The direct reason for this article is a question from cyclist Eddy who also runs. His bike is equipped with a power meter. For running he uses the Stryd running powermeter. He shares all his sports performances with his trainer through TrainingPeaks. Running on power makes a lot of sense to Eddy. In addition to similarities with cycling, he has also noticed differences. That’s what we’re going to explain.

Normalized power P_n

PowerCenter from Stryd does not give a value for the normalized power P_n (in Watts) of a workout or race. TrainingPeaks will give this, also in the non-paid version of TrainingPeaks. TrainingPeaks originates from the cycling and triathlon world.

As everyone will understand, it always takes extra energy when the speed is not constant. This is related to the fact that you have to deliver considerable more power at higher speed. This costs more energy than you save at lower speed.

TrainingPeaks calculates P_n from the power differences during training and defines this as the weighted power with which you would have run (or cycled) without variations.

Normalized power P_n is therefore a value that differs from the average power.

In cycling, the difference is usually greater than in running. The reason is understandable. For example, downhill you don’t necessarily have to pedal on a bike. In running this is not possible. Also downhill your legs have to provide power. There are also differences between running and cycling caused by the wind resistance. The difference between the normalized power P_n and the average power P_ave is generally smaller in running than in cycling.

Ron has looked at the differences for his training sessions in the month of January 2021. The main differences between P_n and P_ave occurred in training sessions with hills or during interval sections. An endurance run or climax run showed less difference.

Interval training

We have worked out the normalized power in an example for an interval training of our Marathon Man, the example figure from our books that weighs 70 kg and runs the marathon in 3:30.

Marathon Man in this example runs alternately for 1 hour at a pace of 10 km/h and 5 minutes fast at a pace of 16 km/h. In the table we have shown the required power numbers, the average power P_ave and the normalized power P_n. You can see that P_n is always higher than P_ave. In this example, the ratio is 1.08. Here you can already see that variations always take more effort and power than maintaining constant speed.

Normalized intensity I_n

TrainingPeaks gives another parameter for the intensity of your training. This is an important parameter as it is necessary to include intensive, high speed training forms in your training schedule.  In TrainingPeaks, you can look it up and use it to check if you’ve met your training goal.

For the concept of intensity, the normalised power is compared with the FTP. FTP stands for your Functional Threshold Power, the power you can maintain for an hour.

The normalized intensity I_n is defined as follows: I_n = P_n/FTP.

In the simple example that you run at constant pace during the entire workout at the level of your FTP, the intensity is defined as 1. An intensity of 1 or more corresponds to a very heavy training session or race. It goes without saying that it is physically impossible to maintain a training or race with an intensity above 1 for more than 1 hour.

So you can just look up the intensity of your training in TrainingPeaks. The following classification is used:

• I_n < 0.75: light (recovery) training
• I_n 0.75 – 0.85: normal endurance runs
• I_n 0.85 – 0.95: pace training, intervals, longer road races
• I_n 0.95 – 1.05: short intervals, road races
• I_n 1,05 – 1,15: speed workouts
• I_n > 1,15: speed workouts or short track race

TSS and RSS

In addition to the degree of variation and intensity, there is another factor in TrainingPeaks that determines the strain of a training or competition, namely the duration. The 3 parameters P_n,I_n and T are combined in the so-called Total Stress Score (TSS), which is defined as follows:

TSS = 100*T*I_n²

In practice, values of TSS above 150 correspond to very heavy training sessions or races.

Usually the following classification is used:

• TSS < 50: light (recovery) training
• TSS 50 – 100: normal workouts, from which you no longer suffer the next day
• TSS 100 – 150: strenuous workouts, requiring 2 days of recovery
• TSS > 150: very tough races (marathon), requiring a longer recovery

TSS and (RSS) are used to check whether your training load meets the objectives and that you are not overloaded.

In PowerCenter, Stryd defines the TSS slightly differently and calls it RSS (Running Stress Score). The classification is the same as that of the TSS. Stryd explains this in a blog. They use our book ‘The Secret of Running’.

In short, the severity (stress) in cycling and running is fundamentally different. Stryd therefore uses a different coefficient in the formula for RSS than the squared one in the formula for TSS.

For the comparison, Ron put the TSS values of his training courses in January 2021 together with the RSS values of the same training sessions according to Stryd PowerCenter in the graph below. It is clear that depending on the content of the training, it depends on whether the RSS is a bit higher or lower than the TSS.

What is the best race strategy?

It follows from the theory of the human engine and the above that your performance is mainly determined by 2 factors:

1. Duration

In a short track race you can perform well above your FTP with an I_N N of 1.15. With a longer race, this drops to your FTP (at 1 hour) and even lower (0.88*FTP at 2 hours).

So you will have to make a good estimate of the power you can maintain throughout the race. If you start too fast, you will inevitably hand in a lot of time later. If you start too slowly, you still have something left in the tank at the finish and therefore do not perform optimally.

2. The degree of variation

As we saw from the concept of normalized power, it is usually best to divide your powers in such a way that you can complete the entire race with a constant power.

On a flat course without wind, a constant power also means a constant speed. However, if conditions vary along the way, for example due to hills or wind, then your speed at a constant power will not be constant.

Uphill you will slow down and downhill you will run faster. This is no problem. Just stick to the strategy to keep your power constant. Trust that maintaining a constant power is the best way to reach your fastest time!

In fact, the above means that it is almost always the best strategy to make a good estimate of the pace and power that you can maintain throughout the race. Stick to that strategy!

The only exception to this has to do with the fact that your total time on a course is always more determined by the slowest parts than by the fastest parts. It may therefore be wise to turn on a little extra power on the heavy parts (uphill or against the wind). You can then use the time savings you book there to recover something on the less heavy parts.

Be careful, however, because you can easily run in the red on the heavy pieces and that backfires. If you choose the right power for the whole race, you will be close to your anaerobic power threshold and so you only have a small margin before lactate accumulation becomes a problem. If you choose this strategy, do it just before a descent in which you can recover, or join a group that keeps you out of the wind.

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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’.

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