Imagine this scenario: You’re out on an easy recovery run in the summer, and you’re wearing a heart-rate monitor. You’re on an out-and-back course and you’re planning to run for 40 minutes. It’s pretty hot and humid, just a bit more intense than the weather has been in previous days. You turn around at the 20-minute mark, check your heart rate and it’s at 150 beats per minute (bpm).
That’s a bit high for an easy recovery run, but you know you’re running easy, so you assume the heart-rate monitor is just off a bit. You run just as easy on the way back, and right before you end the run you look down and see that you’re now at 162 bpm. Something must be wrong because you know you didn’t run any faster on the return trip, yet your heart-rate monitor shows a 12 bpm increase in the last 20 minutes of the run.
Is your heart-rate monitor broken? Probably not.
What you experienced on that run is a phenomenon called cardiac drift. When you engage in aerobic exercise, your body has to get oxygen-rich blood to working muscles so that the mitochondria in your muscles’ cells can produce energy to contract the working muscles.
Blood is made of cells and plasma, and the plasma is over 90 percent water. When you exercise and start to sweat, you loose some of that water from the blood plasma traveling through your body in order to get oxygen to the working muscles.
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.
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.
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.
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.