Gravity vs Inertia

Traditional strength training is generally done by lifting some sort of weight – such as a barbell, dumbbell and weight stack machines – against gravity to create resistance.

In flywheel training or isoinertial training, resistance is created through the inertia of a flywheel, which is accelerated or decelerated with muscle force. The level of inertia of the flywheel rather than the weight determines the force needed. The principle is equivalent to that of a traditional yoyo device.

The resulting force in a flywheel training exercise depends on a combination of the inertia and the intensity you work with. With a higher inertia, the motion will be slower and more controlled, and your muscle will be able to produce a higher force. With a lower inertia, the motion will be faster which also puts higher demands on skill, balance and technique.

Inertia on the kBox, kPulley and SingleExx

Our kBox, kPulley and SingleExx devices can be equipped with two or four flywheels simultaneously depending on the device model. Additionally the Advanced Flywheel Knob can help to boost that capacity by two flywheels of a compatible size. Our flywheels come in five different sizes with inertia ranging from 0.005 kgm² to 0.070 kgm².

• Extra Large: 0.070 kgm² (only compatible with the kBox4 Pro, LegExx, LegFlexx and all kPulley devices)
• Large: 0.050 kgm²
• Medium: 0.025 kgm²
• Small: 0.010 kgm²
• Extra Small: 0.005 kgm²

Choosing Inertia

In flywheel 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 the ideal choice of inertia depends on your exercise purpose, also coined the Flywheel Workout Zones.

Learn More

• The Flywheel Workout Zones: How to Pick the Right Inertia
• See also Wikipedia about inertia and kinetic energy