Flywheel vs Weights for Eccentric Overload in Strong Athletes

If you have seen any athlete do eccentric overloaded squats on the kBox you will realize that there aren’t any research papers covering that type of training. Question is, do these targeted eccentric loads provide a sufficient, or even greater, stimulus than weights in elite athletes?

To bring some clarity into this we asked a national champion weightlifter to squat in our gym and compare the eccentric load in the deep portion with a barbell vs the kBox to see what happened. Which modality generated the highest eccentric loads?

Key points:

  • kBox provides a high eccentric load and eccentric overload even for really strong athletes and good barbell squatters, even more so than weights.
  • Eccentric overload means eliciting a higher force (in that motion or angle) eccentrically than you can elicit during a concentric movement. For a whole lift this is > 1 RM or in a part of a lift ECC force > CON force in that range.
  • It’s not only about the type of exercise, the amount sets, reps or the inertia you use. It’s about HOW you do it.
  • Read up on methods for eccentric overload on the kBox.
  • How to use the kMeter for assessing eccentric overload. Avoid the pitfalls.

Eccentric Overload and the kBox

I’ve written about this before so if you want to skip the rant and dig into the results skip this section. The physiological background to all of this eccentric training is mainly that the muscle can produce higher forces during an eccentric action versus a concentric one. By using a weight for resistance, the load in the eccentric phase is set by your concentric strength. You can only lift weights up to your concentric 1 RM and with a 30-50% stronger eccentric action you aren’t working as close to your eccentric max as your concentric max using gravity as resistance.

The term ‘Eccentric training’ is used for many things and the very general and vague description “training with focus on the eccentric action” can translate into many different things, with the most common one being the super slow eccentrics you see from time to time. Other things can be more isolated eccentric actions like when we see with the Nordic hamstrings exercise (NHE) for example. NHE has been proven to elicit both improvements in neuromuscular strength, trigger structural adaptations and to reduce the incidence and re-occurrence of hamstring injuries.

When we talk about eccentric overload we mean loads beyond your max concentric strength (> 1 RM). However, strength is also specific to the exercise, speed and range of motion. So the overload can also target a certain part of the range of motion, being overall within a 1 RM load but still considered as eccentric overload. On the kBox for example you can generate max force in the concentric phase and put a certain amount of rotational energy into the flywheel. By absorbing it evenly over the eccentric portion you get a 1:1 relationship CON:ECC. You can also delay the eccentric action and absorb all that energy in for example the deep portion, but then you have to do it in a much shorter period of time. Since Power is Force times Velocity, you can also say Power is Force times Distance divided by Time. If you accelerate or decelerate a load in half the time, you would have to double the power. So without doing 2-1, using spotters etcetera, you can easily and safely overload exactly where you want in the range of motion.

Pilot Project: National Weightlifter Using kBox for Overload

The backstory here is that we came in contact with the former Swedish National champion in WL -77 kg. He trained two sessions a day, 6 days a week. He was a student back then who was about to get his degree and start working so he realized he would only make it to the gym once a day from that day forward. Being a former champ and now ranked #2 he of course wanted not only to use the kBox as a replacement but also to improve his performance and catapult back into the top position again. He has been using the kBox at home since then and we have been lucky to be able to bring him in to Exxentric HQ from time to time to do a few test sessions. One of the things we wanted to test was to see how much eccentric overload he can get by using the kBox. About 6 months ago we compared heavy barbell (BB) squats with normal kBox Squats 1:1 and those were fairly similar. We ran out of time and he ran out of juice that day so this latest session was focused on maximal eccentric load. To do so we added more inertia and a delayed eccentric action to make him work harder in the deep portion of the eccentric phase.

Set up

Aim: compare eccentric peak forces between heavy barbell squat and high inertia kBox squat with and without delayed eccentric action (i.e overload).

Subject: National elite male weightlifter, bodyweight 80 kg (176 lbs), back squat 1 RM 195kg/430 lbs.

Measurements: two Pasco force plates, 250 Hz. MuscleLab. Video.

Barbell load: 82%, 88%, 92% of 1RM for two maximal reps (ie maximal intent and velocity in concentric and eccentric phase).

kBox 1:1 load: 0.100, 0.150, 0.200 kgm²

kBox overload load: 2+2 reps of kBox squats with inertia 0.200, 0.240 and 0.280 kgm² with delayed eccentric action. As a note if you look at all flywheel studies published until today max inertia used is around 0.110 kgm². This means we used x2.5 more than the inertia seen published in the literature.

Our subject Erik was told to delay the eccentric break on the kBox as long as he felt he could, without being pulled into the hole. The concentric actions were maximal on both exercises.

RESULTS Heavy squats vs 1:1 kBox

When looking at peak forces we wanted to look at the deep portion of the squat so the peak force value is the highest value in the range 60 degrees (ECC) – 45 degrees (bottom) – 60 degrees (CON), ie the turn.

From the earlier session you can see in the three first columns vs kBox 1:1 that peak forces increase with increased BB load. However, the eccentric load (in terms of peak force) on the kBox doesn’t increase as much with increased inertia in the 1:1 setting. Due to our subject getting tired we don’t know what numbers a maxed out inertia would look.

To get a closer look at this and really stress the overload we continued this at a later session with two higher inertia settings, 0.240 and 0.280 kgm², and with the delayed eccentric action. Now he is increasing the load quite a bit in the deep portion of the squat both looking at the best rep and the average force between the two reps. In the table you can see the comparison vs the “best” barbell set, 92% of 1 RM. A win for the kBox with +14% peak force in best rep and +11% in the average for two reps.


From this n=1 trial we can’t draw any strong conclusions or present “new facts”, you don’t need to be a scientist to know that. However, since no study has looked at elite squatters and compared their BB squat and flywheel squat we have nowhere else to get any info at all. So what did this experiment show? Now we have an indication that the kBox Squat 1:1 provides a fairly high eccentric load without “trying” to stress that specific load, this is a regular CON-ECC movement. However, when increasing inertia and trying more actively to force an overload we get quite high overload vs weights even with an elite weightlifter that is really an expert in training with high loads on the barbell. Even for him you can see the practical differences in the video. With the heavy barbell squats we have a spotter but on the kBox he trains unsupported and he is, as I told you earlier, doing this at home in his apartment two or three times a week during some cycles.

I’d suggest that the average athlete and barbell lifter would get an even higher overload vs their maximal squat since they have a limited technique, strength and experience in the barbell squat (at least vs a elite weightlifter). This also tells us that we can’t just focus on sets, reps or inertia but it is also about HOW the exercise is performed. By delaying the eccentric action you can target this area of the range of motion. In the same way you can target the end range power by absorbing the flywheel energy very early in the ECC action. So, it’s not only about WHAT you do but HOW you do it. Let the kMeter assist you and think about what effect you want and where you want your athlete to be loaded. All squats aren’t equal, especially not on the kBox.

Happy DOMS

/Fredrik Correa, M.D., co-founder



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