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Physical Principles

Strength Training Basics

Strength training can be defined as a limited number of repeated muscle contractions against some form of resistance, producing high force.

This stimulates hypertrophy (muscle growth) and increased activation, by the nervous system. These effects together give an increase in strength over time, if the exercise is repeated regularly.

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.

In flywheel 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 kinetic energy loaded in the flywheel in the concentric phase of the motion determines the force needed in the eccentric phase. This has important advantages when it comes to resistance training.


In traditional gravity-based strength training, a weight is placed on an extremity (arm or leg) and the extremity is put through a range of motion. While the direction of the motion varies for many important exercises, the direction of gravity remains the same, leading to that maximum force is achieved only at the point where the weight has the highest momentum on the active joint, in other words, “the sticking point”. At your 1 RM (1 Repetition Maximum) you just barely pass the sticking point but still, you would be able to lift a heavier weight in the other angles of the motion due to a lower momentum.

Flywheel training exercises, on the other hand, are isoinertial, meaning that they maintain constant inertia throughout the range of motion, facilitating a constant resistance and maximal muscle force in every angle. Compared to the weight, you don’t have any sticking point and can produce maximal force in every angle, in other words, through the whole range of motion (ROM) and therefore increase more in strength and mass with flywheel (isoinertial) training.

Isoinertial exercises strengthen both the muscle being targeted and the synergistic muscles. This type of exercise also helps to strengthen ligaments and tendons throughout the range of motion.

Further reading


kBox4 Active

The Newest model, the kBox4 Active is built on proven, reliable technology. The perfect tool in strength and rehabilitation at our lowest price point.

  • Newest Model
  • Holds 1-2 Flywheels
  • kMeter Compatible
  • Weighs 15 kg
  • Starting at €1 410 / $1 585
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kBox4 Lite

Often chosen for its smaller foot-print, the kBox4 Lite model is of a lower weight, portability and not to mention its competitive entry cost level.

  • Portability
  • Holds 1-4 Flywheels
  • Built in kMeter
  • Weighs 9.5 kg
  • Starting at €2 300 / $2 575
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kBox4 Pro

The kBox4 Pro is our premium model with the greatest versatility. It’s ideal for sports teams and larger training and physiotherapy facilities.

  • Largest Floor Space
  • Holds 1-4 Flywheels
  • Built in kMeter
  • Weighs 15.5 kg
  • Starting at €2 990 / $3 345
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The kPulley is the ideal horizontal solution for improving strength by offering flywheel training for rotational and pull exercises.

  • Horizontal Model
  • Holds 1-2 Flywheels
  • Built in kMeter
  • Easy to assemble
  • Starting at €1 750 / $1 955
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About us

Exxentric was born out of a real need

Exxentric AB is a company developing innovative, science-based training equipment and methods for strength and conditioning, including the kBox, and the kPulley used by performance coaches, personal trainers and physiotherapists worldwide since 2011.

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