An Examination of Flywheel Training Means & Methods Part 1 by Jenna Burnett


[This is a guest blog from Jenna Burnett. Jenna has a Bachelors degree in physics and mathematics from Purdue University and is working on her Masters in kinesiology at Iowa State University. She is currently working as a Sports Science Research Intern at Athletic Lab.]

[Note from Mike Young: This post is the first in a 3 part series on the Exxentric kBox3. The kBox3 is a critical training tool for us at Athletic Lab and, as with any new sport technology we integrate, we test the equipment to not only ensure the data it produces is accurate, valid and reliable but also to determine best practices for its subsequent use. We regularly conduct in-house testing both for the companies and for ourselves and have published some of this data on this and other blogs. In this series, Jenna looked at various loading protocols for the kBox on the most commonly used exercise: the Kbox Squat.]

squatTraditionally, athletes focus their strength training on mostly concentric motions, such as a weighted back squat or a deadlift. However, recent studies have documented several additional benefits when eccentric only loading or overloading is included in training plans. For example, when athletes have trained with eccentric loading or overloading, decreased injury risk, increased flexibility and range of motion, and increased muscle size and strength, as well as many other benefits has been seen (O’Sullivan, McAuliffe, & Deburca, 2012). As a result of these benefits, many athletes have begun to turn to eccentric overloading to give them an edge over their competition. While several different eccentric loading methods are available, such as only doing the eccentric movement of an exercise like a squat, the method studied here involved the Exxentric kBox3. The kBox3 is built and created by a company in Sweden and has been available commercially for many years, allowing several years of research to be conducted. As a part of my internship with the Athletic Lab, I collected data using the kBox3 looking at the absolute peak power in the concentric and eccentric motions of a back squat during several different protocols.

To understand the way in which the kBox3 works, the first important piece of information was to learn how the kBox3 creates the eccentric loading. The kBox3 is a flywheel inertial device. This simply means that the kBox3 has a flywheel that is given an inertia based upon the amount of energy the athlete generates in an exercise. The general idea of inertia is that once an object begins to move, it does not want to change or stop. The force needed to stop or change the motion is considered the object’s inertia. The inertia for the kBox3 is determined by the number and size of the flywheels the athlete chooses, as well as how much force they use to complete the exercise. As an example of how the kBox3 functions, suppose you have an athlete who wants to do squats using the kBox3. They put on the vest or hip harness, attach the cord to the harness, and choose a flywheel. They, or another individual, begins the squat by spinning the flywheel lightly, winding the cord around the axel and the athlete descends into a squat. The first squat or two will most likely not be at full depth, as the velocity and inertia of the flywheel is not yet high enough to keep the cord from getting slack while the athlete descends to full depth. Once the athlete hits approximately a quarter or half squat, they begin to drive upward in the concentric motion of the squat. But the flywheel wants to keep spinning in the same direction, so the athlete must generate enough energy to first stop the wheel, and then get it to begin spinning in the other rotational direction and allow the cord to begin unwinding. The faster they can do this, the more power they can generate in the concentric direction. They continue to push upwards until full extension is reached, speeding up the rotational velocity of the wheel. The flywheel’s rotational velocity determines the resistance for the eccentric overloading. At the top of the squat, the cord is completely unwound and now begins to wind back around the axel in the other direction, pulling the athlete down while they try to control their descent into a full squat. The better they can control and slow the flywheel motion, the higher their eccentric peak power will be and the higher their eccentric overloading will be. The basic idea of the kBox3 is that the harder the athlete pulls on the cord, the more the flywheel will ACCELERATE, and the more ANGULAR MOMENTUM the flywheel will generate, creating a higher resistance for the eccentric part of the movement.

cintroduction_to_clinically_oriented_anatomyffi_20The eccentric overloading is not the only benefit of the kBox3 over traditional training. The resistance in the full motion of an exercise will vary depending on how much force the athlete can generate on the cord and allows individuality that is not always available by a traditional barbell back squat. In general, the weight an athlete can use on a traditional back squat is limited by the point in the squat motion at which the muscle force is weakest. For most individuals, this weak point occurs at a specific knee and hip angle determined by their individual muscle strengths. The kBox3 however, allows the resistance to vary throughout the range of motion, creating full exertion at every point of the exercise. This means that the athlete will get stronger at every angle rather than just their weakest angle, as in traditional lifting. This also implies that the athlete will increase their peak power even further, as the full range of motion gets stronger.

This variable force is an important part of the peak power equation, as power is the product of force and velocity. This means that an individual can reach their peak power by either moving an object quickly or generating a high force upon that object, implying that maintaining one component and increasing the other will allow higher power generation. We were interested in studying power generating capabilities. Therefore, several different kBox3 squat protocols, as well as two different harness systems, were tested on individuals at Athletic Lab. These protocols included a baseline or familiarization test, an isometric hold or pinch test and a slack or lag cord test on the kBox3. As a result of the data collected, we hoped to document not only what situation generated the peak power within an individual but also whether specific training protocols may be an important consideration for getting the most out of a session. As a whole, this study will be released as a three part sequence, with this introduction the first part, the methods and results will be part two, and conclusions will be discussed as part three of this blog sequence.