Your Athletes Might Not Be Acclimatized — Even If the Calendar Says They Are

Every coach knows the rule: get through the first two weeks of preseason in the heat and your athletes are acclimatized. It's standard practice. Most states mandate it. And it's based on real science.

The problem is that the calendar tells you how many days have passed. It doesn't tell you how much actual adaptation happened inside each athlete's body.

Those are not the same thing.

What the Research Actually Says

The science on heat acclimatization is unusually precise. We know the adaptations, the magnitudes, and the dose required to produce them. A 2025 Bayesian meta-analysis of 211 studies — the most comprehensive quantitative review of acclimatization research ever published — puts the numbers in sharp focus.

After a full acclimatization protocol, the average athlete sees resting heart rate drop by 5 bpm, end-exercise heart rate drop by 17 bpm, and core temperature at the end of exercise drop by nearly a full degree Fahrenheit. These aren't marginal changes. They mean the same practice that was hammering an unacclimatized athlete is now significantly less taxing. The cardiovascular system isn't working as hard. Core temp rises more slowly. The athlete is genuinely more resilient in the heat.

The research also shows that heat acclimatization reduces exertional heat illness risk — one study found that states mandating the 14-day preseason protocol saw roughly 55% fewer heat illness incidents during preseason compared to states without the mandate.

So yes — acclimatization works. The science is solid. The question is whether your athletes are actually getting it.

The Dose the Science Used

Here's where it gets important.

The studies that produced those results didn't just put athletes outside in August for two weeks. The controlled protocols kept each athlete's core temperature at or above 101.3°F (38.5°C) for a minimum of 60 minutes per session — typically across 8 to 10 sessions. That's the thermal dose the body needs to trigger the adaptations.

The 101.3°F (38.5°C) threshold isn't arbitrary. Every single controlled-hyperthermia study in that 211-study meta-analysis uses this same target. It's where the body starts signaling the adaptations that matter: plasma volume expansion, earlier sweating, lower cardiovascular strain at a given work rate. Most of the meaningful adaptation happens within the first 5 to 14 days of consistent heat exposure — but "consistent" means qualifying doses, not just practices.

The Gap  

Here's the disconnect: a typical preseason practice in August heat might get an athlete's core temperature above 101.3°F (38.5°C) for 15 to 25 minutes. A controlled lab protocol gets them there for 60 to 90 minutes. That's a three-to-four-times difference in dose per session.

Run 14 calendar days of typical practices and an athlete might accumulate the equivalent of 3 to 5 qualifying doses — not 8 to 10. The calendar says day 14. The body may be at day 4 or 5 of actual adaptation.

This isn't a criticism of how coaches run practice. It's just biology. Conditioning drills, water breaks, shade, position group work — all of it affects how long core temp actually stays elevated at the threshold. Some athletes run hotter than others. Some sessions are more conditioning-heavy. Two athletes standing in the same practice have different thermal experiences inside their bodies.

You Can't Know Without Measuring

This is the honest truth the calendar can't tell you.

Heat acclimatization also decays — and faster than most people realize. Research shows roughly 2.5% decay per day without continued heat exposure. An athlete who earned full acclimatization and then spent a week at a cool training camp or took time off for injury isn't fully acclimatized anymore. Re-acclimatization takes about half the original time — but only if you know you need it.

A coach managing a 50-person roster can't intuit which athletes are truly adapted, which ones are still inducing, and which ones earned acclimatization three weeks ago and have since lost half of it. Not without data.

The only way to know how much qualifying heat dose an athlete has actually accumulated is to measure their core temperature in real time and track how long it stays above the threshold that drives adaptation. That's it. Everything else is an estimate.

What Measuring Changes

When you can see the actual thermal dose — not the calendar, not the ambient temperature, not how hard practice looked — you can make real decisions.

You know which athletes are on track for full acclimatization and which ones are lagging, before an incident tells you. You can see which sessions are actually delivering qualifying doses and structure accordingly. You can catch the athlete who came back from a week off and looks fine in practice but whose acclimatization state has been quietly decaying.

None of this requires changing how you coach. It requires being able to see something that's been invisible.

What's missing is the ability to measure it in the field, for every athlete, every session. That gap is finally closable.

Sources

• McDonald et al. (2025). Influence of Exercise Heat Acclimation Protocol Characteristics on Adaptation Kinetics. Comprehensive Physiology 15:e70017. https://pubmed.ncbi.nlm.nih.gov/40442924/
• Daanen, Racinais & Périard (2018). Heat Acclimation Decay and Re-Induction. Sports Medicine 48(2):409–430. doi.org/10.1007/s40279-017-0808-x
• Kerr et al. (2019). Mandated Preseason Heat Acclimatization Guidelines and Exertional Heat Illness. Environmental Health Perspectives 127(4):EHP4163. doi.org/10.1289/EHP4163
• Périard, Racinais & Sawka (2015). Adaptations and mechanisms of human body temperature regulation. Scandinavian Journal of Medicine & Science in Sports 25(S1):20–32. doi.org/10.1111/sms.12511