You've got a hot race coming up and you train somewhere cool. The usual fix is expensive and awkward — book a heat camp, fly out a week early, sit in a purpose-built heat chamber you don't have access to anyway. Most riders just skip it and hope the first hot lap of the race is where the acclimatisation happens, in real time, while it's costing them watts and possibly the result. The alternative that actually gets used is worse: showing up cold to the heat and finding out on the start line how your body handles it — heart rate climbing for no wattage gained, legs feeling heavy an hour earlier than they should, a pacing plan built in cool conditions falling apart by the first feed zone. Zurawlew et al. (2016) tested a version of this problem that fits in a bathroom. No chamber. No travel. Just a hot bath, straight after an easy session, six days running.

Section 01What six days of a hot bath actually did

The design here is worth understanding before the numbers, because it's the thing that makes the result credible rather than a spa-brochure claim. Seventeen non-heat-acclimatized men completed six consecutive days of a 40-minute treadmill run at 65% VO2max in temperate conditions, followed immediately by 40 minutes of immersion — 40°C hot water for ten of them, a 34°C thermoneutral bath for the other seven as controls (Zurawlew 2016). Before and after the six-day block, everyone ran a 5km treadmill time trial in both temperate (18°C) and hot (33°C) conditions. The hot-water group improved their time trial in the heat by 4.9% (P < .01), with no equivalent gain in the temperate trial or in the thermoneutral control group (Zurawlew 2016). Resting core temperature fell by 0.27°C, and exercising core temperature dropped by 0.28°C in temperate conditions and 0.36°C in the heat (all P < .01) (Zurawlew 2016) — the textbook signature of heat acclimation, produced without a single minute spent training in the heat itself.

:::statblock value="4.9%" eyebrow="Time-trial improvement, heat trial only" cite="Zurawlew 2016" Six days of a post-run hot bath — no heat chamber, no travel — measurably improved 5km time-trial performance at 33°C (P<.01).

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:::chart title="5km time-trial change after 6 days, hot-water immersion group" source="Zurawlew et al. 2016, Scand J Med Sci Sports" Heat trial, 33°C: 4.9:dark Temperate trial, 18°C: 0:light Control group (thermoneutral bath), both trials: 0:light

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Section 02Why a hot bath produces a heat chamber's adaptations

Classical heat acclimation works by repeatedly raising your core temperature under load until your body starts protecting itself against it — earlier sweat onset, a lower core temperature for a given effort, better perceived comfort at a given heat load. The usual way to trigger that is to train in the heat itself, which is exactly what most riders training for a hot race can't easily do. Zurawlew's protocol gets to the same trigger by a different door: finish a normal, temperate-conditions session, then immediately raise core temperature anyway, in a bath, while your body is already warm and primed from the exercise. The stimulus your body responds to is the elevated core temperature itself, not the environment that produced it — which is why sitting in 40°C water after training produced the same resting- and exercising-temperature reductions you'd expect from a genuine heat camp, without ever running a single interval in the heat.

The timing detail matters more than it looks. This isn't "have a hot bath at some point in your day" — it's immediately after exercise, while your core temperature is already elevated from the session and your cardiovascular system is already working to dissipate heat. Stack a second heat load on top of that window and the total thermal strain, and the adaptive signal it produces, is larger than either the exercise or the bath would produce alone. A hot bath taken cold, hours after training with your core temperature back to normal, is a different — and by this protocol's logic, weaker — stimulus.

:::glossary eyebrow="Glossary · Terms in this piece" title="The terms that matter" Heat acclimation | | The physiological adaptations that let you exercise in hot conditions more efficiently — lower core temperature, earlier sweat onset, better thermal comfort at a given effort. Core temperature (Tre) | Rectal temperature | The gold-standard internal-temperature measurement used in heat research — more reliable than skin temperature for tracking true heat strain. Thermoneutral | | Water or air close to body-neutral temperature — used here as the control condition, since it raises heart rate slightly without triggering true heat-acclimation signals.

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:::pullquote attribution="The six-day takeaway" You don't need a heat chamber. You need a bath that's actually too hot, six days running.

