A follow-up to How to Master Running in Heat & Humidity

If you’ve read my previous article on running in heat and humidity, you know the basics: slow down, stay hydrated, respect the conditions. That’s table stakes. But what if I told you that the heat — the very thing you’ve been trying to survive — could actually be one of the most powerful performance tools in your arsenal?

This article is about the next step: deliberately using heat as a training stimulus.

Not tolerating it. Not just managing it. Harnessing it. Because done right, heat training is one of the most accessible performance tools in endurance sports, and you don’t need any special equipment to get started. You just need to understand what’s happening in your body, and what signals to pay attention to.

Why Heat Training Works

When you repeatedly expose your body to heat stress during exercise, it adapts. And those adaptations are significant.

Here’s what the research shows happens after a proper heat training block:

• Increased plasma volume and hemoglobin mass. More blood plasma means better oxygen delivery to working muscles and more efficient cooling. Studies have shown hemoglobin mass can increase around 2.5%, with both VO2max and power at lactate threshold improving by up to 8%.
• Earlier onset of sweating and higher sweat rate. Your body starts cooling itself sooner and more aggressively, which means better thermal control when it matters most.
• Lower heart rate and perceived effort at a given intensity. After adaptation, the same workout simply feels easier. Less thermal stress means more resources available for forward motion.
• Better fluid balance. Adapted athletes lose less sodium through sweat, which improves hydration and endurance.
• Improved performance in hot conditions. You stop surviving hot races and start executing them.

The part that surprises most athletes: these adaptations improve performance in cool conditions too. The expanded plasma volume increases total blood volume, which benefits you regardless of the weather. Think of heat training the way you’d think of altitude training — but far more accessible. Real altitude requires flying somewhere and spending weeks above 6,000 feet. Heat training can happen in your garage, your local sauna, or outside on a July lunch run.

The Basic Protocol

You don’t need much to get started. Athletes have been doing this for decades with nothing more than willpower and warm clothing.

The research-backed target is 10–14 heat sessions within a 2–12-week timespan, each one targeting around 45–60 minutes of sustained effort under heat stress. The quickest adaptations come from training daily or every other day. Two to three sessions per week works too, just more gradually. Most athletes start feeling a difference after 4–5 sessions.

Important: it often takes 20–40 minutes of exercising to generate significant heat stress in your body. You don’t just step outside in July and immediately start adapting. Plan your sessions accordingly — budget 60-90 minutes total when you account for warmup and the ramp to real heat stress.

Once you’re adapted, 1–3 sessions per week keeps you there. And if you stop entirely, adaptations fade at roughly 2.5% per day, disappearing within 5–6 weeks. That’s not a reason to panic — it just means you can time a heat block intentionally before a target race.

To generate heat stress, you’ve got options:

Train outdoors in the heat of the day. Getting out between 10am–2pm in summer, especially once temps hit 86°F (30°C) or above, is the most natural approach. High humidity helps at lower temperatures too.

Layer up. In cooler conditions, overdressing creates a warm, humid microclimate around your body. The goal is to trap heat and prevent sweat evaporation — think waterproof layer on top of a base layer, not just a sweatshirt. A nylon base followed by a rain jacket is a simple, effective combination. It feels absurd. It works. You can also grab a paint suit and sweat it up that way.

Close the garage and kill the fan. An indoor trainer session with no airflow generates heat stress quickly. Turn off every fan, close every door.

Post-workout sauna or hot bath. Do your training session, then go straight into 20–40 minutes in a sauna (aim for at least 80°C / 176°F) or a hot water bath (around 40°C / 104°F). Since your core temperature is already elevated from training, the combined stimulus is highly effective. Build tolerance gradually.

Personally? I love a good lunch run in the Ohio heat and humidity to do my heat training, but I also layer up on my indoor trainer as well.

How Do You Know It’s Working?

Great! Now you know what to do, but now the real question is how do you know if it’s working or not?

I think one of the best ways to go about answering this is by using a three-prong approach with information most of us already have available:

1. Subjective feedback. How hot do you actually feel and how does that match up with the forecast? Are you sweating heavily? Is your effort perception rising even as your pace holds steady? Perceived exertion correlates meaningfully with core temperature — your body is giving you real information. A session that felt “comfortably warm” probably didn’t generate enough stimulus. A session where you felt genuinely, uncomfortably hot for a sustained period probably did.

2. Heart rate drift. At a fixed pace or power output, your heart rate should gradually climb during a heat session even if you’re not working harder. This is called cardiac drift, and it’s your cardiovascular system diverting more blood to the skin for cooling. If you start a 60-minute run at 140bpm and finish at 158bpm without touching the pace, that’s heat stress showing up in your data. The more dramatic the drift, the more thermally stressed your body is.

3. Pace or power decoupling. The flip side of cardiac drift: your pace or power drops as you try to hold a target heart rate. You’re going slower at the same effort level. That’s your body pulling resources away from the working muscles to prioritize cooling. It’s a clear signal that heat stress is elevated.

Together, these three signals give you a working picture of your thermal state. Subjective feedback tells you how it feels. Heart rate tells you the cardiovascular cost. Pace and power tell you the performance cost. You can use all three combined to triangulate the effects heat is having, and an experienced athlete can learn to read their heat response pretty accurately over time.

The limitation is that you’re still estimating. You’re inferring what’s happening inside your body from external signals, none of which gives you the number you actually need: your core temperature. You might be in the adaptation zone, or you might be slightly short of it, or past it — and you won’t know for certain how well your heat training is going.

Adding a Fourth Layer: Core Body Temperature

What the three-prong approach is missing is objective feedback on our thermal state. That’s where CORE comes into play. CORE came out with their wearable core body temperature sensor in 2020 and it gives you real-time feedback on your thermal status. Adding it as a fourth layer to the three prongs mentioned above is the perfect way to deepen your understanding of your heat training.

