New Advanced Functions in kMeter 2.3

The latest version of the kMeter App has now rolled out, including several new features solely based on user request. Fredrik Correa explains:

Summary of new functions:

  • Discard reps before saving
  • Import users
  • Delete users

This release is special since all new features are solely based on user feedback and suggestions. After talking to different users I came across various interesting questions such as:

– “My athlete stopped one rep short of the set so the last rep was spoiled, how can I edit that?”

– “I want to do a unilateral drill but work both sides in one set but during the switch I loose a couple of reps, is there a setting for this?”

– “Our facility has multiple kBoxes and we would like to spread the user data (not the training data) over multiple iOS devices, how can we do this?”

Given that my answers would have normally been either “there’s no such feature” or “this would need to be done manually”, these questions made me realise our app’s limitations and how it could be improved.

Before you continue reading and start pulling your hair. We understand these new features are slightly more complicated than previous ones, so we compiled a .zip file containing instruction and templates for importing users here if you need it.


After completing your set and viewing your summary you can now press ‘EDIT’ and you will get a list over every rep (not the pre-reps) in a list. Here you can select the reps you want to discard and they will be taken out of the summary and your metrics will be re-calculated based on the data without those reps.

So if your athlete stopped one rep too short or lost balance in the 2nd rep you can easily just click that one away and save the data like before.

As for the second question, if you want to do 10 single arm rows on each side, you can set up the app with two pre reps and 22 training reps. You start with your two pre-reps followed by 10 training reps and then you switch over and do two (pre-)reps with the another arm and the following 10 training reps. Before saving you will have to discard those two extra reps in the middle of the set (rep 11 and 12) after you switched arms. Watch video.


If you are using the kBox with a whole athletic department of a bigger team and use multiple kBoxes and iOS units, the user data would have been quite difficult to set up. Not anymore.

Now you can set up a .csv file with all you users and import it from an email, google drive or iCloud. You still have to approve every user but it’s much faster and you can send out the .csv to the other coaches for them to add in their devices. When doing this we also added the function to delete users to clean up your user list. Here is a step-by-step guide how to set up the .csv file (or download our templates) and how to import them. Watch video.


We had our multisystem users in mind when we added the import function but sooner or later deleting users would be required too, so we added that feature in the Set Up > User menu. Just select the users you no longer need to track and press delete. Watch video.

Existing kMeter 2.2 Features

These improvements come on top of the current kMeter features that were already available before today’s release:

  • Real time data of average power, peak concentric and eccentric power.
  • Graphic, numerical and voice feedback.
  • Set limit based on reps, time or power drop-off% for traditional, metabolic or VBT based training.
  • Set summary with power, force, displacement, average and peak speed, relative peak power (W/kg BW), rep time, kcal expenditure and more.
  • Multi user, create different users with DOB, gender, height, weight and switch easily from the front screen.
  • Editable data, change drill, user, VAS score from previously saved data.
  • Landscape mode in device or on TV via AppleTV.
  • Share screen of summery to camera roll.
  • Database with possibility to share all or selected data in a CSV compatible file.
  • Filter data in the database based on user.
  • Research mode (send raw data from a set).
  • Edit exercises (change the order they appear in, insert new exercises or delete existing ones).

For a quick demo of the new features check the full video below.


Wrapping Up

Thank you for your feedback, keep it coming! Now, go download version 2.3 of the kMeter now on iTunes.

Happy DOMS!

/Fredrik Correa, Co-Founder


Guest Article: kBox and Eccentric Overload in Archery

Today, we have the honor to present a guest article for download from a long-time kBox user, sports performance consultant Antonio Robustelli.

This is the first part of two pieces about the use of eccentric training with archers and how he implemented the kBox leading up to the 2016 Olympic Games in Rio.

Research is a great driver in our field, but the practical experiences shared by coaches and physios are no less important. Our goal is to help our users by offering knowledge from both these areas, and that’s why I’m very happy to present a two part guest article by one of our successful users willing to share his insights.

Antonio is an Italian high level educator and CSCS certified performance coach that has been using the kBox for over two years with all types of athletes (LinkedIn, Twitter). In this first article, he is providing an overview about the importance of eccentrics in sports in general and in Archery in particular. In the second part, he will provide some info on how they programmed their training running up to the Rio Olympics and how it helped his athlete Claudia Mandia to secure the fourth place with the Woman Italian National Team.

