The 4th mile plot - a spy story

I just had a very good run! Despite being a recovery run, the quality of its pace was on par with last nights quality session run at 90secs/mile faster. How was I able to compare 2 runs of widely differing paces?

For each run, I wear my Garmin Forerunner 201 GPS on my left wrist and my Polar S610 Heart Rate Monitor on the right. The Garmin bleeps the mile splits at me and I tap the laps into the Polar, giving me an average heart rate (HR) for each mile. So I have the data for each run. There just remains the interpretation of it.

There is a fitness test called the Conconi test. It's a method of determining Lactate Threshold by running progressively quicker 200m repetitions and recording HR for each rep. It has been discredited in some quarters, but the graph used gave me an idea. The test involves plotting speed against HR. This gives a straight line for paces below the Lactate threshold.

My idea is to use the same plot, but plot each days speed and HR, rather than Conconi's single session. For the same level of fitness, each point plotted will lie on a straight line. As fitness improves the line will move downwards: for the same HR you can run quicker, or for the same pace you have a lower HR. This way you get a visual representation of your fitness and are able to compare runs at different paces.

There are some caveats though. You must wait for a point in the run where your HR has stabilised. The first mile is always at a much lower HR and is not suitable for this analysis. I wait for the 4th mile of the run, although the 3rd may also be suitable.

You must also ensure that the 4th mile is comparable for each run. For me this is fairly easy as there are no hills within 6 miles of my house. However, you must still be aware of the conditions: it's no good comparing a mile with a tail wind and another with a head wind.

On the other hand, you may decide to plot every mile of every run from the 3rd mile onwards. At least this will give you more data and a denser graph.

Note that you should ideally plot speed (mph or km/h) against HR. If you plot pace (mins/mile) against HR, you won't quite get a straight line. However, it's still a curve that gives a useful comparison, so for simplicity I've used pace vs HR.

Here's the graph:


You can see the straight(ish) line for each month's data. As fitness improves, the line moves down to the right. You can see that November was a very good month for me! You can also see how much I've improved since the beginning of the year. Looking at this months data, the red diamonds, you can see how much the illness affected my fitness - there is a big spread - some are on a par with January's yellow triangles - but the latest are back at my best, particularly the one to the right.

This morning's run is the red diamond at the bottom next to the pale blue triangles. This was a recovery run only 16 hours after a tough tempo run. Despite being tired and buffeted by a strong wind (see the caveat above) it is on a par with my best. Good news!

One other thing that this data tells me. Looking at the slope of the curve, an increase of 10bpm give a 40second improvement in pace. Useful information when estimating training and racing paces!

As you are probably gathering by now, I like data!