Pre-Miyakojima Prep: Heat Acclimatization Training with CORE Temperature Monitoring

Tue, 07 Apr 2026 00:00:00 +0000

I’ve had the CORE sensor for a while now. During summer training I run it to monitor core temperature, mainly to check if I’m overheating and need to stop. But Miyakojima will be the first time I bring it into a race.

Miyakojima is late April in Okinawa — island climate plus humidity, daytime highs potentially hitting 27–30°C. Even if actual race-window temps (morning start) sit around 20–25°C, that’s a big gap from my current training environment in Tokyo. Tokyo right now is about 10–20°C; in normal training gear you’re nowhere near meaningful heat stress. If I don’t prepare deliberately, trying to adapt on-site is too late.

So I’ve been doing structured heat acclimatization these past few weeks. This post covers the research background, the gear, and what I’m actually doing.

What Is Heat Acclimatization Training

The core idea behind heat acclimatization (heat acclimation) is simple: repeated exposure to a hot environment forces the body to produce physiological adaptations, instead of just toughing it out on race day.

The main physiological changes:

Increased plasma volume: Research shows plasma volume increases by an average of 10–12% after heat acclimatization^[1]. More plasma means more blood delivered per heartbeat, plus sustained sweat output for longer, keeping cooling efficiency up.

Sweat response improves significantly: Sweating starts earlier and total sweat output goes up — some studies show nearly three times more than pre-adaptation^[1]. This is critical for keeping core temperature in check under heat.

Lower core temperature: At the same exercise intensity, core temperature drops by roughly 0.27–0.28°C^[2]. Sounds small, but when you’re pushing close to limits in an Ironman, that margin is real.

Lower heart rate: The heart doesn’t have to work as hard to maintain the same output, leaving more headroom for late-race fatigue.

These adaptations don’t appear overnight. They require accumulated training.

What the Research Says: How Many Weeks

Adaptation takes roughly 10–14 days: The main physiological changes complete within this window. Each session should be 45–60 minutes, with a common protocol being 5 sessions per week for two weeks. If time is short, at least 6 sessions still provide meaningful benefit^[3].

Benefits start showing after session 4–5: You don’t have to wait two full weeks to feel the difference, but maximizing the effect requires 10+ sessions.

Adaptation decays: This is the part people overlook. Two weeks after stopping heat training, heart rate adaptation has decayed by about 35% and core temperature adaptation by about 6%; by week three, roughly 75% of the benefit is gone^[4]. So you can’t just finish heat training a month before the race and coast. Start 2–3 weeks before race day, stop 3–4 days out — that way you show up with peak adaptation intact.

The good news: re-adaptation is much faster than the initial decay. After 10 days of adaptation plus a 12-day gap, just 2 days of re-exposure restores most of the benefit^[4].

Why the CORE Sensor

A fundamental problem with heat acclimatization training: how do you know you’re “hot enough”? Feel alone isn’t reliable. Wearing heavy training clothes and feeling hot doesn’t mean your core temperature is actually in the adaptation zone. The reverse is also true — sometimes core temp is already elevated but perceived effort hasn’t caught up.

The CORE sensor clips onto a chest strap and monitors core temperature and Heat Strain Index (HSI) in real time. HSI combines core and skin temperature into a single 0–10 score that quantifies the body’s current cooling load. The signal feeds straight to a Garmin or Wahoo.

CORE’s official recommendation for heat training targets HSI Zone 3 (3–6.9) for 45–60 minutes^[3]. My approach is to cap HSI at around 5, hold for 45–60 minutes. After stopping, I don’t strip the gear right away — core temperature and HSI stay elevated for a while, and that continued exposure still contributes to adaptation.

Three use cases:

During training: Confirm I’m in a sufficient heat stress zone — not too little, not too much
Tracking adaptation progress: As training accumulates, HSI should rise more slowly in the same conditions — that’s the signal of real adaptation
Race day: Adjust pacing and fueling based on live HSI data instead of guessing by feel

One honest caveat: a peer-reviewed validation study found that about 50% of CORE’s measurement points exceeded the study’s acceptable error threshold^[5]. So treating it as an absolutely accurate core thermometer isn’t quite right. But as a relative indicator for tracking training trends and as a real-time reference during racing, it’s still practical. Building trend awareness matters more than chasing absolute numbers.