We have arranged for you to present yourself to download the article here:

I hope you will enjoy the reading and I would be happy to share more insights from you, our users and coaches if you want a platform to do so. Sharing is caring.

Happy DOMS!

/Fredrik Correa, M.D., co-founder

Introducing kMeter 2.1 at the CSCCa National Conference

On site today at the National Conference of the Collegiate Strength and Conditioning Coaches Association, we are proud to introduce version 2.1 of the free kMeter app, providing reliable exercise feedback for kBox users. The kMeter now includes editable exercise list, research mode and more:

So, what were we able to add in the new version 2.1? If you are in Orlando this week, just stop by booth 928 at the CSCCa conference for a personal demonstration ? or keep reading:

Edit Exercises

Change name, order of the existing drills or remove or add your own drills. By changing order you can make sure your favorite drills are at the top of the list saving time for you saving data. Create your own drills and no need to write long comments and having to keep track of them.

I think many of you will like this possibility to edit your own exercise list. Let us know if you use it!

Research Mode

By turning this on the share button in the summary turns in to a raw data dump into an Excel-compatible file giving you the rotational data for every 40 ms reading. This is a feature some have asked for, and now you got it.

Minor Fixes

We made a few fixes regarding W/kg in the relative power calculations and file compability when sharing data to Excel.

Existing kMeter 2.0 Features

Before we wrap up, let’s briefly go through the current kMeter features that were already available before today’s release:

  • Real time data of average power, peak concentric and eccentric power.
  • Graphic, numerical and voice feedback.
  • Set limit based on reps, time or power drop-off% for traditional, metabolic or VBT based training.
  • Set summary with power, force, displacement, average and peak speed, relative peak power (W/kg BW), rep time, kcal expenditure and more.
  • Multi user, created different user with birthdate, gender, height, weight and switch easily from the front screen.
  • Editable data, change drill, user, VAS score from previously saved data.
  • Landscape mode in device or on TV via AppleTV.
  • Share screen of summery to photo roll.
  • Database with possibility to share all or selected data in a CSV compatible file.
  • Filter data in the database based on user.

For a run through of the new features, check out the video below.


Wrapping Up

Thank you for your feedback, keep it coming! Now, go download version 2.1 of the kMeter now on iTunes.

Happy DOMS!

/Fredrik Correa, Co-Founder


James Harden’s Hidden Athleticism and How the kBox Improves Basketball Performance

Basketball is a sport where you don’t necessarily have to be the fastest to succeed and James Harden of the Houston Rockets has been demonstrating that all throughout this season as well as his entire career. Here’s how flywheel training on the kBox can help to unlock your potential on the basketball court, as changing speeds is not just about your explosiveness.

To anyone that doesn’t follow the NBA, James Harden has just wrapped up a historical season leading the league in assists, while averaging a stat-line that hasn’t been seen since Oscar Robertson and producing the most points per game for his team since Wilt Chamberlain in 1961-62, all while leading his team to the third best record in the NBA. Simply said, what he’s accomplished this season is amazing.

I remember how at one point during the season I was listening to a podcast where it was quickly mentioned as a side note that P3 Applied Sports Science, who have worked with over 100 NBA players, measured James Harden’s athletic abilities and found out that he was average to below average in most categories. While being one of the best basketball players on the planet, his physique certainly doesn’t stand out as much as that of LeBron James or Russel Westbrook. So, finding out that he’s not one of the most athletic players in the NBA is definitely not a surprise. But knowing this, what is it then, besides the cognitive portion of the game, that gives him such an enormous edge over the rest of his peers in the NBA?

It turns out that James Harden is elite at decelerating, according to Marcus Elliott at P3. His exact words are “Harden is barely average in almost every metric we look at related to athleticism, except for deceleration metrics, and in those he’s one of the best athletes we’ve ever measured in any sport — in soccer, football, or basketball.” Anyone who’s ever played basketball knows how important change of pace is but when striving to become better players, both athletes and coaches often focus on acceleration and explosiveness. Yet by decelerating quicker than your opponents you can always gain an advantage in the open floor as well as in short spaces on the court, which Harden has clearly been demonstrating all throughout his career.

