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


Launching Online Courses in Flywheel Training

The world is a big place and unfortunately we can’t be everywhere at once. To make knowledge about the kBox and flywheel training more accessible, we are now launching our first online courses at Exxentric Academy. Fredrik Correa explains:

Key points:

  • Courses available for potential customers as well as existing users
  • All are currently free of charge, open 24/7
  • Requests for custom courses welcome!

Nothing beats attending a good course in real life. Meeting people, discussing and mixing theory with practice. We have been traveling around the world to do flywheel training introductory courses of all kinds – and we have loved it. I have personally learned a lot from meeting people interested in physical training engaged in different ways on the rehab–fitness–performance spectrum from different sports and settings.

Attending a course is of course a great way to get started with your flywheel training but it is also a way to evaluate the tool and the training before committing to a purchase. So of course, spreading knowledge is vital for us at Exxentric. Why would we support 10+ research projects if that wasn’t the case?

However, with the kBox now in 45 countries, we need to add new ways of sharing knowledge. Writing articles or tweets is good for Q&A, and conducting 1 on 1 courses over Skype is still an important tool for us. But we wanted to start making the knowledge in our field even more accessible. That’s why we are now launching an online learning platform.

Available Online Courses

Today, you can find our first online courses at Exxentric Academy. Like always, we are starting small but hope to develop this depending on your needs. Courses we added to the platform so far include:

  • kBox4 demo [English & Swedish]
  • kBox4 getting started intro (for new customers) [English]
  • kMeter intro course [English & Swedish]
  • Lecture on flywheel training for physios [Swedish]

All of the currently available material is offered for free, so take a look and give us your feedback. It’s far from perfect and you have to be able to withstand a fair amount of ‘Swenglish’ to get through this but I hope it will help you anyway.

Future Online Courses

There are of course a bunch of courses that could be done but instead of deciding what is important for you we hope that you will tell us. We can talk about programing, different ways to load, how to use the kMeter, ways to make your training more specific to your needs. Everything from general to very specific is on the table. If you ask for it and we got the time, we are ready to do it.

1 on 1 Intros

I just want to highlight the possibility for you to get started with your flywheel training over a 1 on 1 conversation through Skype or Google Hangouts. This can be a great way for a dedicated home user upgrading his gym with a kBox to get started and getting ROI from day 1 on your investment. Reach out to us for pricing if you are interested. There are also experienced coaches all over the world using the kBox that might be able to coach you online for more regular sessions and follow-ups. Maybe we can connect you if you are interested.

Real Life Courses

We will of course still do our standard courses in real life when people are interested, just like today. Reach out to me if you want to host a course at your facility or if you have any feedback for us regarding education.

Happy DOMS!

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

BJ Gaddour’s Best kBox Training Programs

As many of you already noticed, BJ Gaddour has gone on a kBox frenzy. Here, Fredrik Correa shares a few of his videos to inspire your personal fitness routine.

Since this summer, the former Athletic Director of Men’s Health magazine, BJ Gaddour, creator of The Daily BJ, has shared a ton of quick and intense programs to his +200k followers with the kBox as as a central piece of equipment.

His programs are perfect for a busy schedule and I wanted to highlight some of them here, both to return the favor and to show the kBox being used in a home gym setting as opposed to most of the other media we share.

Maybe a few of these programs will fit as recurring workouts for you, or maybe you just want to take one or a few of these drills and put them into your own existing program. As BJ points out, just as you can do lots of traditional free weight exercises on the kBox for increased effect and efficiency, most of these programs can be done with traditional equipment as well.

