Flywheel Training is a Gamechanger

In the strength world, a new machine or new training protocol comes out almost weekly. As coaches we get so used to gimmicks that sometimes it’s hard to see when something is actually of value. I am going to start out by saying the Kratos Flywheel has been nothing short of amazing. I have documented increased vertical leaps, personal records in the squat, and direct correlations to personal records in the snatch and clean & jerk. These benefits are related to improvements in joint elasticity, strengthened tendons, and Type II muscle hypertrophy. I have no doubts that the Kratos Flywheel would improve sprint times, change of direction capability, road jumps, and of course vertical leaps. Petré, H., et al., 2018 showed statistically significant increases in the areas of hypertrophy, absolute strength, power production, horizontal movement, and vertical movement. For all you science nerds like me, the effect sizes were for hypertrophy, CSA 0.59; volume/mass 0.59; maximum strength 1.33; power 1.19; horizontal 1.01 and vertical movement 0.85.


What does this mean in layman’s terms? Simply put, you are going to get more jacked, stronger, more powerful, faster, and improve your hops. The best part is that it only takes 4–6-week bouts to get these improvements. Therefore, you can spend the last half of training preparing for individual sport. For my team, we can spend time focusing on snatch and clean & jerk. Lately, we have discovered or improved three elements of our program that have all yielded improved results: velocity-based training, athlete data collection, and the latest being our flywheel. I know most of you coaches aren’t going to apply this, and you are going to say that you don’t need this. It makes me sad for your athletes, but at the end of the day, I love winning. Therefore, by all means, keep doing what you’re doing, but you have been told. One thing that no one can ever say about me is that I try to keep our training a secret. I learned from one of my mentors and friends, Louie Simmons, that the only way to effect change is to pass on knowledge. That’s what I am trying to do. By the way, I have received zero compensation for this article. I am simply trying to help all of you.



We ordered our Kratos Flywheel from Kabuki at the middle of last year. I have been using it ever since with my top athletes and the data is undeniable in that it clearly works well. There are some exciting upgrades coming that include velocity and force readouts, and they will be available for existing units in case you want one now. In this article, I am going to briefly explain the concept, go over the benefits, and I will give you a quick way of implementation.


What is Flywheel Training?


Flywheel training started back in 1913 by some Swedish researchers, and then again in 1994 some Swedish exercise physiology scholars researched the idea of using flywheels, since they produce resistance independent of gravity, for Astronauts in space to prevent muscle atrophy. Flywheels produce a type of isoinertial resistance meaning a continuous resistance throughout the range of motion regardless of joint angle. This type of resistance causes an eccentric overload and increase in the velocity of the eccentric contraction, which is a beautiful thing for athletic performance that we will explain in just a bit.


In normal resistance training, the external load is created by gravity acting on the mass of the object being lifted. Therefore, the resistance experienced at each joint is dependent upon the horizontal distance perpendicular to gravity from the specific joint in question the line of action of the external load. That means the total load experienced at each joint changes throughout the range of motion because the horizontal distance to the line of action changes. For example, when you are at the parallel position of a back squat, the horizontal distance from the hip to the line of action perpendicular to gravity of the barbell is at a maximum distance. Therefore, the hip is experiencing a maximum resistance. However, when you stand up, that distance is shortened each inch/centimeter on the way up making the resistance less and less.



A flywheel uses inertia to form the resistance. Newton’s first law is considered the law of inertia. Put simple, inertia is the property of an object to resist change. If an object is at rest, it will stay at rest until acted upon by an outside force. If an object is in motion, it will stay in motion unless acted upon by an outside force. With a flywheel, we are actually talking about rotational inertia, and the resistance required to alter the direction of a spinning wheel aka angular momentum. Angular momentum is equal to angular velocity (how fast the object is spinning) multiplied by the moment of inertia. The moment of inertia will help understand how we alter the resistance of a flywheel. Let’s look!


