Karhu Fusion 4 Review
Back on the road this week, we have our Karhu Fusion 4 review. The Karhu Fusion 4 is a neutral running shoe designed for everyday training that is versatile enough for more than just your easiest running. Rather than a performance shoe that you can do the miles in, which is becoming popular with many brands, like the New Balance Rebel v4 or the Hoka Mach X, the Karhu Fusion 4 is more of a training shoe that is still responsive and light, so it feels good at quicker running speeds.
That means the Karhu Fusion 4 is a great shoe for those who only keep one running shoe at a time. It doesn’t feel lazy going quickly, or too firm going slowly.
Karhu – Land of the ‘Flying Finns’
Karhu have been making running shoes for over a hundred years and were the first brand to put air cushioning into the midsoles of their shoes. They realised that the bounce you get from the midsole isn’t good if it’s not pushing you forward. It lead to the development of the first propulsion system in running shoes, that Karhu call the ‘Fulcrum’. Despite still bring a relatively unknown brand, Karhu is one of our best selling brands in our Newcastle store, where customers can test run shoes on the concrete corridor at the back of the store. Karhu has developed a devoted following as the running shoes are comfortable, durable and do the job and they clearly feel good out of the box during a 5min test run.
The current range of Karhu running shoes are all focused on everyday training with varying cushioning levels and level of pronation support. The latest Fusion, now version 4, has added a touch of performance to the range.
Karhu Foot Shape
The ‘last’ (foot shaped form used in production) of Karhu running shoes has been developed using the data from millions of 3D foot scans, supplied by specialist running shoe stores that used a foot scanner when fitting their footwear. The results allowed Karhu to develop a last shape that fits a large variety of foot shapes comfortably. The toe box allows for a straight big toe line, allowing your feet to function best and promoting a strong toe-off. The toe box shape is rounded (not to the extent of a Topo Athletic or Altra) so it doesn’t push your toes into a point and gives them some room to spread.
Fit & Feel
The upper is constructed of a very soft knitted mesh. The heel cup is plastic to provide a good level of support and hold the heel in place nicely. The padding at the heel and around the heel cup is nice and plush. Exactly what you expect from a shoe designed for you to run in it everyday. The tongue is very lightly padded and held in place by a half gusset at either side. It doesn’t slip or move while you are running. The fit at the heel is comfortably secure, but if you feel the need to make it snugger then there is an additional lace loop that you can put the laces through the pull the heel in further or use to butterfly lace the shoes.
The laces thread through the M Lock which are internal tapes that are attached at the base of the shoe and when laced up pull the upper in around the foot. This gives the Karhu Fusion a secure fit at the heel and through the midfoot of the shoe but allows for the forefoot to function and spread. This fit and flexibility in the midsole helps prevent the foot from moving around inside the shoe.
Cushioning
Karhu running shoes have always had a firmer feel to the cushioning, similar to Brooks or Topo running shoes. This lets your feet ‘know’ they have touched the floor and encourages the foot to spread and function correctly. This gives you some natural shock absorption, stability and spring from your feet. It also helps you maintain a more upright posture, which makes you lighter on your feet and helps you move quicker.
The Karhu Fusion 4 has a slightly softer and bouncier feel versus the Karhu Fusion 3.5. This is due to a new midsole foam. The midsole is now made from a Nitrogen Infused version of Aero Foam. When a foam is Nitrogen infused it is essentially ‘sealed’, which prevents air from getting out and dirt, water etc.. from getting in. This gives the midsole more energy return, it feels bouncier and lasts longer as the water and grit can’t get in and degrade the midsole.
The midsole is 28mm in the heel and 22mm in the forefoot. Although not an excessive stack height, that provides a good level of cushioning due to the quality of the midsole foam. The 6mm heel to forefoot drop makes the Karhu Fusion 4 suitable for both heel and midfoot strikers and the Fulcrum works effectively with either foot strike.
