I've owned and used a lot of pieces of exercise equipment in my time.
Of all those pieces, the Exxentric kBox is one of the products I've been most excited to try.
This is a tool that stands out for several reasons: it's small, it's versatile, and it trains the eccentric portion of a movement better and safer than anything else.
Eccentric training has grown in popularity in recent years, and Exxentric as a company has become one of, if not the authority on flywheel devices. Since the company was formed in 2011, they've iterated the kBox now four times and they've created additional flywheel products to create a comprehensive lineup.
Having owned the kBox4 for nearly five months, it has become a tool that I regularly use in my training. It creates a stimulus that is totally different than traditional strength training implements like barbells, dumbbells, etc…
I've not only enjoyed using it just because it's fun and challenging, but also because it's generating real results. My strength has improved, my conditioning has improved, and I overall feel like a more well-rounded “athlete.”
After using the kBox4 and seeing the benefits of flywheel training in general, I see this as a tool that virtually everybody can benefit from.
The biggest downside and barrier to entry, however, is its price.
In this review, we'll take a deep dive into what eccentric training is, how flywheels work, all of the features/qualities of the Exxentric kBox4, and my experience with the product. By the end, you should have an idea of whether or not it's for you.
Let's dig in.
What is Eccentric Training?
We know that weight training consists of two stages: the concentric portion (weight lifting/shortened phase) and the eccentric portion (weight lowering/lengthened phase). We also know that muscles are more capable and stronger in the eccentric/lengthened phase. The problem is that we are limited by the concentric portion because, in order to lower the weight, we must first lift the weight (unless starting at end range and performing singles). Being that a shortened position isn't as strong as a lengthened position, we are essentially leaving gains on the table and being less efficient.
Eccentric training then is an attempt to take advantage of our strongest position by increasing the time under tension in this lengthening phase or overloading the eccentric portion. An example of increasing time under tension would be to curl a 35lb dumbbell and then slowly lower it in order to “train the negative.” An example of overloading the eccentric would be to have a partner help with the concentric and then allow you to lower it solo… or to bilaterally contract the movement and unilaterally lower it… or to cheat using other muscles to contract the movement and lower it using only the desired muscle.
As great as eccentric training sounds, and it's been scientifically proven to increase muscle size, strength, hypertrophy, and help with recovery/rehab, it's not easy to accomplish in the traditional sense. There's inherently more risk involved because again, you have to find ways to get through the weaker (concentric) phase, in order to take maximum advantage of the stronger (eccentric) phase.
That's where flywheels come in.
How Flywheels Work
Flywheels have been around a looong time… the late 1700s, in fact… compliments of Frenchman, Francis Lowndes, who invented the Gymnasticon. After some fitness research in the early-1900s, the flywheel was mainly used in other mechanical applications to harness kinetic energy. In the mid-1990s, however, flywheels came back into the resistance picture as the research community was attempting to slow muscle atrophy and bone loss in space travel. Fast forward to 2011, and after more research, prototyping, and trial and error, the very first kBox1.0 was released.
The flywheel is unique within resistance training in that it relies on inertia and subsequent force responses (i.e. the user) to generate eccentric and concentric stimuli. It's inherently different than traditional weight-based strength training in three primary ways:
- Unlike traditional weight-based movements (barbells, dumbbells, machines, etc…), flywheels don't rely on gravity. Rather, they utilize kinetic energy.
- Because flywheels use kinetic energy, this means the load is always maximal (per user) and totally variable. What you put into the wheel energetically on the concentric, you receive back on the eccentric.
- You aren't actually using “weights” at all. The plates on a flywheel are sized to generate more or less inertia, and they're chosen by the user based on a desired training outcome. Again, this means that the load is unlimited and variable. You can technically add infinite energy into the flywheel system and adjust that intensity across every rep. If you tried variable resistance in traditional weight-based training within a given set, you would need to add/remove weight/accommodating resistance on every rep. Load on a barbell, for instance, is also limited to the bar's weight capacity, the sleeve length, and the user's strength.
How flywheels physically operate is actually very simple. There's a strap that's attached to a drive shaft, which holds one or more inertia plates (more on these plates below). The user will spin the plate(s) to get the movement started. When that happens, the strap wraps around the shaft either fully or partially. When the strap is fully pulled out at the end of the concentric portion of a movement, there is a bunch of energy in the system. This energy and continued flywheel rotation cause the strap to retract via the opposite side of the shaft.
