Karhu Fusion 4.0 Review

The Karhu Fusion 4.0.

The word fusion typically refers to the process of joining two or more elements together, like blending two different cuisines (hello, sushi burritos).

The Karhu Fusion 4.0 is not as tasty as a sushi burrito, but it shines in its own right, combining a nitrogen-infused midsole with an EVA-based fulcrum piece. The result is a balanced ride–a fusion, if you will, of stiff and flexible elements.

In the Karhu lineup, the Fusion is positioned as a propulsive option that fits somewhere in between the Ikoni, their everyday trainer, and the Mestari Run, their max-cushioned shoe.

But can the Fusion 4.0 live up to its energetic reputation? Read on to find out.

Tech Specs

Karhu Fusion 4.0

Weight 7.9 oz (W), 10 oz (M)
Stack height (heel/forefoot) 28 mm/22 mm
Heel-to-toe drop 6 mm
Category Neutral
Surface Road
Price $160
Comparable to... On Cloudgo, Saucony Kinvara 14

Ride

Cushioning

Energy

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New year, new midsole

The fulcrum piece in the midsole of the Karhu Fusion 4.0.

When it comes to all of the “new year, new me” trends, Karhu went all in. The Fusion franchise isn’t going on a crash diet or participating in Dry January, instead it’s sporting a completely redesigned midsole.

The Fusion 4.0 still uses Karhu’s proprietary AeroFoam cushioning, only it’s now nitrogen infused, giving it a more energetic and reactive feel. And it’s done something that most New Year’s Resolutioners can only dream of doing–bulked up while losing weight.

No, the Fusion didn’t need any protein powder for this bulking. Instead, shoe designers added an extra 4 millimeters of midsole foam underfoot for an even cushier ride. And somehow, the Fusion managed to lose an ounce in the women’s model and a half-ounce in the men’s.

“I’m not sure if it’s the nitrogen-infused foam, the slight rocker shape or both, but either way the Fusion feels smooth and firm with just the right amount of energy return I’d expect from a daily trainer,” says Nate, who tested the Fusion during a seven-mile run with his dog, Nora.

The rocker shape Nate refers to is the fulcrum unit, and it’s another part of Karhu’s proprietary technology. Every Karhu shoe features a fulcrum piece in the midsole, which functions like a rocker to guide your feet through the gait cycle. Picture the fulcrum like a see-saw; as you land, it uses your body’s natural momentum to roll forward into the next stride.

“I did a few lunchtime runs in the Fusion that weren’t speedwork per say, but time was of the essence so I could get back to work,” says Mandy. “Thankfully, the Fulcrum pushed me forward and each strike of my foot snapped back up when it hit the ground, turning over my legs quicker and helping me get back to my desk on time.”

Most of our reviewers enjoyed their runs in the Karhu Fusion 4.0, but I couldn’t help but notice the fulcrum piece digging uncomfortably into my arch as I ran. I’ve been struggling with chronic foot pain due to plantar fasciitis, and the shape of the midsole didn’t agree with me.

While most running shoes are pancake-flat on the inside, Karhu’s midsole contours to the shape of your arch for added support–but this makes it tricky if your arches don’t exactly match up with the shape of the shoe. We typically recommend adding arch support by taking out the sockliner of your shoe and replacing it with an insole, like Superfeet or Currex, which come in different arch profiles for a more customized fit.

If you’re not sure about the type of arch support you need, be sure to head into your local Fleet Feet for an expert one-on-one outfitting. Fleet Feet outfitters use 3D fit id® foot scanning technology to gather information about your feet and the support they need by taking precise measurements of your foot length, width and arch height.

3D fit id® technology shapes a perfect fit

A pair of the Karhu Fusion 4.0.

Not only does the Fleet Feet outfitting process help you find the support you need, but it also helps shape Karhu running shoes. That’s right–all of our favorite Karhu models are shaped using data collected from our fit id® scans.

“The Fusion 4.0 really fits like a glove for me,” says Travis. “The heel felt nice and secure and I didn’t have any issues with slippage. The upper material was breathable and fairly light, as well.”