Personally, I was an early-adopter and have found it to be such a valuable tool in my toolbox as an athlete and coach, which is why I want to give a little more insight into exactly what it is, how it works, and some of the best practices developed over the years.

So what is it exactly? The CORE sensor is a small wearable (about the size of a matchbox) that clips to your heart rate strap and measures your core body temperature and skin temperature in real time. It pairs with Garmin, Wahoo, COROS, and other devices, streaming live data to your head unit or wrist while you move.

What CORE Measures (And What to Watch)

Once you’ve got the sensor on, three metrics drive the session:

Core Body Temperature is the direct reading — your internal temperature in real time. This is the number you can’t get any other way. Your body maintains it in a narrow range at rest (96.8–99.5°F / 36.0–37.5°C), and during exercise it climbs. How far it climbs, and how fast, tells you exactly how much thermal stress you’re accumulating.

Skin Temperature provides environmental context. Together with core temp, it completes the picture of your thermal state.

Heat Strain Index (HSI) is the number you’ll stare at most during sessions. It runs from 0 to 10 and tells you how hard your body is working to stay cool at any given moment — think of it as a real-time effort score for your thermoregulatory system. Low numbers mean you’re barely warm. High numbers mean you’re in trouble.

CORE’s Heat Zones

CORE maps the HSI to four Heat Zones, which function like power or heart rate zones:

• Zone 1 – Minimal Heat Strain (HSI 0–1.9): Your thermoregulatory system is barely activated.
• Zone 2 – Moderate Heat Strain (HSI 2.0–2.9): Your body is managing heat stress with no problem.
• Zone 3 – High Heat Strain (HSI 3.0–6.9): The adaptation zone. This is where the gains live.
• Zone 4 – Extreme Heat Strain (HSI 7.0+): Danger territory. Don’t be here.

The sensor’s core value is keeping you in Zone 3. Without it, you’re relying on your three-signal triangulation to hit a moving target. With it, you have a number on your screen that tells you exactly where you are, in real time, the whole session.

Nailing the Protocol With CORE

With the sensor, the protocol from the previous section gets precise.

Your target for each session is 45–80 minutes in Zone 3 (HSI 3.0–6.9). Most athletes land in the 45–60 minute range. Getting to Zone 3 takes time — expect 20–40 minutes of working effort just to reach it, especially early in a block before you’re generating heat easily.

A few hard limits CORE suggests that are worth keeping top of mind:

• Keep core temperature below 39°C (102.2°F)
• Keep HSI below 7.0
• Cap your time in Zone 3 at 100 minutes maximum

Above those numbers, you’re not adapting faster. You’re just digging a hole.

One thing the sensor shows that athletes rarely expect: even at a fixed pace or power, your heart rate will continue rising as core temp climbs. This is exactly the cardiac drift described earlier — but now you can see both the cause (rising core temp) and the effect (rising HR) in real time. Most people end up walking the last portion of a run or backing off power late in a trainer session just to stay in Zone 3. That’s normal and correct.

For passive heat training like sauna: do not wear the sensor in the sauna. It’s not rated for those temperatures and the data won’t be accurate. Log your sessions manually using CORE’s Passive Heat Training Load Calculator in the app so they still count toward your overall heat load.

Tracking Your Adaptation: The Heat Adaptation Score

The CORE app tracks your progress with a Heat Adaptation Score (0–100%) based on your accumulated Heat Training Load from daily sessions. There are four named levels:

• 0–24% – Thermal Rookie: Heat will hurt your performance. Train more frequently.
• 25–49% – Heat Accustomed: Some adaptation is underway but there’s more work to do.
• 50–89% – Heat Adapted: You’re ready to race in the heat. You have a genuine advantage over athletes who haven’t done this work.
• 90–100% – Heat Champion: Maximum adaptation. Maintain with 1–2 sessions per week.

Aim for at least “Heat Adapted” (50%+) before a hot target race. With the decay rate of 2.5% per day without training, you can plan backward from race day and time your block to peak at exactly the right moment — the same way you’d time a fitness peak.

Using CORE to Race Cool

On race day, the sensor shifts from a training tool to a pacing tool.

When conditions are brutal, your decisions should be driven by what’s happening internally. CORE shows you the moment your thermal stress starts climbing toward the danger zone, before your heart rate spikes and before your pace craters. That’s early warning data — and it’s the difference between a reactive response and a proactive one.

With real-time HSI data on race day, you can:

• Time cooling strategies (ice, cold sponges, cold fluids) to when they’ll actually matter, not just on a schedule
• Make smarter early-race decisions, when the damage of overheating is mostly done before you feel it
• Know objectively whether you have margin to push late in the race

I’ll never forget the first time one of my athletes used his sensor for pacing a race. We were aiming for a pretty aggressive 10k goal on the day, and conditions were hot and humid. The real-time insights allowed him to catch the thermal load before it got bad, adjust his pacing and cooling strategies and still come in with a great time. It was a light-bulb moment for both of us for how powerful this metric can be.

And don’t just take it from me: Gustav Iden used the CORE sensor to simulate the exact core temperatures he’d face at Kona before ever setting foot on the island. He won. That’s not a coincidence.

Harness The Heat

Whether you use a CORE sensor or not, heat training is a smart move and can make that upcoming hot race or even just the first hot days of the year much more tolerable. Getting out there and harnessing the heat in your training is 100% a tool worth including in your arsenal and is a great performance unlock whether you’re training and racing in the hot or not.

So what are you waiting for? Get out there and get sweaty my friends!

Have questions about implementing heat training into your season plan? Reach out here — this is exactly the kind of work we do with athletes in our coaching programs.