Why Your Garmin Device’s VO2max Drops in Summer (and How to Fix It)
I always feel stronger after running in the summer heat—until I look at my Garmin Forerunner watch and see that it’s suddenly decided my fitness has declined. If you’ve seen your Garmin VO2 max decline as the temperature rises, you’re not imagining it, and you’re certainly not alone. Every summer, comment sections and Reddit threads fill with people convinced their fitness is declining, or that their expensive watch is broken, or generally panicking about a metric that’s supposed to reflect their aerobic fitness. But don’t panic. Your fitness is almost certainly not going anywhere. What’s really happening is that your body is adapting to the heat, and Garmin’s algorithm is struggling to separate “hot-weather exertion” from “decreased fitness.”
Why does your VO2max seem to drop in the summer?
VO2max calculations are based on your pace, heart rate, and effort during a run. In hot weather, your heart rate rises faster and remains high at the same pace because your body redirects blood flow to your skin to help cool you down, and you’re likely sweating more and experiencing some dehydration. Garmin’s algorithm interprets a higher heart rate at a given pace as a decrease in efficiency, which it interprets as a decrease in fitness. In reality, you’re doing the same physical work; your cardiovascular system is simply working harder to simultaneously cool you down.
This is why many runners experience a decline in VO2max each year in the summer, which then recovers with the onset of cooler weather in the fall.
Some Garmin watches take into account the thermal component.
Newer Garmin models, including the Forerunner 970 , attempt to account for heat (and humidity) when calculating VO2 max, adjusting the estimate to account for the additional cardiovascular strain. Of all the Forerunner models, the 745, 945, 955, and 965 have heat acclimation, while the 245 and 255 have been updated with software updates. And if Garmin’s numbering system is confusing, my colleague Beth Skwarecki can explain it all .
If you have one of these new devices, you may notice that the oscillations are less dramatic than with older models. Older Garmin models typically don’t make these adjustments, which is one of the main reasons why this phenomenon is so widely discussed among long-time Garmin users.
How to Correct Your VO2 Max Trend
If you understand what’s happening and are willing to ignore these numbers for a few months, that’s my recommendation. Your workouts are still working, regardless of what Garmin’s algorithm suggests. But if you want to stop the fluctuations completely, you have several practical options.
The simplest solution is to change your activity profile settings so that certain activities are not included in your VO2 max calculation. In the Garmin Connect app, you can go to your activity profile and completely disable VO2 max calculations for it. A popular approach is to leave the road running profile as the primary VO2 max calculation, as road running paces tend to be more consistent and comparable. However, for trail running, disabling VO2 max calculations is preferable, as elevation changes, terrain, and varying effort levels make VO2 max calculations less reliable.
You can take it a step further by creating a dedicated activity profile for “hot runs.” Simply copy an existing profile, rename it, for example, to “Hot Run,” and disable VO2max calculations in this copy. Then, on very hot or humid days, switch to this profile before heading out for a run. Your run will still be recorded as usual, with pace, heart rate, and all your usual data, but now it won’t distort your overall VO2max trend.
Finally, if you truly want your VO2 max to remain accurate and stable year-round, you may need to run on a treadmill in an air-conditioned gym. Controlled temperature and humidity mean your heart rate response will better reflect your actual fitness rather than heat stress, giving you a more consistent number to track over time. However, by aiming for the ideal number, you’ll sacrifice all the mental and physical benefits of training in the heat . Don’t let your VO2 max number fool you.
Result
A drop in VO2 max in summer is one of the most common false alarms in the running world. It’s always disappointing to see a drop in fitness metrics, especially for Garmin fans like me. But in summer, the body expends energy adapting to the heat, which is a form of fitness in itself. Whether you choose to ignore this drop, adjust your activity settings, or move your core workouts indoors, the most important thing is not to let a single algorithm-generated metric convince you that months of dedicated training have suddenly been wasted.