Buzz
If you regularly check your “cardio fitness” score on apps like Fitbit, Garmin, or Apple Health, you might have noticed a recent drop—or perhaps the score hasn’t improved as much as you’d hoped. However, if you’ve been consistent with your workouts, this decline might not reflect an actual decrease in fitness.
This is especially relevant for those living in areas experiencing summer heat, where temperatures range from intense to unbearable—like much of the United States right now. As we know, running and cycling speeds decrease in the heat, and heat combined with humidity makes outdoor activities even more challenging.
Since fitness trackers rely on workout data to calculate your cardio fitness score, slowing down during summer can impact the results.
How Your App Calculates Your Cardio Fitness Score
To grasp what’s happening, it’s helpful to understand how cardio fitness scores are determined. While devices like Fitbits estimate scores using resting heart rate when GPS-enabled workouts aren’t available, most trackers rely on outdoor runs to assess fitness. If you run faster at the same heart rate or maintain speed with a lower heart rate, the device assumes you’ve improved. (Some devices, like Garmin, also factor in cycling, provided you have the necessary equipment.)
This approach is logical and provides a reasonable estimate of your VO2max, which typically requires lab testing. However, the algorithm doesn’t account for all variables affecting speed. While some devices measure fitness only on flat terrain, none seem to consider weather conditions.
Heat slows us down, and our heart rate can increase by up to 20 beats per minute as our body works to cool itself (e.g., by expanding capillaries near the skin, causing flushed faces). This can skew your tracker’s fitness calculations.
I began monitoring my fitness score in spring and watched it rise steadily with increased cardio. But on the first scorching summer morning, I realized, This will ruin my score, won’t it? Sure enough, as temperatures soared, my numbers dipped. My Apple Watch showed averages of 3 in April, 34.6 in May, and 34.9 in June, but it dropped to 34.5 as the heat set in. The upside? Once fall brings cooler weather, my scores are likely to rebound.
More importantly, this serves as a reminder that the numbers from your fitness tracker are merely the data it can easily measure and compute. They aren’t always the most meaningful way to gauge your fitness. Instead, focus on factors like your training duration, workout consistency, and performance in areas that truly matter to you, such as race times. Most importantly, prioritize long-term trends over short-term fluctuations. Consistent effort over time will always yield results.