Moment of Inertia = centre of mass x distance from the axis² or I = Σ miri2


Therefore, if you want to increase the resistance of the flywheel, you can add a heavier wheel or wheels, or you can add a wheel that has a greater circumference. There is one more key that most overlook, and that is producing a greater concentric contraction for as long as possible creating maximal angular velocity of the flywheel. This action will cause the greatest potential angular momentum that will have to be overcome during the end range of motion. This action will also create a greater amount of stored energy that will have to be overcome during the eccentric contraction.


Alright let’s put this simply!


We know that our muscles are capable of creating a greater force during the eccentric contraction. Our goal here is to overload the eccentric contraction. To do that, we can find creative ways to increase force during the concentric contraction. For example, on a squat we could use our arms to stand up at a greater velocity than we could have with just our legs. That will create a greater angular momentum during the eccentric contraction to be overcome. We won’t use our arms to resist the eccentric load, since our muscles are already able to hand eccentric forces more easily. So why is it important to create an eccentric overload? Let’s get into the benefits of the flywheel.


What are the benefits of Flywheel Training?


This is where it gets good. I looked at two meta-analysis that compiled 46 different studies that met their compliance standards. Here are some of the benefits that you can expect:


  • Strength
  • Hypertrophy particularly increases in Type II fibers (Douglas, J. 2017)
  • Muscle Activation
  • Tendon Stiffness
  • Power
  • Athletic Performance (increased vertical and horizontal jumps, faster sprint times, and improved change of direction)


Did I get your attention? I should have because now I can add the data from our athletes confirming increased jump heights, improved elasticity, and increased power production. Before I get into our results, let’s look a bit more at the findings of these studies.


Some of the benefits that triggered my desire to purchase one were Type II fiber Hypertrophy, tendon stiffness, power production, and improved athletic performance. These are all the qualities needed to become an elite weightlifter, and really these are the qualities required to become a great athlete. I can get my athletes stronger with conventional resistance training, but we found that elasticity actually defines the potential of a weightlifter. This is where I am going to focus the rest of this article, but I plan to do a series on the flywheel. I hope to dive deeper into each benefit, the ‘why’ behind the benefit, and most important to all of you is the ‘how’.


The quality that I believe is behind the majority of benefits is the eccentric overload which creates a stiffer tendon and improved elasticity. A stiffer tendon is a tendon with greater potential for producing energy. This is defined as strain energy.




where SE = strain energy, k = stiffness or spring constant of material, and Δx = change in length or deformation of the object from its undeformed position.


Dr. Keith Baar has taught all of us the importance of tendon strength. I found that the flywheel speeds up the process of creating that stiffness. Let me briefly explain the form and function of the tendon. The most important property of a tendon when it comes to performance or potential injury is that tendons are viscoelastic in makeup. That means, one side is fairly stiff and unyielding, which is the side that attaches to bone. However, the side of the tendon that attaches to muscle is a bit more forgiving. Otherwise, if a tendon is too stiff on the muscle side, the muscle becomes subject to a possible tear. This is where it’s important to understand the balance.


A stiffer tendon will have more potential to create strain energy, which translates into higher jumps and faster sprint times. For example, when an athlete with thick and tight tendons strikes the ground with his or her feet, that strain energy within their Achilles tendon, patellar tendons, and other related tendons is expressed against the ground propelling the athlete horizontally through space. So how do we develop thicker and stronger tendons?


It is well established that strength training increases the blood flow and collagen synthesis, and long-term effects lead to tendon hypertrophy and greater potential for strain energy. At the cellular level, studies have shown that fibroblasts (tendon cells) respond with a more superior adaptation when a dynamic load is applied versus isometric. The result of mechanical tension in the form of strain (tissue deformation) led to increased collagen expression and increased matrix stiffness. Of course, this tendon stiffness has to be balanced with a strong muscle able to hold an isometric contraction, or otherwise an athlete will be at risk of muscular ruptures. The moral of the story is a balanced approach to one’s training regimen.


