Fancy an ice slurry? How to keep athletes’ core temperatures down when competing in hot conditions

“Most people can relate in some way to research looking into how heat affects athletes. Everyone has felt too hot, uncomfortable or fatigued from being in a hot environment, which makes the research I do feel important and useful to not just athletes but the general public too.” Shares Stacey when asked what fascinates her about her field of research. She explains that the need for this type of research is becoming more imperative, due to many major sporting competitions and tournaments taking place around the globe in hot environments. “When watching these events, either live or on the TV, I am able to see first-hand how the research I am doing is helping athletes on the playing fields.”

Growing up in countries such as Brunei and Qatar, Stacey played a lot of sports outside in the heat. Unbeknownst to her at the time, this experience was a catalyst for her career and research interests. As a third-year undergraduate at Nottingham Trent University she chose an environmental physiology module. “I got to use an environmental heat chamber for the first time,” she exclaims, adding “the experiences I gained from this module meant that applying for my current PhD was a no brainer when it was advertised a year later!”.

Athletes competing in the heat

So, how do hot and humid environments impair athletes’ performance when competing in these conditions? “There are mixed findings at present in the literature because the level of impairment is dependent on the level of thermal strain experienced in some cases (i.e., how hot the athletes are getting),” explains Stacey. Adding that previous research has found impairments to sprint performance, evidenced through declines in peak and mean power output, as well as cognitive function when competing in hot conditions. This is due to large increases in core temperature (>38.5°C) and greater physiological strain experienced by the participants. “Combined cooling methods have been implemented to combat this,” says Stacey, which is why she was curious to test it.

Based on previous research, Stacey predicted that a combined cooling intervention would improve some physiological and perceptual responses to the heat, for example neck temperature, thermal sensation and thermal comfort. “The neck is an area of high alliesthesial thermosensitivity and closely linked to perceptual feelings. I expected measures such as thermal sensation to be positively influenced when the neck was externally cooled,” she says.

Testing the cooling intervention on cycling sprint performance

“We used a battery of cognitive function tests on participants that they completed pre and post a 40-minute intermittent exercise cycling protocol. The tests assessed four domains of cognitive function, including perception, executive function, memory and attention, which are said to be imperative for sporting performance,” explains Stacey. The cycling protocol involved repeated rounds of two-minute stages of intermittent exercise, where participants sprinted at their maximum capacity for 5 seconds, followed by 1 minute 45 seconds of active recovery (cycling at 35% max power) and 10 seconds of rest.

“During a 30 minute pre-cool and a four-minute drinks break, participants received either the combined cooling strategy (ice slurry and ice collar) or thermoneutral water (~37°C). We took physiological and perceptual measures throughout to investigate the impact of the combined cooling intervention,” says Stacey.

Lowering core body temperature

“We saw no differences in participant’s sprint performance regardless of the cooling method despite lower ratings of thermal sensation, indicating improved thermal comfort, as well as lower rectal and neck temperatures being recorded. We also found no consistent pattern in cognitive function results with response times when completing tasks during the cooling trial and control trial,” states Stacey as she summarises the results.

The combined cooling intervention lowered rectal temperature, neck temperature as well as heart rate, however, it had no effect on forehead temperature. “Despite these positive benefits, this did not translate into any improvements in peak or mean power output during the cycling protocol at the environmental conditions used in our study.” She explains, “This was likely due to the participant’s rectal temperature not reaching a level where declines have occurred previously, therefore it could be argued that sufficient heat strain was not induced to impact power output and require the combined cooling intervention”.

They also observed no differences in whole-body sweat rate between the conditions, which could be another factor contributing to the lack of differences in peak and mean power output. “Higher sweat rates result in the development of hypohydration and higher levels of physiological strain, which in turn, have been associated with reductions in exercise performance,” says Stacey.

Combatting the negative effects of heat to improve performance

At 33°C and 50% relative humidity, the combined cooling intervention showed positive benefits both physiologically and perceptually. While there was no difference in peak or mean power output, the intervention could help to lower rectal temperature when exercising in hot and humid environments.

“This is still quite novel in relation to combined cooling,” says Stacey about their study which adds to the literature on this topic. “Our findings could help athletes cope better with the level of heat strain experienced in hotter environments, particularly when an athlete’s core temperatures might rise to the critical level where cognitive and sprint performance declines have previously been found.”

Stacey and the team are examining this further, investigating the effect of combined cooling interventions on not only cognitive function but also neuromuscular function in the heat. “It will allow us to further understand the effect of this type of cooling method on exercise performance and the ability to produce force in the heat.”

Read the full research article in Experimental Physiology, ‘Effect of internal and external cooling on high-intensity intermittent cycling performance and cognitive function in the heat’.