Product Review: Exxentric KBox

Product Review: Exxentric KBox

Dr. Joel Seedman, Ph.D.

I’m always honored and excited when companies send me new pieces of training equipment to try out.   Even if it doesn’t seem like a piece that will be worth marketing or investing in, I’m always happy to experiment and tinker around just in case it exceeds my expectations.  Sometimes I see the merit in the equipment and other times….. well let's just say in those instances where I don’t see the value I simply don’t post a review or promote it. 

Recently I had the pleasure of demoing a very unique training device from Exxentric known as the kBox.  Exxentric reached out to me as they knew I was a huge fan of eccentric protocols and negative training techniques for enhancing performance, strength, and size.  I have to admit, when they explained the principle of the kBox I was fairly skeptical as I typically am of any new or unique pieces of equipment.  However after having used this piece on a consistent basis for several months with my clients, athletes, and myself, I can honestly say that this is one of the most incredible pieces of strength training equipment I’ve ever used.  With that said here’s a bit more detail on this amazing tool.

Fly Wheel Training

Flywheel training has actually been around for decades.  However it’s recently gained greater popularity due to various forms of research, scientific investigation, and experimental data demonstrating its usefulness as a legitimate strength training tool.  Rather than relying on gravity and total load or weight of an object to produce resistance for the lifter, flywheel training relies exclusively on the variable inertia produced from different size fly wheels.  Although the technology of flywheel resistance is actually quite simple, creating a sturdy piece of equipment that can be easily adjusted to fit a variety of goals and movements is quite ingenious and groundbreaking.  Exxentric has done just that with the kBox.  

Adaptive Variable Resistance

One of the many unique features of flywheel training is that the resistance is completely variable and adaptable.  In other words resistance can be self-selected and adjusted at any time or point during any exercise for any individual.  Simply put I can use the exact same setup for an elderly individual as I could for a football lineman and both will provide as much or as little resistance as each individual desires.  This is adjusted simply by choosing how intensely and forcefully the individual performs the concentric phase of the movement. If he or she decides to produce maximal force and effort then the flywheel will provide maximal resistance.  If the goal is low resistance with minimal tension then the flywheel will match that effort level and force production. 

In addition, the adaptive variable resistance allows the lifter to maximize the hypertrophy stimulus of an exercise as he or she can perform a set of 8, 10 or however many repetitions are desired but each repetition at maximal intensity and effort.  I refer to this method as micro drop sets.  For instance let's assume a lifter has a 1 rep max barbell curl of 135 pounds and can perform 100 pound curls for a max effort set of 10 reps. When performing a set of 10 reps with a standard barbell, really only the last one or two reps of the set will be max effort as this is where the muscles are close to or at the point of failure/fatigue.  With the kBox, however, the same individual can produce maximal torque and force equivalent to his or her 135 pound max in the first rep, the second rep with 130 pounds, the third with 125 pounds, and so on until he or she decides to terminate the set. 

Simply put the kBox enables an athlete to perform a set of 10 max effort reps instead of just one max effort rep (i.e. the very last rep of the set). Again, this is due to the continuously adjusting nature of the flywheel that immediately adapts to the force produced right then and there by the lifter. No adjusting or manual selection of resistance is necessary as each of the phases previously described occurs in one smooth and seamless fashion without the lifter having to give any thought or attention to the process When it comes to building hypertrophy, the ability to produce repeated max effort reps in a micro-drop set fashion is an incredibly powerful stimulus not only for functional size and strength but also for conditioning and endurance.

Here’s one of my awesome figure competitors Kim Schaper demonstrating the squat deadlift with the v-bar grip which is one of my favorite variations as it literally crushes the full body from head to toe.

Eccentric Overload

Perhaps the single biggest selling point of the kBox is the intense eccentric overload.  Numerous forms of research and scientific studies have conclusively demonstrated that eccentric accentuated training is one of, if not the single most potent stimulus for triggering functional strength and size, not to mention injury prevention, and muscle function.  Whether you’re a bodybuilder looking to maximize hypertrophy, a powerlifter looking to increase his or her PR’s or an athlete looking to gain more speed and power, eccentric training can’t be beat. 

