Fitness trackers and summer weather

A double edged sword

Last week, in one day, my VO2max dropped from 60 to 57. Or so my Garmin told me.

Those of us in Wisconsin have had a very strange spring. It was cool to the point that it felt like we weren’t getting spring. Then, suddenly, things changed very rapidly until, on Tuesday, I found myself running in 85° weather with extremely high humidity, direct midday sun and pretty much no wind.

After one run in that weather, my Garmin said I lost a lot of fitness.

Of course, I didn’t actually lose that much fitness in one run. So what happened?

Fitness trackers use some very complex, proprietary formulas to figure out how fit you really are. But, very roughly speaking, they boil down to two things: your running pace and your heart rate. The faster you run with a low heart rate, the more fit you are. The higher your heart rate at slower paces, the less fit you are.

So what happens when it gets hot and why did my Garmin think I suddenly lost a significant amount of fitness?

Because heat, especially when we haven’t had a chance to acclimate to it, puts a lot of stress on our bodies.

Heat can kill. That’s no secret. Well, it’s not a secret to your body either. When it gets hot, it does several things to try to reduce its temperature. Blood is taken away from the muscles and moved toward the skin, where it can radiate body heat out into the air surrounding you. You sweat, sometimes profusely, again as a method of moving heat from your body to the air around you. These are two well known and significant things your body does but they aren’t all.

As your blood is moved toward your skin, your heart has to continue working hard to pump that blood to the skin but less is available for your working muscles. If you sweat to the point of dehydration, your blood volume is reduced and your heart has to work harder again to pump what is available, again prioritizing the skin to cool over the muscles to keep running.

This means your heart keeps pumping hard, maybe even harder than it otherwise would, but your muscles have less fuel to power themselves. As a result, you run slower, even though your heart is working harder.

Then the fitness tracker steps in. It says your heart is beating faster but you’re running slower. It decides that this means you’re losing fitness, even though that’s not true. There’s a reason your heart is working harder even while you’re running slower.

So don’t panic if your fitness tracker says your fitness is going down as the temperature and humidity go up. You should expect this to happen. As long as there is no reason to suspect you’re actually losing fitness, you probably aren’t. You’re just resetting reasonable expectations in the more challenging weather.

Then remember: just like things get harder and your fitness tracker says you’re losing fitness as it gets hotter, the opposite happens in the fall when we get a break from the heat. As long as you properly adjust for the heat in the summer so you don’t push yourself too hard, you’ll suddenly be running faster at easier efforts in the fall. And your fitness tracker will recognize that as gained fitness.

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