Increased heart rate while running in the heat

A thunder storm rolled through last night. It cooled the air down for a few minutes. In fact, when I woke up this morning at 5AM the temperature was 79.6 degrees (yesterday at the same time the temperature was 83). However, the humidity went from, I’m guessing, 95% to 98% - near rain humidity.

Anyway, back to running in the heat. On Monday at lunch I ran 5 miles at just over 8 minutes per mile pace. I was running with a friend and we were trying to determine his training paces for a fall 5 mile race. The heat index was 115 and it felt like it. Then yesterday I ran the same pace on the treadmill in the gym. The treadmill was set at a grade of 1. I understand that this is not an exact comparison of apples to apples but the differences in the run were substantial.

We talked about blood volume increasing as you acclimate to the high temperatures. However, your body still needs to circulate this extra plasma. How does it do this? If you think about it there are only two ways to pump more fluid. You can increase the amount of fluid pumped per beat or you can increase the number of times your heart beats per minute.

We already know that our heart rates increase as we exercise harder. This is to get more oxygen to your muscles. But now we also have to cool our bodies. Over time you can increase your stroke volume but this physiological adaptation takes years. So, just like when we are exercising harder our heart rate increases but now it has to also maintain our core temperatures. So, obviously, if we now have to cool ourselves more and we want to run at the same level of exertion our heart rate will be even higher.

On Monday when I ran outside my heart rate at the end of the 3rd mile was right around 165 beats per minute (I did not stop at 3 miles and my heart rate continued to rise for the next 2 miles). Let’s contrast this to the 72 degree low humidity treadmill run that took place yesterday. Well, first off the run felt much easier. Yes I was sweating but I was not saturated. Second, my heart rate actually went down after the first mile. At the end of the 3 mile run my heart rate was 140 beats per minute.

My heart rate increased by approximately 25 beats per minute for similar paces on back to back days. I would say that is substantial! For me that is going from a recovery run to high zone 3 / low zone 4. It is almost like riding up hill on the bike.

When the temperature drops we should be able to really haul when we get our true gearing back!