Research has also shown that the muscles can handle 20-40% more load during the eccentric phase of movement than during the concentric phase.  Although you can have a training partner press down on the barbell during the eccentric phase or rely on Power Rack Eccentric Potentiation training as I’ve laid out in one of my T-Nation articles, these methods aren’t always easily accessible for every lift. The kBox provides these intense eccentric overload and negative accentuated training options for nearly every movement in a safe and effective fashion.

In essence, the harder the individual pushes or pulls against the flywheel on the concentric phase, the more tension the device gives back on the eccentric phase.  In fact, the kBox matches the resistance you provide and then some by producing even greater eccentric overload (approximately 20% more tension on the eccentric phase) particularly when decelerating the flywheel in a rapid and forceful fashion.

This rapid eccentric deceleration is also incredibly effective for teaching the lifter to absorb force and turn on as many available motor units and muscles fibers as quickly as possible.  In fact it’s somewhat similar to the Rapid Eccentric Isometric Training methods I’ve laid out in prior articles.

Setup and Portability

The setup is quite simple for the kBox as the unit comes fully assembled and ready to use upon delivery.  The only setup needed includes deciding on the appropriate handles or attachments, choosing the appropriate flywheel weight, and adjusting the belt based on size, height, and desired range of motion.

The kBox is also extremely portable and could fit into the trunk of any car making it a great option for on-the-road or in-home training.

Adjustments and Attachments

The Exxentric kBox comes with a variety of attachments.  This includes different size flywheels that provide more or less inertia.  The athlete can also combine flywheels for more options and more tension.

Depending on whether the individual is going to be performing lower or upper body movements, there are also a number of grip and hand attachments including a straight bar, unilateral handle grips, ankle straps, weight belt, and a torso harness.  You can also use most standard cable attachments as well.

The ability to incorporate a harness on the squat is worth the price of the kBox in and of itself as this allows the lifter to perform belt squats with ease and safety.  This represents one of literally hundreds of possible exercises that can be performed on this device.

Besides being very comfortable, this harness setup allows the individual to perform max effort squats and other lower body movements with very little stress to the spine due to elimination of the traditional axial loading component. Exxentric also provides 7 different size harnesses to fit a number of sizes and heights.


The kMeter is another unique attachment that can be purchased from Exxentric.  This unique tool allows the athlete, coach, or trainer to gauge and measure force, power, torque, and work output, on every rep of every set.

Versatility: The "All in One" Gym

One feature that stood out to me after using the kBox several times was how versatile it was.  The device provides hundreds if not thousands of possible exercise options for both the lower body and upper body as well as the core.  In fact, this is without a doubt the single best all-in-one piece of strength training equipment I’ve ever used.  If you’re like me and like letting your imagination run wild dreaming up unique and cool exercise variations, then you’ll love the kBox. 

Surprisingly each variation can be performed with incredibly high intensity due to the adaptive variable resistance previously discussed.  In other words, there’s no such thing as a pansy or beginner version of any movement on the kBox.  With that said, here’s a breakdown of different movements you can perform including 25 unique exercise examples demonstrated in the following 4 videos.

Lower Body Training

As previously mentioned, the ability to use a harness and perform belt squats is a huge selling point.  However, the harness can also be used on lunges, Bulgarian squats, and good mornings.  In addition, the handles can be used to perform deadlifts, RDL’s, hack lunges, calve raises, single arm deadlifts, single arm squats, single leg hip extension/kick backs and more.  As previously mentioned if a single flywheel doesn’t provide enough tension you can always double or triple up multiple flywheels to exponentially increase the levels of resistance.

Single Leg Training

Once you master the bilateral or double leg versions of the kBox you can progress to single leg versions.  This includes single leg RDL’s, single leg squats, lateral lunges, and even single leg bent over rows.  Just make sure you have strong feet and ankles as well as proper alignment in your lower extremities as the balance and stability can be quite tricky on these.  I recommend beginning with either the small or medium size flywheel.

Upper Body Training

As versatile as the kBox is for performing lower body exercises, there are even more options for upper body movements.  This includes, various compound movements such as bent over rows, overhead presses, single arm chest press, renegade rows, upright rows and more.  The kBox is also very conducive for performing isolation movements such as bicep curls, tricep extensions, front raises, single arm lateral raises, bent over lateral raises, reverse curls, shrugs, power shrugs, and more.