The link is clear between an athlete’s ability to decelerate quickly and his or her eccentric strength (i.e. absorbing a large amount of force during the extension of the muscle, e.g. on the way down during a squat). So, anyone striving to gain an advantage on the basketball court would not want to miss out on an opportunity to build eccentric strength in their leg muscles. Of course that’s possible with weights but not always practical and effective in a team environment. On the contrary, basketball players of all sizes can train together and push themselves as much as they can handle by doing squats, lunges, RDLs and hip adduction/abduction on the kBox among other exercises, without having to adjust the weight on a barbell, its positioning in terms of height or worrying too much about technique.

Plus, the kBox will not only help you to improve your ability to stop before your opponent can even react, but it’s also great for improving your sprint speed and vertical jump, it’s widely used to prevent and rehab from injuries and it can help you get better at changing direction quickly, through lateral squats for example.

James Harden’s team, the Houston Rockets, are already kBox users and so are the Oklahoma City Thunder, the Los Angeles Lakers, the Chicago Bulls, the Boston Celtics and the Sacramento Kings in the NBA among other clubs in various pro leagues around the world. It seems that gradually the basketball world is embracing the kBox and how much it could add to your game, so don’t fall behind the rest of your competition by reacting too slowly. Control the pace.

/Bektur Savitahunov, Exxentric sales executive and basketball coach


ETSU Studies of kBox Inertial Setting and Force Characteristics

The choice of inertia and intensity are the main parameters affecting your workout in flywheel training. At the Coaches College event in December, two new studies from ETSU looking at this were presented. What do these add to or reinforce in the Flywheel Workout Zones? Here’s a walkthrough by Fredrik Correa.


  • A small increase in force can increase net impulse (and eccentric overload) much more.
  • Consistency in Force output within a set is high.
  • kBox is a suitable tool for eccentric overload.
  • Hence, these studies reinforce our previous beliefs.


In December 2016 at the last Coaches College, held annually in Johnston City, TN, two kBox studies were presented by the Center of Excellence for Sport Science and Coach Education at East Tennessee State University (ETSU). They started these projects after the summer, which are now gaining steam, and from what I’ve heard from Dr Kimi Sato at ETSU they are filled with ideas for coming projects too.

These first two projects I’d call descriptive since they look at different parameters acute during training at different settings. When presented to a new form of loading for resistance training the obvious question is ”will this be sufficient for adaptation?” and descriptive studies can tell us about that among other things.

To summarize their findings I’d say these two projects validated our previous beliefs and this will be a good platform for further research at ETSU.

Project titles were:




So basically, they wanted to look at force outputs during the #kBoxSquat and see how different inertia affected the outcomes and variability within sets and between users.

What they did

Ten subjects did:

Two sets of 13 reps of squats (3+10) on inertia 0.010, 0.025 and 0.050 where reps 2-9 of the test reps were analyzed.

Two minutes between sets and three minutes between inertia settings

They used force plates to register peak force, net impulse and positive to negative impulse ratio.


Increasing peak force with increasing inertia

  • My comment: as we know from the Force-Velocity curve, more inertia will result in a slow movement with the intensity unchanged and a slower contraction will increase the force production (See pic F-V relationship)

Peak force consistency over a set showed low variability between reps while variability between users was high.

  • My comment: High rep to rep consistency within a set but large variation between subjects and stronger subjects could elicit larger forces. This is also intent and intensity driven so even a stronger athletes could go ”gentle” as all kBox users already know. The benefit with this feature which we call ”variable resistance” is that you can get the load at a specific part of the ROM if you like. For example, you can start from a deep, low to medium intensity squat, just to get the full ROM and then explode at the end-range of the concentric phase. Other way around you can start with 100% at the bottom if the ECC-CON shift is something you want to focus on and as soon as you come up you stop pushing and just follow through in the last part of the concentric phase. The benefit is that you can target the whole ROM or the part where you want to improve and partly unload the rest of the ROM to increase the training volume on a specific part. For a weightlifter that could be improving the catch phase deep while not having to carry all the load through the whole ROM and save some work for the back or keep it the same while increasing the volume on the legs. This also allows for users differing in strength levels training together using the same settings which improves the logistics and the flow of the session, especially in a team setting.