Elasticity Improved


I have written about post activation potentiation for a lot of years. I have used it on myself when I was an athlete with lots of success, and I have used it with a number of athletes with success. It most commonly defined as a phenomenon by which the force exerted by a muscle is increased due to its previous contraction. Post-activation potentiation is a theory that states that the contractile history of a muscle influences the subsequent mechanical performance of muscle contractions. In layman’s terms the theory states that a muscular contraction that is somewhat similar to the movement or movements performed after the particular contraction will positively affect the performance of those subsequent contractions.


Post-Activation Potentiation isn’t 100% understood, but there are two major theories and one other maybe. Here are the three:


  • Phosphorylation of myosin regulatory light chains, which makes actin and myosin more sensitive to Ca2+
  • An increase in α-motoneuron excitability as reflected by changes in the H-reflex
  • Change in pennation angle of muscle fibers


The phosphorylation of myosin regulatory light chains (MLC) making actin and myosin more sensitive to calcium seems to make a lot of sense from my experience, and I will explain why. Like most of the research will agree, PAP seems to work best with people that contain a high amount of fast twitch fibers. Myosin in general is the determining factor between fast twitch and slow twitch fibers. MLC determine the velocity of the contraction along with the biggest determining factor ATPase an enzyme that breaks down ATP and begins the cross-bridge cycle. Calcium sensitivity is key for the binding of myosin to actin during the cross-bridge cycle leading to the power stroke aka muscular contraction. The more efficient this process becomes leads to more efficient and powerful contractions. However, there are mixed reviews on this theory, which makes sense based on which individuals are taking part in the research.


Figure 2 Myosin Molecule




The second theory is that a conditioning exercise causes increased synaptic excitation within the spinal cord, which in turn results in increased post-synaptic potentials and subsequent increased force generating capacity of the involved muscle groups. More precisely an increase in α-motoneuron excitability as reflected by changes in the H-reflex. The H-reflex is an EMG measurement of the excitability of a muscle. We are talking about creating a higher excitability of the neuromuscular system within the particular muscles stimulated by the conditioning exercise. This increased excitability relates to improved muscle spindle output, or what most of us know as improved stretch reflex. Simply put for all of the coaches reading, this leads to faster and more forceful contraction.


The third theory is a change in pennation angle of the muscle fibers, which helps me understand a faster speed of contraction. However, it doesn’t explain an increase in force. A greater pennation angle is normally associated with slower contractions but greater force outputs. A lesser pennation angle allows more force to be distributed straight into the tendons and bones. However, there is little evidence to support this claim, and anatomically I can’t wrap my brain around it.


Overloading the eccentric contraction is a great idea for many reasons:


  1. Residual Force Enhancement
  2. Eccentric contraction requires less energy
  3. And yes PAP


Residual Force Enhancement (RFE) is an effect that has been noted for several years in the research community. RFE refers to the residual increase in force following active lengthening of a muscle, and it is directly proportionate to the magnitude of the stretch. This increase in force is a passive one, which means it’s a force that the athlete isn’t knowingly creating. Passive forces relate to the automatic neurological responses in the muscle created by structures like muscle spindles and titin proteins located in the sarcomeres. Titin is related to the structure of actin and myosin, and is also contributed to the elastic quality of muscle. The theory is that during the eccentric contraction titin increases in its inherent stiffness upon activation and stretch by binding calcium upon activation (versus during a passive stretch without calcium present), and could shorten its active spring length, thereby becoming stiffer, by binding proximally to actin. This is directly related to the magnitude of the stretch, so the amount being handled and the speed. The Kratos Flywheel becomes the perfect too to maximize the velocity during the eccentric contraction and for maximizing the load.



All of these benefits come with less energy being expended. The theory here is that an increase in the number of cross-bridge cycles per ATP hydrolyzed during the eccentric contraction versus 1 for 1 during concentric contractions. That’s right, you get the enhancement without a lot of energy being expended making this the best form of PAP that I can think of. If the extra force produced from PAP is decreased dependent upon the amount of fatigue, then eccentric contractions only seem to be the best way to produce maximal results from PAP.



