Running with Power: Do Running Power Meters Work for Trail and Ultrarunners?

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By Jason Koop,
Head Coach of CTS Ultrarunning

The recent release of the new Stryd foot pod caused me to take a bit of a pause and evaluate how far the running power meter market has come and whether or not it is useful for trail and ultrarunners. Running power has always appealed to me. If applied correctly, it could open up a whole host of training insights, similar to what power has done for cycling training. We would be better able to prescribe intensity and monitor workload, as well as gain insights into the demands of specific events. Running power meters are getting close to being applicable to all runners. Here’s why I think it’s time to proceed cautiously and optimistically with running power.

Primer on Power

Before we get into running power, it’s important to understand the concept of power in general and how it’s derived. If you remember anything from high school physics, you can skip this section. For the remaining 99% of you, power is a product of Force and Velocity (Power = Force X Velocity). It is a measure of the rate of energy transfer (from your legs to the bike, for example) and the units used to describe power are Watts. Exactly the same as your energy efficient lightbulbs, the lower the Watts, the lower the energy output and vice versa. Power in endurance sports has been primarily exploited in cycling by on-board cycling power meters. They have the ability to measure the force being produced by the rider, and also know the velocity of that force production. Insert those into the equation above (Power = Force X Velocity) and viola, you have Power. Same for running, right? Sit tight…

How a cycling power meter works

Most cycling power meters use an array of strain gauges to measure force that is being applied through the crank arms, pedals or hub. The way the gauges measure force is through the change in electrical resistance running through the gauge caused by the slight changes in the material as force is applied. You push on the cranks (assuming that’s where the power meter is measuring in this example), that ever so subtly deforms them in some way, the electrical resistance in the strain gauge changes and that change in electrical resistance is associated with a known magnitude of force. The reason this whole chain of events is important is that what is being measured is being done so at the point of force application. The power meter is still ‘measuring’ electrical resistance technically, but it is similar to a thermometer uses the rise in mercury to indicate temperature (temperature causes mercury to expand and contract, that expansion or contraction move the mercury up and down the thermometer).

How a running power meter works

Two different ways have emerged for runners to determine running power in the field. First, companies like FeetMe and RPM2 have insole devices that, in a very similar way to cycling power meters, ‘measure’ force at the level of the foot. Just like a cycling power meter, you then can take the force measured, combine it with velocity (which the device can also measure) and you once again have all the ingredients to bake the power cake.

 

Figure 1- Picture of FeetMe insoles. www.feetme.com

Another way companies like Garmin and Stryd have tried to tackle the problem is to reverse engineer the power based on the components it takes to overcome various forces when running, namely gravity and more recently with the release of the new Stryd, wind resistance.

Fundamentally, these types of running power meters measure the movement of a foot based pod (through accelerometers, gyroscopes, barometric altimeters and the like), incorporate the weight of the athlete and then apply an algorithm to that movement that corresponds to a power output. They have validated their devices and algorithms (in part) by putting athletes on treadmills at various speeds and measuring their oxygen consumption (which is metabolic power) to see if the difference in oxygen consumption matches the differences in the calculated power output. There is a ton of nuance relating to the differences between metabolic and mechanical power and what mechanical power actually means in running. All of which is outside the scope of this article. So, for all the egg-heads rolling their eyes (I’m one of them), at the audacious simplification of the previous paragraph, sorry-not-sorry.

Figure 2- Depiction of the power needed to run. Credit- www.TheSecretofRunning.com

Whew, that was a lot. For now, remember that there are two flavors or running power meters-

  1. Power meters that measure force at the level of the foot
  2. Power meters that measure motion and reverse engineer power from there.

The case for running power meters: intensity, pacing and biomechanics

Cycling power meters revolutionized the sport. No, I don’t think that statement overexaggerates the case. It is as ubiquitous as the GPS watch in trail and ultrarunning. Manufactures of running power meters will have you think the same thing is going to happen in running. The use case in running, however, is slightly different.

Cyclists use power predominately as an intensity gauge. It is a superior measure compared to heart rate and RPE in cycling as it represents an absolute measure of work output. Cycling power does not care if you are riding with a tailwind or up a steep grade. Power is power (to a certain extent). This use has become so prevalent, that we know with great certainty based on an athlete’s power to weight ratio (the amount of power they can produce divided by their weight) and power profile (how much power they can sustain for a given amount of time) where they rack and stack across different categories and even if they have the potential to be a better stage racer or sprinter.

The use case for running, however, is markedly different. Both cyclists and runners can use power as an intensity gauge (go run/ride at 200 Watts). However, while cyclists are trying to maximize their power output for a given amount of time or workout, runners are at least in part, trying to minimize it at a given pace. Additionally, as all current run power meters know something about the motion of the foot, various biomechanical insights can be gained, particularly as runs progress and running form changes.

Additionally, running power meters can be used for better race pacing (in theory, at least). In this case, a runner would keep their power output in a specified range (much like heart rate) regardless of grade, technical nature of the terrain or even a headwind.

Finally, running power meters could be used to gain biomechanical insights on their athletes and used those finding to (possibly) predict injury, monitor fatigue, improve form and a host of other biomechanically related training insights.

Running power meters for trail and ultrarunners

So, should trail ultrarunners adopt running power meters? Not yet. In both flavors of run power meters, it is currently not possible to account for dramatic changes in surface, or technical nature of the terrain. Furthermore, I’m not sure we could even make sense of what that change means. Meaning, if your power output goes up because you are now running though some gnarly rock garden, how does that translate into training? Insights would be different depending on the flavor of run power meter you are using (are you striking the ground harder or moving your feet more or both).

For ultrarunners who predominately train and race on roads or the track, I would certainly have them use a power meter in training. Would I use it as the sole intensity gauge, absolutely not. I would use it along with pace and RPE to gauge and prescribe intensity. I would also use running power meters for the biomechanical insights such as monitoring stride length/frequency, leg spring stiffness and other changes as fatigue sets in.

I will leave room for the ‘maybe’ category for runners who train on predominately the same type of terrain. ‘Maybe’ as in maybe run power meters would be useful in a training application. If the if the surface and technical nature of the training consistent enough, using a power meter as an intensity gauge and for biomechanical feedback could work. But, you’d end up with issues if you tried to translate your training power to race power if the race surface is different.

Musings for the future

Despite my pessimism on the current use of run power meters for trail runners, I do think it is a space that ultrarunners and coaches should pay attention to. Simply dismissing it at this point, particularly for coaches, would be an error. Cycling power meters took a couple of decades for widespread adoption. The early ones were heavy, expensive, and lacked accuracy and reliability. The hardware, software and algorithms needed constant, relentless improvement. Coaches and athletes were challenged on how to make sense of the data. Over the course of time, and as the utility improved, the early cynicism from cycling coaches and athletes eventually gave way to widespread use. Running power meters will have a similar run of show. The hardware and software will get better. The algorithms will become more accurate. Coaches and athletes will learn what the data means and more relevant use will ensue.

As I look further into this hazy crystal ball, I ultimately think the ‘flavor’ of run power meters that will ultimately be the most palatable will be the ones that can measure force at the level of the foot. This is in juxtaposition to the current pole setters in the space, Garmin and Stryd, who use motion to calculate power. Similar to the now obsolete iBike power meter (which reverse engineered cycling power from the forces that needed to be overcome), anytime you are using movement (movement of the bike, movement of the foot) to reverse engineer the force and power, it’s going to be problematic.

Pay attention to this space. If you train on the trails, don’t jump on the bandwagon just yet. Get your springboard ready though. Running power is here for road runners and it’s just a matter of time before trail runners will need to get on board.


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