Increasing net impulse with increasing inertia

  • My comment: impulse (F * t) increases as a result of the F-V curve described above. A slower speed will increase force production and with the slower speed and the same ROM the time t (Time under tension) will increase, as a result the product of these two obviously will increase a lot. Without knowing the physics, you realize this when you use the kBox, however it is good to understand that producing a higher force for a longer time increases the amount of work you put in a lot, increasing linear with the net impulse. This means 4 sets x 8 reps of kBoxSquats will be much more demanding in terms of rest and fatigue, if done with higher compared to low inertia, and you can’t just compare the number of reps to compare sessions. In the kMeter app you get a reading of the energy expenditure, which is a metric you can use to compare the amount of work between different settings and training modes.

Increasing positive to negative impulse ratio with increasing inertia

  • My comment: Higher overload with more inertia has been shown in previous studies (Gonzalo-Fernando 2016) even if it wasn’t exactly the same for both sexes. If you want to get a higher overload than higher inertia will give you higher total energy in the spinning flywheel at the end of the concentric phase. This higher rotational energy gives you more to play around with during the eccentric phase and also much easier to time it if you want the overload to come for example in the deep catch position. So if you are looking for eccentric overload in part of the ROM high inertia will give you more overload and it will be easier to target that specific range too.


Further on they discuss how a relative small increase in peak force results in a large increase in net impulse and P to N impulse ratio. This is interesting and as I discussed above putting in the same force or a slightly higher force but during much longer time (as a result of higher inertia) will result in much higher rotational energy and, as a result of that, higher eccentric overload (if you want that). As a side note I think their protocol put all load settings in the power spectrum with fairly high velocities and similar studies incorporating higher inertias > 0.1 kgm2 and fewer reps would be interesting. For example my average velocity is around 0.8 m/s when going max with one heavy flywheel (inertia 0.05 kgm2) so that’s quite far from absolut strength spectrum ( < 0.35 m/s), see pic for my own inertia vs average force and peak power curve.

One of their conclusions is that the kBox is a good tool for eccentric overloading, which can be achieved without a huge increase in peak force. Especially good for stronger athletes, which can create a larger momentum in the acceleration phase. I tend to agree with that fully and see this as one huge advantage for athletes and coaches starting to supplement their regular gravity-based training with flywheel training. Looking at the kBox in a wider perspective, the fact that you don’t have to carry all the load through an axial loading (either distributed through the harness or a belt) makes it very accessible for many team sport athletes with back problems that can’t do heavy barbell squats, common among ice hockey players for example. The logistics, instant feedback and variable resistance is of course beneficial for all athletes but also physiotherapists and personal trainers.

Practical implications and a few tips

  • Equal work?- consider the net impulse or total work in kJ for a set / session.
  • Lower inertia gives higher power, lower eccentric overload and less total work (less fatigue with the same volume of reps).
  • The kBox is a good tool for eccentric overload.
  • And lastly, I know ETSU has some interesting things in the pipeline so if you are interested in them continuing their kBox research give them a shout out and let them know you care about their work!

If you have any questions with regards to the choice of inertia, just drop me an email or check out our courses.

Happy DOMS!

/Fredrik Correa, Head of Research & Development


“Almost Impossible to Achieve in Traditional Weight Room Methodology”

PhD and kBox pioneer Mike Young reveals some of his key flywheel training strategies and more for UC Berkeley strength and conditioning coach Joel Smith in his latest podcast. Fredrik Correa thinks you should listen in.

In the last episode of the Just Fly Performance Podcast, coach Joel Smith is having the US kBox pioneer Mike Young on. Mike is well known for his work at his facility Athletic Lab and for his work in MLS, and as a well traveled speaker at high level conferences and workshops world-wide. We were much fortunate to have him at the Exxentric Summit 2016 (you can download that lecture here).

On this episode, Mike talks about developing sprint and speed ability in team sports athletes. The question of how much you should invest into working on mechanics with non-sprinters is interesting and Mike has some great insights in this area. Further, he discusses if there are differences in cuing between sprinters and non-sprinters.

Discussing the kBox

To finish it he gives his take on velocity based training and (from 38:30) the kBox. Mike lets us know how he sees the kBox in a performance setting and what they use it for in power and strength development. We also get to know how he uses it weekly with his track and field group.

Being a long-time advocate of isometric and eccentric training Mike explains that he now appreciates the safety and logistics of doing this with the kBox as compared to traditional methods.

“For many years i had to come up with some very creative and borderline dangerous things to kind of really press the pedal to the metal. I think the kBox provides a very safe alternative to a lot of the things that I was doing, and its much logistically easier.”

