Flywheel Training causing Fatigue – Myth or Fact?

kBox3 Fredrik Correa

Does flywheel training and eccentric overload cause more fatigue than regular strength training for athletes? Here, Dr Fredrik Correa brings some clarity.

  • Flywheel squats don’t equal eccentric overload, it depends on how you do them.
  • There is no significant difference in fatigue between flywheel squats and other squat protocols.
  • The repeated bout effect is strong with eccentric overload.
  • For in-season training: start low and increase the frequency and then volume per session gradually to avoid fatigue.

Fredrik Fatigue One common concern from coaches when it comes to implementing flywheel training is fatigue. For me being on the outside, it feels almost like they see their athletes as really fragile, and like they are under the impression that a few flywheel squats will put their athletes in a catabolic and fatigued state for weeks.

Even if some are overly concerned in my opinion, there is, of course, a reason for being cautious about new loading schemes and ways of training. The goal for any physical preparation, fitness or S&C coach is to maximize adaptation and minimize recovery, nothing strange with “training smarter”. But being afraid of flywheel or over-doing the recovery work might make you miss out on adaptations, and in the mid to long term, it may also increase the risks of injury.

This post will hopefully ease your concerns about fatigue, by answering some questions about the relationship between flywheel and overload, comparisons between different protocols and how we adjust to overload training. Rest assured, incorporating some flywheel squats won’t annihilate your athletes and they will survive.

Let’s look at the facts:

#1 Flywheel training doesn’t equal eccentric overload

Flywheel training on the kBox is excellent for eccentric overload. Well, there is not much competition really. Supramaximal barbell squats are good if you got 15 minutes per set and 3 friends hanging around the gym just dying to spot you. For everyone else, not too much bang for the buck. However, this doesn’t mean the kBox equals eccentric overload.

First of all, you don’t have to go maximal. The kBox is self-propelled and if you hold back, the training can be very gentle. In maximal work, if you decelerate from the top in the eccentric phase, there will be CON ECC 1:1 and no overload. You can even take out some extra strap and after the CON phase, simply just brake the flywheel with your foot, then it is CON > ECC. So you will get overload only if you want. For more information on this, see our different methods for overload.

#2 There’s no significant difference in fatigue between flywheel and regular squats

Raeder et al 2015* looked at this. They let their subjects do five kinds of squats, first three using a Smith machine and the other two were flywheel and jumping protocols:

  • MS: Multiple sets with barbell 4×6 85% 1 RM
  • DS: Barbell drop-set 4×6 85% 1 RM and down 15% for 3 consecutive sets without rest.
  • EO: Barbell Eccentric Overload CON 70% 1RM and ECC 100% 1 RM (borderline overload in my book but that is a separate post)
  • FW: Flywheel squats 4×6 (+2 pre-reps)
  • PJ: Plyometric Jumping (15 jumps from 60 cm box, with 5 seconds rest).

They looked at metabolic demands (Lactic Acid) and RPE. For muscular damage, they looked at our old friend CK (Creatine Kinase).

For fatigue, they looked at jump performance (CMJ and repeated jumps MJP) and MVIC up to 48 hours after the session, can’t figure out why the short time span but that is how it is.

So what happened?

When it comes to metabolic demands, flywheel had significant higher LA (12.2 mmol/L) towards all other protocols with PJ showing the lowest values (3.0). As a side note, I think 4×6 maximal flywheel squats is more intense than for example 6 reps at 85% RM barbell so we are sort of comparing apples and oranges here. Flywheel also has the highest sRPE (9.9) but that was only significantly higher than PJ which was significantly lower than all other protocols (5.1).

Most importantly, is the fatigue in terms of recovery of strength. First of all, no group was back at baseline MVIC after 48 hours. Eccentric Overload (with barbell) had however the greatest deficit at 48 hours but no significant difference from the other protocols. Flywheel came in second but still, no significant difference between other protocols. All protocols showed roughly a 10% decrease in MVIC, CMJ and multiple jumps at 48 hours with the PJ showing the lowest deficit.

When it comes to DOMS and CK levels there was no significant difference between protocols after 24 or 48 hours.

So despite that this study does not prove any big difference, there could be a difference hiding in the 48-96 hours window. If you want to draw any practical conclusions from this study, I would suggest plyometric jumping for stimulus if <2 days before an important competition and maybe not full volume eccentric overload or flywheel training.

However, the difference is not huge and remember you have to compare equal work too. You cannot compare doing barbell squats where you leave a couple of reps in the tank to an all-out maximal flywheel squat set, which is basically coupled 1 RMs and for 4 sets that are 24 * 1 RMs performed consecutively with a non-existent intraset rest. However, in the next chapter, you will see that athletes adapted to eccentric overload and flywheel training can handle this so much better with a solid base.

