Last year, a new systematic review and meta-analysis on the effect of flywheel training vs. traditional weight training made quite an impact on the sport science community. I read and digested the 29-page article so that I could explain the study outcomes a little further beyond the abstract, offer some input, and also give a peek into ongoing research by institutions like CESSCE.
After reading the article, the key points I want to get across are:
- Flywheel training with eccentric overload is consistently shown to be superior to traditional weights for increasing muscle power, strength, hypertrophy, and athletic performance.
- Eccentric overloading in these studies is predominantly done through one method, but there are many other more-effective methods.
- There needs to be more research in the future, as we still don’t know what is optimal.
- Researchers in seven different countries are currently looking at the kBox for physiotherapy, fitness, and performance training.
What Is All the Fuss About?
If you follow and read about flywheel science, this meta-analysis won’t surprise you. If you have hands-on experience with the kBox, even less so. Almost all studies comparing flywheel and gravitational loading (weight stack devices primarily) so far have favored the flywheel, so there’s no news here. Two of the studies did not favor the flywheel; however, those are about the conic pulley version, which is completely different from the type with a symmetrical shaft like the kBox. In one of those studies they compare different drills, too, so you can’t really say anything about flywheel vs. weights there, either.
f you have followed us for a while, you might have come across the meta-analysis on flywheels and their effect on power, strength, mass, and horizontal and vertical force production by Henrik Petré; an unpublished MSc project. It contains 15 studies but it isn’t a comparison against weights, so this new study adds something new.
To begin, I want to clarify what eccentric overload means in articles, what people think it is, and what it really is. In all flywheel training articles, the overload has been of the delayed eccentric action type. This means you accelerate all the way through the concentric phase, but don’t resist until after you passed the first third of the eccentric motion. By doing this, you overload the latter two-thirds of the range of motion since you have to absorb the same amount of energy as you produced over the whole concentric phase, but in a shorter period of time.
If you look when people tweet or post about eccentric overload, you can see all kinds of things. For instance, super-slow eccentrics spending 10-12 seconds in ECC phase, which is basically more isometric than eccentric action, at least if we compare them to the eccentric actions done during athletic performance. So, eccentric overload to me is ECC load >1RM concentric. If you are doing 2-1 (i.e., “2 legs up and 1 leg down”) with a submax weight, I’d say you shifted ratios with more eccentric focus, but if that load isn’t >1RM concentric, it is not eccentric overload training.
If you talk about eccentric training (but don’t say eccentric overload), I think it is a broader term that could permit super-slow chins and push-ups with bodyweight. However, I also think they are a waste of time; instead, increase the load and do (fast) overloaded eccentrics because that is the trigger you are looking for. Chris Beardsley wrote a nice piece on fast vs. slow eccentrics.
However, when you use the kBox, there are more ways you can actually overload than seen in these studies. You can use a stronger movement pattern in CON, like doing a “squat-hinge” as our U.S. friends call it, or the terms I prefer: “overloaded RDLs” or “deadlift into RDL” (as performed by Mike Young). RDL is weaker, so it will be overloaded if preceded by a powerful deadlift. You can use accessory muscles like pushing off with arms in the squat in CON and absorb it with the legs or have a coach pull you up, which adds extra energy for you to absorb.
This study looks at flywheel training with a partial overload in eccentric ROM vs. training with traditional weights, nothing else. The adaptations coming from the more powerful overload methods with higher contraction velocities haven’t been studied head-to-head yet, but if we compare more overload, over the whole range of motion with regular CON:ECC 1:1 using weights, I know where my money is. (Read up on the kBox overload methods here.)
What Did the Study Show?
Now, back to the important new meta-study, “Skeletal muscle functional and structural adaptations after eccentric overload flywheel resistance training: a systematic review and meta-analysis” by the mainly Spain-based team of Sergio Maroto-Izquierdo, David García-López, Rodrigo Fernandez-Gonzalo, Osvaldo C. Moreira, Javier González-Gallego, and José A. de Paz. If you just want the results, you can check the abstract. However, if you are still reading, you probably want a little more information, so here goes.
The authors searched the databases and found 97 studies. Although the flywheel might still have a lot of question marks around it, saying there is no research is wrong. Anyway, based on their inclusion criteria, the analysis included nine studies with a total of 267 subjects. All these studies are flywheel vs. weights, ranging from four to 10 weeks, with healthy young people or athletes between a six and an eight on the PEDro scale, which means all are classified as high quality. The average age for flywheel groups was 25.8 years, with a very asymmetric gender distribution since only one study involved women.
The exercises included in these studies were leg presses, leg extensions, leg curls, and squats from lower limb, with two studies including exercises targeting shoulder abduction, arm extensors, and flexors. In the flywheel devices, the overload was provided with delayed eccentric action as described above.
A Clear Win for Flywheel Overload Training
The results in this systematic review are a clear win for flywheel overload training on all training outcomes. Since an image says more than a thousand words, I’d recommend you take a quick look at the summary in the forest plot to see it yourself.