They reach overload of up to 175% with elite athletes, and doing that with regular weights is both complicated, time consuming and somewhat dangerous in my view.

“We’ve seen #eccentricoverload depending on the movement velocity and the range of motion in the 150 to 175% range, which is really almost impossible to achieve in traditional weight room methodology. So, I love the kBox. It’s been a great tool for us.”

With team sport athletes they do more of metabolic and hypertrophic training and the eccentric overload, being excellent for injury prevention, has made them replace most of the Nordic hamstring work with overloaded RDLs on the kBox.

Thanks for Sharing

I hope you’ll enjoy this podcast as much as I did. There are plenty of performance podcasts out there and you can’t really listen to them all, but this is a good one that I can highly recommend you make some time for. Thanks Mike and Joel for sharing!

Happy DOMS!

/Fredrik Correa, Head of Research & Development

PS For more podcasts relating to flywheel training, check out the media page.

[Swedish] Kurser i svänghjulsträning med nya kBox4

Nu kan du som tränare få både utbildning och utrustning för svänghjulsträning – från 550 kr per månad.

kBoxSquat with coachCoacher och tränare på fyra kontinenter, från elitidrott till fitness och hälsa har redan börjat använda kBox4, den nya generationens redskap för svänghjulsträning. Men som ett helsvenskt företag är vi inte nöjda förrän träningsformens fördelar fått fullt genomslag även här hemma.

Efter årsskiftet kör vi därför igång de första introduktionskurserna på svenska med kBox4. Datumen är:

  • Fredagen den 27/1 kl. 12–16
  • Lördagen den 25/2 kl. 9–13

Kursavgiften är 990 kr inkl moms, men för deltagare som beställer en kBox efter kursen så drar vi av hela avgiften!

kBox4 Lite with FlywheelKostnaden för instegsmodellen kBox4 Lite är ca 18.800 kr plus moms. Numer har vi dessutom tagit fram ett helt nytt leasingerbjudande för företagare, från ca 550 kr/månad vid ett 36-månadersavtal med 5% restvärde.

Om du har några frågor eller vill se en komplett prislista så kontakta oss gärna, eller gå till bokningsformuläret för att reservera din plats direkt.

Henrik Petré

Vi ser fram emot att höra av dig!

/Henrik Petré, Head of Education

Now Shipping: The kBox4 – with a New Accessory

Today, we celebrate the great reception of the new kBox4 by presenting a new bag for your flywheels, making any kBox even more portable.

The initial kBox4 batch has been delivered to the first pre-order customers over four continents, and now the second, larger batch is just about to start shipping. Our website is now updated with some nice new photographs of the kBox4.

Meanwhile, excellent feedback (such as the post pictured to the right) has started coming in from users of the new kBox generation, which makes us really happy. Thank you so much!

New Flywheel Bag

Flywheel BagFor all of you as well as for users of all previous kBox models, we now have the pleasure of presenting a brand new accessory: the Flywheel Bag.

Aimed at further improving the portability of the kBox, the Flywheel Bag provides a convenient way to carry your Flywheels and other accessories between clients or training sites, leaving both your hands for carrying the kBox platform itself, weighing only from 11 kg.

The Flywheel Bag is compatible with Exxentric Flywheels in all sizes and for all kBox versions, and it is now available at EUR 50 plus shipping and local tax.

Learn More

Read more about the kBox4 and all new accessories on our updated equipment pages. To get a full price list (starting at EUR 2,190 / USD 2,570) or discuss a kBox4 order for your facility, contact us or reach out to your reseller today.

Up Close and Personal with the Sollentuna Volleyball Team

Visiting customers is a rare luxury when you are working in a startup with customers spread over six continents. However, recently Fredrik Correa got an opportunity to visit and interview the Sollentuna Elite Volleyball team and reflect:

The implementation of flywheel training in team sports seldom goes from 0 to 100% overnight, but gradually grows into the programing in different ways. In the case with Sollentuna elite volleyball team, one of their players and current Strength & Conditioning coach, Carl Ljung discovered the kBox during his own rehab after a torn ACL. The physiotherapist at a local clinic used the kBox with great success and the interest for flywheel training in rehabilitation and performance grew over the years.