Use this for inspiration! Here we go:

Upper body workouts

Lets start with this post focusing on upper body:

#FlexFriday 💪with some straight Bro-J Gaddour arms, shoulders, and back blasting! I’m using the @go_exxentric #Kbox4 and @concept2inc #SkiErg but just focus on the exercises/movement patterns and use whatever equipment you have access to for a precious pump to kick off the weekend: 1. Single-Arm Lateral Raises @ 3 sets of 8 reps/side 2. Triceps Pushdowns @ 3 sets max reps/distance in a minute 3. Single-Arm Row @ 3 sets of 10 reps/side 4. Overhead Triceps Extensions @ 3 sets max reps/distance in a minute 5. Single-Arm Concentration Curl @ 3 sets of 8 reps/side 6. Reverse-Grip Bent-Over Row @ 5 sets of 15 reps 7. Reverse Fly @ 5 sets max reps/distance in a minute 8. Split Kneeling Overhead Press @ 5 sets of 15 reps 9. Straight-Arm Lat Pulldown @ 5 sets max reps/distance in a minute 10. High Pull @ 5 sets of 15 reps #AskYourGymToBuyThis #TheDailyBJ #NotAPornSite #AllBJNoBS #Back #Arms #Shoulders #BroJGaddour #AllBro #UpperBody #UpperBodyWorkout #Bodybuilding #Muscle #TGIF #BJGaddour #💪

Ett inlägg delat av BJ Gaddour (@bjgaddour)

Another one focusing on arms and deltoids:

💪ARMS, DELTS, & CARDIO! Here was the flow of a recent upper body and cardio workout. I got a savage heart and muscle pump. As always, modify with whatever equiment you have access to and apply the principles for dem gainz. CIRCUIT 1 1a. @go_exxentric #Kbox4 rope hammer curl 1b. #Kbox4 overhead rope triceps extension 1c. Reverse fly on @concept2inc #SkiErg @ max distance in 2 minutes CIRCUIT 2 2a. #Kbox4 rope front raise 2b. #Kbox4 rope upright row 2c. #Kbox4 rope bent-over row 2d. Straight-arm lat pulldown on @concept2inc #SkiErg @ max distance in 2 minutes CIRCUIT 3 3a/b. Side-lying dumbbell lateral raise L/R 3c/d. Standing band lateral raise L/R FINISHER 4a. Blood-flow restriction triceps pushdown 4b. Blood-flow restriction lying biceps curl Join (direct link in my bio) to access my exact daily workouts with sets, reps, rest, etc. #TheDailyBJ #NotAPornSite #AllBJNoBS #Arms #Delts #Cardio #MetabolicBodybuilding #Gainz #Bodybuilding #Muscle #ArmsWorkout #Biceps #Triceps #Shoulders #EccentricTraining

Ett inlägg delat av BJ Gaddour (@bjgaddour)

Lower body workouts

Working them legs:

LOWER BODY WERK! Wearing my @pedestalfootwear barefoot training socks to maximize the lower body gainz. As always, modify with whatever equipment you have access to and focus on the movement patterns and principles: 1. Front-Foot-Elevated Split Squats 2. 1-Leg Leg Curls 3. 1-Leg Hip Flexions 4. Constant-Tension Fingertip-Assisted Russian Leg Curls Moves 1-3 are using the @go_exxentric #Kbox4 fly wheel trainer for eccentric overload- the faster you go up, the faster it comes back down. I did 6 sets of 20 reps/side for move 1 and 3 sets of 20 reps/side for moves 2 and 3. Move 4 uses the #SpeedBot floor glute-ham developer but you can modify by hooking your feet into a lat pulldown seat or having a partner hold your feet down. Keep constant tension on those hammies by staying in the bottom half of the range of motion, using only as much fingertip assistance as needed to execute the reps. I did 3 sets of max reps to finish the workout. I post my exact daily workout at (direct link in my bio) under THE DAILY BJ category. Check it out! #TheDailyBJ #NotAPornSite #AllBJNoBS #Legs #LegDay #TheHamstringGuy #EccentricTraining #RussianLegCurls #NordicCurls #LegCurls #BJGaddour #Gainz

Ett inlägg delat av BJ Gaddour (@bjgaddour)

Two of my favorite drills on the kBox, Rows and RDLs:

💪TRIPLE BACK STACK!🍑One of my favorite ways to hit any muscle group is stacking 3-competitive moves back to back to back. It’s a great way to get a lot of work done in a very short period of time, extend time under tension, and create massive muscles pumps and metabolic stress to spur growth. Plus, you can accomplish all of this using lighter loads which is great for longevity. Here are 3 sample triple back stacks using different equipment that will also hammer your glutes and hamstrings. They incorporate a “mechanical drop set” where you start with a weaker position and move to a stronger position as you fatigue while using the same load. 1. Barbell Overhand-Grip Bent-Over Row/Underhand-Grip Bent-Over Row/Hip-Hinge 2. Dumbbell Overhand-Grip Bent-Over Row/Hammer-Grip Bent-Over Row/Hip-Hinge 3. @go_exxentric #KBox4 Flywheel Trainer Overhand-Grip Bent-Over Row/Underhand-Grip Bent-Over Row/Hip-Hinge I use a mini-band wrapped above the knees on setups 1 and 3 to increase hip activation and put the lower back in a more stable position. I block my feet together on setup 2 so the dumbbells don’t hit my thighs and to increase the range of motion through my hips and hamstrings. To see the full instructional video with sets, reps, rest, etc. visit (direct link in my bio) under the GAINZ category. #TheDailyBJ #NotAPornSite #AllBJNoBS #Gainz #Back #BackWorkout #BackCircuit #DropSet #MechanicalDropSet #Bodybuilding #MetabolicBodybuilding #BJGaddour #TransformationTuesday #Glutes #Hamstrings

Ett inlägg delat av BJ Gaddour (@bjgaddour)

This Zercher variation rightfully stimulates a lot of the core and arms since you have to stiffen up the whole chain from the bar to the foot to be able to produce force:

EXERCISE ORDER ON LEG DAY FOR MAX GAINZ! Most trainees will do the heavier bilateral moves first in a lower body workout. I used to do it that way too when strength and load was more of a priority. But now that performance, longevity and muscle gain is my main focus, my approach is quite different. I do unilateral moves first to optimize muscle activation, joint positioning, and mobility. By the time I get to the heavier bilateral work, my nervous system is primed, my joints are lubricated, and my muscles are moist. With this approach over the last year or so I’ve never had better leg days. I get precious, pain-free pumps and my recovery is dramatically faster. Here was the exercise order at a recent leg day sesh: Circuit 1: 10 reps each 1a. Single-leg band hip thrust- left side 1b. Single-leg band hip thrust- right side 1c. Single-leg, single-arm hip-hinge- left side 1d. Single-leg, single-arm hip-hinge- right side Circuit 2: 10 reps each 2a. Zercher Bulgarian split squats- left side 2b. Zercher Bulgarian split squats- right side 2c. Russian leg curls Circuit 3: High reps (20-30+) 3a. Constant tension Bulgarian split squats- left side 3b. Constant tension Bulgarian split squats- right side 3c. Mini-band (hip circle from @mbslingshot) hip-hinges I’m using the @the_hip_thruster for 1a and b, the @go_exxentric #KBox4 fly wheel trainer for 1c, 1d, 2a, 2b, and 3c which provides eccentric overload (the faster you pull up, the faster it goes back down), and the floor glute-ham developer for 2c is from #SpeedBot. But use whatever equipment you have access. I love finishing with high-rep bodyweight moves, especially if I did that same move earlier in the workout with heavier loads. Start re-thinking the traditional approach to #LegDay if you’re looking for better long-term #gainz. I post my exact daily workouts from the week before complete with sets, reps, rest, etc. at (direct link in my bio). Check them out! #TheDailyBJ #NotAPornSite #AllBJNoBS #Legs #Quads #Glutes #Hamstrings #Fitness #Fitspo #Fitspiration #ReturnOfTheMack #Bodybuilding #MetabolicBodybuilding #BJGaddour #Circuit #CircuitTraining #CircuitWorkout

Ett inlägg delat av BJ Gaddour (@bjgaddour)

Have fun!

