Reflections on the kBox Power Test: ROM vs Peak Power

Following our post earlier this week presenting the new kBox Power Test protocol, some debate was stirred up on Twitter about the lack of correlation between self-selected squat depth and peak power. Here is Fredrik Correa’s reflections on the good feedback we got – in more than 140 characters!


A few days ago we published the kBox Power Test protocol for 4 RM squats on our webpage. The protocol allows for free range of motion, and I got engaged in some discussion about this. The main reason for the misunderstanding is that we are mixing up average and peak power. Since I am limited by the tweet maximum of 140 characters, I thought I would elaborate more here instead:

In the kBox Power Test, we decided to let the users select the squat depth where they feel they can generate the most power. There are two main reasons for this:

1. People train differently and strength is specific so we do not want to disqualify anybody from using their preferred depth.

2. We want to collect as much kBox Power Test data as possible, and by limiting depth we will limit the amount of test data eligible for this database.

In our own tests, we have not seen any correlation between self-selected range of motion and peak power and we also use this to advocate our rationale for letting users choose their own squat depth. This was the graph I showed earlier at the Summit. Here, 24 academy soccer players (15-18 yrs, all male) with a long background of doing kBox Squats did a power test with 3 trials and the best peak power was picked. This is their peak power vs ROM (in centimetres) and there is no correlation at all.

Comment: this first graph is plotted with ROM on Y-axis and peak power on the x-axis and the others are plotted the other way around). Could change this to this post but I wanted to show you exactly what I showed the participants at the Summit.

When we released the kBox Power Test protocol online, I also received some valid opinions regarding what is the range of motion to get the peak power and how different squat depths will affect that. Since the Summit, we got some more data from female elite gymnasts and also elite bandy players so I made a new graph with this larger dataset including them too. Now we have a larger dataset with both male and female athletes in ages 15-29 yrs, I was a bit anxious to see if this would look like the preliminary graph. Still, when we compare peak power vs ROM, there is no correlation in this dataset either. This is not saying that there might not be a relationship between peak power and self-selected depth, we just don’t see it in our data.

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The response I got was some couldn’t believe this to be true and also that ”end range peak power is higher”. One good suggestion was that different anthropometry (especially height) among the subjects would disrupt the ”existing” correlation between peak power and squat depth that we don’t see in our data. Going forward looking at the relative squat depth (depth cm/height) vs peak power would be interesting since depth is in centimetres and of course a 50 cm squat is quite different for a short person or a tall person and this might make this relationship disappear in our data. We don’t have that data today so I tried to by-pass this by only looking at the gymnasts. Again, they are not very different in height or mass and their graph looks the same, only a bit more scattered since it’s a very small dataset. Not a strong argument maybe but we only have the data that we have and the non-existing relationship between self-selected squat depth and power is consistent in all our subgroups anyway.

So let’s break this down

I think (or hope) we can agree after this.

First of all. What does our peak power in the kMeter stand for? At any given moment in the concentric action, the flywheel contains rotational energy and between two moments, there will be a difference in energy depending on whether the user is accelerating or decelerating the flywheel. The difference in energy over the time between these moments can give us power during this time. We can then compare all these timeframes and see ”which frame has the highest power value?” and that is our peak power. So the kMeter breaks down the concentric action into blocks of 20 ms (1/50 of a second) into blocks and compares all the blocks.

This means that by going into a deeper squat you can still get the peak power value in the top position. kMeter will just pick the best 20 ms from the whole range of motion. So if you get a peak power of 1000 W by doing a 1/4 squat we know that this is the highest power you can produce somewhere in this range of motion. The question is then, can the peak power be higher (in the same range) but with accelerating from a deeper position. You will then enter your 1/4 squat range with speed instead of standing still which might increase your peak power (in that end range squat). If you are looking at the average power then the ROM definitely plays a big part since we are looking at the power over the whole contraction.The last test I did was 6 trials of the Power Test myself, 2 trials on 3 different depths full, parallel and 1/4 squats with 3 minutes rest. I randomized the order as follows: Full, 1/4, Parallel, 1/4, Full, Parallel

Screen Shot 2016-06-02 at 22.25.21As you can see to the right, my best value was in the (almost) parallel squat which is also my self-selected squat depth. The 2nd trial on the same depth is a bit lower in power but was also the 6th trial in 20 minutes (not including warm-up set) so it felt pretty decent anyway and much better than the preceding full squat. Doing an end range squat (1/4) does not seem to give me any benefit in terms of peak power, at least in these trials. For me, trying to go outside the range where I can absorb the energy in a powerful manner (ie full squat) has a detrimental effect on my peak power value in the following concentric action. I am just not able to load the spring in an efficient way when I go too deep. Fatigue also affects this since at the end of the set if you are tired you will be pulled down further than you have planned which might increase average ROM in that set.

It is also much more imagetaxing and the drop-off during the set is clearly higher in the full ROM squat compared to the 4 trials with shorter ROM. (see a graph showing the average peak power between the two trials at all three different ROMs to the right). If you look at the first 2 repetitions the full squat isn’t that much behind in terms of peak power despite range is almost double (75 cm instead of 38-39 cm) but it is clear that in the individual the squat depth can affect peak power depending on fatigue, specific strength, eccentric strength and other factors. But with subjects that are familiarized with the kBox Squat and are being told to go as deep as they want and you want them to be able to be as powerful as possible, we don’t see that correlation in the group. This is like in my own trials we seem to be able to pick our optimal or close to optimal depth pretty good.

Conclusion

So we are not saying that you are not the most powerful in the end range. We are saying that from our data, in order to get there with a self-selected range of motion, we do not see any correlation between the squat depth and peak power. We are also not telling you where in that range of motion your peak power is. You can still have your peak power in the end range of the squat even if you are doing a parallel or deep squat since kMeter takes the best value from the whole range of motion. My advice is that you should do the kBox Power Test using the depth that suits you best. If you train or compete in the end range and think this is specific for your sport, you should do your test there. If your focus is on full ROM squats, do the test there as well. Be as powerful as you can be, measure it, train, re-evaluate and correlate to your performance to build your own understanding for the numbers and he they affect your performance.

Thank you for your feedback, I learn just as much from you in our community as I do from reading papers so keep giving feedback. And bring your data to us, let’s compile the biggest database of peak power values together and let the data tell us more!

For the full protocol of the kBox Power Test, Click here.

Happy DOMS!

/Fredrik Correa, Head of Research & Development

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