Here's where the fun begins. ALL of that energy must be absorbed and subsequently reversed to continue the exercise. Whatever energy you put into the system on the concentric is 1:1 what you receive on the eccentric. You have to decelerate that energy FAST. Once you do, you apply a concentric force again and the sequence continues, always variable, always unlimited, and always a heck of a ride.
Because the flywheel works the way it does, it allows users to more effectively, more easily, and more safely train the eccentric portion of a lift. And because of that, it has become an increasingly popular mechanism to combine with traditional resistance training in order to enhance sports performance, increase strength, etc…
Whether you're in recovery/rehab from an injury, you're a bodybuilder looking for hypertrophy, you're a powerlifter looking for strength, or you're a sport-specific athlete, flywheels are excellent tools.
The Exxentric kBox4
The kBox4 comes in three different versions: the Active (reviewed here), the Lite, and the Pro. Each version will be described in more detail below. As mentioned above, the kBox has been refined now four times since its initial launch in 2011. It's noticeably well-made, it's heavy-duty while also being relatively light-weight, and it's just a joy to use.
The below sections will detail specific features and qualities of the kBox4.
The kBox4 Active platform is constructed with formed and welded steel from top to bottom. The total footprint of the platform measures 20″ deep, 30″ wide, and 8.5″ tall. Talk about getting a ton of versatility in a small space!
The top of the platform is slightly smaller at 17.25″ deep and 27″ wide, which gives users plenty of room to perform a variety of movements. On the top of the platform and underneath the feet you will find a rubber material that helps provide traction to the users' feet and to the ground.
In the middle of the platform is a rectangular opening, which allows access to the main drive shaft and provides space for the belt to travel. On the lower left of the rubber encasing surrounding this opening is a red button that allows the user to adjust the slack of the belt. When the button is pressed (and held), it releases the belt to be drawn out or retracted. This is a very important feature because as you perform different movements, the end range will be at different elevations. For instance, I would need to draw out more belt for a biceps curl than I would a bent-over row. Once you have the end point (concentric termination) set, release the button to lock it into place.
On the underside of the platform, you will find the wheel that holds the belt and that which allows it to auto-retract. It also helps to mostly maintain an untwisted belt. I say mostly because on occasion the belt may twist (think of a seatbelt latch). This has happened very rarely in my experience and it doesn't impact the movement really at all. It can also easily be remedied after the fact just by untwisting and threading through.
You will also find a place for the kMeter (more on that below) as well as a convenient home for two provided allen keys (used to change the belt, adjust tension, etc…). There's a shaft cover that protects the shaft as well as an inner and outer flange to keep the belt routed nicely.
At the end of the belt where it connects to the user is a very nice and high-quality pulley block. This is one of my favorite features of the kBox4 because it functions flawlessly and it feels extremely secure. The block uses a snap shackle to fasten around whatever attachment you're using (belt, harness, bar, etc…). The operation is seamless and very easy to use.
The kBox4 can be loaded with five different types of plates:
- Extra Small – 0.005 kgm²
- Small – 0.010 kgm²
- Medium – 0.025 kgm²
- Large – 0.050 kgm²
- Extra Large – 0.070 kgm² (only fits the Pro)
I own two of the large plates as well as one each of the small and medium. The kBox4 Active can hold two plates at a time with the standard knob for a maximum inertia capacity of 0.100kgm². You can, however, purchase the Advanced Flywheel Knob, which allows you to expand the capacity by two plates. This brings the total capacity of the Active to 0.200kgm².
The plates themselves have a hexagonal center hole, which allows them to slide onto the shaft with total precision. They have a gray finish and they either have a black or a color-coded border around the center which identifies the plate and its associated inertia. Each plate comes with a bag for protection when not in use.
The knob secures the plates onto the shaft by sliding it into the groove and pressing in towards the platform. Once secure, you will feel and hear a satisfying click, letting you know it's time to rock and roll.
When it comes to picking the plates, it's important to note the different flywheel training zones. Exxentric has a great article on this, but in a nutshell, remember that lower inertia leads to greater speed and lower force and higher inertia leads to slower speed and greater force.
- Low inertia is great for warm-up and power movements.
- Medium inertia is great for technique work as well as strength and eccentric overload.
- Higher inertia is great for strength and eccentric overload.
One of the great features of the kBox4 is that it's built to accommodate several on-platform accessories. Exxentric also sells several quality attachments, harnesses, belts, etc… which I will explain below.
