Why Your Garmin VO2max Number Drops in the Summer (and How to Fix It)

We may earn a commission from links on this page. I always feel tougher after running in the summer heat—until I look at my Garmin Forerunner and see it thinks my fitness has suddenly gotten worse. If you, too, watch your Garmin VO2max number slide down as the temperatures climb, you're not imagining it, and you're definitely not alone. Every summer, comment sections and Reddit threads fill up with people convinced their fitness is falling apart, or their expensive watch is broken, or generally panicking about a stat that's supposed to reflect their aerobic capacity. But there’s no need to panic. Your fitness almost certainly isn't going anywhere. What's actually happening is that your body is adapting to heat, and Garmin's algorithm is struggling to separate "hot weather effort" from "declining fitness." Why it looks like your VO2max drops during the summer VO2max estimates are built from your pace, heart rate, and effort during a run. In hot weather, your heart rate rises faster and stays higher for the same pace, because your body is diverting blood flow to your skin to help you cool down, and you're likely sweating more and dealing with some degree of dehydration. To Garmin's algorithm, a higher heart rate at a given pace looks like reduced efficiency, which it interprets as lower fitness. In reality, you're doing the same physical work; your cardiovascular system is just working harder to keep you cool at the same time. This is why every year, so many runners…