Bodyweight Exercises

Besides having the ability to perform squats and lunges the kBox also provides numerous options for more unique bodyweight movements by simply using the belt or harness attachment.  This includes pushups, inverted rows, squats, pullups, and dips.  I recommend starting with the smaller fly wheel on these as the amount of tension you’ll feel in your targeted musculature is incredibly high.

Practical Application

The practical application for the kBox could not be more simple.  Two tips to remember are that each time you begin an exercise you’ll need to start the movement with your foot by pushing the wheel or have a training partner push the wheel.  Think of it as similar to starting an old fashioned airplane minus the risk of cutting off a limb. 

Once you begin the set and the flywheel begins building up momentum make sure you start off smooth and gradual.  I made the mistake of performing deadlifts as my first movement and giving max effort on the very first repetition which nearly blew out my back.  After 3-5 build up reps gradually start producing more force on the concentric and be prepared physically and mentally to decelerate the flywheel as it requires intense effort and focus.

The learning curve of the kBox is very quick and most individuals will only need a few sets at most to understand the nature of the device and figure out the natural rhythm of the machine. 

With that said the kbox can be used on a number of populations ranging from advanced athletes, general populations and even clinical populations provided lower levels of force are employed.

The Ultimate Belt Squat

Here I have NFL quarterback & GSP sponsored pro Taylor Heinicke performing a belt squat with the Exxentric Kbox as we strengthen his body as he recovers from elbow surgery. Besides helping to build his arm strength back up in a safe and functional manner, performing belt squats while pinch gripping dumbbells, plates, or kettlebells provides several benefits.

1. While belt squats variations are becoming more popular, a common problem is that lifters fail to maintain full body tension and forget to brace their upper body and core. This results in energy leaks, compromised force production, & degradation to body mechanics. The pinch grip method helps remedy that. That’s because it forces the lifter to grip the daylights out of the dumbbells which in turn activates their hands, grip, and forearms to near maximal levels. Research shows that this creates something known as concurrent activation potentiation or irradiation which are simply fancy terms describing how squeezing muscles around the grip, feet, hands, or face helps promote full body tension via enhanced neural drive to the working extremities. This is often why you’ll hear powerlifters tell each other to “stay tight” as this helps maximize full body tension, strength, spinal rigidity, power output, and motor control, not to mention decreased risk of injury and joint trauma.

2. This is a great way to kill 2 birds with 1 stone. When working with pro athletes you may only get to see them sparingly throughout the year due to the season and various team roles. With that said it’s critical we maximize our time in the gym and work as many physical components as possible. The pinch grip belt squat allows us to crush lower body while also hammering the daylights out of the grip, hands, and forearms, something that any athlete can benefit greatly from.

3. The Kbox provides incredibly high levels of eccentric force & impact that the lifter must decelerate. The concurrent activation potentiation produced from the tight grip helps the lifter brace more during the eccentric phase thereby reinforcing a tighter & crisper eccentric deceleration.

Additional Benefits

The degree of muscle damage, mechanical tension, metabolic stress, and muscular pump produced from the kBox is enormous making the hypertrophy stimulus quite noteworthy. In addition, the caloric expenditure produced from each set is inordinately high due to the extreme intensity and quick turnaround between the eccentric and concentric phases of movement.  That’s because there is greater ATP expenditure and hydrogen ion accumulation with high force repetitions of movement that involve a quick turnaround phase. In other words this is a great tool for size and strength as well as cardio, conditioning. fat loss, and body composition.


The kBox can range anywhere from $2500-$5000 depending on the specific model and options you choose.  While you may initially think this is semi pricey, to replicate the same number of movements, and exercise variations with other traditional equipment would cost well in excess of ten times this amount as you could create an entire gym and still not have access to the same variety provided from the deceptively versatile kBox.  So if you’re looking for a unique piece of equipment to invest in that not only provides incredibly variety but is also highly effective for producing serious functional strength and size, you owe it to yourself to try out the kBox.  I guarantee that you and your athletes/clients will be hooked provided you don’t mind a bit of therapeutic pain.

To learn more about the Exxentric kBox visit