Garmin watches also have a built-in heat acclimation metric, but it’s calculated from GPS and weather data from a paired phone — not direct core or skin temperature measurement. Accuracy is a step further removed. Fine as a reference, but not something to base heat training decisions on.

Gear: How to Create a Hot Environment

Training at Tokyo room temperature on the indoor trainer or running outside isn’t hot enough. You need to actively create heat. I’m using two options:

CORE Heat Training Suit

CORE’s official training suit, 100% Polypropylene, 65 g/m² fabric weight, designed specifically for heat acclimatization. The material is tight-fitting and holds heat well. I’m 177 cm and got size M — CORE says M fits up to 180 cm, but it actually feels big on me. S might be better (S is listed for up to 172 cm; I’m right on the boundary).

3M 4510 Protective Coverall

This one was an accidental find. The CORE suit package didn’t arrive initially, so I went looking for a substitute and bought this industrial protective coverall. Turns out the heat retention is very similar — same Polypropylene material, 47 g/m² (lighter than the CORE suit’s 65 g/m²), but in actual training the thermal effect is about the same. Much cheaper too. Solid cost-performance alternative.

Running outdoors in a white industrial coverall gets you some very confused stares from pedestrians. I guess that’s part of the acclimatization too — you get used to not caring.

How Training Actually Feels

Most of my heat acclimatization sessions happen indoors on the trainer. The usual approach: finish an intensity workout while core temperature is still elevated, hop off, throw on the heat training suit, and get back on for 45–60 minutes of low-intensity Z2 or even Z1. During the ride I monitor HTL (Heat Training Load) and wrap up once I’m close to the day’s target. If I don’t strip the gear right away after getting off, the residual heat exposure still contributes some adaptation effect.

As sessions accumulate, HSI rises more slowly in the same gear. That’s the adaptation signal, but it also means maintaining enough heat stress requires adding more layers. For indoor trainer sessions toward the end of the block, I layer a long-sleeve jersey and jacket over the training suit to force HSI into the target zone.

Outdoor running feels different. Wind carries sweat away faster, making heat stress harder to sustain, but running also generates heat more quickly than cycling.

After a few sessions, the sweat response is noticeably faster and heavier. That subjective change is pretty clear.

If you’re in Taiwan, regular outdoor training already builds natural heat adaptation at current temperatures — you probably don’t need to go out of your way. Tokyo’s conditions right now don’t offer that, which is why the deliberate setup is necessary.

Things to Watch

Core temperature target zone: 38.5–39.5°C. Do not exceed 40°C. Beyond that it stops being “adaptation” and starts being “damage.”
Hydrate more than usual: Sweat rate goes up, so fluid needs follow. Don’t wait until you’re thirsty.
Stop signals: Abnormal HSI spike, dizziness, heart rate staying well above expected levels — stop immediately.
Timing: Start 2–3 weeks before race day, stop 3–4 days out.

Wrap-up

Heat acclimatization isn’t some obscure training method. The physiology behind it has plenty of research support. For someone training in Tokyo and racing in Miyakojima, this is the most practical prep strategy available right now. The CORE sensor’s role is to make the training quantifiable and trackable, rather than relying on “I feel like I sweated a lot.”

I’ll follow up with a race-day report after Miyakojima.

References

[1] Racinais, S. et al. (2019). Physiological Responses to Heat Acclimation: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Sports Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC6543994/

[2] Goosey-Tolfrey, V. et al. (2019). Mixed Active and Passive, Heart Rate-Controlled Heat Acclimation Is Effective for Paralympic and Able-Bodied Triathletes. Frontiers in Physiology. https://pmc.ncbi.nlm.nih.gov/articles/PMC6763681/

[3] CORE. Building Heat Training into Triathlon Training. CORE Help Center. https://help.corebodytemp.com/en/articles/10447170-building-heat-training-into-triathlon-training

[4] Moran, D. et al. (2018). Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC5775394/

[5] Niederer, D. et al. (2021). Reliability and Validity of the CORE Sensor to Assess Core Body Temperature during Cycling Exercise. Sensors. https://pmc.ncbi.nlm.nih.gov/articles/PMC8434645/

Heat Acclimatization on Tri Labs | Data & Endurance