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Extreme heat, and its effect on heart rate.

Tuesday, July 10, 2012 by  
Filed under Daily Blog

July
10
2012

I’ll kick this off by saying today’s blog is very non-scientific, and is almost purely based on my own research, anecdotal evidence and (non-controlled) experiences.

I’ve been thinking a lot about the 202 BPM peak heart rate that I hit last week, and why I had such a large increase over any of my other previously recorded maximum heart rates. I think the answer may have a lot to do with the very hot and humid outside temperatures I’ve been riding in lately.

Last night I was reading an excerpt from the book Heart Rate Training by Roy Benson and Declan Connolly. This paragraph caught my attention:

“An extremely important factor affecting exercise heart rate is temperature. Warmer temperatures cause the heart to beat faster and place considerable strain on the body. Simply put, when it is hot, the body must move more blood to the skin to cool it while also maintaining blood flow to the muscles. The only way to do both of these things is to increase overall blood flow, which means that the heart must beat faster. Depending on how fit you are and how hot it is, this might mean a heart rate that is 20 to 40 bpm higher than normal.

The so-called “feels like” or “real feel” temperatures have been extremely high around here lately. The heat and humidity combined with direct sunlight (all of which were affecting me when my heart rate hit 202 BPM) have produced “real feel” temperatures around here as high as 117­° (F). I don’t know what the “feels like” temperature was when I was riding that day, but according to my Garmin Edge 500 (which records temperature data once per second data along with GPS, heart rate, speed, etc.) the outside temperature was just shy of 94° when I hit 202 BPM. I can pretty much guarantee the “feels like” temperature was well over 100°.

So let’s look at yesterday’s ride compared to a similar ride in cooler weather.

Yesterday I rode at Mt. Dora, and was focused on practicing my cornering technique. That doesn’t mean I was loafing, but I was not trying to break any speed records, either.

Let’s focus on the 1.5 mile loop segment. Yesterday I rode my 3rd best time of 9 minutes flat. My average speed was 9.9 miles per hour and my average heart rate was 190 BPM (click to zoom):

My time yesterday was 09:00 with an average heart rate of 190 BPM. Temperature was about 90 degrees, with a much higher "feels like".  It was extremely humid, I was very hot and sweating profusely.

My time yesterday was 09:00 with an average heart rate of 190 BPM. Temperature was about 90 degrees, with a much higher “feels like”. It was extremely humid, I was very hot and sweating profusely.

 

Now let’s look at my personal record on that same loop, which is also the second fastest time ever ridden on that segment. Michael, the current KOM, turned in an absolutely sick time a few days before I set my PR. That’s going to be really, really tough to beat. I digress…

My personal record is 8m22s. My average speed was 10.7 miles per hour and my average heart rate was 187 BPM (click to zoom):

My personal record time is 08:22 with an average heart rate of  187 BPM. Temperature was about 90 degrees, but it was not humid and the "real feel" temperature was likely also about 90°.

My personal record time is 08:22 with an average heart rate of 187 BPM. Temperature was about 90 degrees, but it was not humid and the “real feel” temperature was likely also about 90°.

 

It’s interesting that yesterday I was not putting in a maximal effort, yet my average heart rate was 3 BPM higher than it was when I completed that same segment 38 seconds faster!

One could argue that perhaps I was simply in better cardiovascular shape a couple of months ago, but I don’t think that’s the case. My cardiovascular stats (which are updated every Sunday) have actually improved since May 24th. Heck, I set three new personal records last week alone!

Anyway, as I said in the opening paragraph this was far from a scientific look at the issue. I just found the subject interesting, and thought the heat could be a possible explanation for the abnormally high peak heart rate the other day.

I’ll also add that I find the extreme heat pretty fatiguing. Even riding with less than a maximal effort yesterday, I was pretty burnt at the end of a fairly short ride. I don’t think any more records are going to be falling until things cool down a bit.

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