Understanding And Using Strava's Relative Effort Score

Strava's "Relative Effort" score, which is available to those of us dorky enough to subscribe to their premium membership, is an interesting piece of data to think about.

While the exact formula for calculating Relative Effort is one of Strava's proprietary secrets, they readily acknowledge that it is based on the athlete's heart rate during exercise. When you click on your Relative Effort score for a particular activity (from the browser-based user portal), you are taken to an analysis of heart rate. Specifically, you're given a bar graph, by Heart Rate Zone, of percentage of time spent in each Zone. You're given extra points for time spent "in the red," which any time you spend in Heart Rate Zones 4 or 5. (This is all based on a 5-zone approach.)

Those of you familiar with the fitness industry will recognize this principle immediately. It's the "theory" behind Orange Theory Fitness, ie., you'll get a more worthwhile workout if you spend time in the "orange" zone, which is usually designated Heart Rate Zone 4. (Zone 5 is usually shown in "red," and constitutes the athlete's maximum effort.)

Needless to say (I hope), neither Strava nor Orange Theory Fitness innovated this approach to working out. Targeting Heart Rate Zone 4 a few times per week has been a regular part of heart rate zone training for as long as people have been grouping their heart rates into "zones."

A year ago, as I built up my aerobic capacity, I noticed that over time workouts that covered the same distance and speed were getting "easier" from the standpoint that my average heart rate was getting lower. I might have run 5 miles at 6:45/mile pace every day for a month, but at the beginning of the month I'd spend 10 minutes, say, in Zone 4, whereas at the end of the month I might have only spent 1 or 2 minutes in that zone. This signifies an increase in my aerobic fitness level, but not the fitness of my legs. More on that in a bit.

In order to get at that information last year, I had to watch my Heart Rate Zone diagrams on my Garmin Connect app. Every day, I'd check the bar graph and visually confirm where my average heart rate was going. It was imprecise, but close enough for rock and roll. To improve on that, I may have graphed maximum heart rate during exercise over time, grouping by similar distances, and checked for a downward-sloping trend line. That's not impossible, of course, but it is a little convoluted.

Strava's Relative Effort score works much better. Charting that one number over time gives you insight into how your training is progressing. Indeed, this is the graph that Strava calls your "Fitness" graph. Remember, Relative Effort involves the total amount of time you spend in each Heart Rate Zone, so even if you only spend all your time in Zone 1, 10 minutes is worth more than 5 minutes. So it combines both aerobic effort and total time spent training, which also functions as a proxy for weekly mileage.

Thus, if your aerobic fitness improves, but you do not increase your weekly mileage, then you could plausibly see your Fitness level drop, since you'll either spend less time in higher Heart Rate Zones, or less total time exercising (since you'd be running the same number of miles faster). The only way to increase your Fitness level (that's capital-F Fitness, ie. Strava's "Fitness" number) is to run more miles or run the same number of miles in higher Heart Rate Zones (or both).

Strava's website isn't good at explaining this at all! They give you general information about Relative Effort, heart rate zones, and "Power" (for cyclists), but they don't have any explanation that brings it all together so that you can use your personal Fitness chart. If I hadn't have figured it out, I likely would have cancelled my premium Strava subscription, since without a good explanation, the data is essentially useless. Chalk another one up to the uselessness of the way our data is served back to us by the tech industry.

No, that's not quite fair. Relative Effort is a really good number -- for once, we've been given something that is actually highly useful. Unfortunately, it's just not presented in a way that is readily usable to anyone except people like myself, data geeks who are already quite accustomed to probing the data deeply.

Hopefully the above explanation helps you more than Strava's online materials.