Physics problem

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 adaption you get. Together with your workout intensity, the inertia chosen decides your Flywheel Workout Zone. Fredrik Correa explains.


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.