Shoe Review: Karhu Fusion 2021

A pair of Karhu Fusion Ortix 2021 running shoes

The updated Karhu Fusion 2021 is versatile and comfortable for all of your training.

To build the Fusion, Karhu used data points from more than 1.3 million 3D foot scans taken during Fleet Feet’s fit id® outfitting process. The scans measure a runner’s foot length, width, arch height and more to create a 3D model that can be used to find the best running shoes.

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Armed with that knowledge, Karhu engineered the Fusion 2021 with different attributes for men and for women. But both models have the same cushioned ride and propulsive performance.

Fleet Feet reviewers tested the new Fusion to see how it feels on their everyday runs. After a few hundred miles on the road, here’s everything you need to know about the latest Karhu Fusion 2021.

Tech specs

Karhu Fusion 2021


8.8 oz (W) 10.5 oz (M)


6 mm




Everyday training


Road, track

Breathable Mesh Creates Comfortable, Secure Fit

The mesh upper on the Karhu Fusion 2021

The new Fusion 2021 sports a comfortable and breathable mesh upper that delivers a snug fit.

The first thing testers noticed about the updated Fusion is its soft step-in feel. Karhu used a padded tongue that’s coated in a smooth fabric, which gives the shoe a plush feel and soaks up the pressure created by the laces.

“I really noticed the cushioned tongue and heel collar when I first stepped into the shoe,” one reviewer says. “They’re both covered in the same soft, silky material, which feels great.”

Designers used a two-layer mesh system through the midfoot and forefoot. The lightweight outer layer uses perforations to vent heat and moisture during your run, and the thin inner layer creates a smooth, seamless fit for chafe-free performance.

Karhu used its signature M-shaped overlay—called the M-Lock—on the midfoot to create additional structure for a more locked-down fit, while traditional lacing creates a familiar feel.

On the back of the shoe, designers included a rigid heel counter. The counter helps create a snug feel that secures the overall fit of the shoe.

Fleet Feet testers praised the shoe’s dialed-in fit.

“I love how snug the heel and midfoot are, and then it opens up in the toe box,” says one tester.

Another tester agrees: “It fits true to size and length, and the toe box feels just right for my foot.”

Responsive Cushioning, Fulcrum Tech Propel Ride

The side of the Karhu Fusion 2021 running shoe

The cushioning beneath the Fusion gives you a stable platform to log all your runs.

Karhu used its AeroFoam cushioning to power the Fusion 2021. The AeroFoam is protective on hard pavement, and it has a quick response.

The defining feature of Karhu shoes is the fulcrum technology embedded in the midsole, and the Fusion uses the same.

The fulcrum helps turn the energy from your landing into forward momentum to carry you farther. The Fusion’s version of the fulcrum is shorter relative to some other Karhu shoes, which encourages a quick turnover.

One reviewer says the Fusion 2021 is a game-changer.

“This is the most eye-opening experience I've ever had with a Karhu shoe,” he says. “I really feel the fulcrum propulsion experience. I feel the benefit of my foot propelling through the midfoot strike. You combine that with the lightweight nature of the shoe then you find yourself running smoothly and efficiently.”

Karhu’s AeroFoam midsole adds to the efficient ride. The foam is lightweight and bouncy without being too soft or mushy. One reviewer says that’s his sweet spot for an everyday training shoe.

“The midsole makes the Fusion pretty versatile,” he says. “It has enough cushion to feel protective on longer runs, but it’s not heavy, and it doesn’t completely dampen the feel of the ground. It won’t be the fastest shoe you own, but it can push the tempo a bit, too.”

Extra blown rubber covers high-wear areas—the heel and big-toe side of the forefoot—but the rest of the foam is exposed. A Fleet Feet tester who put more than 350 miles on the Fusion says he didn’t see any problems with the shoe wearing out.

Difference Between Women’s and Men’s Fusion 2021

The heel of the Karhu Fusion 2021 running shoe

Data shows men and women have differently shaped feet, on average. So, Karhu designed its men’s Fusion and women’s Fusion to accommodate those differences.

After studying the shapes of men’s and women’s feet from their 3D scans, Karhu found the two most significant differences were the circumference of the instep and the forefoot. Designers then tailored each shoe to those differences.

Instead of taking the same last as the men’s model and making it smaller for women, Karhu made the Fusion with an instep and forefoot to match a woman’s specific foot shape. Karhu calls it HeraFit.

Final Thoughts

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By combining signature Karhu technologies with more data, the new Karhu Fusion 2021 is a step forward for the iconic Finnish running brand.

Karhu designed the Fusion using data points from more than 1.3 million 3D foot scans, which means it’s shaped to fit a majority of people. Plus, engineers used the data to highlight differences in men’s and women’s foot shapes to ensure a good fit for both.

The fulcrum inside the midsole creates a propulsive feeling, and the lightweight AeroFoam midsole creates a responsive, stable ride. Fleet Feet reviewers say the newest Fusion is ideal for everyday training, recovery miles, and long and mid-tempo endurance workouts—it’s even great for walking the dog.

When you need a versatile shoe with a tailored fit, the Karhu Fusion 2021 checks all the boxes.

Still not convinced? Don’t sweat it. Fleet Feet's return policy means you can test drive your shoes and gear without risk. If you’re not happy with the way your gear performs, looks or fits, we’ll take it back within 60 days. Plus, you’ll get free shipping on orders over $99 and free return shipping on all orders. That's our Happy Fit Guarantee.

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