Roughly 15 months ago I tried to sell Gear Fisher, the programming whiz behind TrainingPeaks, on a concept that excited me. It was a vision for a data analysis feature to be built into TrainingPeaks WKO+ software that would assign a score to each and every run workout based on a comparison of heart rate and pace data downloaded from a run speed and distance device. “How cool would it be,” I told Gear, “if runners could obtain a numerical rating of their fitness level in every single run, and compare their performance between any two runs no matter how different they were in terms of distance, pace and structure?”

I did not have the slightest clue how exactly heart rate and pace data could be compared to produce such scores, but I was confident that it could be done based on the simple fact that the fitter you are, the lower your heart rate is at a given pace and the faster you can run at any given heart rate. As it turned out, my speculation was so perfectly correct that the feature I envisioned had already been created and was available to be used by any runner who desired it.

Running Index is a feature that is exclusive to Polar speed and distance devices and does exactly what my concept feature did: on the basis of heart rate-pace relationships, it calculates a rating for each run that can be compared apples-to-apples to the rating of any other run.  It is essentially an estimate of a runner’s current VO2max, which is determined largely by genetic inheritance but is also affected by training.  Therefore tracking Running Index is a useful way for Polar users to monitor the effects of their training on their fitness level.

Polar’s product literature does not explain the science behind the Running Index in any detail, so yesterday I called up Hannu Kinnunen, the manager of physiology research at the company’s headquarters in Finland, and asked him to explain it. Here’s what I learned.

Again, Polar speed and distance devices calculate Running Index by comparing heart rate and speed.  The device notes your heart rate and speed when you reach the 12-minute mark of a run.  It converts your heart rate into a percentage of heart rate reserve.  Your heart rate reserve is the difference between your maximum and resting heart rates.  This requires that you enter your maximum heart rate into the device before using it.  The more accurate this number is, the more accurate your Running Index scores will be.  The following formula is used to calculate your current running heart rate as a percentage of reserve: (current HR – resting HR) ÷ heart rate reserve.  The device then divides your speed by your percentage of heart rate reserve to yield an estimate of the maximum running speed you could sustain for 12 minutes, which will be close to your VO2max running speed.  Finally, the device converts this value into an estimate of your current VO2max, with scores typically ranging from 40 to 80 (the higher, the better).  After 12 minutes, the device continuously recalculates your Running Index as a rolling average up to the present moment, so that your final Running Index score at the end of the run may be slightly different from your initial score at 12 minutes.

For example, suppose that, 12 minutes into a run, your heart rate is 130 and your speed is 7 mph.  The device knows that your maximum heart rate is 186 bpm and your resting heart rate is 55 bpm, hence it also knows your heart rate reserve (186 – 55 = 131 bpm).  At 120 bpm, your heart rate is at 48 percent of reserve ([130 – 57] ÷ 131 = 0.55).  Your estimated speed at VO2max is your running speed divided by your percentage of heart rate reserve at that speed (7 ÷ 0.55), or 12.72 mph in this case.

Heart rate normally drifts slowly upward as any given pace is sustained, and the faster you run, the faster the drift.  Polar speed and distance devices filter out this drift as best they can in calculating Running Index scores.  They also try to filter out the effects of hills on pace.  However, they are not able to filter out the effects of environment conditions such as heat and terrain on heart rate, and this limitation affects the comparability of Running Index scores from different runs.

Nevertheless, testing has shown that Running Index scores are reasonably accurate predictors of actual VO2max.  But they are even better predictors of maximum running speed at VO2max, which is an even more important variable because it factors together both aerobic capacity and running economy and is therefore a better predictor of actual running performance than VO2max.  High Running Index scores result from having a relatively low heart rate at relatively high running speeds, and running economy makes a significant contribution to this capacity.  Indeed, tests have shown that Running Index scores predict running economy separately almost as well as they predict VO2max.

If you don’t use a Polar speed and distance device, you cannot get Running Index scores but you can still calculate estimated VO2max speeds from training data in individual runs.  Just note your speed and heart rate at an appropriate point in a run and perform the calculations above exactly as illustrated in the example given.  For the best results, note your speed and HR on level, smooth terrain at a relatively early point during running at a given speed.  During sustained, moderate-intensity runs, the 12-minute point is ideal.  When doing short, high-speed intervals, it’s actually better to note your heart rate at the end of a given interval, as heart rate tends to lag behind speed in such circumstances.

Personally, though, I would rather get the real thing, and after speaking with Mr. Kinnunen I am so jazzed up about Running Index that I am strongly considering replacing my Garmin Forerunner 405 with a Polar RS800CX. You can do such cool things with Running Index. For example, you can compare Running Index scores from workouts performed at different intensities to isolate strengths and weaknesses in your fitness. For example, if your high-intensity Running Index scores tend to be lower than your low-intensity Running Index scores, you probably need to do some more work at higher intensities.

A few weeks ago I found a Hong Kong-based Italian runner named Roberto Veneziani, who has done some very cool things with Running Index. Check it out here.

This entry was posted on Thursday, February 12th, 2009 at 2:27 pm and is filed under Running, Sports Science. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.