not notice at all while running, but may provide some mild resistance to early medial motion. The Fusion 4.0 is a truly a neutral shoe, while the Ikoni line is a better choice for those who prefer a stable neutral shoe.
Thoughts as a DPT: A Nice Balance
By David Salas
When working with neutral shoes, it is important to not overdo things. The ultimate goal of any neutral shoe is to make a shoe that gives you a natural feeling and comfortable experience. If there is too much going on that takes away from the word natural. The Karhu Fusion 4.0 does a good job of having additional components without it taking away from the experience. The shoe introduces a very gentle rocker profile that still operates with a very natural toe off. The way they get away with this is their heel geometry and use of the 1/4 length fulcrum.
RECOMMENDATIONS
David: I really enjoyed my experience in the Karhu Fusion 4.0, though I do have a couple of recommendations. Because of some of the biting I would get from the laces, I do feel some padding to the tongue would be beneficial. I think that would also round out the rest of the shoe, as it does feel like a relatively plush upper outside of the tongue. The geometry and forefoot also provide a pretty fun ride. You almost want to pick the pace up, and it feels like it if even lost 1-2 ounces this could really make this a versatile shoe for workouts too.
Andrea: The Fusion 4.0 further affirmed my love of the fit and ride of Karhu shoes. It is a no-nonsense daily trainer that lets you feel the ground and use your mechanics to do the work. It could be a great do-it-all shoe if it could lose some weight. This shoe is technically in the same category as the Saucony Kinvara 14 and the Brooks Hyperion, but it is over 3oz heavier than both of those shoes, which limits its use as a performance trainer. The thick upper must be a big part of it, because there isn't that much to the relatively low stack midsole or partial rubber coverage outsole that would make the shoe weigh this much. I hope that Karhu will figure out how to further pare down the upper to make this a shoe that could be used for workouts in addition to easy runs, because the geometry would lend itself well to faster running.
WHO IS THIS SHOE FOR?
David: The Karhu Fusion 4.0 is a low to moderate stack height training shoe that gives you gently rockered yet natural feeling ride. I would recommend this for someone that likes lower riding neutral shoes, but may struggle with some of the flexibility and lack of stability that usually comes with them. This shoe does give you small amount of guidance and structure for how natural it feels at the stack height.
Andrea: I would recommend it to runners without stability needs who are looking for a comfortable, lower stack shoe for easy runs. Be sure to follow Karhu's recommendation to go up 1/2 size in this shoe. This is also a nice option for runners and walkers who need a shoe with a less aggressive rocker profile. At $160, it is priced similarly to other higher end daily trainers, but its higher weight does somewhat reduce its value. I hope that Karhu is able to improve future versions, because they will have a real winner if they do so.
GRADES
David
Fit: A- (Good dimensions throughout with a nice hold on the foot. A little bit of biting from the laces.)
Performance: A- (A very pleasant ride for a lower riding neutral shoe. Natural transitions with a small amount of guidance. The shoe feels like it wants to pick the pace up sometimes but weight does seem to hold it back from higher versatility. Training mileage great though.)
Stability: B (Definitely neutral, but the fulcrum and wide base in the forefoot make for a nice grounded yet rolling experience.)
Value: B+ (The experience feels comparable with other shoes at this price point. Long term durability with so much exposed midsole will need to be tested, as well as weight reduction to put it one more shelf up. For daily miles though this is a nice trainer.)
Personal: A- (Smooth and natural transitions for daily mileage. I just wish I could push the shoe a little bit more.)
Overall Design: A- (A good example of low riding neutral trainer with some elements of guidance)
Andrea
Fit: A (fits well after following recommendation to go up 1/2 size, with a comfortable toe box and secure lockdown. Only negative is thick upper may be warm in the summer.)
Performance: A- (A classic neutral trainer with some pop from updated midsole. Could make a great performance trainer with significant weight loss.)
Stability: (neutral) C (A very neutral shoe with minimal stability features.)
Value: B+ (High weight reduces the shoe's otherwise high value)
Personal: A- (I have really enjoyed using the shoe for easy runs, but would love to have a lighter version to use for workouts. 2025?)
Overall Design: A-
SHOP | SUPPORT DORRunning ShoePrice: $160 at Karhu
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FURTHER READING
On Cloudmonster 2 - A wider upper marks the big change for this versatile max cushion offering
Karhu Mestari Run - Karhu brings their first max cushion offering in a very stable package
Monday Shakeout: What is Drop and Why It Doesn't Always Matter
Asics GEL-Cumulus 26 - A fairly large update unveils a new outsole and new setup
Puma Velocity Nitro 3 - A little more cushioned in v3, this workhorse remains a solid all-arounder
Monday Shakeout: The Science of Sole Width
Saucony Endorphin Speed 4 - Saucony's premier performance trainer gets aggressive again
Hoka Cielo X1 - Hoka's new super shoe hits the market with an aggressive rocker
Hoka Arahi 7 - The stability favorite gets an upper update for the seventh iteration
Monday Shakeout: Challenges with Large Toe Spring
Mizuno Wave Inspire 20 - A streamlined update to Mizuno's guidance based stability trainer
Altra Timp 5 - A long distance, zero drop trail running shoe
Monday Shakeout: Phases of the Swing Gait
Deckers X Lab NBK 2.0 - Deckers lightens up their street sneaker, making the comfortable more so
Altra Lone Peak 8 - A lower stacked, protective shoe built for technical trail running
Find all Shoe Reviews at Doctors of Running here.
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NEXT: On Cloudmonster 2