So, here’s what I’m thinking…

I just posted a video on Youtube about running power meters. It’s quite a complicated topic, and it’s hard to explain the concept in such a short video.

AND THEN, I got an idea: from time to time, I could send you some extra material along with my Youtube videos. It might be my notes about a certain topic. Or it might be my journal entries from an experiment. Or it might be some data from my workouts. As long as it adds value to the Youtube video, I can add it to these letters.

And so, here are my notes about running power. Enjoy✌️

What is running power?

For most, the simple goal of running is to get faster. This means increasing speed.

Running speed = Running Power + Running Economy (RE)

In other words, speed is about how much power you can produce and how efficiently you can convert that power into forward movement.

Here we'll only focus on power.

There are two types of power:

• Metabolic power = input, i.e. how much energy you're using

• Mechanical power = output, i.e. how much of that energy is converted into movement

Under normal circumstances, the human body is able to convert only about 25% of metabolic power to mechanical power (but the efficiency varies based on circumstances). Most of the metabolic power you produce actually dissipates as heat.

Metabolic power is what we want to estimate, as it ultimately determines how fast and how far you can run.

How is running power measured?

Currently, the most accurate way to estimate metabolic power is by analyzing the amount of oxygen and carbon dioxide when you breathe in and out (that's what the mask and other equipment during a VO2max test are for).

Of course, power meters are not measuring gas exchange. Instead, they try to estimate metabolic power by measuring mechanical power.

In cycling, this works well, as there is a tight correlation between metabolic and mechanical power: your metabolic power is about four times the amount of mechanical power you produce. So if you're pushing 250 watts of mechanical power on the pedals (that's what you see on your power meter), you can be reasonably sure that you're using about 1,000 watts of metabolic power (with about 25% efficiency). Hence, the number on your power meter is a proxy for metabolic power and can be used to estimate intensity.

But in running, the relationship between mechanical and metabolic power is constantly varied.

For instance, when you run up a steep hill, there's less bounce (or elastic recoil) in your step, which means you need to exert more metabolic power to keep producing the same amount of mechanical power.

So how, then, do running power meters measure mechanical power? The simple answer: they don't.

Hey👋

Instead, running power meters use mathematical models to reverse engineer an estimate of mechanical power. Here's a good explanation by Alex Hutchinson, who interviewed the co-founders of Stryd (one of the leading running power meter companies):

What that means is that

1) on level ground, the power meter is measuring positive (but not negative) mechanical power

2) on hills and downhills, the power meter is using an algorithm to estimate what mechanical power should be based on a lab-tested metabolic power

So all in all, running power meters are not actually measuring power. They spur out a rather arbitrary number using algorithms and indirect inputs.

So... why should you use a power meter then?

Even though the number of your power meter is somewhat arbitrary, it doesn't mean it can't be useful. Several studies have analyzed the validity and repeatability of power meters against VO2 testing and found that power meters do a good job estimating metabolic power (especially Stryd). In other words, they are good at estimating intensity.

Steve Palladino looked at 8 different studies, and concluded that "Stryd power does correlate highly with oxygen utilization" and that "Stryd power, as a consumer device worn in the field, does a reasonably good job at reflecting metabolic demand."

It also seemed that Stryd was superior to other power meters. For instance, in a study that compared four of the leading power meters, Stryd power meter had a 4.3 percent coefficient of variation (or about 12.5 watts), compared to 7.7 percent for Garmin, 14.5 percent for Polar, and 14.8 percent for RunScribe.

In the graph below (which is from this study), you can see how the data from Stryd is much less scattered and correlates better with VO2 compared to the other power meters.

Pros of running power meters

• Power meters can provide faster feedback on the intensity of effort.

  • Unlike HR, power is not affected by factors like caffeine, dehydration, sleep deprivation.

  • Unlike HR or RPE, power has no (or minimal) lag.

  • Unlike HR or pace, power is less affected by hilly terrain.

• Running power meters might help quantify your running form. But this should be approached with caution. For instance, Hans van Dijk's experiment showed that running with a higher cadence seemed to improve running efficiency based on power, but when measured with VO2, a higher cadence actually lead to an increased energy cost (and thus, lower efficiency). Read the last part of this article to learn more. Some other metrics from a power meter might be used to assess improvements in running form (GCT, LSS, VO) over a longer time period.

• Can increase the accuracy of pace measurement for running (like pretty much any other footpod).

• Can be used to run on Zwift (like pretty much any other footpod).

Cons of running power meters

• What we ultimately want is to get faster, i.e. we want more speed. In running, pace is an excellent metric for that: faster pace means faster speed, no matter the terrain or circumstances. On the contrary, more power does not necessarily mean more speed. You could create 300 watts and be running in place.

• Power measurements (or estimates) are currently not very accurate in soft sand, soft snow, very icy/slippery surfaces, severe grades (>20%), and very technical downhill trails.

• Grade-adjusted pace (GAP) achieves similar results as power in many scenarios. This is readily available in most sports watches, and some new watches even have real-time tracking of GAP.

Training applications

• As long as you understand the limitations of running power meters, they can help you estimate the intensity of workouts (especially on hilly terrain).

• Can be used as an aid for pacing in races.

• Can be useful in uphill interval training to get a better estimate of the intensity of the intervals.

• Can be used to create a power-duration curve, which helps pinpoint weaknesses and strengths, and directs your training. However, you could also create a similar curve using pace and duration.

• If you want to give a go at running power meters, your best bet might be a Stryd power meter.

Read more:

• Alex Hutchinson's articles about running power (one of the few journalists I trust):

  • Rethinking What Power Meters Mean for Runners

  • Who Makes the Best Running Power Meter?

  • Is Power the Next Great Metric for Runners?

  • What Is “Running Power,” Anyway?

• Understanding powermeter metrics

• Guidelines for training with power

• How Stryd measures "power"

• Thoughts and criticism on running power meters

Shoot me an email or leave a comment on this post :) It's an easy way to give feedback or suggest content ideas!

Unlazy regards,

Jonne