Is Eccentric Overload on the kBox3 Safe?

Safety is a key concern in heavy strength training in general, and in eccentric overload training in particular. How does flywheel training differ from conventional methods in this regard – and how is it safer? Read M.D. Fredrik Correa sharing his thoughts on this important subject.

Super Strength Show badgeWith more and more kBoxes out there the number of people using it increases at a fast rate. Some of those use it for eccentric overload training which is a great purpose for the kBox and a very important feature of flywheel training. Eccentric training is a known promotor of hypertrophy and strength (Roig et al 2009) and it’s also injury preventive. Eccentric overload training also has been shown to be more effective to transform strength gains into performance gains in trained subjects compared to regular CON:ECC 1:1 training (Friedmann-Bette et al 2010).

Coming on the Super Strength Show with Ray Toulany recently we talked about this type of training and one of the questions was, is it safe? This is a valid question but being on the show with a limited time to answer it basically boiled down to me saying “yes”. However, with the show just released I want to take the time to elaborate on this subject here.

Eccentric overload with kBox Squat by PhD Mike YoungCompetent high speed #kBoxSquat’s performed by PhD Mike Young, using his arms and the squat rack in the concentric action to achieve #eccentricoverload on the descent.Learn more: #flywheeltraining

Posted by Exxentric on den 7 juni 2015

First of all, heavy resistance training and basically all maximal or near maximal training do pose a risk, however you do it. Training can also be maximal in different ways with maximal speed, load or time (to exhaustion or fatigue) and all of them pose different risks of injury. In the short perspective you can have a trauma or acute overload like a muscle tear. Maximal training also poses a risk in the long term with constant heavy loads or repeated trauma that could lead to overuse injuries, stress fractures and other nasty stuff. So to say if the eccentric overload training with the kBox really is safe we have to establish a baseline risk of complications related to that type of training with standard protocols.

The next problem, what is the standard protocol? Comparing the rather fast eccentrics with the kBox is problematic since traditional supramaximal eccentrics is done (if done at all due its impractical nature) using spotters and often very slow. Forced reps is often faster than the supramaximal eccentrics but also submaximal in terms of load in comparison with supramaximal kBox eccentrics. In the end all those eccentrics can be done in different ways, kBox eccentrics can be slow too, barbell eccentrics can be faster and so on. So in terms of load and speed it’s difficult to say what is standard and what risk it does pose, but logic tells us that higher loads and higher speed would increase risk of injury, but how much is unknown.

Overloaded squat-assisted single leg RDL:s by PhD Mike YoungPhD Mike Young doing some overloaded squat-assisted single leg RDL:s using the #kBox3.By pulling with two legs in the concentric phase using a stronger movement pattern, Mike “loads” the flywheel for a massive eccentric overload in the single leg eccentric phase. This is great for performance enhancement, and research strongly indicates that eccentric focused hamstring training helps prevent injuries.Learn more about #flywheeltraining:

Posted by Exxentric on den 25 juni 2015

Moving on to the things around the training, the setup. With the kBox you can be connected with the harness which to some looks dangerous. However, if you stop pulling in the CON phase you are immediately unloaded since the force depends on you pulling. The flywheel will continue to spin and unwind the belt from the shaft. If you are close to the end of ROM in CON or even in beginning of ECC you can get and instant unload as well just by pulling the quick release on the snap shackle connecting the pulley to the harness. The flywheel will continue to spin and rewind the strap and pull in the pulley but you are disconnected and unloaded. Quick release doesn’t need a spotter, you can do it yourself and you don’t have to rely on a – sometimes unfocused – spotter.

Comparing to heavy barbell lifts like the squat when you most definately need spotters, sometimes even multiple spotters, to lift the weight and probably in ECC too for safety. That involves more people, communication and timing in help and to my experience this is posing a higher risk of injury if you need to suddenly abort your lift.

Back on the kBox, when doing deadlifts using a bar you can just drop the bar at any point and basically the only injury you can get is dropping the bar on your foot, which is less than 1 lbs from about 59-100 cm height and if you get seriously hurt from this you are probably not looking at playing professional rugby in the future with that glass foot. You could theoretically get the strap around your foot and have it squeeze your foot but that would require a very strange positioning and a wreckless technique. If you have a spotter using the kBox it’s even safer since your spotter can at any time just put his or her sole against the flywheel and stop it in a second.

To sum it up I think the kBox offers SAFER overload despite that you can work at supramaximal loads at higher speeds. This is because you aren’t lifting any weights that can fall down and that you can quick release from the pulley or drop the bar at any time or have a spotter that can easily stop the flywheel. Is it completely safe? No probably not since your dealing with high loads. So take necessary precautions and make good preparations. Warm up, know your drills, prepare yourself with what you should if you need to abort and increase overload gradually over time. Then you will do alright.

Happy DOMS,

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


If you have comments on this or other opinions on eccentric overload training and/or flywheel training, please feel free to comment this on twitter (@fredrikcorrea) or contact me privately at DS