Tracking Fitness With the "Heart Rate-Running Speed Index"

Finnish scientists propose a simpler alternative to lab testing.

Saturday, February 1, 2014, 4:10 pm

This post isn't as complicated as the graph below might seem at first glance. The basic point is this: as you get fitter, your heart rate at a given running speed should decrease. (Or, conversely, your running speed at a given heart rate should increase.) That seems like a convenient way to track your progress -- but the problem is that there's too much day-to-day variation in heart rate, thanks to factors like hydration status, weather, terrain, and so on -- not to mention pace and intensity in any given run.

The solution proposed by researchers in Finland, in a paper just published in the Journal of Strength and Conditioning Research, is a "Heart Rate-Running Speed Index" that uses the data processing powers of heart-rate monitors and GPS watches to compute the average relationship between heart rate and speed, and track how it changes over time. Here's a diagram of how the index works:


The index basically represents the difference between the average speed of your run and the speed that you theoretically "should" have run based on your average heart rate. If the actual speed is faster than the predicted speed, it suggested that you've gotten fitter since the baseline value of the index was calculated.

To calculate the baseline value, you start with two data points: your standing heart rate at rest (which corresponds to a running speed of zero), and your heart rate and running speed at VO2max. Then you draw a straight line between those two points, because research shows that speed increases as a roughly linear function of heart rate (as long as you're not going too close to all-out). That gives a straight line that tells you, in theory, what speed should correspond to what heart rate (and vice-versa).

To use the index, you record the average speed and average heart for each of your runs. Then you compare the average speed to the theoretical speed predicted by the straight line for that heart rate. The difference between those two speeds is your HR-RS Index. (Hopefully looking at the diagram above makes that seem a little less like total gobbledygook!)

The Finnish researchers put 45 runners through a fairly rigorous 28-week training program (that's a very long study, which allowed them to track long-term trends). They also did four VO2max tests during that time, in order to provide a "gold standard" reference for how the subjects' fitness was changing. Here's what the data looked like for a sample subject:

The square data-points represent percentage improvement in running speed at VO2max, which is considered a very reliable gauge of improvements in running fitness (it's better than VO2max itself, because it also incorporates any changes in running economy). The continuous line shows a running average of the individual HR-RS Index values for each run (which are represented by the bars); the running average helps to smooth out some of the random day-to-day variation. The upshot: there's pretty good agreement (for the group overall, there was statistically significant correlation). It appears that this fairly simple non-invasive technique can give you a pretty good gauge of how your fitness is progressing, without the need for expensive maximal treadmill tests.

So how should you expect your HR-RS Index to behave? When you first start training, or if you increase how much you're training, you should expect to see steady improvements. Other studies that have tracked submaximal HR find that it tends to plateau after a few months. In this study, the subjects kept improving (on average) for the entire 28 weeks; that's probably because the training program was designed to keep getting progressively more difficult. Still, if you've been running a long time, you shouldn't expect to see big jumps. If anything, you might be more sensitive to decreases -- it might give you some early warning that something is wrong with your training.

Is this something that people will actually use? I don't know. But if it's the kind of data that you're already collecting automatically because you wear a GPS/HR watch when you run, and if you're the kind of person who loves breaking down and analyzing data, this seems like it might be a fun parameter to play around with.

[UPDATE: As several people have pointed out, this index is very similar to the "Running Index" that Polar offers: there are some minor differences in how the baseline is calculated, but the general principle is the same. So that makes it even easier to play around with this parameter!]

