We regularly receive interesting practical questions from our readers. We are happy to answer them, so don’t hesitate to contact us at www.thesecretofrunning.com. Last week, Marcel van Boxtel e-mailed that his track coach designs the pace of his interval sessions based on his 10-K time.
This is very common. Depending on the interval distance, the coach will indicate the pace by the number of seconds per 100 meters. Marcel prefers to switch from pace-based to power-based interval training. Because that is better and more scientific, Marcel rightly says. We agree with him, also because the power (in Watts) is immediately visible. You don’t have to wait until the first 100 meter mark to check if you are on schedule.
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, VO2 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 paper we will 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 a balanced runner the FTP is generally the same as your Stryd Critical Power (CP), as determined as auto CP in the Stryd app. You can also calculate it on the Stryd website by using a fast 5 K or 10 K-time.
In the table below you can see the percentage of your FTP/CP 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 also 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 47 minutes. Their coach specifies their 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 a in a previous article on Stryd.com.
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 tabel clearly shows that Hans needs less power than Ron and still runs faster. Obviously, this is related to Hans’ lower body weight.
This paper has discussed power-based interval training. For other training forms you can also use power based training by using training zones theory. If you want to know more about this, please refer to our book The Secret of Running.
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’.