Can You Quantify Fitness?

Part 3 of a series on training data. Part 1 covered the 5-zone heart rate model and part 2 covered zone 2 specifically.

If you use Strava or a fitness-oriented smartwatch like a Garmin, Coros, or Apple Watch, you’ll know that one of the central features these brands want to sell you on is their ability to evaluate your fitness holistically and present it to you in the form of a neat and tidy number. The brands in the smartwatch/wearable category claim to be able to tell you VO2 max—your maximum aerobic capacity—and Strava has invented its own Fitness score which supposedly helps you track your progress relative to your past self. But how useful are these measures, and to what extent are they grounded in reality?

VO2 max scores, when performed by smartwatches, are inherently squirrelly. They seem tidy: they’re just one number, after all, and they have been shown to generally correlate to athletic performance and longevity. But the number represents the maximum volume of oxygen your body can use during exercise, measured as milliliters of oxygen per kilogram of body weight per minute of exercise. There’s no way to reliably estimate this with the data available to a smartwatch.

What the watches actually do is compare your effort at a given pace over time paired with the quality of your recovery. If your average HR when running at 9:00/mi is 154 in February but 148 in April, your watch assumes that you’ve gotten fitter and that your VO2 score must have increased in the interim, so it adjusts its estimate accordingly. This also has issues because the watches have trouble accounting for heat, elevation, and so on: my watch has recorded marked drops in VO2 max when I travel to Colorado, and then equally large jumps back up when I land back in Chicago. However, just because the VO2 max score you see on your watch is unlikely to be accurate, what can be useful is comparing your scores to themselves over relatively large periods of time. Just be aware of the pitfalls mentioned above.

(Here, I will also give a shoutout to Garmin’s training load feature which gives you relative scores for your Low Aerobic, High Aerobic, and Anaerobic training load over the last 4 weeks. I find that while the watch has a tough time estimating my total fitness, it does a great job at visually laying out the overall training effect that my body is experiencing at any given time.)

Strava is a slightly different case. Its fitness score is purely relative, meaning that it’s not measuring anything like VO2 max or lactate threshold, which are actually measurable in a lab environment, but rather just how fit you are compared to past you. My main gripe here is that Strava simply does not respect easy running (or easy exercise of any kind). The only way to increase your Strava fitness score is to run hard—typically only a structured workout, tempo run, or race will increase my score. I could run 100-mile weeks back to back at low intensity, and my fitness score would drop simply because I wasn’t spending time in zones 4 or 5. Not only is this annoying, but I think it’s borderline irresponsible because it may encourage folks to run too hard too often to appease the Fitness Score Gods. Don’t fall for it.

One final note on the last feature that many of these services have: race time estimates for different distances. Garmin, Coros, and Strava will all let you know what they think you could run for a 5K, 10K, half marathon, and marathon based on your recent runs. While this is a fun feature to look at, treat it like the VO2 max scores: they’re only even remotely reliable relative to themselves, and over longer periods of time. It’s important to trust your own body and your training. Don’t let Strava gas you up and convince you to go out harder than you’re ready for, but also don’t let it convince you that your goal time isn’t feasible if you’ve put in the work. Case in point: right now, with the exact same inputs, my 5K estimates for Garmin and Strava are almost a minute apart, and my marathon estimates are a whopping 21 minutes different.

As with all things technological, use these features like the tools they are and don’t let them rule you.