Interview with Fredrik Correa from Exxentric

By Milan Veselinović & Robert Simonič

“I don’t think we are in a very eccentric world really. We just appreciate and highlight the eccentric portion of training in the way it deserves.”

This two part interview with co-founder of Eccentric company and kBox and others devices. Learn and Enjoy.

1. Hey Fredrik can you introduce yourself at first so everbody can appreciate from where you’re coming in your work?

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Hi, well I have sort of a fairly mixed background. My base is in sports, predominantly ice hockey that I played until late adolescensce and then turned coach for about 15 years. Did a few other sports to as a kid, soccer, track and field mainly. As an adult I’ve mostly been into strength training and some martial arts on a hobby level. My interest for natural science brought me to the Royal Institute of Technology in Stockholm but I dropped out within 1.5 years because of my ice hockey coach dream became to strong. I studied the coaching program at University of Sport and Health Science (GIH) in Stockholm while training elite juniors and working for the Swedish Ice Hockey federation some. Coach at heart but my years at GIH really opened up my eyes to physiology and studied a BSc in Physiology at the Karolinska Institute (KI). I came into contact with flywheel training there and decided between going into research or to medical school and decided to pursue medical school and work with flywheel more from the angle of product development instead of research. My work with flywheel and product development became like a parallel career to my medical work and after receiving my specialist MD title I decided to focus on training and product development again so now it’s Exxentric only, 100%. Sort of back to the roots now with training but from another perspective than being an athlete or coach.

2. When and how did you start with exxentric firm?

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I came in contact with flywheel training around 2000–2001 when I studied physiology at KI. I worked in the lab where they developed flywheel devices and did studies looking at training in microgravity (ie space). After a few smaller projects I decided to work on the product development side, it was easier to combine with studies. Me and my study mate from GIH, Mårten Fredriksson (also a hockey coach) developed the YoYo Squat but after that we and the researchers had quite different opinions of the future development. We wanted to develop, improve and form training devices that were really good AND affordable and they didn’t share that interest so me, Marten and 3 more people started Exxentric in 2011 to be able to take our own path.

Fredrik Correa

3. What is your main objective with eccentric “world”? Where dou you want to “push” this in a future? Do you have interest developing new devices or doing more researches for elite sports, recreational or rehabilitation with eccentric training on your mind?

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I’m not sure if I understand the question correct but here goes. First of all, we aren’t really only focusing on flywheel training. In the beginning we wanted to solve a problem and our problem as coaches was that in our elite club we got a lot of talented players from small clubs but their training age was basically zero. We had to spend a major part of the “golden years” between 16–19 to teach them the basics when it comes to training, especially lifting weights. We lost a lot of time when we were held back with their lack of technique. So, we saw that a “squat device” that allowed for many of our key movements (squats, pulls, deadlifts, lateral squats) would make the training much more efficient and less technique dependent and a perfect combo with the Olympic lifting and powerliftning drills we tried to teach them. So, flywheel was a means to an end for us. After developing our first device we realized there is so much potential in the device and development needed and we are still trying to improve it. However, looking forward we are still looking at solving problems for people around training so future devices might not have a flywheel, you never know. We aren’t religious about it. Maybe apps, sensors or other devices can solve other problems better by themselves or in combination with flywheel.

That’s my first point, the second I don’t think we are in a very eccentric world really. We just appreciate and highlight the eccentric portion of training in the way it deserves. In the beginning with the kBox we pushed the eccentric work a lot to create our niche and some awareness when today I actually highlight this a feature for sure but also many other features, maybe even more, like the variable resistance, possibility to train or overload a part of the range of motion and the measurements in power instead of kgs on the bar etc. The variable resistance for example gives a great benefit for the individual and also for the team in a team setting. You can cycle through more athletes in a shorter period of time and training efficiency is so important. It also helps to create a strong metabolic stimulus. Being able to unload or load max in different part of the range of motion is also a very unique aspect that you don’t get with weights. I think this is something that people don’t realize until they start using it. This is one of the main reasons for powerlifters liking it for example, at least the majority of those that I met. The portability is nice but many don’t realize how this can impact training and adaption. It opens up a unique possibility for PAP and also in-season training with complementary heavy strength training after games for example, a stimulus that is hard to fit into a program for team sport athletes with a lot of games and traveling.

When it comes to devices I can see a bunch of devices with flywheel that can be good. Of course, horizontal devices but also exercise specific ones.

