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