We regularly get practical questions from runners who run with the Stryd running power meter. Last week we answered the question about improving your critical power when training with Stryd training plans.

Actually, that explanation is valid for most training plans. Typically, you run many miles at a slow pace. The purpose of such workouts is to train you to run for a longer period of time.

With intervals and running in blocks at a higher pace you accustom your body to a high speed. With clever combinations of speed, rest, and repetitions, your coach ensures that your running economy improves. For middle and long distance runners, the energy sources are mainly aerobic glycogen and fatty acid burning.

Therefore the power requirements for the intervals in this article are not intended as a complete workout. They are building blocks for a workout.

**Interval training makes you a better runner**

Many serious runners train at least once a week with their athletics club on the track. On another day of the week, they train for themselves, or with a group, and do a bit longer intervals, speed work or hills according to the design of their training plan.

The added value is that you can train at higher intensity with intervals. It improves your speed, VO_{2 }max and anaerobic threshold. The goal can also be to train your different energy systems (you have four!). Or to get your body used to lactic acid. Because in between the intervals you are allowed some recovery, this type of training is not as stressful as a race.

Depending on the goal, the coach varies the length of the intervals and the intermediate rest in the training scheme. The coach also varies the pace during the intervals. Usually, coaches define a base intensity as 100%. In a rest week this can be reduced to 95% or even 90%. In the race period, the intensity can be 105%, or even 110%.

The coach divides the group based on the 10 K-times of the runners. You run in a group that is about as fast as you are. The number of seconds per 100 meters is different depending on your level, the interval distance and the intensity. This all seems to be the coach’s secret. If you train with a power meter, it needs no longer to be a secret to you.

**Power-based interval training **

In this article we explain how you can calculate the required power of your intervals. First you need to know your Functional Threshold Power (FTP), which is the power (wattage) that you can sustain for 60 minutes. In practice, for an average balanced runner the FTP is generally about the same as your Stryd Critical Power (CP), as determined as auto CP in the Stryd app. You can also read your FTP in the power duration curve in PowerCenter from Stryd at 1 hour.

It can be even simpler as you can also have your FTP determined with the calculator on our website www.theSecretofRunning.com. You enter the distance and time of a fast race and find on the second to last tab your calculated FTP in Watts/kg. This number you still need to multiply by your body weight. For Ron this is currently 3.25 Watt/kg at 80 kg, so 260 Watt.

In the table below you can see the percentage of your FTP that you should use as Target Power for different interval distances.

The percentages increase for short interval distances because your anaerobic energy systems kick in to supply extra energy. However, your anaerobic fuel supply is limited. For this reason high powers and fast speeds can only maintained for short periods. In an earlier article at Stryd.com you can read more about this. We fully explain this in our book The Secret of Running.

The figure below also gives these percentages. In the graph we have drawn a line that indicates the maximum theoretical power supplied by the energy systems of the human engine. That is the limit of your human power. In an interval work-out it is of course not intended to run that fast. It’s no race. If you do, you will miss your training goal and may get injured.

**Practical examples Hans and Ron**

As practical examples, we use the interval data of authors Hans and Ron. Hans runs 10 K in 38 minutes. Ron currently needs about 51 minutes. Their coach specifies the required pace in terms of a number of seconds per 100 meter for the various intervals. The table also shows their corresponding power targets for the intervals, which we have calculated with the same method as we explained in another article.

For the average runner this can be easily done with the formula:

P = 100/t*1.04*m

In the formula you enter for t the number of seconds per 100 meter and for m your body weight (in kg) to get the power P (in Watts).

The table clearly shows that Hans needs less power than Ron and still runs faster. Obviously, this is related to Hans’ lower body weight.

*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’*