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Interval running - an analysis

Updated: May 19, 2020

So what does interval running look like and how do I interpret my Garmin / Strava (or similar) results for any given effort?

In this article we break down one example of an interval running session to better understand the data and what it means.

First off, it is important to note that wearable devices have a huge variation in accuracy. Usually the distance/GPS settings are pretty good, but the heart rate (HR) statistics can be way off... sometimes up to 25%. So we can still use HR, but only as a rough guide.

Therefore we need to add in something else in order to "triangulate" the data and that is RPE (rating of perceived exertion). This simply means, how did you run feel out of ten? Some days you will do a run and feel like you are running in quicksand, your RPE might be 8-9 out of 10. Other days you will finish and feel like you could go again and you might give the run an RPE of 5. This is an important metric and we will go into more detail on how to use it later on.

For this run, I chose to perform a timed interval with a 1:2 ratio, which means 1 minute of high-speed running (HSR) followed by 2 minutes of recovery jogging. Remember that the point of the 'work' interval is to push yourself (RPE 7-8) and the purpose of the recovery section is just that, get your heart rate down and recover for the next work interval.

So let's look at the data.

Fig 1.

I did a 5 min warm up jog (not recorded) with 10 x intervals of 1 minute effort. Add on a 5 min warm down (recorded on the graph) and that totalled a 40 min running session and turned out to be 7km exactly.

What can we see?

In Fig 1, you can easily see the increases in pace for the work interval and the decreases for the recovery periods. You can see that across the 10 intervals I maintained a fairly consistent maximum speed in the work intervals, starting a little conservative maybe, then peaking at intervals 6, 7 and 8, with 9 and 10 starting to drop off.

What does this mean?

You might be led to think, "ok great he has the ability to maintain speed over 30 minutes," which looking at the basic data is true, however what can you see in my recovery periods?

In Fig 2 you can see that the difference between my recovery periods and my HSR isn't actually that large a difference. I didn't need to go too slow in order to recover. However, if you compare that with Fig 3, the second half of the run, you will notice that my HSR and my recovery rates are significantly different. In other words, I was gasping for air during the 2 minute rest interval, but still managed to maintain my consistent max speed in the work intervals.

So what does this say of my performance? It probably means that this session is appropriate for my level. It was not too easy (I didn't feel super fresh at the end, trust me) but likewise it wasn't so difficult that I had to stop after only 6 intervals. I could complete it by challenging myself, at an overall RPE of about 8. The first few intervals the RPE would have been a 5 or 6, the final two almost certainly an 8 or 9.

The other data most relevant to this type of training is heart rate (HR). My Garmin does track HR, although as mentioned, being a wrist watch as opposed to a chest strap, the accuracy is questionable. But it is close enough to give me a guide of where I am at and to inform some general training decisions.

Fig 4

If we look at the HR data in Fig 4 compared with running pace, we can see that in the beginning my HR was fairly stable, even in the work periods. This is a good thing. In the long term, we would want to see this continuing further into the workout. That would be a marker of progression and improving fitness. You can even see here that just before the half way mark I was starting to get into a groove and feel really comfortable, my HR was actually going down...and my RPE at that point would have been a 5 or 6. But all good things must come to and end! The 8th interval tipped me over my threshold and my HR was unable to stabilise in the recovery period. In fact, the 7th interval, where I was starting to feel strong, was my fastest and that 1 min effort appears to have started a chain reaction from which I couldn't recover fast enough. From there you can see two things, a steady decline in the pace in my recovery periods as I am gasping for air and also a decline in max speed in the work period. My guess is that if there was an 11th or 12th interval it would have followed this pattern and the the work interval itself would have been significantly slower and heading toward that 4.00min per/km line. That's certainly how I felt, the final interval was an RPE of 9. I couldn't have given more effort safely.

See here the benefit of comparing your RPE with the HR data? One is a subjective rating of how you feel, the other is a numerical figure of +/- 20% accuracy, but both tell us how we are coping with the stress of the run. The running pace/distance covered are our "results" but the RPE and HR are how difficult it was to achieve those results.

The cool down period shows that my HR was able to stabilise relatively quickly, back to the same as the start of the run. This is the purpose of a cool down! Once again this is a positive aspect of my data, my RPE had dropped considerably after 5 minutes and I felt ready to jog a little further or continue a different activity.

How can I use this to structure further sessions?

Current level: At the moment this structure appears to suit my training goals (HSR endurance) at an appropriate level of intensity. If I repeat this session then I can compare my performance and see whether a) my max pace increases / decreases, b) I have better recovery in the final intervals and c) if my subjective RPE improves.

Progression: If I start to work through these intervals at an RPE of 5 or 6, then I might need to make it more challenging, perhaps by reducing the rest period or increasing the number of intervals. While it is still a valuable running session, it isn't challenging my system to an appropriate level for adaptation and improvement. We want the RPE to be 7+ ideally.

Regression: If my performance drops off consistently, something isn't quite right and I will need to modify some of the variables in my sessions, for instance the number of intervals (see this article for detailed information about structuring your Interval Training).

Does your data look different to this?

Perhaps you tried a 10 interval session and by number 6 you were totally wrecked and had to stop. That's fine. Your graph will probably show a very high HR that just won't drop with an obvious reduction in max speed and RPE of 10, meaning you couldn't continue. If this is the case the interval is not quite right for you at the moment. Not a problem! We just need to modify the variables; number of intervals, intensity of work, duration of recovery or duration of work (only modify one at a time).

So perhaps you could make your work effort 30 sec and your rest 120 sec, a 1:4 ratio instead of a 1:2 ratio. Perhaps you lower the intensity in your work period so you can complete the entire session without your HR getting out of control.

Anything else?

There is always more data to look at. Today we just took a look at 3 of the key components we can use to gauge our performance in order to make decisions on how we are training. The variables are of course endless...

To get some help analysing your data and structuring your running training, get in touch by clicking here and sending me an email.

Don't forget about hydration, nutrition, recovery and sleep as four of the most important aspects in your training! Click here to read more on nutrition related to match day performance.


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