- Hip Belt – This is my preferred way to squat on the kBox since it simulates a belt squat nicely (but with eccentric overload as mentioned above). The belt is very comfortable and it comes in two sizes: Small-Medium & Medium/Large. I'm 5'8″ & 165 lbs – I wear the Small-Medium. It has a high-quality seat belt-like clip with a fabric ring that connects to the pulley block.
- Harness – The harness is another option to perform squats and other movements that you would with the hip belt (lunges, calf raises, etc…). Personally, I like the hip belt more. The harness works well too, but it takes a little longer to set up and I prefer the feeling of the pure vertical load directly under the hips. I do like the dual metal rings that are used to connect to the pulley block. The harness comes in 7 sizes ranging from XXS to XXL. Exxentric has a helpful sizing chart here.
- kBar – The kBar is another attachment that I use frequently. It's a traditional straight bar attachment that measures ~23″ in length and gives you a lot of freedom of hand position. It's coated in a very comfortable rubber material, which feels great in the hands. The swivel attachment allows it to rotate to easily perform biceps curls, rows, and other movements efficiently. You can of course use this as a cable attachment on any machine you have access to and it's also a great option for the kPulley on things like triceps pushdowns, straight arm pulldowns, etc…
- Decline Board – The decline board is another favorite of mine. It can be used to perform decline squats which elevates the heels and makes it easier to get into proper position for those with limited flexibility/mobility in the ankles, knees, and/or hips. It can also be useful if you're coming off an injury, specifically in those areas. If you flip the board around and use the flat portion at the top, you can easily use it as a great calf raise pedestal. You can also use the decline board for other movements unrelated to the kBox.
- Foot Block – This is a unique attachment that utilizes the side holes in the kBox4 platform. Its main purpose is to serve as an angle limiter in the ankle when performing a lateral squat where one foot is off the platform and the planted foot pushes off the block. When using the foot block, you may need to place a heavy object (KB, DB, etc…) on the opposite side to keep the kBox firmly on the ground.
- Angle Adjuster – This is a unique accessory that attaches to the platform to more easily perform movements that are more horizontal in nature (think rows, etc…). It comes standard with your kBox4 purchase. When using the angle adjuster, you may need to place a heavy object (KB, DB, etc…) on the opposite side to keep the kBox firmly on the ground.
- Elevation Blocks – As the name implies, these are blocks to elevate the feet. If you want to perform deficit work or if you're looking for additional depth/range of motion, these are a great tool. They measure 13.7″ long, 6.3″ wide, and 1.9″ tall and they're made of a durable eco-friendly rubber.
- Advanced Flywheel Knob – Discussed above, this allows for two additional inertia plates, which adds 100kgm²-140kgm² of inertia depending on which kBox4 unit you have.
- KMeterII – the kMeter II is an on-board device that wirelessly connects to an integrated app to track and record various metrics including power, ROM, force, eccentric overload, and energy by measuring the rotation of the wheel. The device sits beneath the platform and it's battery-operated (AA batteries). The app is easy to use, allowing you to connect the device, select your movement, and add your inertia plates. It then plots your results, which can be viewed in the app or even exported into Excel for further analysis. If you're a competitive athlete/coach or you're someone who really cares about the finer details, this is an awesome and unique feature of the kBox4. I don't personally use it much, but it's a valuable tool for some users. The kMeter comes equipped as standard on the kBox4 Lite and kBox4 Pro. It's an add-on for the kBox4 Active.
The kBox4 Versions
The Exxentric kBox4 is currently available in three versions:
kBox4 Active – The kBox4 Active is the version reviewed here, and it's what I would recommend for most people, especially in the home gym community. It cannot produce as much inertia as the other two models, but if you add the Advanced Knob, it can generate up to 200kgm², which is very, very challenging. Another reason I recommend this version is that it's over $1,000 cheaper than the Lite and nearly $2,000 cheaper than the Pro. One potential downside of the Active is there is only one color choice, Bronze.
kBox4 Lite – The kBox 4 Lite is made of aluminum, which makes it lighter than the other models. If you travel a lot or if you just prefer a lighter-weight unit, this is one to consider. It can hold 4 plates as standard and up to 6 with the advanced knob, taking the maximum inertia up to 300kgm². It can be purchased in two colors: midnight blue or jet black. It's the mid-priced model in the kBox4 lineup and it comes with the kMeter as standard.
Box4 Pro – If you're looking for the top dog, the pro is your unit. This version has a larger surface area for additional movements and it can produce more inertia. It's the only version that allows for the use of the extra-large plate, bringing its standard inertia capacity to 280kgm². With the advanced knob, it can generate up to 420kgm². The pro also comes equipped with the kMeter, and it's the most expensive model.
