The physiology of summer running: why heat is the "poor man's altitude."

If you have ever stepped outside in July for a routine easy run and found yourself gasping for air at a pace that is usually a jog, you are not alone. And you are not losing your fitness.

As a coach to NCAA All-Americans at the University of Oklahoma, US Olympic Marathon Trials qualifiers, and World Team athletes, I spend every spring having the exact same conversation with my runners: you cannot out-tough the heat. You have to out-smart it.

To train effectively in the summer, you have to stop looking at your GPS watch and start understanding the biological negotiation happening inside your body. Here is the science of why summer running feels so hard, and how we use it to actually get faster.

The physics of sweat: why relative humidity is a trap

The biggest mistake runners make is looking at the "relative humidity" percentage on their weather app. Relative humidity is a ratio of how much moisture the air holds compared to its temperature. It does not tell you how your body will react.

For that, look at the dew point.

Dew point is the absolute measurement of moisture in the air. The human body cools itself through the evaporation of sweat, not the sweating itself. When the dew point climbs above 65°F (18°C), the air is so saturated with moisture that your sweat has nowhere to go. It drips off you instead of evaporating.

When sweat does not evaporate, your body cannot dump the heat it is generating. That sets off a physiological chain reaction.

Thermoregulation and the 102°F threshold

When you run, your working muscles generate a massive amount of heat. In perfect 50-degree weather, that heat radiates off your body. In summer, it gets trapped.

As your core temperature climbs toward 102°F (39°C), your central nervous system enters a protective state. To prevent your organs from overheating, your brain commands your cardiovascular system to divert oxygen-rich blood away from your working leg muscles and push it out to the capillaries in your skin to try to cool you down.

The result: cardiovascular drift

Because you now have less blood delivering oxygen to your legs, your heart has to beat significantly faster to maintain the same pace. This is called cardiovascular drift. A 7:30/mile pace that normally requires 140 BPM might suddenly require 165 BPM.

If you try to force your normal paces during this physiological shift, you push your body past its lactate threshold. What was supposed to be an aerobic base run accidentally becomes a grueling anaerobic threshold workout. You aren't building fitness. You are digging a hole of overtraining.

To prevent this, we use the Combined Score Method (air temperature plus dew point) to gauge true environmental stress.

Combined Stress Score (air temp + dew point)

How to calculate your Combined Stress Score

To find your true environmental stress, add your local air temperature to your local dew point.

Real-world example. A humid morning in North Carolina:

• Air temperature: 80°F
• Dew point: 70°F
• Combined score: 150

Once you have your score, use this scale to determine the percentage you need to slow down:

Pace adjustments by percentage slowdown

The elite secret: plasma volume expansion

So if summer running is so destructive, why do elite athletes routinely train in the heat? Because when managed correctly, heat acts like a "poor man's altitude."

When you subject your body to consistent, controlled heat stress (by slowing your pace to keep your heart rate in check), it triggers a survival adaptation called plasma volume expansion.

Within a few days of heat exposure, your body literally creates more fluid volume so it has enough blood to send to your skin for cooling and to your muscles for running. When the weather finally breaks in October, you still have that expanded blood volume. Suddenly, your heart does not have to work nearly as hard to deliver oxygen, and you experience a measurable drop in race times.

The 14-day heat acclimation protocol

To get the benefits of plasma volume expansion without ruining your season, you have to acclimate strategically. Whether I am prepping an athlete for a World Championship or a local runner for a fall marathon PR, we use a strict protocol:

1. Days 1–5 (Shock phase). Run 100% by perceived effort or heart rate. Drop your ego. Keep runs under 60 minutes.
2. Days 6–10 (Adaptation phase). Your sweat rate will naturally increase, and your sweat will become less salty as your body learns to conserve electrolytes. You can slightly increase duration, but maintain the adjusted, slower paces.
3. Days 11–14 (Full acclimation). Plasma volume has expanded. You will still run slower than you do in the winter, but your heart rate will stabilize and the "heavy" feeling in your chest will subside.

Train with the science

The heat is only your enemy if you try to fight it. If you respect the dew point, slow down, and let your body adapt, the summer becomes the ultimate training tool for a fall personal best.

Written by

Coach Johnny Crain

2:12 marathoner. 4× US Olympic Marathon Trials qualifier. 4× NCAA national champion and 4× NCAA runner-up. Spoke at the RRCA National Convention. Founder and head coach at RunFitCoach, where he coaches every athlete personally.