Fulcrum Propulsion vs. a Rocker
Within all Karhu running shoes is the Fulcrum. This is a firmer piece of midsole in a very specific shape, unique to the different Karhu styles. When your foot lands on the midsole and moves through the gait cycle, this has the effect of pushing you forwards instead of bouncing upwards. This is what happens in shoes that have a bouncy midsole with no propulsion system. A lot of shoes therefore use a rocker. This is where the midsole of the shoe has a curved shape. When your foot hits the floor in a shoe with a rocker the rocker guides the foot backwards. The difference that I experience in Karhu shoes is that the sensation I get when my foot hits the floor in a Karhu shoe is to lift my heel of the floor. It promotes a quick cadence, upright posture and good running form. Shoes with rockers often just guide the leg backwards, which doesn’t promote as much flexion at the knee and makes me feel like I am ‘riding the shoe’, rather than running.
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The outsole rubber is placed just on the areas where you would expect to get most wear. It grips well to wet roads and the disused railway lines around me.
Due to the fulcrum increasing your cadence and making you lighter on your feet, Karhu shoes do tend to feel lighter when running in them than they are on the scales. The men’s UK8 weighs 302g and the women’s UK5 is 250g, which is about standard for everyday training shoes in the current market.
Summary
At easy running paces the Karhu Fusion 4 feels light on your feet and nicely cushioned. Different to the ‘plodding along’ feel that you get from a lot of lazier feeling everyday training shoes. They don’t ‘urge’ you to run any quicker, they just feel light and easy to run along in. On steady runs/tempo runs I find the Karhu Fusion easy to cruise along in. They are not a speed shoe, so that’s a pleasant surprise. If you’re not looking to purchase a speed shoe and an everyday training shoe, it makes it a great option.
A great everyday training shoe, from a brand really ‘on the up’ in our opinion, that feel great and help improve posture and cadence with the Karhu Fulcrum. I will enjoy putting them on for everything apart from smashing a 5k PB, so they’re great for a huge range of runners. For a bit of support, go see the Karhu Ikoni or the most support go see the Karhu Synchron.
Other Considerations
New Balance 880v14: Doesn’t feel quite as light and zippy at quicker paces. More feel for the ground makes it feel nimbler and it’s more flexible in the forefoot. Available in three width men’s and two width women’s. https://www.northernrunner.com/blog/new-balance-880v14-review/
Hylo Impact: A much softer feel to the cushioning makes them a bit too soft for anything quicker than steady running. The rocker gives me the same feeling of lifting my heel rather than riding the curve. Which gives similar improvements to your posture. https://www.northernrunner.com/blog/hylo-impact-review/
Topo Phantom 3: A much broader more rounded toe box designed to stimulate foot function. A firmer less bouncy ride with more ground feel. Promotes good posture and the firmness of the midsole makes it feel good on short hills and short fast efforts as you don’t sink into the midsole. https://www.northernrunner.com/blog/topo-athletic-phantom-3-review/
Altra FWD Experience: A broad rounded toe box with a 4mm heel to forefoot drop and a rocker. The rocker pushes you a long and makes it difficult to run at easy slow pace. At quicker paces it feels smooth. https://www.northernrunner.com/blog/altra-fwd-experience-review/
Brooks Launch 10: Available in two with men’s and two width women’s as well as a neutral and pronation support version (GTS). The EVA midsole isn’t as bouncy and loses energy return on longer runs but it is versatile enough to be used at all paces including really fast stuff. https://www.northernrunner.com/blog/brooks-launch-10-launch-gts-10-review/
Hoka Mach X: The plate in the forefoot gives the Mach X a noticeable bounce. This makes them nice to run in at a variety of paces but takes away from the my rhythm at faster than 10km pace and then I feel like I am fighting the shoes. https://www.northernrunner.com/blog/hoka-mach-x-review/
Topo Athletic Specter: The rocker and PEBA foam combination help push you a long effortlessly in these shoes. Great for long runs, tempo runs and steady runs. They aren’t easy to run slow in and feel a bit bulky at really quick speeds. They have the same broad rounded toe box of all Topo shoes which stimulates foot function and provides room for your feet to spread. https://www.northernrunner.com/blog/topo-athletic-specter-review/
Altra Escalante 3: A broad rounded toe box and zero drop stimulates good running posture and foot function. These feel light and responsive. If you want a road shoe that feels like your old racing shoes, fell shoes, cross country shoes or orienteering shoes but has enough cushioning for regular road runs then these are worth a try. https://www.northernrunner.com/blog/altra-escalante-3-review/
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