Carl and a few other coaches attended the Exxentric Summit this year and we started discussions on how to implement flywheel training using the kBox in their setting. After demos for the coaches and familiarization training with the players, they have now been using the kBox regularly for slightly over 4 weeks. I decided to visit their team training and see what questions they may have and how they implemented the kBox in this early stage and their plans going forward. Obviously, I wanted to interview a couple of the players too, to hear their story and what they think about training on the kBox.


Just walking into training hall itself, I noticed that the women’s team had just replaced the men’s team Sollentuna trainingon the court and started their volley training, while the mens team started their S&C training. A hall full of action, energy and positive attitude. kBox was going hard all the time, working 3 and 3 in a circle of kBox power squats, med ball throws and sprints. Right there, I could see one of the big benefits of the kBox. While having access to a gym only two nights a week they can now add two strength and condition days when training at this facility by having the kBox there. There is absolutely no transition time to move over to another facility, and instead they can just get right on it in the court.

Cycling through a whole squad, it would be more efficient to have another kBox but it actually worked pretty well. Doing the S&C after volley training had a practical aspect since players can leave after they finished their training. When doing it before training, resting time and chatting increased and affected the start of the volleyball training. Now, everybody trained hard and didn’t lose any time messing around, both training wise and moving into competition phase. This is also probably beneficial from a physiological perspective.

Interview with KOSMA

I also sat down with two of the players after the training, 30 year old Kosma Smiechowski and 19 year old Jacob Zachrisson. Kosma had an interesting background being of Polish descent and coming from a strong Volleyball background with both mom, dad and grandfather playing on a national elite level. He grew up in Australia and has been playing there.

However, six years ago, his career almost ended due to a knee luxation. After ten hours of weekly rehab, he got back and has been playing in Sweden for three years. The knee joint isn’t giving him any problems playing volleyball but his flexibility has decreased in the gym. He also had back issues when lifting and previous patellar tendinitis and I was really interested to hear his thoughts regarding using the kBox.

I am glad that Kosma is very happy about the new addition to their training as he gained almost 5 cm in his flexion in the left knee and he experienced no problem whatsoever from the patellar tendon or back so far during pre-season training.

Interview with JACOB

On the other hand, Jacob is in the beginning of his senior years as a player. He has played for Sollentuna since he started as a kid but attended the National Volleyball programme in Falkoping in high school and have just returned back to Sollentuna to play for the A team.

During his years in Falkoping, they did a lot of weightlifting but last winter he had some back issues that prevented him from doing cleans and back squatting. So far this season, he has had no problems with the back on the kBox. During the years when he had Volleyball on the schedule in school, they spent more time in the gym while now there is more focus on the play itself. He thinks he was stronger last year before the back issues so he likes the new pre-season programme with heavier focus on S&C. Being a rather short player, he knows his athleticism is important so the training off the court is an important part.

I’m glad that maybe the kBox can do this type of training a little safer, efficient and more convenient for the team and coaches in Sollentuna.


During the interview, the upcoming season premiere was discussed, where they met one of the favourites Linköping and the game was lost 3-1. However, Sollentuna won the home premiere against Habo in the recent games.

Currently, Sollentuna elite men’s volleyball are currently ranked 5th in Sweden and we wish them all the best in subsequent games. Warm welcome to the #Exxentricfam!

Happy DOMS!

/Fredrik Correa, Head of Research & Development

Study Overview: Effects of Inertial Setting

The choice of inertia and intensity are the main parameters affecting your workout in flywheel training. A new study just came out looking at how flywheels with different inertia affects work, power, force and eccentric overload in both genders. Here is a walkthrough of the study by Fredrik Correa, who stresses individualisation and revisits the Flywheel Workout Zones.

Still young but already a seasoned flywheel researcher, Rodrigo Fernandez-Gonzalo, together with first author Luis Manuel Martínez-Aranda have published their latest work and this time it’s a descriptive flywheel study, “Effects of inertial setting on power, force, work and eccentric overload”. It came out published-ahead-of-print in JSCR the other week.

I have been saying that we should leave this area of research and look more into training studies and programming. I would love to see some head-to-head studies looking at protocols instead of lab-based stuff that coaches cannot implement. However, that being said, I think this work adds some understanding to how inertia works and can be helpful in programming your training. What’s missing in the ahead-of-print version is graphs, tables and unfortunately a readable formatting so we might have to go back to this one when it’s published if I missed something.

