Login | August 02, 2025
Heat and humidity impacting HR
PETE GLADDEN
Pete’s World
Published: July 28, 2025
When we talk about the training variables that can raise our heart rate (HR) during workouts, we typically talk in terms of speed, resistance (work load), volume and even verticality. Increase any one or a combination of these variables and boom, your HR will more than likely move upwards.
But lately, what with the hot, steamy weather we’ve been experiencing here during June and July there’s two additional variables we have to consider, variables which by themselves can cause significant increases in our HRs.
The variables I’m talking about of course are heat and humidity.
So as of late I’ve had to readjust my cardio workouts to compensate for these heat/humidity variables which can severely impact my HR response to exercise…in addition to dulling my performance level (speed and power output).
The end result is that I go slower than normal at my prescribed training zones - at an increased perceived exertion level.
And that physiologic response pretty much agrees with most of the research on this phenomenon.
Take for instance a research piece published in the Oct. 15, 2021 issue of Medicine Science & Sports Exercise, where the researchers determined that, “Conditions of 7.5°C–15°C WBGT [45.5 degrees Fahrenheit to 59 degrees Fahrenheit Wet Bulb Globe Temperature (or 10°C–17.5°C air temperature) increased the likelihood for peak performance. For every degree WBGT outside these optimum conditions, performance declined by 0.3%–0.4%.”
Now for you mathematically challenged folks, that means when the outside temp hovers in the 88-90-degree Fahrenheit range you could experience a 7-plus percent decrement in your performance level.
That’s pretty big.
Therefore our running, cycling, walking, jogging speeds inevitably decrease in hotter/more humid conditions, this despite the fact that we could be perfectly dialed into a particular target HR zone. Not only that, but our perceived efforts in said zones will increase despite the fact that we’re going slower than normal.
So let’s take a bit of a deeper dive here and explore what’s going on from a physiological standpoint.
Whenever we exercise heat is generated, which in turn increases our body temperature.
As our body temp increases the heart has to work harder in order to increase circulation such that the body can cool down.
Now when you add high heat and humidity, well, the body has to work that much harder to maintain that cooling down process.
And it does so by increasing the sweat rate and increasing blood flow to the skin.
When this happens the HR will inevitably increase to expedite the cooling down process.
Now the good news here is that your heart is communicating to you - via increased HR - such that you can make the needed adjustments to your controllable workout variables - speed, load, volume, verticality, etc. - in order to remain in the safe zone and avoid heat related maladies.
And as I’ve learned from decades of training and racing in the heat, I’d bloody well better take heed of both my HR and my perceived exertion level instead of letting my frustrations about slower speeds and harder efforts persuade me to stoically push through it with a “no-pain-no-gain” attitude.
Having said this, here are a couple of precautionary steps to take when you’re doing your training - run, jog, bike, hike - amidst heat and humidity.
1. Reduce your average speed. This rather than battling through your frustrations concerning slower pace and higher than normal HRs. Remember, the whole concept of HR training means you’re actually listening to your heart!
2. Increase your electrolyte and fluid consumption during bouts of heat and humidity. Dehydration and electrolyte imbalances can cause further increases in HR. Research has determined that just a 1.5% loss in bodyweight due to dehydration can increase one’s HR by seven beats per minute. That’s darned near a whole HR zone.
3. Reduce your volume. When heat and humidity become added stressors, you can also compensate by backing off the time/distance just as you do with speed, load and verticality.
4. And finally, if your HR gets unusually high while working out amidst heat and humidity, check with your healthcare provider to ensure you don’t have any preexisting heart conditions that could be exacerbated by these two stressors.
So enjoy this sultry summer workout weather - but always make sure to listen to what that heart of yours is trying to tell you.