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


In Ice Hockey, The Legs Feed The Wolf

With customers like the Detroit Red Wings, New York Rangers and Los Angeles Kings, it comes as no surprise that our co-founder and former hockey coach Mårten Fredriksson looks forward to the upcoming ice hockey season.

With the start of the NHL pre-season, as well as the SHL season in Sweden, we’re happy to finally indulge in our hunger for good hockey.

As some of the best athletes in the world step out on the ice, we know that many of them now have an advantage over their competition. They can carry this advantage with them on their team planes, buses and any other vehicle for that matter. You don’t actually need to be in the NHL to possess it. It’s just a piece of science based, innovative training equipment that offers variable resistance and easy access to eccentric overload training for quicker gains in strength and mass.

By now you’ve probably guessed that I’m talking about the kBox, a tool that can help you improve your stride length and stride rate on the ice, whether you’re a NHL superstar like Kris Letang of the Pittsburgh Penguins (pictured) or an aspiring young athlete without much experience in the weight room. You can avoid injuries by using the Harness to unload your back and train like a beast. Heavy squats, explosive squats, lateral squats, split squats, you name it. As Coach Herb Brooks used to say, “the legs feed the wolf” and you can feed your legs with some kBox training.

Happy Hockey Season, Folks!

Mårten Fredriksson/Mårten Fredriksson, co-founder of Exxentric, former Ice Hockey Coach and S&C Coach at Junior & Senior Elite levels


Exxentric Ups Its Product Development Game

Earlier this year Exxentric started a search for a product development engineer. The response from a broad range of highly competent candidates was overwhelming and an intense interview period followed. Now, a couple of months later it’s time for Erik Lindberg to present a new team member.

Exxentric may be known as a flywheel training company, but in our vision for the future we think much more freely than that. In order to extend our technological lead we will continue to work closely with external consultants, subcontractors and trusted suppliers in order to leverage cutting-edge expertise wherever it can be found.

In order to further improve our product development process, we will now also add a new engineer to our own technical staff. Salli Carlfjord, who is a highly qualified engineer with a degree in Mechanical Engineering and Exercise Physiology from the University of Halmstad, will co-ordinate all activities in this area.

Salli is also a competitive athlete with no less than five wins of the Swedish Waterpolo Championship under her belt. Last week-end, she also won the Engadin swim-run competition in Switzerland (see picture).

In her role as Product Development Engineer, she will report to Fredrik Correa, Head of R&D.

We all welcome Salli to the #exxentricfam!

Erik Lindberg/Erik Lindberg, CEO

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


Assessing Your Eccentric Overload

Since the launch of the kMeter we have received a lot of positive feedback. It’s very satisfactory that so many of you see the benefit of direct feedback in your training and that you want to go beyond counting sets and reps. Here, Fredrik Correa dives into the subject of assessing your eccentric overload.

Screen Shot 2015-09-23 at 21.08.15Key points:

  • Main point: don’t just look at the overload % in the kMeter.
  • While not requiring the investment dual force plates, the kMeter needs some workaround to fully assess eccentric overload.
  • Do a max test (4 or 5 RM) and use average numbers in summary (peak power, force) for higher reliability.
  • Look at all numbers in total for comparison and relate your overload sets to your 4RM.
  • Re-test regularly, adaptation is rather fast with flywheel training, make sure you calibrate your reference values regularly (every 4-6 wks).

