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Pre-cooling: how cooling before the start helps you run in the heat

An ice vest, cold water, and ice slurry before the start create a heat reserve and push back overheating. We break down the 2024 meta-analysis and what an amateur can actually apply at a hot start.

AL
Andrey Leskov

You're standing in the start corral, it's +30 Β°C, and there's still half an hour to the gun. Your body is already starting to overheat, even though you haven't taken a single step. This is exactly where pre-cooling β€” pre-start cooling β€” comes in. The idea is simple: if you lower your core body temperature before the effort, you gain a reserve, a "heat buffer," which in the heat pushes back the moment of critical overheating. And overheating is one of the main reasons why on a hot day your pace falls apart in the second half of the distance.

How it works

During prolonged work, the muscles produce an enormous amount of heat. The body removes it through sweating and a rush of blood to the skin, but in heat and high humidity these mechanisms get overwhelmed. Core temperature creeps up, the brain registers the threat and begins to consciously and unconsciously reduce power output β€” you slow down so as not to cook from the inside.

Pre-cooling intervenes in advance. By lowering the starting body temperature and the subjective thermal sensation, we:

  • increase the heat-storage reserve β€” the body needs to accumulate more heat before the critical point, which means more time;
  • reduce the load on the heart β€” heart rate at the same speed is slightly lower, stroke volume higher;
  • make the effort more comfortable β€” perceived exertion (RPE) drops and thermal comfort improves.

A key nuance: the effect appears specifically in the heat and during long aerobic work. The longer you run and the hotter it is outside, the more each saved "degree of reserve" matters.

What the research shows

A fresh systematic review and meta-analysis (Yu et al., Nutrients, 2024) gathered 15 randomized controlled trials β€” 236 people in cooling groups versus 175 in controls. All experiments were conducted in the heat, at temperatures >26 Β°C. They examined two types of task: time trials (cover the distance faster) and time to exhaustion (hold out longer).

Results:

  • Time trial: overall effect size SMD βˆ’0.37 (95% CI βˆ’0.60 to βˆ’0.14; p = 0.002). This is a small but statistically significant improvement in performance β€” both in running (βˆ’0.41) and in cycling (βˆ’0.37).
  • Time to exhaustion: SMD 0.73 (95% CI 0.41 to 1.05; p < 0.00001) β€” already a medium-large effect. Simply put, cooled athletes held the load noticeably longer.

The most interesting part is the difference between methods. "External" cooling (ice vests, cold baths, head cooling) worked more strongly: in the time trial SMD βˆ’0.43, and in time to exhaustion an impressive 1.01. But "internal" cooling (ice slurry, cold drinks) in this analysis gave a zero effect on the time trial (SMD 0.01; p = 0.96) and only a borderline one on time to exhaustion (0.44; p = 0.06).

The authors rated the level of evidence as moderate for time to exhaustion and low for the time trial β€” that is, the direction is clear, but the exact figures will still be refined.

What an amateur can actually apply

Translated into the language of a mass start in the heat:

  • An ice vest or cold on the warm-up. Since external cooling proved stronger in the studies, put on a cooling vest 20–40 minutes before the start or apply ice/cold water to your neck and forearms during the warm-up.
  • Ice slurry ~30 minutes before the start. A slush (finely crushed ice with a little drink) is convenient for a runner: you don't carry it on you, and it also provides fluid. A rough guide for volume is about your body mass (roughly 1–1.5 g per kg). The effect on performance per the meta-analysis is more modest than the vest's, but it improves comfort and thermal sensation β€” and that also helps hold your pace.
  • Per-cooling along the course. Cooling doesn't end at the start: douse yourself with water at aid stations, put ice under your cap, down your collar, in your hands. This extends the effect while you're already running.
  • Combine, don't replace. Pre-cooling doesn't cancel out heat acclimatization and proper rehydration β€” it works on top of them. The calculator below will help you estimate your fluid losses in the heat.

Mistakes and limitations

  • "A cold shower in the morning is pre-cooling." No. Hours pass between the shower and the start, and the body has long since returned to its own temperature. Cooling must be immediately before the effort.
  • "The colder, the better." Also no. Overcooled muscles lose power, so before explosive, sprint work there's no need to freeze your legs.
  • At a cool start there's no point. The whole effect hinges on heat (>26 Β°C). In comfortable weather you'll just waste time and ice.
  • Short efforts barely benefit. The mechanism is about a heat reserve over a long distance; for sprints and short intervals the influence is weak or absent.

The bottom line

  • Pre-cooling is cooling the body before an effort in the heat; it creates a heat reserve and pushes back overheating.
  • Meta-analysis of 15 RCTs (Yu et al., 2024): significant improvement in both the time trial (SMD βˆ’0.37) and time to exhaustion (SMD 0.73) in conditions >26 Β°C.
  • External cooling (vests, cold water) proved stronger than internal in the studies; ice slurry is convenient and improves thermal comfort.
  • Practice: vest/ice on the warm-up, ice slurry ~30 min before the start based on body mass, dousing and ice along the course.
  • It doesn't replace acclimatization and rehydration; useless in cool weather and for short efforts; there's no need to overcool the muscles before explosive work.

Sources: Yu L, Chen Z, Wu W, Xu X, Lv Y, Li C. Effects of Precooling on Endurance Exercise Performance in the Heat: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2024;16(23):4217. https://doi.org/10.3390/nu16234217

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