Before you get started, you’ll need a .gpx or .tcx file on your computer’s hard drive. (This is actually a security feature. There’s no log-in on GraphMyRun.com and your files are never uploaded anywhere. That’s why you need to have them on your computer.) Pick a run where you didn’t pace yourself well and were forced to take walk breaks because of fatigue. If you’re like me, those training runs aren’t too hard to find.
From the web browser of your choice, navigate to www.GraphMyRun.com, click on the Analysis tab, and choose your file, and then open it. (That’s 3 clicks so far.) Use click #4 to select Run/Walk Heart Rate from the pull-down button to generate a graph like the one at the beginning of this post (that’s real data from my run the Eugene Marathon in 2013). If needed, choose a smoothing value that creates a table with values in the “HR Recovery Difference” column somewhere around 5 and 15 bpm. (That’s click #5 – and it’s optional).
Look at the values in the column “Ave HR Before Break”. These are the values for your heart rate right before you had to take a walk break. To optimize your run, you’ll need to adjust your pace to stay below this heart rate. In my experience, you should aim for about 10 bpm below the “I can’t run anymore” heart rate. That’s how to determine your functional threshold heart rate in just 5 clicks.
For me, it turns out that my functional threshold is about 150 bpm. This was about the heart rate where I felt able to start running again (as shown in the “Ave HR Upon Resume” column). Co-incidentally, this is right around the tipping point on the Pace vs. Heart Rate graph that can also be created from the drop-down button on the Analysis tab (shown below):
Overall, this graph shows that over the entire marathon, an increase in heart rate of 5 bpm resulted in a slowing of my pace by 5 sec/mile. That makes sense. When I ran too fast and my heart rate climbed, I had to walk to recover and I ended up slowing down. But by clicking and dragging on the graph, we can zoom in on just the data below my threshold heart rate with this surprising result: whenever my heart rate was less than 150 bpm, I increased my pace by 3 sec/mile for every increase in heart rate of 5 bpm. 150 bpm is the tipping point. Moving the cutoff above 150 bpm results in a net slowing with every increase of 5 bpm. When the cutoff is below 150 bpm, there is a net gain in pace.
Simple and practical, knowing your heart rate threshold will help you run a faster race.