Karhu Fusion 3.5 Review

The Karhu Fusion is a neutral shoe designed for everyday training. They are well cushioned and durable. Exactly what you would expect from a brand that has been making running shoes for over 100 years and who’s shoes have been worn by the world-dominating Finns of yester year.

Karhu has become one of our most popular brands in store where customers can test run their shoes on the corridor at the back of the shop. Karhu’s have a straight big toe line so that you can push off from the end of your big toe and your feet can function inside the shoes. This is a noticeable difference to the pointed toe box of the more fashion orientated brands. Pointed toe boxes push the big toe to the inside which puts pressure on the big toe joint this can lead to bunions and inhibits the function of the other toes. This means that your foot can’t spread on impact to stabilise you and absorb the shock. The toe box isn’t as rounded or roomy as an Altra or Topo shoe so if you are used to snug pointed toe boxes then you won’t find the toe box of a Karhu shoe excessively roomy. You will just notice that your toes are free to wiggle in the toe box.

 The women’s UK 5.5 weighs 252g and the men’s UK 8 301g. The feeling of the cushioning is solid, durable and dependable. Initially they felt quite firm but, they have softened up a touch after 50 miles. There is some bounce to the midsole but, it is far from excessive, so it doesn’t affect your ability to comfortably trot a long in the shoes. If you prefer a soft feel to your shoes then I would suggest considering the Karhu Ikoni, which has a softer midsole.

Karhu’s have become my morning run shoes. When I fall out of bed at 5.30am I want a shoe that looks after my feet and allows me to plod a long for the first few miles while my legs wake up and then allow me to gradually flow along. The original Karhu Fusion did this job perfectly and due to the firm durable midsole lasted over 1,000 miles. The Karhu Fusion 3.5, although initially a bit too stiff has now bedded in to be that go to shoe for my morning runs. The current pair has over 150 miles on them, and the outsole and cushioning are untouched. The midsole uses the same Aero foam midsole and has a heel to forefoot drop of 6mm. Which I find allows me to easily midfoot strike. The Fusion would work equally well for a heel sticker. There is plenty of durable rubber on the heel and the midsole is firmest and therefore most durable at the heel too.

Karhu was the first brand to use Air in the midsoles of their shoes to increase the level of cushioning back in the 1970’s and quickly realised that the rebound from bouncy midsoles pushed the runners up rather than forward. They developed the Karhu Fulcrum in the 1980’s which help the runner maintain cadence and move forward. The Fulcrum is still used in Karhu shoes today as it has proved to be a very effective propulsion system. The Fulcrum is roughly speaking a hard piece of foam and plastic in the midsole of the shoe. It is positioned just behind the forefoot and is in the shape of an upside down triangle. As your foot rolls over the Fulcrum and compresses the softer foam in the forefoot. The firmer foam gives your foot a push. I find that this push keeps me lifting my heels up and stops me leaning forward at the waist and over striding. Which makes me lighter on my feet.

Other brands often use a rocker which is where the midsole is curved. This does a similar job but, I find that I have a tendency to get lazy in shoes with a rocker. I rely on the shoe to move my foot through the gait cycle. When I have been reviewing shoes with a rocker for a while and then run in a shoe without one. They are initially quite difficult to run in. As my hamstrings and glutes have got used to the help that the rocker gives them. As the Karhu shoes encourage me to lift my feet off the floor I don’t notice the same issues.

It’s quite hilly where I live which is another reason that I have got on well with the Karhu Fusion. The midsole is firm so you don’t sink into it when you are running up a steep hill which makes it much easier to trot up the hill. The fulcrum also helps you maintain a quick cadence and stops you leaning forward at the waist which gives you the most efficient uphill running posture. On the downhills the cushion absorbs the shock well and the fulcrum encouraging me to lift my feet up means that I am light on my feet. Which makes me fly down the hills.

Although the Karhu Fusion is designed for everyday training it is still responsive enough to allow you to pick up the pace. They don’t have the zip of a super shoe or racer trainer but, if you like to run fartlek’s or put picks into your longer runs then the Karhu Fusion will do the job.

The difference between the Karhu running shoes is the length of the Fulcrum. A longer Fulcrum gives you more propulsion and controls the foot more through the gait cycle. The Fusion has the shortest Fulcrum followed by the Ikoni and the Synchron has a full length Fulcrum, making it a neutral shoe.

In summary the Karhu Fusion 3.5 is a neutral everyday running shoe that will cope well with your everyday training, long comfy runs and fartlek’s. I really enjoyed my time in the new Fusion and it grows in popularity in the shop, once on people’s feet.

Other Considerations

Hoka Clifton 9: Much softer cushioning and a rocker means that the shoe does more of work for you. https://www.northernrunner.com/blog/hoka-clifton-9-review/

Altra Torin 7: Broad rounded toe box and zero drop allows you to run a more natural midfoot strike and gain a good level of foot function. Also available in a higher volume version. https://www.northernrunner.com/blog/altra-torin-7-review/

New Balance 880v13: Much softer and more pointed toe box. https://www.northernrunner.com/blog/new-balance-fresh-foam-x-880v13-review/

Topo Phantom 3: Broad, rounded toe. Similar responsive feel to the cushioning. 

361 Centauri: Lighter, softer feel to the cushioning. Rocker gives a good level of propulsion but isn’t over controlling. https://www.northernrunner.com/blog/361-centauri-review-361-centauri/

Brooks Ghost 15: Similar firmness to the midsole. Higher heel to forefoot drop makes the Ghost more of a heel strikers shoe. https://www.northernrunner.com/blog/brooks-ghost-15-review/

On CloudGo: Higher heel to forefoot drop, soft cushioning and a rocker. https://www.northernrunner.com/blog/on-cloudgo-review/