Many see the eccentric overload as one of the biggest advantages there is with the kBox and I agree, it’s really a key feature. Eccentric training with long time under tension and low loads is inefficient and higher loading % of 1 RM is beneficial [ref] and transfers more to performance, especially in well-trained subjects [ref, ref2]. The overload zone (red) with an eccentric over-capacity (supra 1 RM loads) is well-known but not used mostly because of practical reasons. Release hooks, spotters and really slow drills do not work if you want to incorporate eccentric overload training to a larger extent, at least before flywheel training and the kBox. Loading up the eccentric phase, not only in isolation exercises but also in the more heavier lifts, and doing this in a safe and controlled manner in a self-contained and self-operated system is unique for the kBox. Mike Young, founder of AthleticLab, posted some really nice vids showing this [example]. We know people all around are experimenting with this on the kBox and that leads to a new question, how do we assess the overload? After writing a piece earlier on safety during eccentric overload training I think it’s time for the next phase, assessing your overload.

The gold standard would be using two load cells on top of the kBox since this would give you left, right and total force during the whole rep. However, you can do without the hassle of starting up your laptop for some squats and you probably don’t have those load cells lying around anyway so lets see how we can use the kMeter for this feedback instead.

Throughout your set the kMeter gives your real-time feedback of power (average and peak) and force. In the summary you get the % eccentric overload and the average force, we’re going to use these numbers. For more info on the numbers, check the kMeter manual.

Some focus only on the % overload number but that really doesn’t tell you much if you don’t put it in context with the rest of the numbers. Think about it, if I said I had done dumbbell biceps curl with 100% eccentric overload (i.e ECC load = 2 x CON load), would you be impressed? I hope not because that could mean that I had lifted a 3 lbs dumbbell and lowered 6 lbs. Not too impressive but still a 100% overload. So, to get some meaning out of this you need to get some absolute numbers to go with to that so you can relate the percentage. My suggestion is that you use the average force. Average force is a calculated number, not measured, so it’s far from research grade so you can’t use it for comparing athletes or different exercises. But it is close enough to give you a number on your intensity during the set.

Eccentric Training: The Force-Velocity Relationship

First I would recommend you assess a 4 RM for that exercise. Why a 4 RM? Because your maximal rep in a maximal set on the kBox is usually around rep 2-4 and then you decline. This is because of the stretch-shortening cycle and the first maximal reps high eccentric load stimulate your muscles to produce higher forces in the consecutive reps and usually max out around rep 4 and then decline due to fatigue. If you still increase by rep 5 and 6 or even level out I would think that you hadn’t maxed out the first 3-4 reps and for some reason you are holding back. If your athletes show this they probably need more familiarisation with the kBox and/or that exercise to be able to perform maximally or they might be struggling with some pain or something holding them back. In the summary you get the peak power and that’s an average over your 4 RM. Looking in the graph you can see peak power rep by rep but using that would decrease the test-retest reliability so I would suggest you go with the peak power values in the summary. If you are familiar with our 4 RM kBoxPowerTest you will notice I used a higher inertia for this 4 RM. This is because that is a power test and this is a max force test similar to a traditional 1 RM so I want to place myself out to the left on the Force-Velocity curve, closer to my MVC (Maximal voluntary contraction).

As an example I’ve done some #kBoxBicepsCurl which is a good exercise for demonstrating the different ways to do eccentric overload. [Comment: Used previous data from an earlier version of the kMeter. That’s why the screen dumps looks a bit different from today]

If you are unfamiliar with the methods for eccentric overload on the kBox here is a video showing the curl variations below in one video.

So I started with a 4RM with inertia 0.120:

4RM (1:1)Foto 2015-09-19 21 10 21

  • ECC PEAK POWER 349 (-29%) W

The kMeter tells me that the average rep time was 3.5 seconds so I think that puts me pretty far on the left on the force-velocity curve and I don’t think the average force will go up very much with higher inertia. If you think you might then do another 4 RM with higher inertia and compare.

[As a comment I went with a little longer ROM in the overload settings which isn’t recommended for comparisons but you will still get the point. When you test the athletes you should of course use the same ROM and technique as much as possible between different types of tests or overload if you want to use the numbers for comparisons]

On with the eccentrics, here is the first one, Delayed Eccentric Action , same drill but with half the inertia so the rep time drops to 2.8 seconds i.e faster motion. Now I’m doing 8RM so the total time under tension is a bit longer here in the 8RM. You can look at the energy expenditure (kJ) to assess the total work and compare between to sets.


