citius5000 wrote:
I think you answered your own question. Heat and humidity doesn’t affect everyone the same. Poor mans altitude! Maybe his routes are hillier? Summer running should be a lot by feel especially with the heat and humidity. Make sure his paces are based on current ability. Make sure he is hydrating properly as well. He is running more and for some it takes time to adjust.
Exactly. People have different heat and cold responses. This is obvious when you go outside and see someone in a tank and shorts and someone else in pants and a heavy jacket. This was also obvious when all the top runners dropped out of boston when it was too cold.