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Physiological Advantages

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


Eccentric Training

Your muscles are stronger in the eccentric phase of its motion, but it is difficult to reach your maximal eccentric power in traditional weight training. In flywheel training, you can use the kBox to achieve eccentric training with overload safely without help.

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Variable Resistance

Among the advantages of flywheel training, variable resistance may be the most important, resulting in proven gains. The resistance spectrum of a kBox is completely unlimited, as it depends solely on the force you use. Hence, flywheel training is not just solely for elite athletes or rehabilitation. The resistance is also variable between repetitions and you can easily work maximal repetitions all through the set, increasing your hypertrophic response.

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Scientific Evidence

Multiple academic studies have provided support for flywheel training and eccentric training. Here is a selection of some of the more important studies in the field.

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Advanced Methods

To develop the basic exercises, you can apply a set of methods to secure optimal eccentric and concentric loads, as well as isometric training and power testing.

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Harness Ergonomics

The pressure over the shoulders when doing traditional barbell squats is a big limitation for many. Often, serious bruising can be seen after heavy sets. Furthermore, the discs in the lumbar spine are under a lot of pressure, even if the athlete has good technical skill. For tall people, individuals with a weak midsection or persons with back problems, squatting is almost impossible and may be accompanied with a lot of discomfort or pain. With the kBox, you are able to work out with maximal or closer to maximal intensity, since the harness equalizes the pressure over the shoulders and also relieves the lumbar spine of pressure. With a less perfected technique, the harness puts the force over the lower back, instead of the shoulders, which shortens the lever and the momentum drastically on the lumbar spine without compromising the training effects in the muscles of the lower body. The kBox is also almost completely silent and the risk of bumping into others is low.

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kBox4 Active

Our entry level model for the serious personal trainer or demanding home user. The perfect tool in strength and rehabilitation at our lowest price point.

  • Entry Level Model
  • Holds 1-2 Flywheels
  • kMeter Compatible
  • Weighs 15 kg
  • Starting at €1 410 / $1 635
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kBox4 Lite

Offers the lowest weight and best portability which makes it perfect for frequent travel without compromising the training.

  • Enhanced Portability
  • Holds 1-4 Flywheels
  • Built in kMeter
  • Weighs 9.5 kg
  • Starting at €2 300 / $2 665
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kBox4 Pro

Our premium kBox model with the greatest versatility and largest performance area. It’s ideal for sports teams, premium fitness clubs and physiotherapy facilities.

  • Largest Performance Area
  • Holds 1-4 Flywheels
  • Built in kMeter
  • Weighs 15.5 kg
  • Starting at €2 990 / $3 465
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The kPulley2 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 €1 890 / $2 190
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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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