  • Foto 2015-09-20 21 02 55CON PEAK POWER 366

As you can see I now get a 32% overload in the ECC phase (ECC power / CON power) and that overload is also clearly visible in the graph showing CON and ECC peak power. However, since the average force drops about 37% compared to 4 RM I would say the eccentric load is about the same as in the 4 RM even though the power is higher. ECC power is a bit higher but this is only during a short time of the ECC phase due to the features of the delayed ECC action, you don’t do any work or low work through most of the ECC phase and brake hard in the end of ECC. So this is a way to emphasize the ECC overload in the end of the ECC phase but as a total overload this setting doesn’t work very well.

Delayed Eccentric Action is a good way to get started with eccentric overload for familiarisation and to incorporate it gradually in your training program but not for heavy overload in well-trained subjects looking at specific effects. The benefit is that it works in all exercises since you can always delay the deceleration phase and work a bit harder in the end. In some exercises the end of the ECC phase is also the sticking point, for example the deep portion of the squat. Here the delayed eccentric action can make you work harder in the deep portion and help you build strength in this weak spot and improve your lifting and even the catch-phase in the olympic lifts.

So I moved on to the Overloaded Concentric Action which means adding energy in CON using legs (squat) and resist that energy in ECC phase using only the arms. Compared to the Delayed Eccentric Action I work harder but also during a longer portion of the ECC phase. Here are the numbers:


  • Foto 2015-09-14 21 49 12CON PEAK POWER 1060

In the graph you see that the Eccentric Overload % is -41% but don’t let this fool you, the work is much harder. Look at the average force and you see this 8 RM set is closer to the average force in the 4 RM. Also the ECC peak power is +29% compared to Delayed 8RM and +79% compared to 4 RM. Besides that this setting gives you a high ECC load over the whole ECC phase, not only in the end like in the Delayed Eccentric Action. And think about it, you are actually reversing the energy added with your legs using your arms, of course the overload is higher here, just don’t let the graph fool you, look at all the numbers, there are more to this than only the Eccentric Overload %. For total work, look at the energy expenditure again, 7 kJ compared to the 3.8 in the delayed eccentric action with the same number of reps.

A graphic overview comparing the delayed eccentric action and the overloaded concentric action looks like this. Eccentric load is definitely much higher in the second setting even though eccentric overload % is lower if you look at the peak power values and average force.


As I mentioned in the comments this wasn’t a perfect example since the 4RM had a shorter ROM, however, I hope you got the message. Overload should be related to your 1RM concentric max but with flywheels the 1RM doesn’t exist so we use a short set for max testing. If we don’t have force cells we have to work with the numbers presented in the kMeter app and use all the numbers and not only go with the % overload since that is an intra-set comparison and not comparing to your max. So, in short, here is how to do this:

  • Make a max test (4 or 5 RM) and use average numbers in the summary (peak power, force) for higher reliability.
  • Do your training. Look at ALL numbers in total for comparison and relate your overload sets to your 4RM.
  • Don’t overdo the overload training. Start with a low volume and progress fast instead of starting with too high volume and getting destroyed.
  • Re-test regularly, adaptation is rather fast with flywheel training, make sure you calibrate your reference values accordingly.


Happy DOMS!

/Fredrik Correa, M.D., 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


Flywheel Training vs Weights: What Does Science Say?

Last week a new systematic review and meta-analysis on the effect of flywheel training vs traditional weight training made quite some impact on the sport science community. Fredrik Correa has taken some time to read and digest the 29-page article to explain the study outcomes a little further than beyond the abstract, offer some input and also a peek into ongoing research by institutions like CESSCE (pictured).

