Heat Acclimation and blood volume

Summer heat brings fall PR’s - I have been saying that for years. But why does running in the heat make you so much faster when the weather turns cooler?

Here in South Mississippi the temperatures have been in the mid to upper 90’s. On top of that we always have humidity. The weather channel said that yesterday’s heat index was 115 degrees. We ran at noon during the hottest point of the day.

There were several of us and we each had specific workouts. I ended up running 5 miles at a seemingly comfortable pace – comfortable for temperatures about 10 degrees cooler. However, in these conditions my heart rate was elevated and my perceived exertion was higher. The run was harder than it should have been. But why?

There are several methods that your body uses to combat the heat. The goal is to keep your core temperature in a range that allows for your body to operate most efficiently. One way to cool off, your body pumps blood close to the skin to dissipate heat. To cool more it pumps more blood. The only way to pump more blood is by increasing the number of heart beats or increase the volume of your blood. And this is exactly what your body does.

More blood is circulated at the surface of your skin to cool you off. However, this oxygen rich blood is no longer available to your muscle tissue, so your heart beats more to get needed blood to your muscles. It is harder to run at high temperatures. In addition, to further cool the body, at the same exertion level your heart rate is several beats higher to get even more blood to your skin.

The most dramatic adaptation derived from heat acclimation is an increase in blood volume. Your body produces more plasma that it can use to cool you in hot temperatures. I have read that your blood volume can increase by as much as 2 liters. That is an enormous increase in circulating blood volume.

Now regarding fall race PR’s. Although your body will reabsorb the increased blood volume when the temperatures drop you do have a window where your races can benefit. You have two to three weeks where your blood volume remains high after the weather becomes milder. What this means for your race times is that you now have an abundance of blood volume with which to fuel your muscles valuable oxygen. You have effectively blood doped naturally.

The secret is scheduling your races around the first cool snaps of the season.

NOTE: Running in high heat and humidity is dangerous. The chance of heat related illness and heat stroke are very real concerns. Don’t try to be a hero and ‘tough out’ these workout sessions. If you are not able to maintain your workout goals it is time to stop. You need to head indoors to cooler temperatures. Suffering through a hot and humid workout is not smart training. Nothing good can come from it!