The shoe’s glove-like fit can also be attributed to Karhu’s M-lock technology, a series of two side panels on either side of the shoe denoted by the M-shape on the upper. The panels secure the shoe around your foot while an internal bootie construction adds extra support.

“The upper of the Fusion reminds me of a cozy sweater,” says Mandy. “As I slide the shoe on, it wraps my foot in a warm, fuzzy hug.”

Don’t worry, the upper isn’t actually warm and fuzzy. While the material is called Ideal Knit, it’s actually a mesh material made with breathability zones to keep your feet cool and dry. Be sure to pair the Fusion 4.0 with a pair of performance running socks for the best experience.

Karhu Fusion 4.0 vs Karhu Fusion 3.5

The heel of the Karhu Fusion 4.0.

Tech Specs

Karhu Fusion 4.0

Karhu Fusion 3.5

Weight

7.9 oz (W), 10 oz (M)

8.8 oz (W), 10.5 oz (M)

Stack height

28 mm/22 mm

24 mm/18 mm

Heel-to-toe drop

6 mm

6 mm

Price

$160

$150

The Karhu Fusion 4.0 has managed to add cushioning while shaving weight, allowing the shoe to feel more nimble while also more protective, thanks to the higher stack height underfoot.

I think the Fusion 4.0 is definitely an upgrade,” says Travis. “Runners who have liked the Fusion in the past will still like this version, since it doesn’t feel drastically different. However, I think the new nitrogen-infused midsole foam will entice some new fans as well.”

And, since we’re all about knowing our worth in 2024, the Fusion 4.0 has updated its price tag from $150 to $160. While we’d love to say this is a result of a newfound sense of self-worth, it’s more likely due to industry expenses increasing across the board.

How does the Karhu Fusion 4.0 compare?

We took a look at some comparable models to see how they stack up against the Karhu Fusion 4.0. Here’s what we found:

Tech Specs

Karhu Fusion 4.0

On Cloudflow 4

Saucony Kinvara 14

Weight

7.9 oz (W), 10 oz (M)

7 oz (W), 8.2 oz (M)

6.2 oz (W), 7 oz (M)

Stack height

28 mm/22 mm

31 mm/23 mm

31 mm/27 mm

Heel-to-toe drop

6 mm

8 mm

4 mm

Price

$160

$160

$120

Thanks to its relatively light weight and moderate stack height, we found the Karhu Fusion 4.0 to feel most similar to the On Cloudflow 4 and the Saucony Kinvara 14. All three shoes offer a firm underfoot feel and a rocker shape that’s great for uptempo runs.

While the Fusion isn’t quite as light or as cushioned as the other two shoes, the updates have brought it closer to a true speedwork shoe that can handle uptempo paces.

A pair of the Karhu Fusion 4.0. One shoe is facing upward, and one is upside-down with the sole facing up.

Who is the Karhu Fusion 4.0 best for?

Based on our test runs in the Fusion 4.0, we recommend it to runners who like a firm cushioning. The AeroFoam midsole offers a stiff underfoot feel, making it ideal for those who like a rigid platform to push off from while picking up the pace.

Its stack height of 28 millimeters in the heel and 22 in the forefoot is relatively low (most of the shoes we test hit at least 30 millimeters in the heel), making it a great choice for runners who tend to feel unstable in higher shoes.

“I gravitate towards shoes with a firmer underfoot experience,” says Nate. “I like the sense of stability I get and I also appreciate a stronger connection with the ground. The midsole of the Fusion 4.0 is on the firmer side while still having some life, making for a fairly energetic run.”

However, we all agreed that we’ll be reaching for something with more cushion for our long runs, like the Karhu Mestari Run.

The Fusion 4.0 is available in both medium and HiVo sizing. Karhu’s HiVo (high volume) sizing accommodates volume as well as width. Karhu also recommends this shoe to runners with high, rigid arches. If you’re not sure about your width, volume, or arch height, don’t forget to stop by your local Fleet Feet for an expert one-on-one outfitting.

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