Key points:

  • Flywheel training with eccentric overload is consistently shown to be superior to traditional weights for increasing muscle power, strength, hypertrophy and athletic performance.
  • Eccentric overloading in these studies is predominantly done through one method but there are many other, in my opinion, more effective methods.
  • Future research needed. We still don’t know what is optimal.
  • Research using the kBox is under way from researchers in five different countries regarding physiotherapy, fitness and performance training.

What’s all this fuzz about?

If you have been following and reading flywheel science until now, this meta analysis won’t surprise you. If you have been using the kBox and have hands-on experience even less so. All studies so far comparing flywheel vs gravitational loading (weight stack devices primarily) have been in favor of flywheel so no news here. There are one or two studies not favoring flywheel, however those are about the conic pulley version which is completely different from the type with a symmetrical shaft like the kBox. In one of those studies they are comparing different drills too, so you can’t really say anything about flywheel vs weights there either. If you been following us for a while you might have come across the Meta-analysis on FW and effect on power, strength, mass and horizontal and vertical force production by Henrik Petré, an unpublished MSc project. It contains about the same number of studies but that isn’t a comparison against weights. You can read about it here if you missed it.

Before digging into the recent publication I want to just set the baseline here with explaining eccentric overload to clarify what it means in articles, what people think it is and what it is in reality. In all flywheel training articles the overload has been of the delayed eccentric action type. This means you accelerate all the way through the concentric phase but don’t resist until after you passed first 1/3 of the eccentric motion. By doing this you are overloading the latter 2/3 of the range of motion since you have to absorb the same amount of energy as you produced over the whole concentric phase but in a shorter period of time.

If you look around when people tweet or post about eccentric overload you can see all kinds of things like super slow eccentrics spending 10-12 s in ECC phase, basically more isometric than eccentric action, at least if we compare to the eccentric actions done during athletic performance. So, eccentric overload to me is ECC load > 1 RM concentric. If you are doing 2-1, ie. “2 legs up and 1 leg down” with a submax weight I’d say you shifted ratios, with more eccentric focus, but if that load isn’t > 1 RM concentric it is not eccentric overload training. If you are talking about eccentric training (not saying eccentric overload) I think that is a broader term which could permit super slow chins and push ups with bodyweight. I think they are a waste of time however, instead increase the load and do (fast) overloaded eccentrics because that is the trigger you are looking for. Chris Beardsly wrote a nice piece on fast vs slow eccentrics.

In reality however, when using the kBox, the ways you can overload are much wider than what is seen in these studies. You can use a stronger movement pattern in CON like doing a “squat-hinge” as our US friends call it or the terms I prefer “overloaded RDLs” or “deadlift into RDL” (as performed by Mike Young in the pic above). RDL is weaker so if preceded by a powerful deadlift it will be overloaded. You can you use accessory muscles like pushing off with arms in the squat in CON and absorb it with the legs or have a coach pulling you up adding extra energy for you to absorb. So this study is looking at flywheel training with a partial overload in eccentric ROM vs training with traditional weights, nothing else. The adaptations coming from the more powerful overload methods with higher contraction velocities haven’t been studied head-to-head yet but if we compare more overload and over the whole range of motion with regular CON:ECC 1:1 using weights, I know where my money is. You can read up on the kBox overload methods here.

What did the study show?

Now, back to the important new meta-study by the mainly Spain-based team of Sergio Maroto-Izquierdo, David García-López, Rodrigo Fernandez-Gonzalo, Osvaldo C. Moreira, Javier González-Gallego and José A. de Paz. If you just want the results you can check the abstract. However, if you are still reading you probably want a little more info so here goes.

The authors searched the databases and found 97 studies so flywheel might still have a lot of question marks around it but to say there is no research is wrong. Anyway, based on their inclusion criteria, 9 studies with a total of 267 subjects were included in the analysis. All these studies are FW vs weights, ranging from 4-10 weeks, with healthy young people or athletes ranging from 6-8 on the PEDro scale meaning all are classified as high quality. The average age for FW groups was 25.8 yrs with a very asymmetric sex distribution since only one study involved women. Exercises included in these studies were leg press, leg extension, leg curl, squat from lower limb and two studies included exercises targeting shoulder abduction, arm extensors and flexors. In the flywheel devices the overload was provided with delayed eccentric action as described above.


