The kBox4 is a tool developed by Exxentric and distributed in Australia by in2performance_hq. This blog is endorsed by both brands to help provide practical strategies to make the most out of your investment.

Eccentric Overload Training; What is it?

Eccentric overload training — that is, training that emphasises the eccentric component of a muscle contraction (usually the downward portion of a movement in the direction of gravity’s pull) is one of the most heavily researched and validated areas for the reduction of injury, increase in muscular size and strength, and, maintenance of muscular function. It also happens to be a critical component of athleticism — before force is expressed, it is usually first absorbed, and that ‘absorption’ phase can be thought of simply as the eccentric phase of a muscle contraction (made up of the stretch reflex and the stretch-shortening cycle). It is well understood that the more force you are able to control and decelerate, the more force you can produce; a fundamental underpinning of great athleticism. Training in a manner which emphasizes the eccentric portion of a muscle contraction isn’t new; it has been popular since we understood the importance of time under tension (TUT) and tempos in strength training popularised by the late Charles Poliquin. However, actually overloading the eccentric phase of a muscle contraction is no easy feat; because most athletes are 20-40% stronger eccentrically (compared to concentrically), their eccentric rate of force development is rarely ever challenged with traditional strength training or if its attempted, it often misses the mark.

This is in part because we have conflated eccentric overload training with tempo training (while they can be the same thing, they aren’t always), but also in part because it’s commonly associated with muscular stiffness and soreness, something a lot of athletes (and coaches), think they can’t afford to have. 

These conflations and concerns (no matter how well-intended) have meant that a vast majority of athletes are not getting the exposure they need to extract the well documented protective benefit of eccentrics as they relate to injury mitigation, muscular strength, muscular size and muscular function. This blog will help clear up the two most common fallacies surrounding eccentric overload training  and then discuss how true overload can be achieved with the use of the Exxentrix kBox4.

Eccentric Training goes beyond slow reps

Commonly, eccentric training is synonymous with ‘slow’ yielding repetitions. We are often told that if you want to emphasise the eccentric component, then simply instruct your athletes and clients to ‘slow down’. This was popularised by Charles Poliquin in the 90’s under the umbrella principle of Time Under Tension (TUT); inferring that the more time the muscles are under tension, the more likely a stimulus would be achieved.

Eccentrics are not simply slow and heavy repetitions as commonly seen by bodybuilders seeking to hypertrophy their muscles as efficiently as possible. Landings, sprinting, throws and other ballistic exercises also contribute to high rates of eccentric contractions (because the muscle first yields before it contracts and produces force), and, is what is commonly seen in athletic endeavours. 

To divide up eccentric training simply, we’ll use Carl Valle’s breakdown of different forms of eccentric training:

1. Bodyweight Strength Training: Very low speed of muscle contraction with very low load

2. Traditional Strength Training: Low speed of muscle contraction with moderate-high load

3. Traditional Loaded Jumps/Throws: Moderate speed of muscle contraction with low-moderate load

4. Depth Jumps/True Plyometrics: High speed of muscle contraction with very high load

5. Sprinting: Very high speed of muscle contraction with high load

As you can see, eccentrics exist along a continuum, with some activities demanding a higher rate of eccentric force development, while others demand less. If your eccentric training exists only to help an athlete absorb heavy loads, slowly, you may be missing out on critical exposures to higher speeds of muscle contraction, and, to high internal/external loads. Having a system that allows you to deliver a moderately high-speed eccentric contraction with an external load that bridges between loaded jumps and true plyometrics is a wonderful and useful asset to have in your toolbox (where the kBox4 comes in; more on that later).

Eccentric Training does make you sore and stiff, but that isn’t reason enough to avoid them.

Perhaps the most common fear with eccentrics is the concerns for athlete discomfort following a new stimulus. This is commonly called Delayed Onset Muscle Soreness (DOMS), and it still carries a significant stigma; some athletes we’ve met flat out refuse to get sore. 

Unfortunately for them, avoiding DOMS or stiffness as a result of a new, challenging training stimulus hurts an athletes chances from taking full advantage of what the weight room has to offer. While the avoidance of the eccentric portion of a lift has utility in some contexts, avoiding it altogether will ultimately lead to an athlete that becomes fragile and vulnerable.

Like anything good in life, effective eccentric training is dose-dependent in the same way that in small doses, the poison can be the cure. Eccentrics as a strategy for strength training should be integrated intelligently; starting with the minimally effective dose. A few quality sets now will lead to an even greater amount of quality sets in the future; which will set up your athletes for success when they reach a period in training (like the off-season) where they can really go to town.

There is another benefit to enduring the transient stiffness/soreness that often results from overloaded eccentrics — you’re likely to increase your fascicle length and flexibility. Greater fascicle lengths are associated with quite a few desirable athletic attributes: faster muscle contraction velocities, greater peak torque, and a protective benefit against injury. This is explored in this article here; long (increased flexibility with more sarcomeres in series) and strong (greater eccentric strength) hamstrings, provides a significant protective benefit to hamstring injury in elite football. The inverse is true — short and weak increases the likelihood of lower limb injury. Given the prevalence and priority need of athletes to stay uninjured, trading out transient stiffness and tightness for a protective benefit seems like a worthwhile risk-reward.

