STRATAHeat acclimatization: the preseason plan that prevents exertional heat illness
The first days of preseason are the deadliest stretch of the athletic year for heat. A heat-acclimatization plan is the single best thing you can do about it, and it costs you nothing but patience.
Almost every exertional heat-stroke death in sport follows the same script: a fit, motivated athlete, in the first few days of preseason, in full equipment, on a hot afternoon, before the body has had any chance to adapt. The athlete is not the problem. The schedule is. And the schedule is the one thing you control completely.
Why the first week is the danger zone
The opening days of preseason stack every risk factor on top of each other. Athletes show up unacclimatized, coming off a summer of air conditioning and on-and-off training. Equipment sports put them in the most gear they will wear all year, and that gear traps heat and blocks evaporation, which is the body's main way to cool itself. Motivation is running high, so athletes push through warning signs to make an impression or make the team. And the schedule often crowds conditioning tests and long installs into those very first sessions, right when the body is least ready for them.
Put those together and you get the reason most sudden heat-stroke deaths in organized sport happen in the first days of a new season, before anyone is ready for the heat. The one saving grace: this window is predictable. And predictable means preventable.
What acclimatization actually is
Heat acclimatization is the set of physical changes the body makes when you expose it to heat and exercise, over and over, across about 10 to 14 days. It is a real, measurable shift in how an athlete handles a hot afternoon, and it does not happen in one session. The main adaptations:
- Earlier, heavier sweating. An acclimatized athlete starts sweating sooner and sweats more, which cools the skin far better.
- More dilute sweat. The body learns to hold on to sodium, so acclimatized sweat carries less salt. That protects electrolyte balance over a long session.
- A lower working heart rate. The same drill costs the heart fewer beats per minute once the cardiovascular system adapts and blood plasma volume expands.
- A lower core temperature. The athlete runs cooler at the same workload, and that is the whole point: more room before the dangerous zone.
- Less strain overall. The work feels easier, and the athlete simply handles heat better.
Plasma volume expands within the first few days, but the full set of adaptations takes the better part of two weeks of steady exposure. Miss several days in a row and the adaptations start to slip, which is why a stop-and-start preseason is riskier than a steady one.
The preseason plan, principle by principle
You cannot rush the biology, so the plan is built around easing athletes in while their bodies catch up. What follows are the principles behind the heat-acclimatization guidelines the NATA published, the ones most state associations have adopted in some form. The catch: your state's own day counts and hour caps are what actually govern you, so pull that policy and follow its specifics to the letter. The principles underneath them do not change from place to place:
- Phase in over about two weeksTreat the first roughly 14 days as their own ramp, not business as usual. Intensity, duration and equipment all build a little at a time.
- Single practices before doublesThe first days are one practice a day. Do not open the season with two-a-days.
- Add equipment in stagesIn football, that usually means helmets first, then shoulder pads, then full pads across successive days, not all of it on day one.
- Cap practice duration earlyKeep the early sessions short, and count warm-up, walk-throughs and conditioning inside the daily total, not on top of it.
- Protect recovery daysBuild in days off and lighter days. The adaptations happen during recovery, not just during the work.
- No back-to-back two-a-day daysOnce double sessions are allowed later in the ramp, follow a double day with a single-practice or rest day, never another double, and keep the two sessions several hours apart in a cool space.
The through-line: build load gradually, and give the body time between hard sessions. Every principle above is a version of that one idea.
One practical note: a walk-through is not a practice, and a practice is not a walk-through. If athletes are in helmets running drills at full speed, that is a practice, whatever you write on the schedule. Logging a hard session as a walk-through to squeeze in more work is the exact shortcut these guidelines exist to stop.
Acclimatization is only half the plan
A phase-in schedule assumes the heat itself stays within reason. It will not always. Run the acclimatization plan next to two other things:
- WBGT monitoring. Measure Wet Bulb Globe Temperature on your own field before practice and during it, and modify or move the session the moment you cross a threshold. A phase-in day still has to bend to an extreme-heat day. Our WBGT and heat-safety guide walks through the thresholds.
- A hydration plan. Unacclimatized athletes lose more salt, and many show up already behind on fluid. Weigh them before and after, replace what they lose, and keep an eye on the heavy sweaters. Here is how to calculate sweat rate.
Acclimatization, WBGT and hydration are three layers of one defense. Run all three, not just one.
Have the emergency plan ready anyway
Even a careful plan can run into a bad day, so the emergency piece has to be in place before the first whistle, not thrown together after someone goes down.
Exertional heat stroke is survivable, and survival comes down to one principle: cool first, transport second.
Suspect it when you see a very high core temperature along with changes in how the brain is working: confusion, collapse, loss of coordination. Start on-site cold-water immersion immediately and cool the athlete before EMS transport. Survival is tied to how fast core temperature comes down, not how fast the ambulance arrives. Activate your Emergency Action Plan at the same time.
In practice, that means a cold-water immersion tub, ice, and a rehearsed plan on-site through preseason. In the first weeks of the year, the tub is not optional equipment.
When to modify
The plan is a floor, not a ceiling. Slow down further, or hold an athlete out entirely, when:
- The WBGT reading crosses your region's threshold, no matter which day of the phase-in you are on.
- An athlete is new, back from illness, or coming off missed days. They are effectively unacclimatized again and need to restart lower.
- Someone shows early warning signs: cramping, dizziness, an unusually high heart rate, or a drop-off in coordination and performance.
- An athlete is sick, short on sleep, or came in dehydrated at weigh-in. Any of those erases much of the margin acclimatization buys you.
You can always run the plan more conservatively than the minimum. You can never safely run it faster.
Know exactly where each team is in the ramp
Strata logs your field's WBGT and tracks which day of the acclimatization plan each team is on, so the phase-in becomes a record you can see and share, not a schedule taped to a clipboard. If a decision ever gets questioned, the timeline is right there.
See it with your school's branding →Frequently asked
How long does heat acclimatization take?
Most of the adaptations develop over about 10 to 14 days of steady exposure to heat and exercise. Blood plasma volume expands in the first few days, but the full set of changes, including more dilute sweat and a lower core temperature, takes the better part of two weeks. Miss several days and you lose some of that adaptation, so consistency matters.
Why is preseason more dangerous than midseason?
Because athletes are unacclimatized, often in the most equipment they will wear all year, and motivated enough to push right through warning signs. That combination is why most sudden heat-stroke deaths in sport happen in the opening days of a season, before the body has adapted to the heat.
Do the exact hour and equipment limits come from Strata?
No. Your state athletic association sets the specific day counts, practice-length caps, and equipment schedule, and that policy is what governs your program. This guide covers the principles behind those rules. Always follow your state association's current heat-acclimatization policy and your team physician.
Sources & further reading
- NATA Position Statement: Exertional Heat Illnesses, and preseason heat-acclimatization guidelines. nata.org
- Korey Stringer Institute (KSI), heat-acclimatization resources. ksi.uconn.edu
- Gatorade Sports Science Institute (GSSI), heat acclimatization and hydration. gssiweb.org