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Power of Inertial Training: A Case for the Substitution of Core Barbell Lifts with Flywheel Training

By Paul Cater


Are barbells the best way to resistance train athletes?  Recent technological developments may say otherwise.

I am finding that inertial training, which I first discovered more than a decade ago with the Versapulley conical flywheel, and more recently the kBox by Exxentric not only substitutes nicely for the higher rewarding barbell resistance, but is now replacing traditional gravity based resistance training entirely in many situations.

Flywheel training is both a compliment to barbell training, as well as a stand alone method to build massive levels of athletic strength

As longer missions into space became more of a focus for NASA in the late 80’s and early 90’s, different proposals for maintaining bone density and lean muscle tissue during long absences of gravity astronauts would endure came into focus. Research surrounding the simple properties of a spinning flywheel was conducted and has led relatively small following of those prescribing to inertial training.

Relative to space flight, there are often stretches of sport seasons, where an adequate overloading of the muscle-tendon unit and skeletal system takes place. Forces needed to provide the balance and structure of the joint while handling high velocity deceleration encountered in game and match play to maintain high performance are often misgiven for a myriad of reasons. Most often facilities do not allow for the application of high force and velocity exercises, but it is the strength and conditioning- or often administrative and sports medicine- staff that does not prescribe adequate stress during a long competitive cycle. 

My experience in Major League Baseball, professional Rugby and Soccer in Europe, as well as amateur youth sports in the United States, has led me to believe that the lack of adequate progressive high force and velocity loading has not matched the rise in playing demands leading to greater instances of soft tissue noncontact injury. The shift towards anterior dominant, non-axial loading such as bench and leg presses in season exacerbates the downward spiral of areas such as:

  • Joint pain
  • Length tension relationships
  • Shifts of shoulder and hip skeletal access
  • Eccentric utilization ratios whereby the athlete’s tendon usage becomes les optimal

In a lengthy baseball or soccer season, or even the nearly year round youth softball and baseball seasons repetitive high velocity deceleration patterns lead to instance of injury if there is not adequate strength and stability surrounding the joint.

Facility limitations may be the most excusable, whereby there is limitation in throwing, dropping or lifting weight. Resistance training, at any load or velocity is a simple schematic of overloading the muscle-tendon-skeletal system to increase efficiency of power production for specific sport.

Overall, weightrooms need to enable dropping and racking higher loads on a barbell, and walls, ceilings and floors for ballistic exercise. Furthermore, as in the case of baseball and golf, whereby travel may prohibit the use of high force, high velocity exercises may not be able to be perform unless equipment and kilos are transported

Further to the lack of facility or expertise for administering what amounts to Olympic lifting for in season sport training, there does lay the limitation of the barbell exercises themselves that may create performance degradation rather than increasing instance of maintenance- or even increase- of power in-season.

The barbell lifts demand longer amounts of setup and progression for desired neural stimulus. Moreover, there does lay a risk of increased joint stress and therefore inflammation/injury-even for the saavy professional strength coach- when it comes to repeated movement for metabolic effect and or illicit eccentric overload.

Some sports demand non-running conditioning whereby repeated barbell movements are prescribed for conditioning effect. Likewise, rapid deceleration of barbells-which are limited by the properties of gravity and momentum shift- produce overloads which may be beneficial to the nervous system but expose the joint articulations to un due stresses.

Because of the instances of repetitive, rotational and random torques are the nature of in-season, there has to be adequate stresses in season to protect the joint, tendon and truly maintain power production, and the ability to reproduce power over time.


Strength coaches have a growing list of machines and exercises to circumnavigate the core barbell lifts. The sport performance industry has seen the popularity of elastic bands, and pneumatics rise, as well as accommodated resistance via different cambering to increase muscle activation and adaptation. Every time I have tried to avoid the barbell, I have found myself lacking in some basic elements most beneficial to the in season athlete. Aside from a lacking global strength and power, the lack of confidence and seems to wane as I don’t feel strong and in control. The psyche of the barbell in my mind can never be replaced by pneumatics, rubber bands or kettle bells.

Flywheel training better than barbell training?

“The psyche of the barbell in my mind can never be replaced by pneumatics, rubber bands or kettle bells.”

Yet it is in fact the axial loading principles of the barbell that are the reason why many have to replace the simple squat and Olympic Lifts. When dealing with lumbar spine, knee and shoulder issues it is often justifiable to find loading alternatives to the squat and clean, in order to maintain leg strength and power in season. Unfortunately, coaches and athletes will forgo the benefits of the squat all too soon, in favor of watered-down loading methods that offer little to the needs of the competitive athlete.

There is some validity in avoiding reproduced HIIT via the barbell as the momentum shifts alone, exposing weak links of the hand, wrist, shoulder and lumbar spine, are avoided.  The advent of turf space within gyms, sleds, kettle bells and High Intensity Interval Training (HIIT) machines have all attempted to produce higher work rates at the neural and metabolic level, with less risk. Whether the clean and jerk and snatch mechanics were meant to be reproduced over extended repetitions is debatable, but yet is now normative even within the general population as a means for metabolic conditioning.  In many college and professional sport cultures the risk of repeated barbell bouts is not worth the risk for the reward.


Though metabolic changes may take place with the ability to reproduce loaded multi-joint complex movement, and muscle groups, it is lack of adequate compression and eccentric overloading that may in fact work against the athlete in the attempt to replace the core lifts.

Inertial resistance via flywheel training, the loaded cone and cylinder, provides maximum voluntary contraction in concentric (or shortening movement) and overload in eccentric (lengthening) movement patterns. Because this form is not gravity dependent, one is able to reproduce these MVC’s and high velocity eccentric overloads without great shifts in momentum where the joint is unloaded.

