Thursday, July 15, 2010

Cardiac Creep

Yesterday I headed out for a recovery 4 miler.  I had my heart rate monitor in tow because I'm a big proponent of using heart rate training zones, especially during recovery runs (otherwise, I inevitably wind up running too hard).

When heart rate training I normally use the Karvonen formula (%intensity/100*(Max HR - Resting HR) + Resting HR) so I can base my training levels off of my heart rate reserve .  

Since this was a recovery run I was targeting an intensity of <70%... plugging my personal heart rate numbers into the formula I get  .7*(193-43)+43 = 148... so the goal for this run was to keep my heart rate under 148.  

So I'm going along for about 5 minutes at a perceived pace that seemed excruciatingly slow but when I look down at my heart rate monitor it says that my heart rate is already over 150... so I slow down a tad and focus on really taking it easy but then I look down at my heart rate monitor again and my heart rate is approaching 160!

At this point, I'm going about as slow as I'm willing/able to go.  Not only that but I feel great!  So I keep a level effort and finish out my run... but by the end my heart rate is approaching 170!!!

Here's the caveat to this story: it was 92 *F out with heat index of 101 *F, and subsequently I was experiencing an acute case of cardiac creep...

Cardiac creep is a phenomenon that happens during hot weather conditions when your heart works extra hard to increase blood flow to your skin so that you can sweat and cool off.  Essentially, it's a biological radiator.  Additionally, as your body expels water in the form of sweat, your blood volume reduces and your heart needs to work even harder to get the required amount of oxygen to your exercising muscles... or to put in in layman's terms:  a little man aka "the Cardiac Creep" with a cartoon sized sledge hammer was bludgeoning away at my heart!!!

So what does this all mean???  Basically it means that heart rate training zones are meaningless in extreme weather conditions and that you're better off sticking to perceived effort... It also means that I wasn't over exerting myself during my recovery run (which my final pace seemed to corroborate)... on the other hand I'm skeptical that doing anything outside during this heat wave can really be considered recovery?!? 

Final lesson learned:  I'm running today's tempo run inside on the treadmill!


  1. Very interesting read. Been noticing higher heart rates from my monitor than my winter training, without significant differences in effort, and assumed it was a function of the heat/humidity, but didn't know (and was too lazy to look up) the physical factors contributing. Thanks for the info.

  2. It's crazy, right? I don't wear a monitor, but when I felt like I was going to die at one point yesterday, I checked my pulse and in 10 secs I had 31 beats. Walked for 30 seconds and it came back down to a reasonable rate. Our bodies are intriguing machines.

  3. I'm a big believer in perceived effort b/c of that. We can become too big of a slave to the HRM sometimes!

  4. Just going outside laetley is a chore let alone running in it. Glad to see you're aware of the cardiac creep

  5. I run in the 5-6pm heat of Florida. It is usually between 88-92 degrees with heat index over 100. When comparing my runs on the same route in cooler weather, and the same pace, I average about 5-7% higher heart rate on the hot runs.
    Can't wait to see how much faster I will be able to go in the winter after running all summer!

  6. OK - if you're going to keep putting math in your posts I am going to have to find another Champion!

  7. Hey cool, at least you don't need to bring along your HRM or sledge hammer on hot days.
    Travel light!

  8. FORTY THREE resting HR? Wow man. I'm at something like 70. (although, my max is around 220... *shrugs*) Do you know what that 'means'? I honestly don't

  9. My treadmill and I are bff's in the summer. And the winter. And the spring b/c of my severe allergies. Sometimes in the fall if I have separation anxiety.

  10. If you think that is extreme, you should see what happens when it's humid! Haha! HR Zone training is not very productive during hot summer weather. Much more helpful during winter.