Basically, all studies are placed under the respective outcomes they wanted to look at: strength, power, hypertrophy, jump, speed. The standard mean difference on the far right shows the difference between flywheel and weight training groups, with an average to the right of the vertical bar meaning difference between groups favoring flywheel (i.e., more effect). Studies are weighted, so a larger study has more impact than a smaller one. All studies are weighted and put together in the row with the big black diamond. As you can see, all studies favor flywheel on all outcomes, with power and strength being the most obvious.
Naturally, the results from any systemic review and meta-analysis depend on which studies you choose to include and what outcomes you look at, creating room for debate. Covering a relevant subject, the publication of this study inspired an interesting discussion and additional work by another research group. This other group published both a letter and a study (Vicens-Bordas et al., 2017) showing no significant benefit in strength with flywheel training.
This difference depends mainly on a different selection of studies, where they included only 76 and 71 subjects in their primary and secondary analysis, versus 267 subjects in the first one I review here. They also set the cut in November 2016, which excluded an interesting paper from Maroto-Izquierdo (2017) on professional handball players that got really good results on performance outcomes in the flywheel group and is very relevant for sports performance.
Further on, the second meta-analysis included a paper from 2005 (Caruso) where they mainly studied bone osteogenesis in obese women (and a few men) on hormone replacement therapy. In this study, both groups showed very poor strength gains of 7% vs. 12% (no significant difference) over 10 weeks of training on a seated leg press. With such small gains in strength in both groups for very untrained subjects, I think you can argue for the whole intervention to be suboptimal for strength gains and, as a result of that, it’s also not ideal for comparisons between modalities, especially if you train a younger and more athletic population. If you want to form you own opinion and dig into this flywheel vs. weights “beef,” you can read the letter and the reply from Maroto-Izquierdo et al., and the second meta-analysis.
The current article by Maroto-Izquierdo et al. that this review is about provides a good discussion around the results and mentions a few other interesting studies (not included in the meta-analysis), so I recommend you read the full text if you want to get more details.We have no reason to believe flywheel training wouldn’t benefit women, but there’s no evidence yet. Click To Tweet
Did the meta-study prove that flywheels are better than weights? Looking at the results, the flywheel is definitely more effective than weights—at least if you train young, healthy male athletes. However, as already discussed, this result will depend on the included studies. When it comes to women, we have no reason to believe they wouldn’t benefit from flywheel training, but all the evidence isn’t there yet. We need future studies to include women. This meta-analysis only included three women out of 276 subjects in total. Sport science must do better than that.
How can you apply this knowledge? These studies are basically single-exercise drills and not a part of a training program. Therefore, we actually don’t know (in a scientific sense) how flywheel overload training works in an environment with a much higher total training volume and with parts of concurrent training. Still, it’s hard to see how there would be a negative effect if you add flywheels to a well-designed program. Coaches afraid to train their athletes too hard by adding flywheel training can use this as evidence that they should replace some of the training with barbells with more effective flywheel training.
We are also missing more closed chain exercises involving multiple joints like squats, deadlifts, and split squats. What we see from our users are also different types of overload with a higher degree of overload than in the studies and over the whole range of motion. This is probably an effective stimulus for adaptions in these outcomes but future studies must quantify it. Adding flywheel will cause some muscle damage and fatigue early on, but adaptation is fast and muscle markers for damage don’t seem to have a detrimental effect on adaptation. You can read more on this subject here.It’s hard to see how there’d be a negative effect if you add flywheels to a well-designed program. Click To Tweet
Last, but not least, the flywheel device is only a tool. You need to use it properly for strong positive effects. I usually say that training on the kBox doesn’t get you strong if you don’t train with that intent. Lousy training is still lousy on a flywheel device. The benefit of the kBox is that it makes it easy to train really, really hard and that is what triggers the adaptation—the overload.
What More Do We Need to Know?
I’d like to see more studies looking at specific populations so we can prescribe training more effectively, depending on training age, strength, sport, etc. As mentioned above, we need studies on more closed chain drills and realistic and complete training programs to help us with periodization. Flywheels might be better, but we don’t yet know what is optimal.
Besides this, I think physiotherapy can benefit a lot from using flywheel training. Patients need to get stronger and more powerful with good timing, since time saved is important for good flow in the health care system and getting people back to work. However, before we see a massive surge of flywheels in physiotherapy clinics, we need more clinical studies on specific diagnoses and conditions.
There Is Work to Be Done
There are studies using the kBox being done right now in Canada, the U.S., the U.K., Sweden, Holland, Italy, and Australia. The topics involve all three main groups: athletes, patients, and the general population. I know at least one publication on physiotherapy that is supposed to come out this spring. Without revealing too much, I can say the kBox was in favor over the gold standard treatment for a common problem among athletes in sports involving a lot of jumping.
In addition to these research projects, we are in discussions with other researchers, so the list will be longer later this year. We try to understand the problems or questions our users have, and I’m tasked with trying to get the researchers to look for answers. I hope that we see more studies we can apply in the field that will help us with protocols, periodization, loading, and in-season training.