The Flywheel Workout Zones: How to Pick the Right Inertia

In flywheel or isoinertial training, choosing inertia means more than deciding between light and heavy. The inertia resulting from the size and number of flywheels impacts the training adaptation you get. Together with your workout intensity, the inertia chosen decides your Flywheel Workout Zone. Fredrik Correa explains.

Flywheel Workout Zones - new

  • The kBox offers training in all zones from warm-up and technique to power and strength training.
  • The inertia chosen affects velocity, force output, power, volume, total work, the degree of eccentric load and ultimately the training adaptation.
  • For maximum intensity (intended max velocity), the inertia sets the speed and decides where you end up on the force-velocity curve.
  • For eccentric overload, the speed going into the eccentric phase is as important as the total energy.


The other week, I had a discussion on flywheel programming with one of our power users, Jim Ferris. We mainly discussed inertia and I shared this graph on Flywheel Workout Zones with him. We have put it out before but I think I should spend some time to explain it. I hope you will find it helpful in your training.

Gravity vs Inertia

With a regular weight (barbell, weight stack) you have to overcome gravity to get the weight moving, for a 10 kg dumbbell that is roughly 100 N. With a flywheel or many flywheels, more inertia will add resistance but the flywheels are not lifted so you are not working against gravity. Instead, you are accelerating the flywheels and you are working against the inertia of the flywheel, not its mass.

When a weight is standing still, there is a force acting on it- gravity, trying to pull it down to Earth. In order to hold a weight still, you have to apply a force to counter-act the gravitational pull. With flywheels, there isn’t any counteracting force to overcome so all the force you put in, whether small or large, will accelerate the flywheel. At the same time, there is no upper limit for how fast a flywheel can rotate, so basically the force applied to a flywheel can be everything from minimal to unlimited.

Hence, I try not to talk about light or heavy load since the number of flywheels is not that relevant. You can load up with four heavy flywheels and go really gentle and that will be easy but really really slow. You can’t do a 500 lbs barbell deadlift by going gentle but slow. However in every day talk, when someone says low or high load with flywheel, I suppose that means low inertia and high inertia.

Also, remember that inertia is fixed, like a weight on the bar but it can be different in load depending on which exercise you are doing. A weight when doing heavy biceps curls is probably not a heavy weight when squatting. So you have to consider the amount of muscle you are using in the exercise. With that being said, High or low inertia plays a lesser role in flywheel training on the kBox compared to weights on a bar when 5-10% of weight added can set you off completely.

low speed Zones

So now to the graph, The vertical axis indicates speed and the horizontal one is for inertia. Let’s start in the bottom left corner with low speed, low inertia. This is labelled as warm-up. Load and intensity is low and it is a good way to get started feeling out the range of motion and preparing for more intense work. Personally I could put a warm-up label in the whole column of low speed and I usually do at least one set on high inertia but with low speed before I go into full intensity sets. That would however make the graph more complicated so we left it out. What I like with going slow with a high inertia is the dynamic stretch you get, especially at the end of the eccentric phase which is good for subjective assessment of how the body feels that day, be it stiff, sore or suffering joint pain. Maybe let the load pull you down and stretch out until you reached your target ROM for a deep squat for example.

I labelled slow with medium inertia technique since increased movement speed is one factor making a movement more difficult. Everyone can do movements fairly accurate at low speed, it’s doing it fast that is the challenge. So if you are coaching someone that is new to the kBox, I suggest using a medium inertia to keep speed and intensity fairly low. This will make the movement controlled and easier for the user. Moreover this will simplify things for the coach who may be spotting the user or providing general feedback & instructions.

Medium and High Speed Zones

Eccentric Training: The Force-Velocity RelationshipMoving on, increasing speed on a low inertia will put us first in more intense warm-up zone and also conditioning zone if we go for longer sets. Increasing speed further will put you in a power zone. Power output and speed is high, force is high but maybe not maximal in absolute values. The delay between concentric and eccentric phases (CON and ECC) are short here (almost direct), since CON phase is fast and also shorter in time with the same ROM as compared to having higher inertia.

By applying a (lower than maximal) force for a shorter period of time, the amount of rotational energy gets lower in the end of CON compared to a maximal repetition in the same drill with heavier inertia, which can affect how you should load. Going for maximal CON power output, inertia should be low and speed maximal (see below). But if focus is ECC power production, I would go for higher inertia and maximal speed. This will put you in the strength/overload zone. CON peak power is lower here but going back to force applied over a longer time there will be more total energy in the motion of the flywheel, hence more energy to absorb. This will create a longer, tougher ECC phase but it will prime you better for the large ECC forces we see in landing, change of direction and vertical forces during sprints.

Inertia vs force

Generally inertia is low at the peak power. For the kBox Power Test, we see optimal range from 0.025 (some females) to 0.075 (male national level weightlifter) but the U-curve is much less pronouned on the kBox compared with weights. For finding a optimal inertia zone, plotting a power/inertia graph is recommended.

Here is my power/inertia graph for the kBox Squat. Not as nice as the schematic ones but this is what reality looks like when you are sore as hell and have to do 10 sets of maximal squats. What you notice is that power peaks at a lower inertia (blue curve). Probably my max is close to 0.05 but on this day, I couldn’t really deliver.

Average force gradually increases with increased inertia (red curve). It might look like it stops increasing after a while but considering this is from a day when I was really sore and also that I did 5 RM on every inertia and on the really high inertias, that means a really long time under tension and high average force is difficult to maintain for a long time. For max force, I had go for fewer reps at the higher inertias and equalize time under tension instead of reps and not do as I did here.


Working on strength and hypertrophy, you probably end up in the middle. From research in later years, we know low load training to momentary muscular failure can promote strength gains and mass as much as traditional loading but I think most people tend to do 5-12 reps if they are looking for strength and mass. Using a kBox, that will put you in mid-range inertia going fairly slow, creating high forces over the whole ROM for a longer time under tension compared to power training. With a medium inertia, it is quite easy to add some extra energy in the CON phase at the end of the set, you get a little extra ECC squeeze of your muscle for a nice finisher. Try adding a shallow squat movement in any upper body drill and absorb that in the ECC phase by only using the upper body and you will know what I mean. If you want to see an example of this, check our page about eccentric overload for all variations.

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





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