International Journal of Sports Physiology and Performance, 2015, 10, 655 -663 http://dx.doi.org/10.1123/ijspp.2014-0281 © 2015 Human Kinetics, Inc.
ORIGINAL INVESTIGATION
Effect of Thermal State and Thermal Comfort on Cycling Performance in the Heat Emiel Schulze, Hein A.M. Daanen, Koen Levels, Julia R. Casadio, Daniel J. Plews, Andrew E. Kilding, Rodney Siegel, and Paul B. Laursen Purpose: To determine the effect of thermal state and thermal comfort on cycling performance in the heat. Methods: Seven well-trained male triathletes completed 3 performance trials consisting of 60 min cycling at a fixed rating of perceived exertion (14) followed immediately by a 20-km time trial in hot (30°C) and humid (80% relative humidity) conditions. In a randomized order, cyclists either drank ambient-temperature (30°C) fluid ad libitum during exercise (CON), drank ice slurry (–1°C) ad libitum during exercise (ICE), or precooled with iced towels and ice slurry ingestion (15g/kg) before drinking ice slurry ad libitum during exercise (PC+ICE). Power output, rectal temperature, and ratings of thermal comfort were measured. Results: Overall mean power output was possibly higher in ICE (+1.4% ± 1.8% [90% confidence limit]; 0.4 > smallest worthwhile change [SWC]) and likely higher PC+ICE (+2.5% ± 1.9%; 1.5 > SWC) than in CON; however, no substantial differences were shown between PC+ICE and ICE (unclear). Time-trial performance was likely enhanced in ICE compared with CON (+2.4% ± 2.7%; 1.4 > SWC) and PC+ICE (+2.9% ± 3.2%; 1.9 > SWC). Differences in mean rectal temperature during exercise were unclear between trials. Ratings of thermal comfort were likely and very likely lower during exercise in ICE and PC+ICE, respectively, than in CON. Conclusions: While PC+ICE had a stronger effect on mean power output compared with CON than ICE did, the ICE strategy enhanced late-stage time-trial performance the most. Findings suggest that thermal comfort may be as important as thermal state for maximizing performance in the heat. Keywords: thermoregulation, core temperature, pacing, internal cooling, power output Exercise in hot conditions leads to increases in heat storage and core temperature.1 Thermoreceptors located throughout the body detect the thermal change, relaying this information through afferent channels to the brain,2,3 which integrates such information and lowers motor output.4 This response reduces the rate of heat production from the contracting skeletal muscles to minimize possible thermal injury.5 From an applied perspective, it has been found that performance in the heat is most effectively enhanced with aerobic-fitness development and heat acclimation.6 However, 2 acute methods that increase performance in the heat involve a focus on counteracting the bodily reactions to heat development and include preexercise cooling7 and consuming cold fluids during exercise.8,9 Precooling involves the lowering of core body temperature, or the thermal state of the body, before competing in the heat. Cooling can be administered externally, with cold-water immersion or iced garments, or internally, through the ingestion of cold fluids.10 The combined procedure of using both internal and external techniques, such as the cyclic dousing of iced towels on the legs and torso while drinking ice slurry over the 30-minute period before exercise, has emerged as a practical and effective method.7 Ingesting cold fluid during prolonged exercise in the heat can lower core temperature and the perception of the body’s thermal Schulze, Daanen, and Levels are with MOVE Research Inst Amsterdam, VU University, Amsterdam, The Netherlands. Casadio, Plews, Siegel, and Laursen are with High Performance Sport New Zealand, Auckland, New Zealand. Kilding is with Sports Performance Research Inst New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand. Address author correspondence to Emiel Schulze at [email protected].
state and enhance exercise performance.8 A beverage formation with high cooling capacity is ice slurry, a mixture of water, ice, and sugar/electrolytes. Ingesting ice slurry has been shown to lower core temperature before exercise10 and is considered a practical strategy for use in warm conditions,7 as it also provides fluids to replace water lost from sweating.11 To our knowledge, only 2 studies have investigated the effect of ice slurry ingestion during exercise on performance and physiological responses in the heat. Using an Olympic-distance triathlon model, Stevens et al12 showed that the ingestion of ice slurry (10 g/kg body mass) during a race-simulated variable-intensity 60-minute cycle phase led to enhanced performance during a subsequent 10-km running time trial.12 Furthermore, Burdon et al13 found that ice slurry ingestion (260 ± 38 g every 15 min) during a 90-minute cycling phase (62% VO2max) enhanced performance during a subsequent 4-kJ/kg body-mass cycling time trial (~19 min).13 Notably, while no differences in rectal temperature between trials were found, rinsing of the mouth with ice slurry in 1 of these trials was also found to result in a faster time trial compared with thermoneutral (32°C) fluid ingestion. The latter finding suggests that even the presence of a cold substance in the mouth, which solely altered the perception of temperature, may be beneficial. That is, the perceived comfort of one’s body temperature, as much as one’s actual body temperature per se, may itself influence power output during exercise in the heat.14 Perceived thermal comfort is therefore an interesting variable to measure, since it indicates how the body’s thermal state is interpreted and whether exercising in a specific environment with a particular bodily condition is tolerable, not merely whether one recognizes a warm or cold sensation.14 Such perception may be more important for regulating exercise in the heat than the actual thermal state. For example, Schlader et al15 showed that exercise in 655
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the heat commencing with a low skin temperature reduced thermal perception and subsequently improved cycling performance. This was in contrast to an alternate trial where participants finished exercise with a low skin temperature; mean skin and core temperatures were similar in both conditions. This highlights the importance of investigating novel ways of altering thermal comfort (compared with thermal state) to maximize performance in the heat. In the previously described studies by Stevens et al12 and Burdon et al,13 participants had to cycle at a fixed intensity during the preload phase and drank fluids at set rates and were without forward-facing convective wind, all factors that do not take place in the field setting. In light of these considerations, our aim was to use 2 practical cooling strategies that had the potential to alter thermal comfort, thermal state, and performance in the heat—namely, practical precooling and ice slurry ingestion during exercise. As endurance-performance outcomes in field settings can have different “critical periods,” we compared effects over a long (~90-min) cycling trial under self-paced conditions, including a finishing 20-km time trial, using ecologically valid wind speeds and ad libitum drinking conditions. The different cooling strategies allowed us to compare the effects of thermal state and thermal comfort on cycling performance in the heat.
Methods
highest power output reached during the test and in a pro rata manner when participants ended the test partway through a step. VO2peak was defined as the highest VO2 measured during a 30-second period. Approximately 20 minutes after termination of the progressive exercise test, the first familiarization trial was performed.
Familiarization The experimental trials consisted of self-paced exercise, divided into 2 phases. The first phase consisted of exercise at a fixed rating of perceived exertion (RPE),17 while the second phase consisted of a 20-km time trial. As consistent fixed-RPE performance can be achieved within 3 trials,18 2 familiarization trials were performed before the first experimental trial. The first familiarization trial involved 30 minutes of cycling in thermoneutral conditions on a cycle ergometer (IndoorTrainer, SRM, Jülich, Germany) at a fixed RPE corresponding to a score of 14 on a 6-to-20 Borg scale, between somewhat hard (13) and hard (15).19 The second familiarization trial identically replicated the experimental trials (see next section). For the familiarization trial, participants drank 15 g/kg body mass of cool sports drink (4°C) during the 30-minute preexercise sitting period and ad libitum during exercise. Participants were briefed on the use of the RPE scale by the same investigator and in a similar fashion, to ensure familiarity with the scale throughout the study.
Participants
Experimental Trials
Seven experienced (>1 y competitive) and well-trained (>200 km cycling/week) male triathletes (mean ± SD age 33 ± 8 y, body mass 73.1 ± 3.3 kg, height 179.5 ± 4.9 cm, sum of 8 skin folds 63.7 ± 13.3 mm, peak oxygen uptake [VO2peak] 61.7 ± 3.0 mL · kg–1 · min–1, maximal aerobic power [MAP] 399 ± 38 W) volunteered for this study. All participants provided written informed consent before their first trial, and the study was approved by the Auckland University of Technology Ethics Committee.
To begin all trials, body mass was measured with the participant wearing only cycling shorts after voiding the bladder. Afterward, participants were equipped with a heart-rate monitor; skin-temperature thermistors (DS-1922L, Maxim Integrated, San Jose, CA) on the chest, arm, thigh, and calf; and a disposable rectal thermistor (Monatherm Thermistor, 400 Series, Mallinckrodt Medical, St Louis, MO), which was self-inserted ~12 cm past the anal sphincter. Participants entered an environmental chamber (Design Environmental Ltd, Gwent, UK) set to 30°C and 80% relative humidity, equal to a wet bulb globe temperature of 34°C. In a randomized order, the 3 experimental trials involved (1) a control trial where participants drank an ambient-temperature sports drink (30°C) during the 30-minute preexercise sitting period and during exercise (CON), (2) a trial where participants drank an ambient-temperature sports drink (30°C) during the preexercise sitting period and ice slurry (–1°C) during exercise (ICE), and (3) a trial in which participants were precooled during the 30-minute preexercise sitting period16 and drank ice slurry (–1°C) during exercise (PC+ICE). The participants were seated on a chair in the heat for a 40-minute period. The first 10-minute period was used to achieve stabilization, while during the remaining 30 minutes participants ingested 7.5 g/kg body mass of fluid during two 15-minute periods, so that 15 g/kg was consumed during the preexercise period.16 The drink contained electrolytes and carbohydrates (Gatorade 02 Perform Instant Powder Mix) and was identical in content for all conditions. The practical precooling routine consisted of ingesting ice slurry and being doused with iced towels on the legs and torso.16 Each 30 seconds, a towel was replaced with one from a container with ice and water and the warm towel was returned to the cool container. This format allowed a towel to remain on the torso or legs for 60 seconds before being replaced. After this preliminary phase, participants performed a 10-minute warm-up on a cycle ergometer (IndoorTrainer, SRM, Jülich, Germany), which included
Preparation Before the experimental trials, participants visited the laboratory on 2 separate occasions. The first visit involved a progressive maximal exercise test and a short familiarization trial, while the second visit involved a complete familiarization trial. After these preliminary tests, 3 experimental trials were performed, making a total of 5 separate visits to the laboratory separated by 3 to 7 days. Participants were asked to maintain their normal training regimen and to replicate this each week throughout the study. They were asked to replicate their typical daily dietary consumption for each trial, to avoid long (>2 h) or strenuous physical activity (>70% HRmax) the day before trials, and to avoid consumption of alcohol and caffeine.
Progressive Test The progressive maximal exercise test was performed on a cycle ergometer (Lode Excalibur Sport, Groningen, The Netherlands) in thermoneutral conditions (22°C, 50% relative humidity). Participants were equipped with a heart-rate monitor (Suunto M5, Suunto Ltd, Finland), and oxygen uptake was measured with a calibrated diagnostic system (TrueOne2400, ParvoMedics, East Sandy, UT). The test started with a warm-up of 5 minutes at a power output of 100 W, and thereafter intensity increased in a stepwise fashion by 25 W every 60 seconds until the participant reached volitional exhaustion.16 The MAP of each participant was determined as the
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3 minutes at 25% MAP followed by 5 minutes at 60% MAP and 2 minutes at 80% MAP.16 Frontal-facing convective wind movement was simulated with 2 industrial fans (FS-75, FWL, Auckland, New Zealand) generating a wind speed of ~33 km/h, an appropriate wind speed for reducing heat storage and comparable to outdoor cycling.20 Wind speed was measured using an anemometer (3000 Wind Meter, Kestrel, Sylvan Lake, MI). Four minutes after warming up, participants began the 60-minute steady-state phase at the RPE of 14. RPE was reaffirmed every 5 minutes, and participants self-adjusted power output to maintain the required RPE. Ingested drink volume was measured every 10 minutes by recording the weight of the drink bottle (Floe Bottle, Teknicool Ltd, Auckland, New Zealand). The bottle was specifically designed to keep ice slurry cold over a longer period of time. As the density of ice slurry is different than that of water, change in bottle mass was used to report fluid ingestion as opposed to volume. On completion of the fixed-RPE phase, participants were given 2 minutes of rest before they commenced the 20-km time trial. They were asked to perform their best and finish the trial in the shortest time possible. During the time trial, power output, heart rate, elapsed time, and thermoregulatory measures were monitored and recorded but blinded to participants. Participants were informed of their progress every 1 km of the trial elapsed, but no direct encouragement was given.
Measurements Every 10 minutes during a trial, participants were asked to rate their thermal comfort on a 1-to-4 scale and thermal sensation on a 1-to-7 scale.21 Before and after a phase, participants were asked to rate their gastrointestinal comfort, measured on a 4-point scale, varying from very comfortable to very uncomfortable22; thermal comfort and sensation21; and thirst sensation, measured on a 9-point scale, varying from not thirsty at all to severely thirsty.23 Body mass was measured after exiting the environmental chamber and toweling dry, while only wearing cycling shorts. Rectal temperature (Tre) was recorded with a data logger (Grant Instruments, Shepreth, UK), skin temperature (Tsk) was recorded on the thermistor (DS-1922L, Maxim Integrated, San Jose, CA), heart rate was measured using a heart-rate monitor (Suunto M5, Suunto Ltd, Finland), and power output was measured using the SRM power meter linked wirelessly to the PowerControl 7 unit (SRM, Jülich, Germany). All data were recorded at a sampling frequency of 1 Hz. Mean Tsk was calculated using the formula24 Tsk = [0.3 × (Tchest + Tarm)] + [0.2 × (Tthigh + Tcalf)], and body-mass loss was determined as body mass loss = Δ body mass + total fluid ingestion.
Statistical Analyses Data are presented as mean ± SD or with 90% confidence limits (CL) as appropriate. The differences in mean cycling power output during the steady-state phase, time-trial performance time, and power output, as well as physiological and subjective measurements, were analyzed using a magnitude-based inference approach.25 This method is used to indicate the possible benefit, in terms of beneficial or harmful effects, of each condition. The magnitude of differences in the changes between trials is expressed as standardized differences (Cohen effect sizes, ES). The criteria to interpret the magnitude of the ES were 0.8, large.25 ESs, with uncertainty of the estimates shown as 90% CL, were determined using a custom-made spreadsheet (Post Only,
Sportscience).26 Data were log-transformed and, after calculations, back-transformed to obtain differences between trials. The smallest worthwhile change (SWC) in performance measures was set at 1% for mean power output.27 For physiological measures, these changes were set at 0.2 multiplied by the between-subjects SD.28 To calculate this, the between-subjects SD from CON was used for analyses between CON and ICE or PC+ICE, while the between-subjects SD from ICE was used for analyses between ICE and PC+ICE. Quantitative chances of being beneficial or harmful were assessed qualitatively as 99.5%, most likely or almost certainly.25 When an effect was both beneficial and harmful for >5%, the true difference was described as unclear.26 Furthermore, correlations (r) between measures were determined and the magnitude of correlation (r [90% CL]) was assessed with the following thresholds: SWC). Notably, this enhanced performance occurred despite a lower fluid (and therefore carbohydrate; –18 ± 19 g, –20% ± 26%) ingestion in ICE than in CON and with only a trivial effect on Tre (Figure 4). While ice slurry ingestion independently of precooling had little effect on overall thermal state during exercise, it improved thermal comfort in both experimental conditions compared with CON (ICE ES = –0.67 ± 0.68, PC+ICE ES = –0.43 ± 0.42) and likely contributed toward the differences in power output shown between these trials. A decoupling of thermal state and thermal comfort, as shown between the ICE and CON trials in the current study, has been documented previously. Burdon et al13 showed that ice slurry ingestion during exercise elicited a higher power output during a finishing cycling time trial. While ratings of thermal comfort were lower with ice slurry ingestion than in the control trial (37°C fluids), core temperature was similar.13 As with the current study, the comparable levels of thermal state between ICE and CON trials, despite the
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enhanced performance in ICE, suggest that thermal comfort is at least as important as thermal state for maximizing performance.14 It has been proposed that thermal receptors in the mouth29 and gastrointestinal region10 are important for the regulation of body temperature during exercise in the heat.11 Ingesting a –1°C ice slurry likely lowers the temperature of these regions,30 potentially influencing thermal comfort. Evidence for this in the current investigation was shown through a substantially lower thermal comfort during the first 60 minutes of cycling in the ICE trial, despite a similar Tre and power output compared with CON. This suggests that ingested ice slurry cooled regions near internal thermoreceptors and likely lowered heat sensation, despite no detectable change in the global thermal state of the body measured through Tre and Tsk. Indeed, Levels et al31 recently showed that reduced body-heat content and sensation of coolness is beneficial for pacing during later exercise stages. In the current study, the improved thermal comfort with ICE likely allowed for the higher power outputs achieved in the time trial,14 with the additional metabolic heat produced from the higher power outputs in ICE likely offsetting any potential cooling effect, resulting in the similar Tre shown.10 The combination of practical precooling and ice slurry ingestion during exercise led to the highest power output during the fixed-RPE phase compared with both CON (4.3% ± 4.3%, 3.3 > SWC) and ICE (3.3% ± 6.2%, 2.3 > SWC) conditions. While higher power outputs were achieved during this initial fixed-RPE phase in the PC+ICE trial, this aggressive pace resulted in a reduced time-trial performance compared with ICE (–2.8% ± 3.0%, 1.8 < SWC; Figure 1). Indeed, it has been shown that starting Tsk has a large influence on anticipatory pacing in the heat compared with thermal perception.32 Quod et al33 showed that when well-trained cyclists were precooled using an ice jacket, they tended to start aggressively during a performance trial, irrespective of Tre, and resulting overall performance was similar to a control condition. In the current study, the lower Tsk and Tre measured near the start of the fixed-RPE phase in the PC+ICE trial likely enabled the higher work output and associated heat production, resulting in a greater rise of core temperature and fatigue accumulation, leading to equivalent time-trial performances (Figure 1). Notably, when Tre in PC+ICE became similar to, and above, that of CON and ICE, after ~30 minutes during the fixed-RPE phase, the power output remained higher until the end of the phase (Figure 3[C]). This occurrence again suggests that one’s thermal state is not the only regulator of power output during exercise in the heat.11,13 Since ratings of thermal comfort remained lower throughout the fixed-RPE phase of the trial, these perceptions likely influenced power output and performance, as shown previously.14 Two final observations worthy of mention are the decreased thirst shown in PC+ICE compared with the CON trial and the strong correlation between overall power output and thirst sensation. Compared with CON, both volume of fluid ingestion and weight loss were similar in PC+ICE during the trial. The lower sensation of thirst may have been a result of decreased skin blood flow due to the (external) precooling, thereby increasing water content to central circulation and lowering plasma osmolality and ultimately thirst.23,33,34 This in turn may have contributed to the enhanced overall performance effect shown.35
Practical Implications Precooling and cooling during exercise may be useful strategies to employ during prolonged cycling performance in the heat,36 but caution with administering such strategies should be taken, as the
perceived cooler state can elicit inappropriate pacing, especially when an end spurt is tactically key to the overall performance outcome. While increased heat-storage capacity (thermal state) can enhance performance in the heat, methods of improving thermal comfort during exercise are also effective.
Conclusions While PC+ICE had a stronger effect on mean power output compared with CON than ICE did, the ICE strategy enhanced late-stage time-trial performance the most. Despite equivalent thermal states between ICE and CON, performance was enhanced in the ICE condition. These findings suggest that thermal comfort may be as important as thermal state for maximizing performance in the heat. Acknowledgments No sources of funding were used to assist in the preparation of this article. Paul Laursen is a minority shareholder in Teknicool Ltd, the company that produced the ice slurry bottle used in the current study. The authors express their gratitude to all participants in this study. We would like to thank Professor Will Hopkins for his expert opinion and advice on statistical analysis.
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11. Siegel R, Laursen PB. Keeping your cool: possible mechanisms for enhanced exercise performance in the heat with internal cooling methods. Sports Med. 2012;42(2):89–98. PubMed doi:10.2165/11596870000000000-00000 12. Stevens CJ, Dascombe B, Boyko A, Sculley D, Callister R. Ice slurry ingestion during cycling improves Olympic distance triathlon performance in the heat. J Sports Sci. 2013;31(12):1271–1279. PubMed doi :10.1080/02640414.2013.779740 13. Burdon CA, Hoon MW, Johnson NA, Chapman PG, O’Connor HT. The effect of ice slurry ingestion and mouthwash on thermoregulation and endurance performance in the heat. Int J Sport Nutr Exerc Metab. 2013;23(5):458–469. 14. Schlader ZJ, Simmons SE, Stannard SR, Mündel T. The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior. Physiol Behav. 2011;103:217–224. PubMed doi:10.1016/j.physbeh.2011.02.002 15. Schlader ZJ, Simmons SE, Stannard SR, Mündel T. Skin temperature as a thermal controller of exercise intensity. Eur J Appl Physiol. 2011;111:1631–1639. PubMed doi:10.1007/s00421-010-1791-1 16. Ross MLR, Garvican LA, Jaecocke NA, et al. Novel precooling strategy enhances time trial cycling in the heat. Med Sci Sports Exerc. 