#3 The repeated bout effect is strong with flywheel and eccentric overload

Eccentric training gives more DOMS compared to more concentric focused training but it also has a repeated bout effect, which means it prevents DOMS in future sessions. This effect can last for weeks or even months. You do not even have to get DOMS from the training to have a preventive effect. I wrote about it before.

To prove this, a few sadistic researchers (Coratella 2015*) talked 13 young (and easily fooled) men into doing a 100 flywheel squats with eccentric overload (10×10). This protocol is no joke. Afterwards, they looked at Creatine Kinase (CK; muscle damage), DOMS (soreness) and peak torque (neuromuscular fatigue). What is interesting is that they let these poor fellas do that same protocol once more, 4 weeks later. What happened with the same parameters?

  • Time to baseline Session 1 Session 2
  • CK 72 hours No increase (!)
  • Soreness 96 hours 72 hours
  • Peak Torque 72 hours 48 hours

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So the conclusion is: Your athletes might say that they are sore when they actually are fit for fight, soreness lasts longer than measurable muscular damage and lowered peak torque. The most interesting finding here is the strong effect that one session of overload has on muscular damage and fatigue after a similar session 4 weeks later! Seeing this should make you feel confident in the repeated bout effect that your athletes will eventually adapt.

From other studies (more in line with the Geneva convention), like Gonzalo-Fernandez 2014*, with a group of 16 men and 16 women doing flywheel squats 2-3 sessions a week (4×7), similar findings have been observed. There is an elevation of markers of muscle damage early in the study but that is attenuated over a few weeks. Most importantly, the increased levels of muscular enzymes don’t seem to interfere with the gains made in strength and power, roughly 25% increase in 1 RM over 6 weeks together with increases in power, performance (jump) and a 5% increase in CSA for both sexes.

#4 Monitor intensity to lower risk of fatigue

An interesting study just came out from a Spanish group (Pareja-Blanco et al 2016*) with many other interesting publications in the area of fatigue and VBT. I recommend you to take a look at what they did in this latest study. They let two groups train with the same load with maximal speed and they cut off the set when dropped with 20% or 40% in their rep-speed. After the training period of 8 weeks, the 20% cut-off group had done 40% fewer repetitions in total. The results? The lower volume group with a higher ratio of “high speed” reps had better adaptation and also increase in performance outcomes as compared to the 40% / higher volume group. The only benefit in the higher volume group was slightly higher hypertrophy.

So limiting your sets with a focus on the quality reps done with maximal intensity with a lower volume can be a way to have good adaption and less recovery. Furthermore, the same group has previously shown* that cut-off earlier in the set dramatically shortens recovery time, half the training volume showed recovery within 1/4 of the recovery time. So this low volume group with 20% limit could probably have compensated with higher training frequency (more volume equated) and my guess is that there would be even greater changes in adaptation and performance than compared to the higher volume group without adding extra fatigue.

kMeter 1.3 Set UpTo help you do this on the kBox, we have developed a new function in the kMeter feedback system for the kBox. This allows you to choose other endpoints for your set than just the number of reps. Now you can also set a time limit (for AMRAP or metabolic training), a fixed effect threshold in watts or a relative drop-off percentage from your max. By allowing a drop-off % of only 5-10%, you can easily squeeze the best reps out of your athletes and get an objective cut-off when they start losing power and eliminate that low-quality volume. Hope you will find this useful!


Don’t worry about incorporating flywheel training, but make a plan. You can start gentle and get a feeling of it, your athletes won’t be devastated if you don’t want to. Even if you go hard but do it smart, they will recover and due to the repeated bout effect, they will be able to handle more load quickly.

Use VBT tools or our kMeter app with the kBox to limit fatigue and recovery time in an easy way without losing the adaptions.

To end this, here is an example on how to incorporate it slowly when you have athletes with a medium to high workload and there will be concurrent training when you add it (like most team sports). If you do it in pre-season for high strength athletes or can accept some fatigue in the first weeks, you can probably progress much faster (from Hoyo et al 2015*).

  • Get your athletes to know the exercise(s) in the first 1-2 sessions.
  • Keep the total volume of squats to about 25% for the first 2 weeks, increase frequency (number of sessions) and the number of sets to reach 100% of prescribed volume over 7-8 weeks and you will be safe.
  • Target: 2 sessions/wk with Squats 6×6

Skärmavbild 2016-03-29 kl. 11.27.20

  • Week I-II: 1 session/wk. 50% of target volume/session (3 sets) (25% of total volume)
  • Week III-IV: 2 sessions/wk. Same volume/session (3 sets). (50% of total volume)
  • Week V-VI: 2 sessions/wk. 4 sets/session. (67% of total volume)
  • Week VII-VIII: 2 session/wk. 5 sets/session. (83% of total volume)
  • Week IX-X: 2 sessions/wk. 6 sets/session. (100% of total volume)

Have fun and happy DOMS!

/Fredrik Correa, Head of Research & Development


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