4. Which are by your oppinion most positive “side effects” using FW devices?

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Besides the harder endpoints like strength, power and local endurance I like that it teaches control. See people struggling with this in the beginning when they are absorbing the eccentric load. People with low degree of control don’t resist at all and when they do they just hit the brakes and stop the flywheel completely. They can’t assess the energy in the pull and smoothly absorb. With a fast concentric-eccentric transition this is even more obvious. kBox teaches control and I think this control is really important when it comes to injury prevention and performance, especially in team sports is it rare that you have to have max output, you need to have enough output. Compare with a boxer, imagine all his punches was max effort and power, he would be fatigued in one round. When needed he will go for the knock out, until then it’s a mix of fast and heavy punches and everything in between.

Fredrik on his last Powerlifting Meet few weeks ago. Broke some PR. Practicing what he preaches.

5. Dose FW have any negative effects?

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Short term you can fatigue your subjects, especially if you don’t adjust your programs or have a lot of concurrent training or additional load in the athletes training week. You have to consider the total load. Unwanted fatigue is not something we want obviously. It will increase risk of injury and also give us poor performance, both in the gym and in competitions. Another side effect, at least from my personal experience, is that you can get a different set point for your training. For me I never liked high rep training and have barely gone beyond 10 reps in my whole life. After doing more kBox however, I don’t really feel like I’m training if I go below 3 RM weight, everything lighter feels like warm-up. I think that when you get used to that constant load and the struggle it means by using the kBox max out that feeling is hard to replicate with weights. Depends on mindset I guess, I know a few coaches have problems with their athletes “cheating” on the kBox, not going as hard as they should. This is easier to do since it is not loaded with a specific weight, but your intensity is the load. However, if your athletes are cheating you have bigger problems that can’t be solved by switching training devices. That is what coaching is about in my opinion, motivate them, set goals, help them create a vision for themselves and give them the push in the direction to get there.

6. Can we train with FW devices for same reasons as we do with weights? (Strength, power, hypertrophy, endurance, rehab…)

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I’d say yes. We see positive outcomes and all strength training related metrics in the studies: strength, power, horizontal and vertical speed and mass. Today more and more studies have made the old paradigm of “rep-range” obsolete. You don’t need to lift heavy to get strong. This has highlighted other ways to quantify the work and how that effect adaptation. Besides load we have training volume, intensity, effort and training density for example and kBox is a great tool to work with all those parameters. With the right exercise, inertial load and intensity I think kBox can be a good tool for all those areas you mention, rehab, strength, power, hypertrophy. Maybe not endurance meaning aerobic capacity but definitely for local capacity and metabolic conditioning.

7. Since it has metabolic aspect, is performing exercises on kBox good for endurance sports like: rowing, running? Do you have any feedback of owners using it for such reason?

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We do have a few elite runners using it but I haven’t got feedback enough to know how they implement or program their kBox or strength training. On a general level adding strength training is beneficial and can improve performance for endurance athletes on all levels. It seems that high load low volume training is the best for their performance (Berryman et al 2018). This is the type of training I guess they are least prone to do so I can see how the kBox can be an easy and safe way for them to allow for this type of training. When it comes to flywheel specific studies there is one Spanish study on eccentric overload training and running (Sanz-López et al 2017). Their conclusion was that six weeks of eccentric overload training before running had favorable impact on measurements related to tendon quality vs CTRL. I’d love to hear more from the endurance athletes and their coaches how they program and what their results are and eventually seeing some studies too. Both aspects are important since you can’t draw to many conclusions from one setting to another. Coaching and training is working with individuals and laboratory studies something else where we look at groups, we need both pieces of the puzzle. Going back to the question if the metabolic aspect of kBox training can improve performance for endurance athletes I don’t really know. I can see how kBox training can be suitable for rowers both for strength and the metabolic aspect but how much they contribute individually I don’t know. For running my guess is that the metabolic aspect is less important than the strength gains and improved tendon health but you should really ask a experience running coach about this.

Fredrik Correa with Milan Veselinović in Belgrade May 2018.

End of part 1.

Where can people reach you for more information?

People can contact me on:

or @fredrikcorrea on Twitter. (

For media or info I’d suggest or (web)

Thanks Fredrik, I really appreciate your time, willingness to do this interview and share with us your wisdom. If anybody from our part of the world (Serbia & Slovenia) would like to experience Fly wheel/eccentric training can contact us and we can arrange session, just contact us on social media:

Serbia — Milan Veselinović:

Slovenia — SuperTrening: