Ice slurry ingestion during break times attenuates the increase of core temperature in a simulation of physical demand of match-play tennis in the heat

ABSTRACT

This investigation assessed the effect of ice slurry ingestion compared to that of cold water ingestion during break times on thermal strain and perception in simulated match-play tennis in the heat. Seven male recreational athletes (age = 22 ± 2 yr, height = 1.72 ± 0.08 m, Body mass = 64.8 ± 6.8 kg) performed two trials in a climate chamber, each time completing 4 sets of simulated match-play. During International Tennis Federation-mandated breaks (90-s between odd-numbered games; 120-s between sets), either ice slurry or cold water were ingested. The rectal temperature, forehead skin temperature, heart rate, rating of thermal comfort and total sweat loss were measured. The change in rectal temperature in the ice slurry trial was significantly lower than that in the cold water trial by game 3 of set 3 (p = 0.02). These differences in Δrectal temperature persisted throughout the remainder of the “match” (p < 0.05). Forehead skin temperature, heart rate and rating of thermal comfort were significantly lower in the ice slurry trial than in the cold water trial by the second half of the experiment (p < 0.05). Total sweat loss in ice slurry trial is significantly lower than cold water trial (p = 0.002). These results suggested that ice slurry ingestion was more effective than cold water ingestion in mitigating the development of heat strain during simulated match-play tennis in the heat.

Introduction

It is common for major annual international sport competitions to be held in severe hot conditions. Annual events such as the tennis Grand Slam tournaments at the Australian and US Open tournaments are often held at ambient temperatures of greater than 35°C. Bergeron 2014 [1] reported that core temperature (Tc) of professional tennis players during hard court tennis match play reaches approximately 39°C in these conditions. Exercise-induced increases in Tc could negatively impact performance and promote the heat-related health issues [2].

Various cooling interventions are used to mitigate heat stress during mandated rest periods in match-play between games [3]. Schranner et al. 2017 [4] and Lynch et al. 2017 [5] reported that cooling by ice towels or a fan with moisture applied to the skin during break time in simulated match-play tennis were effective for attenuating the increase in rectal temperature (Tre) and mean skin temperature (Tsk) in hot environments. However, a previous review noted that these methods of external cooling, such as fans are ineffective because they may also reduce muscle temperature [6].

Many recent studies focused on cooling by the ingestion of ice slurry to reduce the Tc and improve sports performance in the heat [7]. For example, Stevens et al. 2013 [8] showed that 10 g/kg body mass (BM) of ice slurry ingestion during the cycling leg of a simulated Olympic distance triathlon decreased the gastrointestinal temperature, and subsequently improved the 10-km running performance time by 2.5% compared to the effects of warm water ingestion. However, ad libitum ice slurry ingestion during exercise tends to be lower than that of cold water ingestion [9]. Therefore, the decreased intake of ice slurry will not attenuate the increase in Tre when compared to cold water. Ice slurry ingestion may be somewhat difficult to ingest during intense exercise with high associated ventilation, and intake may cause participants additional discomfort during exercise compared with to cold water, which is more easily consumed even though it is less effective in lowering Tc. However, it is possible that cooling by the consumption of ice slurry could be effective and simple in the short in-play breaks (90–120 sec) at the bench between odd games and sets permitted by the standard structure of tennis play.

The following two mechanisms could explain how ice slurry ingestion during break times is effective in attenuating the rise of Tc in hot conditions. One potential rationale is that due to the high humidity present it is likely that a greater percentage of sweat secreted during match play would drip off the skin rather than evaporate. As such, creating a large internal heat transfer could be beneficial even with a subsequent reduction in sweating because the sweat are secreting is not able to evaporate anyway. Another potential rationale is that self-generating airflow stop by moving quickly through air particles leading to a rapid reduction in sweat rate when athletes stop exercising. This reduction in sweat rate during break times provides an opportunity to accelerate them potential cooling through the ingestion of ice slurry as athletes become less dependent on cooling by evaporative means. Therefore, we thought that ice slurry ingestion during break times may be effective strategy in order to cool your body in hot conditions.

The purpose of this study was to determine whether ice slurry ingestion during break time attenuates the increase Tc in a simulation of physical demand of match-play tennis in the heat compared to cold water ingestion. We hypothesized that ice slurry ingestion would attenuate the rise in Tre during the second half of the exercise compared to cold water ingestion as reported by Takatori et al. 2002 [10], who demonstrated that cold beverage ingestion during exercise attenuates the rise in Tre during the second half of exercise in the heat.

Methods

Participants

Seven male recreational athletes (age = 22 ± 2 yr, height = 1.72 ± 0.08 m, BM = 64.8 ± 6.8 kg) volunteered for the study. The participants currently completed a minimum of 8 hours (h) of training per week. The number of participants was determined by a sample size calculation using data from a previous study [11], which reported differences in the reduction of the Tre after ice slurry ingestion compared to that after cold water ingestion. Based on an α-level of 0.05, a power (1-β) of 0.95, and the expected difference in Tre between ice slurry and cold water ingestion, at least seven participants were required. The experiments were approved by the Ethics Committee of the Department of Sport Science, Japan Institute of Sport Sciences (2017–024), and all participants read and signed an informed consent form before the experiments began.

Experimental design

Participants performed two different trials in counterbalanced order as follows: ingestion of ice slurry (SLURRY) or cold water (WATER) at each break. In all session exercise was performed in a climate chamber to simulated the peak environmental conditions reported at US Open: 36.5 ± 0.5°C ambient temperature, 50 ± 3% relative humidity. During the 24-h period before the experimental trial, the participants were instructed to avoid strenuous exercise as well as the consumption of alcohol, caffeine, nonsteroidal anti-inflammatory drugs, and nutritional supplements. Throughout the study period, all participants were asked to keep their normal lifestyle activities at a stable level, including their physical activity and nutritional habits. Each participant arrived at the laboratory after having refrained from eating for 6 h and drinking any type of beverage for 2 h. They were instructed to drink 500 mL of plain water 2 h before all tests to help promote euhydration prior to the start of each trial. The trial was started at the same time for each participant to control for circadian variations in the Tc, and all trials were separated by 48 h [4].

Upon arrival at the laboratory, urine samples were collected and nude BM was measured before entering the climate chamber. A rectal thermistor (ITP010-11; Nikkiso-Therm CO., Ltd., Tokyo, Japan) was inserted approximately 15 cm into the rectum. Four skin thermistors were affixed using hypoallergenic polyacrylate adhesive tape (ITP082-24; Nikkiso-Therm CO., Ltd., Tokyo, Japan) to the forehead, chest, forearm, and thigh. These thermistors were covered with temperature insulation pads (P252, Nihon Koden, Tokyo, Japan) in order to remove the influence of the air temperature. A heart rate (HR) monitor (WEP-3214, Nihon Koden, Tokyo, Japan) was then fixed to each participant’s chest. After the remaining instrumentation was affixed, participants entered the climate chamber.

Tennis simulated running test

The exercise included a running protocol using a treadmill (BM-1200 Biomill, S & ME, Tokyo, Japan) to simulate match-play tennis designed by Schranner et al. 2017 [4]. The protocol consisted of 5-sec sprints at 16 km/h, followed by a 3-sec deceleration phase, a 3-sec walk at 2 km/h, and a standing phase of 5 sec. This duration and intensity were reported to correspond to the average point length for elite male tennis player at the US Open [12]. Each simulated “game” consisted of 6 “points”, and each simulated “set” consisted of 8 “games”. Each session comprised of 4 simulated “sets” spanning a total time of approximately 81 min (Figure 1). Each mandated break of 90 s between odd-games, and the break of 120 s between sets were implemented in accordance with the International Tennis Federation (ITF) rules. Trials were terminated early if the Tre exceeded 39.5°C or volitional exhaustion occurred. After the exercise period, the participants dried themselves with a towel and were weighed again to determine their BM before the collection of a final urine sample.

Figure 1.

The protocol to simulate four sets of tennis match-play. Each set consisted of eight games, with a 120-sec set break and a 90-sec game break upon the completion of each odd-numbered game. Each game consisted of six identical points comprised of 5 sec of 15 km/h running, 3 sec of deceleration, 3 sec of 2 km/h walking and 5 sec of recovery.

Ice slurry (−1°C) was made using a slurry machine (Big Biz1, FMI, Japan), and cold water (4°C) was cooled by a thermostatic chamber (TR-2A, AS ONE, Japan). Both beverages were conventional sports drinks (Pocari Sweat, Otsuka Pharmaceutical, Japan). Participants consumed either 1.25 g/kgBM ice slurry [11] or cold water throughout each game and set break.

Measurements of physiological demands and the calculations

The HR was monitored continuously throughout the trial using an HR monitor and was reported as the average for each break before 30-sec. Urine samples were measured to evaluate the hydration status by urine specific gravity (USG), which was determined using a digital USG scale (PAL-09S, Atago, Japan). Nude BM was measured using a weighing machine (HW-100KGV, A and D, Japan) to the nearest 10 g. Throughout the two trials, the Tre and four skin temperatures (forehead [Thead], chest, forearm, and thigh) were recorded continuously via a data logger (N542R; Nikkiso-Therm Co., Ltd., Japan) and logged intermittently at each break. Onitsuka et al. 2015 [13] mentioned that Thead could be a useful index of brain temperature. The Tsk was calculated using the following formula from Roberts et al. 1977 [14]: Tsk = 0.43 × (chest temperature) + 0.25 × (forearm temperature) + 0.32 × (thigh temperature). The total sweat loss (TSL) was calculated using the following formula: (BM before the experiment – BM after the experiment) + the amount of the ingested drink. Ratings of the subjective thermal sensation [15] (RTS; 9-point scale ranging from 1 = “very cold” to 9 = “very hot”), thermal comfort [16] (RTC; 7-point scale ranging from 1 = “very uncomfortable” to 7 = “very comfortable”) and perceived exertion [17] (RPE; 15-point scale) were recorded at each break.

Statistical analysis

Results are shown as the means ± standard deviations (SD). An a priori sample-size calculation was performed (G*Power3.1.9.2; Dusseldolf, Germany) using p value data. All statistical computations were performed using the IBM SPSS Statistics 24 software package (SPSS, Inc., Chicago, IL, USA). The Tre, Tsk, Thead and HR were average over the final 15 s of every break and the final game. Two-way (drink × time) repeated-measures analysis of variance (ANOVA) was performed to compare the changes in the Tre, Tsk, Thead HR, RTS, RTC, RPE, BM and USG with the different experimental conditions. When a significant main effect or interaction effect was identified, the differences were delineated using a Bonferroni adjustment. The differences in TSL between conditions were examined using a t-test. For all comparisons, significance was set as a p value < 0.05.

Results

The mean volume of beverage consumed during the exercise period was 1555 ± 152 g for all treatments. The hydration states before and after the experiment are summarized in Table 1. The BM and USG values in both conditions were similar after the experiment, and no significant differences in the BM (p = 0.87) and USG (p = 0.53) after exercise were observed between conditions. TSL in the SLURRY trial was significantly lower than that in the WATER trial (p = 0.002). All participants completed the SLURRY trial, but one participant terminated in the WATER trial due to Tre exceeding 39.5°C. Three of eight participants experienced headaches while consuming ice slurry, whereas none experienced this symptom with cold water ingestion. No participants reported any headaches during each trial when exercising.

Table 1.

The hydration state before and after experiment.

	WATER		SLURRY
	Before	After	Before	After
Body mass (kg)	64.8 ± 6.8	64.5 ± 6.8	65.0 ± 7.1	64.9 ± 7.0
Total sweat volume (kg)	1.8 ± 0.4		1.6 ± 0.5*
Urine specific gravity	1.015 ± 0.007	1.021 ± 0.003	1.017 ± 0.005	1.020 ± 0.006

*WATER vs. SLURRY (p < 0.05)

Temperatures

The changes in Tre (ΔTre) during the experiment are shown in Figure 2a. Mean baseline absolute Tre was similar between the SLURRY (37.4 ± 0.3°C) and WATER trials (37.3 ± 0.3°C, p = 0.64). However, ΔTre in the SLURRY trial was significantly lower than that of the WATER trial beginning at game 3 of set 3 (p = 0.02; Figure2A). These differences in ΔTre persisted throughout the remainder of the “match” (p < 0.05).

Figure 2.

The Δrectal temperature (a) and forehead skin temperature (b) under two experimental conditions. The mean values are expressed as mean ± SD (WATER: ○, SLURRY: ●). Vertical lines the end of each “set”. Remaining participants (n) out of seven are indicated for trial. BL = baseline, G = game, * = WATER vs. SLURRY (p < 0.05).

The Thead was not significantly different between SLURRY (36.2 ± 0.3°C) and WATER trials in baseline (36.1 ± 0.6°C, p = 0.87). However, Thead was significantly lower in the SLURRY trial compared with WATER trial from game 8 of set 1 until game 5 of set 4 (p < 0.05; Figure2B).

Tsk (Figure 3a) during both trials is showed that both thermoregulatory responses were not significantly different throughout the match (p > 0.05).

Figure 3.

The mean skin temperature (a) and rating of thermal comfort (b) under two experimental conditions. The mean values are expressed as mean ± SD (WATER: ○, SLURRY: ●). Vertical lines the end of each “set”. Remaining participants (n) out of seven are indicated for trial. BL = baseline, G = game. * = WATER vs. SLURRY (p < 0.05) and † = WATER vs. SLURRY (p < 0.10).

HR

There were no significant in the HR at baseline between SLURRY (78.3 ± 7.5 bpm) and WATER trials (80.1 ± 5.4 bpm, p = 0.44). However, the HR was significantly lower in the SLURRY trial than that in the WATER trial from set 2 to the last set (Table 2; p < 0.05).

Table 2.

The changes in rating of percieved exertion (6 [no exertion] to 20 [maximal exertion]), rating of thermal sensation (1 [very cold] to 9 [very hot]) and heart rate during trials. Data are presented as mean ± SD.

		BL	1 s-8 g	2 s-8 g	3 s-8 g	4 s-8 g
RPE	WATER	−	11.3 ± 1.8	12.4 ± 1.5	14.0 ± 1.3	14.7 ± 2.5
	SLURRY	−	10.0 ± 2.4	12.1 ± 2.1	13.3 ± 0.8	13.5 ± 1.0
RTS	WATER	−	7.9 ± 0.9	8.1 ± 0.9	7.3 ± 2.6	7.8 ± 1.9
	SLURRY	−	7.4 ± 0.8	7.6 ± 1.0	7.3 ± 1.3	7.3 ± 1.4
HR (bpm)	WATER	80.1 ± 5.4	145.6 ± 13.4	150.9 ± 11.9*	158.3 ± 14.4*	165.0 ± 10.6*
	SLURRY	78.3 ± 7.5	133.0 ± 14.0	139.4 ± 8.9	144.6 ± 10.4	146 ± 8.2

*WATER vs. SLURRY (p < 0.05), BL = baseline, s = set, g = game, RPE = rating of percieved exertion, RTS = rating of thermal sensation, HR = heart rate

Perceptions

No significant differences were observed in the RTS and RPE between conditions throughout exercise (Table 2; p > 0.05). However, participants were more comfortable in the SLURRY trial (3.3 ± 1.0) than that of WATER trial (2.3 ± 1.4, p = 0.02) by game 1 of set 3 (Figure 3b). RTC generally remained a lower in SLURRY trial than in the WATER trial (p < 0.10).

Discussion

This is the first study to compare the Tre resulting from ice slurry (SLURRY) and cold water ingestion (WATER) during simulated match-play tennis in hot conditions. In accordance with our hypothesis, ice slurry ingestion during break time was significantly more effective for attenuating the increase in Tre than cold water ingestion at the second half of exercise during simulated in match-play tennis in the heat. In addition, the important results of this study were the following: HR, TSL and RTC in SLURRY trial was significantly lower than those in the WATER trial.

The present findings indicate that ice slurry ingestion during simulated match-play tennis may be a more practical and effective cooling strategy to maintain a lower Tre in the heat. Compared to other sports, tennis has short in-play break times (90–120 s) for cooling. Although the ingestion of ice slurry may be practically difficult during exercise [9], the present study found that ice slurry ingestion across the same timeframe of an actual match did attenuate the increase in Tre. Previous studies reported that the ad libitum ingestion of ice slurry during exercise did not attenuate the rise of Tre compared to those found following warm (30°C) or cold (4°C) water ingestion. [9,18,19] However, in these studies, participants consumed approximately 800 g of ice slurry, half as much as the present study. Therefore, the present results suggest that ice slurry ingestion during rest may be able to attenuate the rise of Tre, if enough ingestion volume is secured, even if exercise-induced heat production is large.

The present study observed for the first time that ice slurry ingestion at rest interval during exercise attenuated the rise in Thead covered with temperature insulation pad compared to cold water ingestion. On the other hands, each other skin temperature (i.e., forearm, chest or thigh temperatures) did not cause the attenuation of rise between SLURRY and WATER trials (p > 0.05) in the present study. Naito et al. 2017 [20] and Siegel et al. 2010 [21] speculated that oral ingestion of ice slurry possibly resulted conductive cooling of the facial skin. There is possible that ice slurry ingestion may cool the blood flowing to the brain via conductive cooling because ice slurry is ingested through the mouth. Onitsuka et al. 2018 [22] recently reported that ice slurry ingestion reduced temperature at the frontal cortex using magnetic resonance spectroscopy (MRS). If ice slurry ingestion can attenuate temperature at the frontal cortex, it is speculated that it would be a more effective cooling strategy that attenuates both peripheral and central fatigue for racket sports such as tennis.

The decoupling of RTC and RTS, as shown between the SLURRY and WATER trials in the present study, was documented by Schulze et al. 2015 [19], who showed that ice slurry ingestion during exercise caused a moderate decrease in RTC compared to that of warm water ingestion, but the difference in RTS was unclear. It is possible that this decoupling of thermal perception may be related to the cooling of body parts, which was inferred from papers reviewed by Bongers et al. 2017 [23] and Stevens et al. 2017 [24]. Indeed, although external cooling of the neck or facial area cooling decreased RTC, RTS and Tsk without attenuating the rise in Tre during exercise, internal cooling by cold beverage ingestion only caused the decrease in RTS despite the reduction of Tre (see Stevens et al., 2017 [24]). These observations suggested that RTS was affected by cold receptors located in the skin, especially in the neck or facial area, which might stimulate a greater density of cold-sensitive thermal afferents. Therefore, the similar RTS between conditions in the present study was probably similar to the Tsk due to the lack of external cooling. In addition, the reduction in RTC may have occurred because cooling of the mouth and gastrointestinal tract by ice slurry ingestion during exercise in the heat is perceived as pleasant, as it protects against dehydration [25]. To summarize these suggestions, the peripheral cold receptors in skin, mouth and gastrointestinal tract are therefore likely to differ in the way they are connected to central control mechanisms in the brain. Finally, Schlader et al. 2011 [26] noted that the interpretation of thermal comfort is more important for performance than thermal sensation. However, ice slurry ingestion, which causes a false sense of lowered thermal perception, may cause an athlete to extend themselves beyond normal thermoregulatory limits, exposing them to a greater risk of heat-related illnesses [21,24].

Greater sweat loss during exercise in the heat leads to dehydration and decreases in cardiac output, which is compensated for by an increase in HR. Position stand in American College of Sports Medicine 2007 [27] has recommended that the goal of drink during exercise is to prevent excessive dehydration (> 2% BM loss from water deficit). The potential heat sink created by ice slurry ingestion may allow for more metabolic heat to be produced without compromising cardiovascular dynamics [28], leading to an attenuated rise in HR and prevention of dehydration. In the present study, it was estimated that the volume of ice slurry might be reasonable because BM in post-exercise was not significantly increased compared to the BM in pre-exercise in SLURRY trial (p = 0.27). In addition, TSL in the SLURRY trial after exercise was significantly lower than that of the WATER trial. This finding has previously been reported by Morris et al. 2016 [18], who noted that the ingestion of ice slurry significantly reduced whole-body sweat loss and evaporative heat loss compared to when ingesting warm water (37°C) during 75 min of cycling at 33°C. It is possible that the lower Tre induced by sufficient ingestion of ice slurry itself reduced cardiac heat stress and prevented dehydration, including the HR and TSL [29].

The intervention strategy using the present study might be difficult to accept that ingesting a similar volume of ice (1.25 g/kgBM) over an 80 min period during high intensity exercise would have been comfortable for the participant. However, we could indicate to have more choice of cooling strategy for sports competitions. If athletes have a discomfort when ingests ice slurry during break times, they should choice pre-cooling by ice slurry ingestion. Naito and Ogaki 2016 [11] showed that intermittent ice ingestion before exercise for a 30 min period decreased Tre on resting. Further study is warranted to compare the effects of pre-cooling versus mid-cooling by intermittent ice slurry ingestion.

Limitations

This study is associated with some limitations. First, the physical fitness level of participants in this study were unclear. The previous study reported that HR in moderate physical fitness athletes was approximately 160 bpm during the second half of exercise [5]. It is possible that the physical fitness level of participants in the present study may be similar to that of previous study because HR observed here was similar to that reported in previous studies.⁵ Second, we did not perform tepid fluid or no-fluid ingestion trials. However, the ingestion of cold water served as a control trial as this is common practice among many athletes. Third, the present study was conducted in a climate chamber but not outside on a tennis court. Outside conditions with solar radiation may be caused different responses of physiological demands in a simulation of match-play tennis. Therefore, further study is needed to determine the effects of ice slurry ingestion during break times on outside conditions. Finally, the present study did not have a performance trial, as its primary aim was to assess the influence of ice slurry ingestion on thermal strain and perception compared to cold water ingestion. As a result, it remains to be determined whether ice slurry ingestion leads to improved tennis specific performance. Note that one participant in WATER trial terminated early due to Tre exceeded 39.5°C, but all participants completed the SLURRY trial.

Conclusions

Ice slurry ingestion during breaks in simulated match-play tennis significantly attenuates the rise in both rectal and forehead skin temperature in the heat and, thereby could reduce total sweat loss, heart rate and thermal comfort. This study indicates that the ingestion of ice slurry was more effective than cold water ingestion in mitigating the development of heat strain during match-play tennis in the heat. Further research is needed to extend the effects of ice slurry ingestion on tennis-specific performance [30] and cognitive function.

Acknowledgments

The authors would like to thank the participants for their involvement in the study.

Abbreviations

BM

body mass

ITF

international tennis foundation

HR

heart rate

RTC

rating of thermal comfort

RTS

rating of thermal sensation

RPE

rating of perceived exertion

SD

standard deviations

SLURRY

ice slurry

Tc

core temperature

Thead

forehead skin temperature

Tre

rectal temperature

Tsk

skin temperature

TSL

total sweat loss

USG

urine specific gravity

WATER

cold water

Disclosure statement

No potential conflict of interest was reported by the authors.