[Highpointer46]

I’m writing this to share an experience conducting HR Drift tests and something that caused a dramatic change in my result. Five months ago, I conducted a series of HR Drift tests which established my AeT at 120bpm -> severe ADS. I’ve been doing between 9-10 hours per week of training just below this threshold for 5 months. During periodic re-testing, I saw some improvement with the distance I could cover at the 120bpm threshold (Ex. My first “pass” in Nov was at 1.4mph @ 120 bpm while I “passed” in March at 1.8mph @ 120bpm). But, I could not pass a HR drift test with a starting HR above 120bpm which was very frustrating to say the least.

Recently, I read a running article about HR drift which pointed out that one root cause of HR drift was an increase in body core temperature while exercising. This got me thinking about several things:
(1) My purpose for the HR drift test was to measure a shift in metabolic process (fat burning to carb burning) using HR as an indicator
(2) If rising core temperature also causes HR to rise this might skew my results due to an unrelated and uncontrolled variable
(3) I personally run ridiculously hot while exercising – I end up hiking in winter in a short sleeve T-shirt once I get warmed up

So, I re-tested and made three changes. First, I ate a light meal an hour before the test (previously I had been testing fasted just because that’s how I prefer to workout in the morning). Second, I pre-hydrated with ~16oz of ice water 15-20 min before the test (previously ~8oz of room temp water). During the test, I consumed an additional 32oz of ice water in small sips every ~2 min and ran a small fan on my face (previously drank ~8oz room temp water). My test results were dramatically different. I immediately jumped from 120bpm to passing a test at 140bpm (and it could be higher).

I had not read anywhere that core temperature could have such a significant effect on the results of a HR drift test. I see no reason to suspect that my new testing protocol is “cheating” in any way and that my AeT is probably (at least) 140bpm. Am I interpreting this correctly and is this a known phenomenon?

Jim

 

 

[Scott Johnston]

Jim:

Good for you to do this experiment on yourself. Definitely core temperature has an effect on heart rate. We see this all the time with athletes who are forced to train in hot weather conditions. I’ve talked about this a great deal in the past, but here it is in short form: as your core temperature goes up your body attempts to dump heat through the skin, the capillaries of the skin dilate so that more blood can be shunted to the surface area where you lose heat via: convection and radiation. This greater volume of blood going to the skin means that there is a lower volume of blood available for the muscle work.  As a consequence, your cardiac output needs to increase to sustain the same level of work. Cardiac output being the product of stroke times heart rate. At this low heart rate intensity, this happens with an increase in heart rate.

Nonetheless, the heart rate drift is a reflection of a shift of metabolism as well.  We see this all the time with folks with ADS using our training protocol and it can be verified in gas exchange test in a laboratory.  As to which mechanism, heat or metabolism is most responsible for the upward drift and heart rate, I don’t have a good answer, but I don’t think it’s quite as simple as the article you referred to. Otherwise, we wouldn’t see the metabolic shifts we do using these methods.

Now, let’s move on to your question: With the above explanation, you can see that it’s unclear which factor has the greatest influence. A faster pace with your higher heart rate will definitely give more training stimulus down at the metabolic level in the muscle because you will be engaging a greater volume of muscle fibers to do this work to run faster. just as I would for any runner forced to train in hot conditions I would recommend using the method you just did as much as possible for your training.  I think you’re going to be putting a lot more stress if you try to train at 140 when you’re hot

Scott.

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