[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.