When you're running on a hot day or worse, a hot and humid day, you sweat more than normal. Part of that water coming out of your body to cool you off so that your core temperature stays stable comes from plasma. So the blood in your body has made a subtle shift from a liquid that's similar in viscosity to water to a liquid that is now a bit more viscous, almost like a watered-down syrup.
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Your body's need for oxygen has not decreased, assuming you're running the same pace throughout your run, so the only way your working muscles get the same amount of oxygen is if the heart pumps faster, moving this more viscous fluid at a faster rate to make sure enough blood gets to the working muscles.
This brings us back to heart rate. When you start an easy recovery run, you might be at 120 bpm, and if you ran in the fall or the spring in a long-sleeved shirt and shorts on a day when the temperature isn't too warm or too cold, your heart rate will stay fairly steady over the course of a 40-minute run.
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But on a hot, humid day where you are constantly sweating and loosing water, your heart-rate monitor will show a slow rise in heart rate throughout the run via the mechanism above. This is called cardiac drift: the slow increase in heart rate over the course of a bout of endurance exercise. While cardiac drift happens on 20-mile runs in the winter as well since you're still sweating during that run, the phenomenon can be pronounced even on easy days in the heat of the summer months.
Now you might be thinking, "Great, but what does this have to do with my running?" Actually, not that much, other than the following two things.
First, it's smart to wear a heart-rate monitor if you are doing a hard workout, such as a threshold or progression run, or track workout, in the summer. This isn't so much to be informed about the workout paces, but the heart-rate monitor can serve as a leash. Your maximum heart rate should be roughly 220 subtracted from your age. For most workouts, you don't want to run at your maximum, yet if you're 40 and you are at 175 beats per minute but still have 20 minutes left in the workout, you might need to stop, or take a five-min recovery jog before you finish the rest of the workout. You don't want to be up near your max heart rate, but it's very easy to get close to it in the summer months.
Second, your hydration level can be monitored during the summer by paying attention to cardiac drift. Again, if you're on a 40-minute easy run and you see a big change in heart rate over the course of the run, such as a difference of 20 to 30 bpm, then you probably aren't hydrating enough. Water is important, but you should also consume a drink with electrolytes, such as coconut water.
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If you wear a heart-rate monitor and are running the same pace and effort on an easy run, and you see a dramatic rise in heart rate over the course of the run, now you know why.
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