Consistency with the Kratos Flywheel– Performing this overloaded eccentric contraction on a regular basis could lead to long term improvements in the physiology of the myosin light chains and long-term improvements in the neuromuscular junction possibly from improvements in titin (thickness, tightness, and elasticity), muscle spindles, and possible inhibitions in the golgi tendon organ. Therefore, I recommend sticking with the Kratos Flywheel activity for several weeks before halting to accumulate maximal results.


However, the difference in Flywheel Training is that studies have shown and my own velocity data shows higher velocities during the eccentric contraction leading to improved neuromuscular junctions in the form of higher muscle spindle outputs and inhibition of the golgi tendon organ. If you read the earlier portions of this article, you learned that muscle spindles create a passive form of concentric muscular contraction. The output of the muscle spindles is directly correlated to the speed and magnitude of the eccentric contraction. The golgi tendon organ (GTO) is located in the tendons, and it’s a part of the neurological system responsive for inhibiting muscular contraction. Its job is to shut off muscles to avoid tears or major injuries. When the GTO senses high forces like from a hard change of direction for example, it shuts off the muscle to prevent tears. The increased velocity induced by the bands also increases the strength and thickness of the tendons and over time desensitizes the GTO allowing for maximal motor unit recruitment. As we learned earlier, the higher magnitude of the eccentric contraction leads to higher levels of RFE as well. Lastly, muscle spindles not only activate agonist after sensing the stretch of the muscle, but they also inhibit the antagonist leading to a better synergistic relationship between agonist and antagonist. This synergistic relationship is crucial for athletes if you think about sprinting for example.


Reactive Strength Index


This is my final point, but probably the greatest discovery so far with my own athletes. I test the majority of my athletes daily on a 45cm depth jump. We record the height of the jump and the ground contact time. We divide the height by the ground contact time to give our athletes a Reactive Strength Index Score (RSI Score). We found that this score is directly proportional to the potential of our athletes. If you think about it, that makes total sense.


The ground contact time is going to show the athlete’s elasticity, which is a look at their tendons and joint make ups. This is their stored energy in the form of strain energy. This has to be high for qualities such as fast sprints, change of direction, and everything to weightlifters. For a weightlifter, it’s the ability to change direction during a dip and drive in the jerk, or to catch a bounce out of the bottom of a heavy clean.


The height of the jump shows the athlete’s ability to produce force in relationship to his or her own bodyweight. Obviously, for an athlete to produce power, this is an important quality. After we test our athletes, we are able to tell if they should focus on strength movements like squatting and pull due to a low jump height. We are also able to see if our athletes need elasticity training due to a long ground contact time. Here’s where the story gets awesome.

Ryan Grimsland at the Arnold Classic earning best lifter in the country, and establishing himself as an Olympic Hopeful.


We had been testing our athletes for months when we received our Kratos Flywheel. After only two weeks of performing a twice per week protocol of no more than 10-20 minutes per session, each athlete increased his or her jump height by an average of 3.5” and decreased his or her ground contact time by 0.08 seconds. Both improvements are massive especially the ground contact time when the mean ground contact time for our team is .47 seconds.


The most amazing story happened with Ryan Grimsland, my top athlete and one of two top runners for the 2024 Olympics. In two weeks, height increased 5” to over 40” from a 45cm depth jump. I have never seen anything like that. His ground contact time dropped by .05 seconds, which was already the shortest ground contact time. We now have two young men with over 40” vertical leaps with zero plyometric work. We only use the Kratos Flywheel, train with weightlifting movements, and use velocity measure by GymAware for our strength movements.




