Acclimatising for the heat? Do it in the water!

Introduction

Most older athletes know that the way to prepare for competition in the heat is to acclimatise by training in the heat. If you live in a cooler part of the world or it’s winter acclimatising can mean finding a heat chamber, an indoor heated pool for an ergo session, wearing tracksuits or even travelling to a place at great expense and effort.

Now there appears a new way to acclimatise. Complete your training session or work out in the cooler temperatures then immerse yourself in hot water after the training session.  This UK study investigated whether daily hot water immersion (HWI) after exercise in temperate conditions (18 degrees C) induces heat acclimation and improves endurance performance in either the temperate and/or hot conditions.

The Study Details

Seventeen non-heat-acclimatized young males (23 +/- 3 years; 69.5  +/- 6.9 kg; V̇O2max of 60.5 +/- 6.8 mL/kg/min) performed a six-day intervention involving a daily treadmill run for 40 min at moderate exercise intensity of 65% V̇O2max in temperate conditions (18 °C) followed immediately by either hot water immersion at 40 °C or thermoneutral at 34 °C immersion for 40 minutes. Before and after the six-day intervention, each participant performed a treadmill run for 40 min at 65% V̇O2max followed by a 5-km treadmill time trial (TT) in temperate (18 °C, 40% humidity) and hot (33 °C, 40% humidity) conditions.

The Results

HWI induced heat acclimation as shown by significantly lower resting rectal temperature (−0.27 °C), and lower final rectal temperature during both submaximal exercise in 18 °C (−0.28 °C) and 33 °C (−0.36 °C). Skin temperature, the rectal temperature at onset of sweating onset and the rating of perceived exertion were all significantly lower during submaximal exercise in 18 °C and 33 °C after 6 days of the hot water immersion. Physiological strain and thermal sensation were also significantly lower during submaximal exercise in 33 °C after the hot water immersion. All these results suggest the body has adapted physiologically to the hot water immersion. Critically, the hot water immersion also improved the 5km time trial performance in 33 °C by 4.9%, but not in 18 °C. Thermoregulatory measures and performance did not change when undertaking the controlled immersion at 34°C.

So What?

Rightfully so, the researchers showed that hot water immersion after exercise over six consecutive days presents a simple, practical, economical and effective heat acclimation strategy to improve endurance performance in the heat. Thus, this method of acclimating means you don’t need access to hot environments to acclimatise. This method also means you can maintain training intensity and duration THEN acclimatise versus having to try and train IN the heat where your training performance might be compromised. Similar to the slogan ‘live-high, train-low’ when it comes to altitude training, these researchers suggest ‘train-cool, bathe-hot’ when it comes to acclimatising for the heat. Another advantage of this type of heat acclimation is that it takes 10-14 days to get the heat acclimatisation benefits when training in heat. This six-day method saves time and therefore money if you have to travel to a hot place to prepare for racing.

Source: Zurawlew,, M. and others (2016). Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scandinavian Journal of Medicine and Science in Sports, 26: 745-754.