Two riders roll up to the same start line on a 32°C afternoon. One has been turning the pedals on rollers for twenty-five minutes, building a slow, steady sweat, because every warm-up protocol he's ever read says longer is safer. The other has spent that same stretch of time sitting in the shade with a bag of crushed ice, eating it by the spoonful like it's a race-day treat, not a race-day tool.

When the gun goes, it's the ice rider who finishes faster. Not because he warmed up less carefully — because he started the effort with more thermal room to spend before his own body started slowing him down.

Section 01What the trial actually measured

This isn't a training-camp anecdote. Ihsan and colleagues put it to a controlled test: seven trained male cyclists completed a 40-km cycling time trial (1,200 kJ of work) on two separate occasions, preceded by 30 minutes of either crushed ice ingestion or tap water at body temperature (Ihsan et al., 2010). The ice dose was standardised — 6.8 g per kilogram of body mass, eaten steadily over the half-hour before the start (Ihsan et al., 2010).

The riders who'd iced down finished the time trial 6.5% faster than when they'd taken the water control — 5,011 seconds versus 5,359 seconds on the same course, same effort (Ihsan et al., 2010). Their gastrointestinal temperature, a validated stand-in for core temperature, measured lower going into the effort after ice than after water — 36.74°C versus 37.27°C — and stayed lower deep into the ride (Ihsan et al., 2010).

Here's the part that should make you sit up: heart rate, perceived exertion, and skin temperature didn't differ between the two conditions at any point in the ride (Ihsan et al., 2010). The riders weren't working any harder or feeling any worse to go faster. They simply had further to go before heat became the thing slowing them down.

Section 02Why cooling the inside beats warming the outside

Heat doesn't fail you all at once. Your brain is running a constant, mostly unconscious calculation — how much heat you're producing, how much you're shedding, how close your core is creeping to a threshold it isn't willing to cross. Once you get close to that threshold, effort starts to feel disproportionately hard, and pace drops whether you've decided to back off or not. That's the ceiling precooling is designed to push back.

A normal warm-up is a trade against exactly this ceiling. It opens up blood flow, primes the neuromuscular system, and gets you feeling ready — genuinely useful in cool or moderate conditions, where "readier legs" is the whole story. In the heat, it's also quietly spending the one resource you can't easily get back once the gun goes: thermal headroom. Twenty-five minutes on the rollers under a hot sun raises your core temperature before you've covered a single competitive metre. Precooling does the opposite — it puts you on the start line with a lower core temperature than you'd have arrived with naturally, buying back exactly the margin the warm-up was spending.

Sports scientists describe pacing in the heat as an anticipatory process rather than a simple redline. Your brain isn't waiting for core temperature to cross some fixed emergency line before it acts — it's continuously adjusting effort in real time based on how fast temperature is climbing and how much further there is to go. That's why fatigue in the heat so often arrives as a gradual, involuntary softening of power rather than a sudden wall. Lower the starting point of that calculation and every part of the effort that follows — the tempo section, the attack with ten minutes left, the finishing straight — gets a little more room before the brain decides to pull the plug.

This is a different lever to the one THRIVE's piece on post-ride hot-water immersion pulls — that protocol is about adapting your physiology over days so you tolerate heat better race after race. Precooling doesn't change your physiology at all. It's a same-day, start-line tactic: bank a lower core temperature right before the gun, whether or not you've done any heat-adaptation work beforehand. The two stack. Neither replaces the other.

Itreplicates — and cooling during the ride adds more

A single seven-rider trial is a real finding, not a settled law. So it matters that the pattern holds up at scale. Bongers and colleagues pooled 28 controlled trials of cooling strategies in hot conditions — 20 using precooling, 8 using percooling (cooling applied during the exercise itself) — restricted to studies run at ambient temperatures of 30°C or hotter (Bongers et al., 2015). Precooling alone improved performance by an average of 5.7% across those trials; cooling applied during the ride averaged an even larger 9.9% gain (Bongers et al., 2015).

That second number is worth sitting with. Cooling once you're already working isn't a lesser tool bolted on after the ice you ate before the start — the pooled data puts it in the same league, numerically the larger of the two averages, even though the review treats precooling and percooling as broadly comparable strategies rather than declaring a clear winner between them (Bongers et al., 2015). Either way, staying cool once you're already working — a bottle of something properly cold, ice tucked into a jersey pocket against the back of the neck, a wet towel at a feed zone — isn't just a comfort measure on a hot day. It's a genuine performance lever, not an afterthought to the precooling step.

The pooled trials didn't all use crushed ice. Some used cold-water immersion of the hands or forearms, some used cooling vests, some used the same ice-ingestion approach as the anchor trial here (Bongers et al., 2015). Different delivery methods, same underlying principle: get to the start line — or stay, mid-race — with a lower body temperature than the conditions would otherwise give you. Crushed ice earns its place in this piece because it's the cheapest, simplest version of the idea available to an amateur without a cooling vest in a kit bag, not because it's the only one that works.

Section 04The protocol

1. Dose the ice properly

   6.8 g per kilogram of body mass of crushed ice, eaten steadily over the 30 minutes before the start — the tested protocol (Ihsan et al., 2010). For a 75 kg rider, that's roughly 500 g, about a large bag's worth.
2. Trim the warm-up, don't skip it

   In genuine heat, a shorter, lower-intensity spin is enough to open the legs. The extra 15–20 minutes most riders default to is banking core heat you'll spend the first part of the race trying to shed.
3. Keep cooling once you're rolling

   Cold bottles, ice against the neck or down the jersey, a wet towel at feed zones. Cooling during the effort produced the largest gains in the pooled data, not just the precooling step (Bongers et al., 2015).
4. Match it to the conditions

   This is a heat-specific tool, tested at 30°C and above. On a cool morning start, a bag of ice is just cold cargo in your stomach with nothing to buy back.
5. Rehearse the dose before race day

   A cold bolus that size is new to your gut. Test it on a hot training day first, not for the first time on the start line.

Section 05Where riders get this wrong

Section 06Applying it with HELIOS

THRIVE's ride planner already knows the forecast for a scheduled ride, and heat is exactly the kind of variable that should change your pre-ride routine rather than just your water bottle count. A hot-weather flag on tomorrow's session is the cue to shorten the warm-up and prep the ice, not an afterthought you remember on the drive to the start.

HELIOS's skin-temperature sensor isn't the tool for judging whether precooling is working, and the research backs that up directly — skin temperature didn't move between the iced and non-iced trials even though the riders finished 6.5% faster (Ihsan et al., 2010). What HELIOS is built for here is the surrounding picture: flagging the hot day coming up, and tracking how much recovery a brutally hot ride cost you afterward, so the next hot start isn't a surprise either.

Section 07Bottom line

On a hot start line, a genuinely high-leverage thing you can do isn't ten more minutes on the rollers — it's lowering your core temperature before the gun goes. Seven trained cyclists rode a 40-km time trial 6.5% faster after 30 minutes of crushed ice ingestion than after plain water, with no change in how hard the effort felt (Ihsan et al., 2010). Pooled across 28 trials, the same idea holds at scale, and cooling during the ride adds a comparable — numerically larger — benefit on top (Bongers et al., 2015). Ice before the gun is the free gain most amateurs skip while quietly overinvesting in the warm-up.