We have found that two days per week is manageable with the training, school work, and lifestyle for our university athletes. We normally add the flywheel to days that are followed by a day off to allow for recovery. Remember, this is eccentric training, which comes along with muscle damage. That’s ok just be smart. I recommend a very simple approach at first. Here what I recommend:


Weeks 1-3– 3-5 sets of 8-10 repetitions. For the first week or two, I recommend simply accelerating through the concentric contraction (the ascent) as fast as possible. That will cause a fairly dramatic eccentric contraction (the descent). After you or your athlete is familiar with the machine, then I recommend overloading the eccentric by emphasizing the concentric. For the squat portion, we use handles to pull ourselves up as quickly as possible making sure to attempt accelerating all the way through the ascent. That will cause an equal reaction of the Kratos flywheel to pull you down with that same amount of force. This is how you overload the eccentric contraction. Remember, your body is capable of producing more force eccentrically. Another example is to perform a bilateral RDL with the handles that Kabuki sells, and then immediately switch to a unilateral RDL during the eccentric contraction aka the downward motion.


Weeks 4-6– 4-6 sets of 4-6 repetitions. Once again, you will want to produce more and more of a forceful concentric contraction to guarantee a higher eccentric contraction. So far, we have only used the overload techniques that I outlined above. I am currently playing around with some other ideas, but I don’t want to fill your head with something that might be dangerous. I will try these out first, and then I will get back to you.


After the six weeks, we simply continue training without the flywheel putting the majority of focus on the technical aspects of our sport. However, the benefits of the flywheel don’t disappear like those gained from illegal drugs. Those type II fibers will still be there along with the strengthened titin protein filaments and improved tendons. Please let me know your results if you take the plunge. Also, if you have any questions, feel free to email me at I would love to help because I am so passionate about the flywheel. You can check out the Kratos Flywheel at:


Chris Duffin’s Kratos Flywheel


I love this thing so much that I will give you anyone of my EBooks with the purchase of this amazing product. Simply email with proof of purchase, and let us know which EBook that you would like.


Also, here is the Kratos Flywheel coaching protocol directly from the team at Kabuki Coaching:


==> Kratos Foundations



I will end with these couple more facts. I didn’t recruit either of these 40” vertical leap young men. They have both been with me since they were in middle school, and now have followed me to the University. Both had standing vertical leaps of 34-35” before starting the Flywheel protocols. There’s my story. I have been meaning to write this article for quite some time. I am glad that it’s finally out there. I hope that all of you are able to begin training with a Kratos Flywheel. It has been a major advantage for us, and I hope that you have that same luxury.


Coach Travis Mash





  1. Douglas J Pearson S Ross A. Chronic Adaptations to Eccentric Training: A Systematic Review. Sports Med.2017;47(5):917-941.
  2. Petré, H., Wernstål, F. & Mattsson, C. Effects of Flywheel Training on Strength-Related Variables: a Meta-analysis. Sports Med – Open4, 55 (2018).
  3. Hody S, Croisier JL, Bury T, Rogister B, Leprince P. Eccentric Muscle Contractions: Risks and Benefits. Front Physiol. 2019;10:536. Published 2019 May 3. doi:10.3389/fphys.2019.00536
  4. Sanz-López F, Berzosa C, Hita-Contreras F, Martínez-Amat A. Effects of eccentric overload training on patellar tendon and vastus lateralis in three days of consecutive running. Knee. 2017 Jun;24(3):570-579. doi: 10.1016/j.knee.2017.03.002. Epub 2017 Mar 22. PMID: 28342723.
  5. Christian Couppé, René B. Svensson, Karin Grävare Silbernagel, Henning Langberg, and S. Peter Magnusson. Eccentric or Concentric Exercises for the Treatment of Tendinopathies?. Journal of Orthopaedic & Sports Physical Therapy2015 45:11, 853-863
  6. Baar K. Minimizing Injury and Maximizing Return to Play: Lessons from Engineered Ligaments. Sports Med. 2017 Mar;47(Suppl 1):5-11. doi: 10.1007/s40279-017-0719-x. PMID: 28332110; PMCID: PMC5371618.

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