How much do they really impact performance?
TLDR – a lot.
If you want more information than this basic answer, then read on.
Okay, maybe calling heat, humidity, and dew point the “Satan’s Trifecta” is a bit dramatic. But we’ve all been out there on rough days when it feels like Satan has opened a portal to hell, and clearly, we must be entering that heat-forsaken place. While most of us acknowledge that this trifecta can impact performance outcomes, I regularly read comments from my athletes regarding less-than-expected outcomes that don’t give credence to the impact that Satan’s trifecta can have on us.
So, this week, we are covering some of the specific impacts of heat, humidity, and dew point to help you better understand what your body is trying to deal with, and why you might not necessarily hit a personal best when race day takes place in Dante’s Inferno.
It’s important to note that our body’s responses to temperature, humidity, and heat are non-linear. This means that the changes that occur between 60 and 70 degrees will be less than those between 70 to 80, 80 to 90, and so on. As such, the impacts listed below are general averages. And, how we feel at 90 to 100 degrees, with high humidity, will be much worse than how we might feel in 70-80 degrees.
Let’s start with temperature.
Temperature
As temperatures rise, the body must work harder to cool itself, which can lead to decreased performance:
- Pace
Higher temperatures typically result in slower running paces. For every 5°F increase above 60°F (15.5°C), runners may see their pace slow by about 2-3%. In certain parts of the country, team members have been working through 90-100-degree days. That could amount to as much as a 12-18% decrease in pace. In practice, this could amount to over 1:00/mile slower at the same or possibly higher energy cost.
- Bike Power
Cyclists often experience a decrease in sustainable power output as temperatures climb. This reduction can be as much as 1-2% for every 5°F increase above optimal conditions. So, again, for a 90-degree day, this could mean a 6-12% drop in output for the same energy cost.
- RPE
Athletes generally report higher perceived exertion at the same workload as temperatures increase. Research finds that RPE assessments could increase by 1 to 2 points in higher temperatures.
- Heart Rate
If we maintain the same pace or power in hotter conditions vs. cooler ones, our HR could be at least one zone higher for that output. So, for example, if you normally run 10:00/mile for a zone 2 HR, trying to hold this same pace when the temps are hotter will result in a higher HR.
Humidity
High humidity can significantly impair the body’s ability to cool itself through sweat evaporation. While in cooler temperatures, humidity may not play as large of a role, when we add hot temperatures to humidity, we are going to see some additional slowdowns.
- Pace & Power
Running pace and bike power are impacted by humid conditions, even if temperatures are moderate. This effect is more pronounced when combined with high temperatures.
Because humidity impacts our sweat evaporation, this may also impact our hydration needs and further ramp up core temperature.
Overall averages show that run pace slows by about 10% when the humidity shifts from 30 to 70%, and a 19.3% increase in pace occurs when humidity shifts from 30 to 90%.
- RPE
Humidity can make workouts feel much harder than usual, leading to increased perceived exertion. The research finds that humidity can increase RPE more than straight-up temperature does, with high humidity alone resulting in about a 2-point increase on a 20-point RPE scale and about 3-4 points on this same scale when the high humidity is tied to higher temperatures.
- Heart Rate
Similar to high temperatures, high humidity often results in elevated heart rates at a given effort level. This can be exacerbated by miscalculating hydration and electrolyte needs.
In practice, humidity + temperature could result in 5-10 bpms higher for the same output.
Dew Point
Dew point is considered a more accurate indicator of how environmental conditions will feel to an athlete than relative humidity alone. The dew point is the temperature at which air becomes saturated with moisture, which results in things such as dew, fog, or frost. A higher dew point might feel “sticky” or “thick.” When the dew point rises, this is more likely to impact us than humidity alone.
- Pace & Power
As the dew point increases, the running pace typically slows. A dew point above 65°F (18°C) can start to feel uncomfortable, while above 70°F (21°C) can significantly impact performance. Some research has found that a dew point between 65-70 degrees can reduce pace by 3-4%. Dew points above 70 degrees could slow the pace by 4-8% or more.
Runner’s Connect provides a general chart that gives an overview of how dew point, along with humidity and temperature, can impact pace targets.
The research doesn’t have specific insights into how dew point impacts cycling power, but there is evidence to suggest dew point takes a bigger toll in higher intensity sessions/races or longer sessions/races.
Some sources suggest adding up the temperature and the dew point to get a sense of how power or pace might be impacted. See below for some examples of how this works.
|
Dew Point + Temperature = |
Percentage of pace adjustment |
|
141-150 |
3.0-4.5% |
|
151-160 |
4.5-6.0% |
|
161-170 |
6.0-8.0% |
- RPE
Athletes often report that efforts feel much harder as the dew point rises, even if the temperature remains constant. Shifts in RPE due to high dew point are similar to what we see for high humidity.
- Heart Rate
Higher dew points generally correlate with elevated heart rates at a given effort level. If a high dew point combines with high humidity and high temperatures, we need to be smart to pay attention to our heart rate to ensure we don’t overheat.
Relatedly, high temperatures + humidity and dew point have also been found to reduce peak oxygen consumption by as much as 12%. So, when you find yourself winded more easily, you aren’t imagining it!
Adjusting for Conditions
To account for the impact of this powerful trifecta, athletes should:
- Modify expectations and adjust pacing or power targets accordingly. Give yourself some grace on especially tough days. If you are racing, pace your race. Use RPE and heart rate to make smart and safe decisions.
- Focus on effort level rather than specific pace or power goals in challenging conditions.
- Increase fluid intake to compensate for greater sweat loss. Be sure to increase electrolytes to match the fluid intake. This is one of the reasons I recommend keeping the sources for carbohydrates/calories, hydration, and electrolytes separate. In doing so, you can dose for what you need on the day.
- In training, consider shorter intervals with more rest in high heat and humidity. If it is really bad, consider training indoors for quality sessions to ensure you can hit the efforts, and stay safe.
- Be prepared to reduce overall training volume or intensity on particularly challenging days.
Heat acclimation can help. So, as we train through hot conditions, our body gets better at working in those conditions. But, acclimation is not a cure-all. Even if your watch tells you that you are 100% heat acclimated, that does not mean you’ll be able to hit the same targets that you do on a cool and dry 50-degree day. We still need to adjust expectations.
In extreme conditions, our safety is #1. Be smart and be sure to understand the RPE and heart rate ranges that allow you to compete safely, without bringing on symptoms of heat exhaustion or heat stroke. These will not only end your race day, but they can also bring on severe health complications once the race or training session is over. Remember that individual responses to these conditions can vary, so it’s essential to pay attention to your body’s signals and adjust accordingly.
Train smart & safe!
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