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Section 03Why this still applies to cyclists, not just runners

The cohort here was runners, not cyclists, which is worth being honest about before you plan around it. Running and cycling raise core temperature by different routes — running engages more total muscle mass at a given perceived effort, which likely produces a faster, larger rise in core temperature than an equivalent easy ride does. That means a cyclist doing "40 minutes easy" before the bath may be starting from a slightly lower thermal base than these runners were. The fix isn't a different protocol, just an honest easy session beforehand — ride at an effort that genuinely leaves you warm, not one so gentle it barely raises your temperature at all. The mechanism the bath itself is acting on — pushing core temperature up further, immediately, while you're already warm — doesn't care what got you warm in the first place. A bike leg that actually raises your core temperature should feed the same adaptation a run did here.

Section 04The six-day protocol

This is a pre-event tool, not a season-long habit — time it to land in the week or so before you race somewhere hot.

:::protocol Days 1–6 | Ride or run easy for roughly 40 minutes in your normal, temperate conditions — nothing race-specific, just enough to raise your core temperature and get you genuinely warm. Immediately after | Straight into a bath at roughly 40°C for about 40 minutes. Not a lukewarm soak — properly hot, and immediately post-exercise while your core temperature is already elevated. Every day | Repeat the same sequence at the same time of day for six consecutive days. Consistency is doing the work here — this isn't about going hotter or longer than the tested protocol. Race week | Schedule the block so it lands inside roughly the two weeks before your hot event. These adaptations are real but short-lived, so timing the block matters as much as running it.

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Treat the bath itself with the same caution you'd apply to any deliberate heat exposure: hydrate properly before and after, never do the session if you're unwell, and don't do it alone. This is a genuinely uncomfortable, real heat-stress protocol, not a spa treatment — if you feel dizzy, nauseated, or unusually unwell at any point, get out of the water. Forty degrees for forty minutes is already a meaningful physiological load; there's no case for pushing past what was actually tested, and every case for backing off if something feels wrong.

Six days is also a minimum commitment, not a suggestion you can shorten. The adaptations measured in this study came from the full six-day block — there's no data here on what two or three days buys you, and no reason to assume it's a proportional slice of the same benefit.

Practically, the water temperature is the part most riders get wrong when they try to eyeball this without a thermometer. A bath most people would call "very hot" for a normal soak — the kind you ease into slowly and adjust to over a minute or two — is usually still well under 40°C. Test it with an actual thermometer the first time, note how it feels, and use that as your reference for the rest of the block rather than re-guessing it by hand each day.

Section 05Where this breaks down

:::misread "Hotter or longer must work better." | The tested protocol is 40°C for about 40 minutes. Pushing well past that isn't the validated protocol and adds real heat-stress risk for no proven extra benefit. "This will make me faster everywhere." | The performance gain showed up in the heat trial, not the temperate one. Treat this as heat-specific race prep, not a general fitness upgrade. "I can do this once and I'm set for the season." | The adaptation was measured immediately after the six-day block. This is a short-lived, pre-event tool that needs to be timed to your race, not banked months in advance. "A cold shower or ice vest does the same job." | Cooling strategies help you manage heat load in the moment, mid-race. This protocol works in the opposite direction — it deliberately raises your core temperature after training so your body adapts to running hotter, not so it stays cooler.

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Section 06Applying it with ULTRA

The hardest part of a protocol like this isn't the bath, it's the discipline to actually run all six days and get the timing right before your event.

:::apply eyebrow="Your call today · Race-day readiness" title="ULTRA tracks the heat-prep window, not just your training load" sub="Log the six-day block in the training planner and Marco times your taper so the adaptation is still active on race morning, not faded two weeks early." btn="See recovery + readiness in the app →" visual_eyebrow="Heat-prep block" visual_title="Day 4 of 6" visual_meta="On schedule for Saturday's start"

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This is squarely a pre-event tool for the rider who has a hot race on the calendar and cool training conditions at home — the gravel racer heading somewhere tropical, the roadie whose target event is a mid-summer classic they've prepared for in a mild spring. If your season is entirely run in the conditions you race in, there's nothing here to bolt on. The value is specifically in closing the gap between where you train and where you're about to race.

Section 07Bottom line

You don't need a heat chamber, a sauna protocol, or a week of pre-race travel to show up acclimatised to a hot race. Six consecutive days of an ordinary easy session followed immediately by a genuinely hot bath produced the core measures of heat acclimation and a real time-trial gain in the heat — with no equivalent gain in temperate conditions, so treat it as race-specific prep, not a fitness upgrade. Time the block to land in the one to two weeks before you race, respect it as real heat stress rather than a spa routine, and you've solved a heat-camp problem from your own bathroom for the cost of a hot tap and six evenings.