Shoe Review: Karhu Synchron 2021

The men's Karhu Synchron 2021 running shoe

⚠️ The Karhu Synchron 2.0 has been released!

Karhu's bestselling Synchron has been updated. We carry a full range of sizes and colorways in the Synchron 2.0.

Women's Synchron 2.0 | Men's Synchron 2.0

By the Fleet Feet Review Team. With over 80 years of combined experience, our review team is dedicated to providing in-depth gear and shoe reviews to help you find the best shoes and gear for your next run.

The Karhu Synchron 2021 brings premium cushioning to every run.

Karhu loaded the Synchron with its plush AeroFoam Nova cushioning for a soft and durable ride, and they topped it with a smooth mesh upper that delivers breathable performance.

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Fleet Feet reviewers logged their daily miles in the new Karhu Synchron 2021 to get a feel for how it handles different paces and distances. Here’s what they thought.

Tech specs

Karhu Synchron 2021


10.5 oz (W), 10.7 oz (M)


8 mm

Stack height (heel/toe)

23 mm / 15 mm




Everyday training



Mesh Upper, Internal Bootie Create Comfortable Fit

The Karhu Synchron 2021

Karhu wrapped the Synchron in its Ideal Knit mesh upper. The lightweight mesh has two layers: a breathable outer layer and a silky inner bootie lining. The outer layer looks sleek and sports vents to release heat and moisture, while the inner bootie is smooth for a blister-free feel.

A padded tongue and heel cup create a plush feel that’s enhanced by Karhu’s luxurious AeroFoam Nove cushioning.

Paired with the soft midsole, one of our testers loved the step-in feel of the new Synchron.

“The upper and the cushion are so plush and silky soft. The heel collar is cushy, and the tongue feels like it molds against the top of my foot,” she says. “They feel amazing on my feet.”

To hold your foot in place, Karhu added a rigid internal heel counter to the back of the shoe. The counter creates excellent structure that locks in the fit of the heel so you don’t experience any slippage.

Reviewers say the midfoot fits snugly, and the forefoot has enough room to let your foot splay as you land.

“I thought the Synchron was going to be too narrow in the forefoot, but it actually worked really well for my foot,” one tester says. “The toe box has a good amount of space. I don’t feel squished at all.”

Plush Cushioning Leads to Smooth Ride

The outsole on the Karhu Synchron 2021 running shoe

The Synchron is the softest ride in Karhu’s lineup. That’s because AeroFoam runs the entire length of the midsole for a consistent transition from heel to toe. It provides soft landings and a stable roll through the midfoot and into the takeoff.

Embedded in the midsole is Karhu’s Fulcrum technology. The H-shaped composite material sits directly beneath the midfoot to help transfer momentum from landing to push off.

While it won’t be the fastest shoe in your closet, one Fleet Feet reviewer says the Synchron powered her through long runs and recovery miles.

“They are a solid, neutral trainer,” she says. “They are best for recovery runs and long efforts that require some protection. I feel very supported in this shoe even though it’s neutral.”

While the foam gives the Synchron a soft ride, it’s also plenty flexible. The rubber outsole has flex grooves in the forefoot to allow it to roll naturally through toe-off and improve grip on pavement.

Everything that makes the Synchron good for running also makes it great for walking. Walkers will love the consistent, stable cushioning and the natural roll from heel to toe.

One tester says she’s been wearing them around the house and at her standing desk.

“These are great shoes for anyone who is on their feet all day, who wants a plush walking shoe or a really durable trainer for long miles and recovery days,” she says.

Final Thoughts

The Karhu Synchron 2021 running shoes
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The new Karhu Synchron 2021 packs a bed of soft cushioning so you can rack up lots of miles.

Designers used AeroFoam Nova midsole to give the Synchron the softest ride in the Karhu family. Then they topped it with a lightweight Ideal Knit mesh upper lined with a silky soft bootie for blister-free comfort.

While the Synchron is designed for running, it also makes a great walking shoe. The comfortable cushioning and accommodating fit provide a reliable ride no matter your pace. In fact, one of our testers says this is her go-to shoe for long walks.

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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