My Experience with the Exxentric kBox4 Active
As of the time of this review, I've been using the kBox for about five months. Never having used a flywheel device before that, I wasn't sure what to expect. Was it going to fit my training style? Is eccentric training all that it's cracked up to be? How would I program it? These were all questions I had leading up to delivery.
After a Zoom call with a couple of Exxentric associates walking me through the device, best practices, etc… I was excited to start implementing it in my training. From my very first use, I could tell it was a totally unique experience that created a very different stimulus compared to traditional weight training.
At first, there is a definite learning curve. Firstly, you have to keep in mind that resting is the exact opposite of traditional weight training. Take the squat for example. Typically when you rest between reps on a squat, you pause at the top, reestablish your breath, etc… before continuing the movement. On the kBox, you MUST remember never to pause at the top. All of that energy is going to try to pull you back towards earth. If you try to stop at the top and resist that energy, you're in for a rude awakening and possibly an injury. It's not intuitive right off the bat – I tried to rest at the top on my first day even knowing not to do so. It's like touching a hot pan as a kid… do it once and you won't do it again.
Secondly, there's a learning curve in just getting the wheel started and getting through the first moments of building up inertia. I like to leave a little slack in the belt and pull with my hands, do a partial squat, etc… to get the wheel spinning. You can also swipe the wheel with your foot or hands to start the movement. Either way works. I'd say it took me a few times before I felt like I really had it down.
Thirdly, there's a learning curve with knowing where to set the belt length. This is something that just requires experimentation and reps. Give too much slack in the belt, and there's a lag in the system that will suddenly hit you and aggressively pull you down. This is actually an overload technique, but if you're not aiming for it, it can be a bit awkward, if not jarring. If you don't get enough slack, you won't get the full range of motion and, unless you're specifically training a partial range, it will immediately feel off and you'll have to reset.
Now that I've been using the kBox4 for five months, it's become a staple in my training. I use it multiple times a week. Whether it's as a warmup, as an active recovery tool, a volume/hypertrophy tool, or a strength tool, I've found it to be an invaluable, fun, and very challenging piece. I mainly use it for belt squats, back rows, split squats, deadlifts, calf raises, and biceps curls, but I'm also experimenting with new movements all the time. Even incorporating traditional weights from time to time. For instance, a flywheel goblet squat with a heavy kettlebell has been a favorite of mine.
All told, I am a huge fan and believer of flywheel training now and the kBox4 has been a joy to use. It works seamlessly and it provides an amazing workout that you just can replicate with traditional resistance training.
Exxentric kBox4 – Pros & Cons
- Exxentric is a pioneer in the flywheel space and their products are very refined to reflect as much.
- The kBox 4 Active has a very small footprint of 20″ x 30″, making it great for home gyms, traveling coaches/trainers/athletes, or anyone who prioritizes space savings.
- With a variety of attachment options, there are dozens upon dozens of possible movements you can perform on the kBox.
- Flywheel training makes eccentric training safer, easier, and more effective than attempting to train the negative with traditional resistance training.
- Four different plate sizes (fifth size, XL, available for Pro) allow users to dial in the inertia to provide the appropriate stimulus for various training goals. With the standard knob on the Lite and Pro Models, users can load four plates on at a time, but with the advanced knob, it's possible to load six. The Active can be increased from two to four by purchasing the advanced knob.
- The physical unit is extremely well-made – the welds, the machining, the attachments, etc… all are high-quality.
- It's expensive. Flywheel devices in general are pricey tools and the kBox is no exception. Starting at $1,750 for the kBox4 Active starter package, it can get as high as nearly $6,300 for the ultimate Pro bundle.
- The kBox4 Active allows for the least amount of inertia among the three available units. While 100kgm² can still be very challenging, purchasing the advanced knob may be necessary for some users.
When you boil it all down, the kBox4 is one of the premier flywheel devices on the market. Based on your needs and goals, Exxentric gives consumers the option to pick from several device options. From my perspective, the Active with the advanced knob is the best option for most people since it's the most economically priced, the smallest, and it still allows for up to 200kgm² of inertia.
I am now a huge fan of flywheel training and I would recommend the kBox4 to anyone looking to incorporate flywheels into their routine.
If you have any questions about the Exxentric kBox4, please leave a comment below. Likewise, if you own a kBox4 and you want to chime in with your own thoughts, please do so!
As always, I appreciate any feedback.
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The bar is loaded,