You don’t really need more than Wattage on your running watch.
A power meter is increasingly common in running. We notice that from the increasing number of questions that reach us directly or through our website TheSecretofRunning.com. The Stryd footpod is the most popular. This is not surprising, because we too found again and again that currently this is the most reliable and accurate running power meter. That the competition is not sitting still was evident in our field test of the Coros Pace 2 multisport watch.
Quite a few questions from our readers are about the use of a power meter for schemes that are not yet been available for power. The direct cause of this article is the question of a reader who likes to train with the Souplesse Method by Klaas Lok. We can answer his question in general because a running power meter is independent of the training method. It’s an instrument, not a method. Neither are a GPS running watch or a heart-rate chest strap.
Quick and easy
We give an example how you can easily calculate the wattage with which you have to run according to your work-out scheme: Suppose you have to run a block in 10 K race pace. You weighs 70 kg and runs the 10 K in 50:00. The scheme then actually requires a pace of 5:00/km. Over 1000 meters (1 km) this is 300 seconds (5:00), so 1000/300 = 3.33 meters/second.
The wattage (power) with which you have to run that particular block can be calculated as:
3.33 m/s*1.04*70 kg = 242 Watts.
Another task could be to run a 400 meters in 28 seconds per 100 meters.
This requires a running speed of 100/28 = 3.57 m/s.
Your corresponding wattage is 3.57 m/s*1.04*70 kg = 260 Watts.
This way you calculate for all situations the wattage that you have to run. Whether you need to run an endurance run, blocks, or intervals over a certain distance or duration.
What is that 1.04?
The 1.04 is the specific energy consumption. And stands for energy consumption (in kiloJoules) per kilogram of body weight per kilometer. The unit is kJ/kg/km. In English literature you will find the term ECOR (Energy Cost of Running) for this specific energy consumption.
The default value you often see, and also used in our book ‘The Secret of Running’ is 0.98 kJ/kg/km. The value 0.98 is for running resistance only. In the 1.04 we also comprised the resistance of the running wind. The 0.98 kJ/kg/km can therefore be used on a treadmill and 1.04 outside at windless days.
Is it always 1.04?
Unfortunately, the value 1.04 kJ/kg/km does not apply to everyone. This value applies to someone who is used to running gait training or is born with an efficient running style. It also depends on your shoes. If you run with shoes with a modern carbon plate, it might be less. If you run as heavier runner with, for example, the well-cushioned Asics Nimbus, it can just be more. You shouldn’t be surprised if, as a recreational runner, you have to use 1.07 or maybe 1.08 instead of 1.04 because of your running style.
How do you know your ECOR?
If you want to know your ECOR exactly, it’s good to check out a few results from your races or work-outs at race pace. Take days with little wind, flat and paved course, and distances of the order of 5 to 15K.
You calculate your ECOR from the average power and average pace at which you have run. If you take more than one road race or fast endurance workouts, you have a good indication of your personal running efficiency (which you might be able to improve through training).
An example for a 70 kg runner: you have averaged 250 Watts at an average pace of 4:53/km, i.e. 1000 meters in 293 seconds = 3.41 m/s.
You calculate the ECOR by dividing the wattage by your weight (250/70 = 3.57 Watts/kg) and by the running speed (3.57 Watts/kg)/(3.41 m/s) = ECOR 1,047 kJ/kg/km. Rounded 1.05.
If you do this calculation for several situations, you have a good indication of the personal value you can use instead of 1.04 in future.
Remember that if there are many height differences in the course, or if there is a strongwind, the ECOR is always higher. After all, it will cost you more energy to run at the same pace under these conditions. The ECOR is lower if you run in a group all the time. That is one of the reasons that Eliud Kipchoge could run the sub2 hours marathon in Vienna. It is also the reason that you have to determine your personal ECOR with little wind on a flat and paved course.
Does it matter 1.07 or 1.04?
This certainly matters. It means that with the same human power you can run just a few percent faster. 3% difference for a marathon in 3:30 means about 6 minutes difference. If you do 4 hours on the marathon the difference is about 7.5 minutes.
Your personal human engine has a certain power. From the calculations earlier in this article you can conclude that you can keep a higher pace when you run more efficiently (with lower ECOR).
Furthermore, you see that your weight is important. If you have gained weight after the holidays, the wattage drops per kg. At the same power, you’ll run slower.
Klaas Lok’s book describing the Souplesse Method has also been published in English. The English version is titled ‘Easy Interval Method‘. Therefore the question from the reader is very understandable: for intervals, a running power meter is the best tool imaginable.
Our book ‘The Secret of Running’ is for sale in our webshop. Also available in German as ‘Das Geheimnis des Laufens’, and in Italian as ‘Manuale completo della corsa’.