Because of the higher neural demand and time under tension without momentum shifts, it has become the gold standard in both pre-activation potentiation and metabolic conditioning. Within the rehabbing athlete population, rotary inertia flywheel training can both circumnavigate the barbell, but allow for adequate stressors to actually gain fitness and strength during the rehab process.


It is during the Rugby World Cup that I am reminded of an athlete I trained who wave recovering from a spinal injury. From the fusion surgery we couldn’t load via the barbell and instead, performed belted squats with the Versapulley. Energy efficiency actually increased during his months out of practice and returned to play with a greater work-rate than prior to the injury. Repeated bouts of eccentric overload are unique to inertial training, and when performed without the sheer forces, the overall tendon utilization results in greater ability to store and release energy. This individual went on to captain the England Rugby team in the 2008 World Cup.

Over the last 4 seasons within professional baseball, I have seen the culture change with bigger and better weight rooms being built. Though the use of the barbell is being utilized in MLB, there is only one team I know that fully enables its use in season and with all positions. Most players are encouraged to take the Magellan Route of accommodative resistance, toying with everything but solid core barbell lifting. I have seen lots of exercise, but at such an expense of time and energy, it is almost self-defeating. With the amount of time on feet and doing lo-fidelity movement intensities, the simple core lifts are the most bang for your buck and save the athlete time.

Training these core lifts with greater frequency and less volume, adds to the aggregate workload with achieving the greater fiber recruitment needed to in fact maintain the strength, without increasing training time.  Busy work on top of the already busy work inherent in the practice and play of baseball leads to mental and neural fatigue and ultimately more joint stress.

Flywheel training, with its portability and time efficient neural recruitment paradigms have proven effective in getting to core of why baseball players need to train often in a 8 month grind. Moreover, I have found that the lack of adequate daily stress- especially in eccentric loading phases leads to not only power degradation, but increased instance of injury in the muscle-tendon unit. This is exacerbated even by the off season stigma that baseball players- pitchers in particular- cannot stress the shoulder and elbow with overhead exercises, where joint stability, time under tension and deceleration capability is at its greatest.

Instance of quad dominant power capability in offseason via means of ‘safety measures’ equate to relatively non-functional force and velocity adaptation, with little means to decelerate. I have to wonder in rise Tommy John, Achilles as well as oblique and groin injuries are a result of the poor eccentric utilization ratios.

Inertial training provides more fiber activation at a higher rate of force development (RFD), and the ability to decelerate that force. The more power athletes are able to develop in offseason training programs, the greater the need to maintain that power in-season or strength imbalances and degradation leading to movement inefficiency may ultimately occur.


Though coaches and athletes may not be on a mission to Mars, they may be in the midst of a very long competitive season where injury or facility limitations may abound. In cases of limited training facilities, travel, knowledge or adequate administrative support, today’s strength and conditioning professionals have options to create adequate neural activation and overloads needed to maintain balance, structure and functionality.  

Because the inertial flywheel forces are determined by the effort of the user.  The velocities, as they remain iso-inertial and constant, and overloading forces remain within the boundaries of the users abilities.  Its self-regulating depending on the user’s ability to sequence ground force production, and store energy around the spinning wheel (via cone or cylinder).  In my experience, I believe this solves many of the excusable scenarios that limit barbell use.

With the inevitable limitations of travel, high player salary and lag time for the barbell to permeate traditional non-barbell sport culture, inertial flywheel resistance becomes a very viable option. Moreover, with Versapulley and kBox’s portability and measurability, precise overloads can be determined to provide the progressive neural and structural stressors needed to maintain high function in season.  When the core barbell lifts cannot or will not be utilized, flywheel inertial resistance training is a great option for athlete performance.


Founder of Alpha Project at Central Coast Athletics

Salinas High School, Varsity Baseball, Football 1995
UC Davis: Studied pre-law while playing UC Davis Varsity Football 2000
NSCA, Certified Strength & Conditioning Specialist 2001
Poliquin Certified Level, 2
Internships include UCLA, San Jose State, San Francisco 49ers
Graduate Degree Exercise Science, Human Performance, Brunel University, London 2010
MSC Strength & Conditioning from Middlesex University, London 2011
Over 15 years of experience as an International strength and conditioning coach working with London Wasps Premier Rugby, Baltimore Orioles, USA Rugby and consulting numerous other High School, College & Professional Athletes
Late Stage Rehab Specialist
USA ambassador for advanced training technology equipment including: Versapulley, Kbox and Smartcoach.


  1. Are the Kbox and versapulley the same thing? If not what are the differences brw them?

  2. Is there anyway make shift way someone can benefit from this method of training with the basic equipment in most equipped gym? Rather than splurging on multi thousand dollar pieces of equipment.

  3. Dennis, I’m hoping that as this type of training becomes more commonplace, more facilities will have it, as well as the average price for a base unit becoming more affordable for the average user. I think that good “replacements” at least in theory, for kBox type contractions would be a large amount of quick drops and small stops with a barbell load though the range of motion, as the mini-stops would cause a similar explosive isometric/eccentric overload to what the kbox has to offer, but it’s still a little different world. A big advantage of the kBox is the harness/belt squat possibility. I can’t think of a way to replace any sort of versapulley resistance, a cable machine doesn’t really come close on the eccentric in any way.

  4. hi,
    because doesn’t the kbox makes you slower I said that because of the time under tension

    a sprint has short time under tension I mean short time on the ground

    would it be good to do the kbox squat for exemple and direct after some plyometric jump ?

    • Steeve,

      The kBox can be done with high or low inertia for resistance, which can make it very slow, or extremely fast, so the specificity is a bit relative. Mixing it with sprinting or jumping though is definitely a good option!


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