Applying an overload stimulus eccentrically is the fundamental premise that underpins the kBox4 and why we made the investment to acquire one. Simply put, the more force you produce concentrically, the more force is thrown back at you eccentrically. Providing our athletes with another opportunity to develop their ability to rapidly decelerate eccentric forces is critical to athleticism; absorbing force quickly, utilising energy throughout the SSC, and then, producing force as quickly as possible, is really the basis of sports performance (a crude but fair assessment of athleticism at a low resolution view).

The kBox4 makes the pursuit of this training quality achievable with simplicity and ease. As you may recall, the pursuit of eccentric overload training is somewhat difficult to realise and implement; additional partner resistance is inconsistent and unreliable; 2-up 1-down training is limited to only a handful of exercises, and accommodating resistance via the use of added band tension will only take you so far. This is where the kBox4 comes into its own; it’s portable, easy to set up, and requires absolutely no prior learning; it can be utilised with the most basic of exercises. Let’s take a look at how we’d progress an athlete through different eccentric overload protocols with the primary focus of increasing the speed of eccentric muscle contraction (inline with moving an athlete toward drills like shock method and true plyometrics).

Here is a progression video along with data from the trials to see the impact on eccentric power when the constraints of the exercise demand a greater rate of eccentric muscle contraction.

Phase 1: Straight Eccentrics

For most athletes initially, simply adding the kBox4 to an athlete’s training during off-season, a phase of injury rehabilitation, or, early on during a training week will provide a sufficient eccentric overload stimulus. Completing 3-5 sets of 10-15 reps of squat/deadlift/lunge patterns, we were observing an average 8-12% eccentric overload stimulus (peak forces). These were done as ‘straight reps’: no instructions were given to do anything differently during the eccentric phase and the athlete was instructed to push as hard as possible during the concentric, and resist as hard as possible during the eccentric phase..

For those new to eccentric overload training, this is more than sufficient and a great introduction to this form of training. Here is an example set below. As you can see, the peak concentric power was 757W, while the eccentric peak power was 843W (an 11% difference as shown). We’d prescribe this for athletes with little exposure to eccentric overload training for anywhere between 2-6 weeks, before progressing to phase 2.

Phase 2: Supramaximal Eccentrics

In the second phase, hand assistance is used to artificially augment your concentric power production to further emphasise the eccentric demands. Taking the squat as an example, you would hold onto a bar supported by j-hooks and pull on it during the concentric phase, increasing your leverage and making the concentric phase faster. This extra force you’ve developed will have to be accounted for eccentrically, adding to the demands that you would otherwise experience with straight reps alone. 2-5 sets of 6-10 reps here is what we’d be shooting for over a 2-4 week training block. It’s important to note here that supramaximal eccentrics will actually yield far more work done than the next progressions so if your goal isn’t to increase the speed of eccentric muscle contraction, then swap this with phase 3.

Phase 3: Delayed Eccentrics

The next step in overloading the eccentric phase is to use a protocol known as ‘Delayed Eccentric Action’. Unlike straight reps, or supramaximal eccentrics, where you resist through the whole eccentric phase, a delayed eccentric action requires the athlete to let the flywheel ‘pull’ them down for the top third of the movement, and then, begin resisting or ‘braking’ as aggressively as possible. This is a progression from phase 1 & 2 in the sense that you have to decelerate the same amount of momentum over a shorter period of time which ultimately, increases the eccentric rate of force development. Keeping sets and reps the same as phase 2, the cue used here is ‘brake and drive’ and the goal is to stop the flywheel as soon as you can by pushing against the resistance as aggressively as possible. This style of training could be continued for another 2-4 weeks. This should significantly ramp up the eccentric stimulus; in our early experiments, it increased the peak eccentric power by 20-30%.

Phase 4: Impulse Overload

The most challenging and final phase of eccentric overload training with the kBox4 is called Impulse Overload, for the reason that a large force is applied over the shortest period of time possible. This is an extension of delayed eccentrics, but with one small distinction: rather than letting the flywheel pull you down momentarily, you want to try and ‘beat’ the flywheel and return back to your bottom position, ready to resist the force of the flywheel as quickly as possible when it hits you. This is the penultimate form of developing eccentric rate of force development, prior to shock method and depth jump variations. Given the intense nature of the activity, 2-5 sets of 3-6 reps would be our go-to set/rep scheme. You can see in the video the dramatic rise in eccentric peak power; close to a 100% increase on the previous trials.


Overloaded eccentrics provide a great means for improving athleticism; from injury mitigation, to strength, muscle size, fascicle length and muscle contraction velocity. Like anything however, this friend can become a foe when used inappropriately. Keep in mind the recommendations in these phases are hypotheticals, and careful consideration should be made as to whether the time frames, sets, reps and prescription are appropriate for your athlete. Proceed with caution and remember to include your athlete in the decision making process; they are usually a lot more intuitive than we give credit for and they will let you know when they feel confident to proceed. 

Otherwise, happy lifting with the kBox4. It’s a fantastic tool in the toolbox.