So, about the study. First of all, it’s a well written introduction giving a good background to the subject and I recommend reading it if you are somewhat new to flywheel or isoinertial training. It also points out an important weakness in flywheel training research since the inertia adopted in studies have always been mentioned without any deep thought and studies looking at how the inertia affect the training parameters like power, work and force. Identifying weaknesses in the research is just as important as looking for evidence in my opinion.

What did they want to study?

The researchers wanted to examine how different inertias affect:

Another good aspect with this study is that Fernandez-Gonzalo is one of those people that understand the importance of having both female and male subjects which is rare in flywheel research. One earlier publication of his on this topic is one of the few and I recommend you to check out that one too. You will then see why studying both sexes here are important, not because from a gender equality standpoint but from a more physiological one.

What they did

Eleven men and eleven women did unilateral and bilateral knee-extensions with a flywheel device at different inertias ranging from 0.0125 to 0.1, that is a factor x8 between “lightest” and ”heaviest”. That range covers the span that has been used in previous research. (As a side note, the kBox allows for inertia ranging from 0.01 to 0.280, or factor x28, with the kBox4 which is industry leading.)

Tests were of course preceded by a standardized warmup and throughout the testing, there was a regular ”vocal feedback” (i.e. test leader screams ”harder!!!” in your ear basically). During the testing session with different inertia, they measured force with a force sensor, power calculated from inertia and rotational measurements, unfortunately with an unvalidated system that never appeared in peer-reviewed-research before. They use electro-goniometry for angular positions.

The test itself consisted of three maximal CON-ECC reps preceded by one pre-rep. I would have preferred two pre-reps myself like in our #kBoxPowertest since we see that affects output. However, they did six test sets in total so I guess they wanted to limit the amount of work the subjects had to do ”outside” the data collection.


In all tests, the men performed significantly higher values (power, force, absolute overload) besides in relative overload (%). No surprise here.

For both sexes, there was a power drop from the lowest inertia to the highest but the men showed a higher % drop than the women (36% vs 29%). Higher inertia resulted in higher work (logical from a maximal contraction and same amount of reps and ”distance” covered but with a longer time under tension and slower speed).

When it comes to force, both CON and ECC forces increased with increasing inertia. Surprisingly, mean forces in both CON and ECC increased more in relative terms in women than in men but men showed larger increases in peak force. Men also showed a tendency towards increasing eccentric overload (%) with increasing inertia from 0.0125 to 0.0375 where they had their max. But from inertia 0.0125 to 0.05, it decreased in women. In absolute terms, the eccentric overload didn’t change for women between the different inertias (and as a result, the % overload decreased when CON force increased) while it increased in men.

Higher SCC use was seen in both sexes with low-medium inertias and men had overall greater SCC usage compared to women. SCC usage was the greatest for both sexes at inertia 0.0375 kgm2.


Power in general is higher in men than in women on all inertias and power drops with increasing inertia, however it drops more in men than in women with increasing inertia. Lean body mass can be one factor explaining this and I agree with the authors that further studies on this subject should take LBM into account. Fiber type composition could be another aspect.

Work is increasing with increased inertia and this is independent of sex. When it comes to force, men and women respond differently to increasing inertia in this study where men increase the peak force more and women increase their mean force more with increasing inertia.

Flywheel Workout Zones - newEccentric overload can be achieved with any of the studied inertias in this exercise. However, men have a greater capacity for eccentric overload than women in absolute terms and it increases with increased inertia. In women, the overload in relative terms was the greatest at the lowest inertia (i.e. higher movement velocities). Looking at our Flywheel Workout Zones model, you might think there is a discrepancy. However, actual overload in absolute numbers are the same for women at higher inertia and overload is higher for men so if you are chasing high forces and overload, I would say go for more inertia.

The take-home message from this study in my opinion is that individualisation is important. Assess your athletes or patients and use this information to guide you on inertia settings and re-assess regularly. Since most flywheel users out there are not doing knee extension but squats, deadlifts and other closed chain exercises on devices like the kBox, I would use this information for what it is and not extrapolate to any drill or device. For example the external moment arms change much during the CON phase in the squat which allows for a higher force and power production in the end range compared to a knee extension which can give a different force-velocity, power and eccentric overload profile with increasing inertia. Lets hope this study can inspire some applied researchers to dig deeper into this area!

If you have any questions with regards to the choice of inertia, just drop me an email or check out our courses.

Happy DOMS!

/Fredrik Correa, Head of Research & Development