Here is the forest plot. If you want to interpret it yourself you can find a guide here.

Basically, all studies are placed under the respective outcomes they wanted to look at, strength, power, hypertrophy, jump, speed. The Standard Mean Difference on the far right shows difference between FW and weight training groups with an average to the right of the vertical bar meaning difference between groups favoring FW (ie more effect). Studies are weighted, so a larger study has more impact than a smaller one. All studies are weighted and put together in the row with the big black diamond. As you can see all studies favor FW on all outcomes with power and strength being the most obvious.


The article provides a good discussion around the results and also mentions a few other interesting studies(not included in the meta analysis) so I recommend you to read to full text if you want to get more details here. I’ll continue with my own comments below.

My comments

So is flywheel proved to be better than weights now?

Looking at this it is definitely more effective than weights at least if you are training young, healthy male athletes. When it comes to women, we have no reason to believe they wouldn’t benefit from FW training but all the evidence isn’t there yet. Future studies including women are needed in my opinion. This meta-analysis included 3 women out of 276 subjects in total. Sport science have to do better than that.

How can this be applied? These studies are basically single exercise drills and not a part of a training program so we actually don’t know (in a scientific sense) how this works in an environment with a much higher total training volume and with parts of concurrent training. Still, by adding FW to a well designed program it’s hard to see a negative effect coming out of this. Coaches afraid to train their athletes too hard by adding FW training can have this as evidence to replace some of the training with barbells with more effective flywheel training.

What we are missing are also more closed chain exercises involving multiple joints like squats, deadlifts and split squats. What we see from our users are also different types of overload with more overload and over the whole range of motion. This is probably an effective stimulus for adaptions in these outcomes but future studies have to quantify it. Adding flywheel will cause some muscle damage and fatigue early on but adaptation is fast and muscle markers for damage don’t seem to have a detrimental effect on adaptation. You can read more about on this subject here in an earlier post.

Last but not least, the flywheel device is only a tool. It needs to be properly used for strong positive effects. I usually say that training on the kBox doesn’t get you strong if you don’t train with that intent. Lousy training is still lousy on a flywheel device. The benefit of the kBox is that it makes it easy to train really, really, hard and that is what triggers the adaption. The overload. So to be more effective, incorporate flywheel and take part of our material and courses and you will be up and running in no time with your FW training and enjoying your gains.

What more do we need to know?

What I’d like to see is more studies looking at specific populations so we can prescribe training more effectively, depending on training age, strength, sport etc. As mentioned above, studies on more closed chain drills and realistic and complete training programs to help us with periodization are needed. FW might be better but what is optimal, we don’t know yet.

Besides this I think physiotherapy can benefit a lot from using FW training since patients need to get stronger, more powerful and with good timing, since time saved is important for good flow in the health care system and getting people back to work. However, before we see a massive surge of FW in the physiotherapy clinics we need more clinical studies on specific diagnoses and conditions.

Work being done

Studies using the kBox are being done right now or starting soon in Canada, US, UK, Sweden and Holland. The topics involve all three main groups, athletes, patients and the general population. I know at least one publication on physiotherapy hopefully coming out this spring. Without saying too much I can say the kBox was in favor over the golden standard treatment for a common problem among athletes in sports involving a lot of jumping.

Besides these research projects we are in discussion with other researchers so the list will be longer later this year. We try to understand the problems or questions our users have and one of my tasks is trying to get the researchers to look for answers. I hope that we will see more studies that we are able to apply in the field that will help us with protocols, periodization, loading and in-season training.

Feel free to reach out to me if you have any thoughts around this you want to discuss.

Happy DOMS!

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

For further reading on the scientific support for flywheel training, look here.