If you’ve gotten used to monitoring your Cardio Fitness level on your Fitbit, Garmin, Apple Health, or another activity tracker or app, you may have noticed that it has been going down lately – or maybe did not increase that much you would have expected. But if you’ve kept your workout, the dip may not mean a drop in fitness at all.
I speak specifically to those of you who live in climates that are currently summer and the heat is somewhere between intense and oppressive. In other words, much of the United States right now. Naturally, We all run and cycle more slowly when it’s hot outside, and even slower when heat and humidity come together to make being outdoors especially punishing.
And since fitness trackers use your workouts to calculate your cardio fitness score, slowing down in the summer can affect results.
This is how the cardio fitness score of your app is calculated
To understand what’s going on, it helps to know how these cardio fitness scores are calculated. While some devices, including Fitbits, use your resting heart rate to give a rough estimate when you can’t do GPS-enabled exercise, most devices use outdoor distance runs to find out how fit you are. If you are able to run faster than before with the same heart rate or the same speed as before with a lower heart rate, the gadget will conclude that you have become fitter. (Depending on the device, other training units also count towards the cardio fitness scores; Garmin z. can use cycling, assuming you have the right setup.)
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This makes a lot of sense, and as a result, you can have one decent estimate of your VO2 maxthat would otherwise require a laboratory test. But the algorithm doesn’t take into account all the factors that can speed up or slow you down. Some only measure cardio fitness on level ground, which is a good place to start, but as far as I can tell, no one takes the weather into account.
We run slower in the heat and our heart rate increases up to 20 beats per minute faster while our circulatory system reconfigures itself to keep us cool (for example, more capillaries open near the skin, which as a side effect leads to that tomato-faced look). As a result, your gadget’s fitness calculations may be discarded.
I started looking at my fitness score in the spring and enjoyed seeing it slowly climb higher as I did more cardio. But on the first really hot morning of summer, while chugging up a hill, the thought occurred to me: This will boost my score, won’t it? I’ve been watching the numbers, and yes they’ve gone down a bit since the temperatures started to rise. According to my Apple Watch, my score was 33.5 in April, 34.6 in May, and 34.9 in June, but it’s now back down to 34.5. The good news is that once the weather cools again in the fall, my stats will likely skyrocket again.
But more importantly, this is a reminder that the numbers coming from your fitness tracker are simply the numbers that the tracker can easily measure and calculate. They are not always the most useful yardstick for understanding your fitness. Instead, pay attention to how much time you spend training, how constant you are in your workouts and how you perform on the metrics that are important to you, such as your times in target races. And most importantly, look for year-on-year trends instead of getting excited about things that change from week to week or day to day. Work hard consistently over the years it will always pay off.
