How Does the Heat & Humidity Affect You?


Posted by Coach John

A couple days ago I went for a short, easy run. Although it was 9:30am and not in the middle of the day, the temperature had already climbed to 90 degrees and with the added humidity made it feel at least 101 degrees. The training plan called for 45 minutes of easy effort running. I knew I had to stay within the plan and keep my pace controlled. Aware of the weather, I decided to weigh myself prior to starting:

177 lbs.

I started off at a comfortable pace and, although it was hot, I was tolerating the conditions. Over the span of about 40 minutes, I went from feeling great to feeling completely over-extended. The smart response was to start walking. I checked my Heart Rate (HR) which showed 171 bpm. Certainly this HR did not match my exertion level. I did a mix of walking and light jogging for about 10 minutes. When I arrived home I rechecked my HR which now was down to 140bpm. This was still high for an extremely low exertion level. I also re-weighed myself:

173 lbs.

It’s helpful for us to be able to train in hot, humid conditions as our races may be in those same conditions. However we also need to respect the effects of the heat and humidity.  The “behind-the-scenes” physiological effects that happened during my easy run were an increased body temperature and profound volume depletion. I hadn’t checked my temperature before and after the run, but others have.  Some studies out there have done this and found runner’s temperatures climbing close to 104 degrees. As far as fluid loss, the change in body weight tells the story.  So between an elevated temp and a volume depleted vascular system, my heart was working overtime to both keep me cool and provide the blood flow demands to my legs as I ran.

Let’s all be smart in these hotter conditions and adjust our training. Here are just a few suggestions:

  • Stay hydrated
  • Run a bit slower
  • Run a bit shorter
  • Run in the shade
  • Walk if you have to
  • Wear cooler colors
  • Consider a fuel belt or hand-held hydration system
  • Start your run earlier in the day

Hope you have a great training day and see you at the races!



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