2011;43(1):123–133. PubMed doi:10.1249/MSS.0b013e3181e93210 17. Tucker R, Marle T, Lambert EV, Noakes TD. The rate of heat storage mediates an anticipatory reduction in exercise intensity during cycling at a fixed rating of perceived exertion. J Physiol. 2006;574(Pt 3):905–915. PubMed doi:10.1113/jphysiol.2005.101733 18. Dishman RK. Prescribing exercise intensity for healthy adults using perceived exertion. Med Sci Sports Exerc. 1994;26(9):1087–1094. PubMed doi:10.1249/00005768-199409000-00004 19. Borg GAV. Psychophysical bases for perceived exertion. Med Sci Sports Exerc. 1982;14(5):377–381. PubMed 20. Saunders AG, Dugas JP, Tucker R, Lambert MI, Noakes TD. The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment. Acta Physiol Scand. 2005;183:241–255. PubMed doi:10.1111/j.1365-201X.2004.01400.x 21. Gagge AP, Stolwijk JAJ, Hardy JD. Comfort and thermal sensations and associated physiological responses at various ambient temperatures. Environ Res. 1967;1:1–20. PubMed doi:10.1016/0013-9351(67)90002-3 22. Lambert GP, Lang J, Bull A, Eckerson J, Lanspa S, O’Brien J. Fluid tolerance while running: effect of repeated trials. Int J Sports Med. 2008;29:878–882. PubMed doi:10.1055/s-2008-1038620 23. Engell DB, Maller O, Sawka MN, Francesconi RN, Drolet L, Young AJ. Thirst and fluid intake following graded hypohydration levels in humans. Physiol Behav. 1987;40:229–236. PubMed doi:10.1016/0031-9384(87)90212-5
24. Ramanathan NL. A new weighting system for mean surface temperature of the human body. J Appl Physiol. 1964;19(3):531–533. PubMed 25. Hopkins WG, Marshall SW, Batterham AM, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc. 2009;41(1):3–13. PubMed doi:10.1249/ MSS.0b013e31818cb278 26. Batterham AM, Hopkins WG. Making meaningful inferences about magnitudes. Int J Sports Physiol Perform. 2006;1:50–57. PubMed 27. Lamberts RP, Swart J, Woolrich RW, Noakes TD, Lambert MI. Measurement error associated with performance testing in well-trained cyclists: application to the precision of monitoring changes in training status. Int SportMed J. 2009;10(1):33–44. 28. Hopkins WG. How to interpret changes in an athletic performance test. Sportscience. 2004;8:1–7. 29. Mündel T, Jones DA. The effects of swilling an L–-menthol solution during exercise in the heat. Eur J Appl Physiol. 2010;109:59–65. PubMed doi:10.1007/s00421-009-1180-9 30. Siegel R, Maté, Brearly MB, Watson G, Nosaka K, Laursen PB. Ice slurry ingestion increases core temperature capacity and running time in the heat. Med Sci Sports Exerc. 2010;42(4):717–725. PubMed doi:10.1249/MSS.0b013e3181bf257a 31. Levels K, Teunissen LPJ, De Haan A, De Koning JJ, Van Os B, Daanen HAM. Effect of warm-up and precooling on pacing during a 15-km cycling time trial in the heat. Int J Sports Physiol Perform. 2013;8(3):307–311. PubMed 32. Schlader ZJ, Stannard SR, Mündel T. Human thermoregulatory behavior during rest and exercise—a prospective review. Physiol Behav. 2010;99:269–275. PubMed doi:10.1016/j.physbeh.2009.12.003 33. Quod MJ, Martin DT, Laursen PB, et al. Practical precooling: effect on cycling time trial performance in warm conditions. J Sports Sci. 2008;26(14):1477–1487. PubMed doi:10.1080/02640410802298268 34. Sleivert GG, Cotter JD, Roberts WS, Febbraio MA. The influence of whole-body vs. torso pre-cooling on physiological strain and performance of high-intensity exercise in the heat. Comp Biochem Physiol A Mol Integr Physiol. 2001;128(4):657–666. PubMed doi:10.1016/ S1095-6433(01)00272-0 35. Dion T, Savoie FA, Asselin A, Gariepy C, Goulet EDB. Half‐marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners. Eur J Appl Physiol. 2013;113:3011–3020. PubMed doi:10.1007/s00421013-2730-8 36. Bongers CC, Thijssen DH, Veltmeijer MT, Hopman MT, Eijsvogels TM. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. Br J Sports Med. 2015;49(6):377–384. PubMed
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ORIGINAL INVESTIGATION
Effect of Thermal State and Thermal Comfort on Cycling Performance in the Heat Emiel Schulze, Hein A.M. Daanen, Koen Levels, Julia R. Casadio, Daniel J. Plews, Andrew E. Kilding, Rodney Siegel, and Paul B. Laursen Purpose: To determine the effect of thermal state and thermal comfort on cycling performance in the heat. Methods: Seven well-trained male triathletes completed 3 performance trials consisting of 60 min cycling at a fixed rating of perceived exertion (14) followed immediately by a 20-km time trial in hot (30°C) and humid (80% relative humidity) conditions. In a randomized order, cyclists either drank ambient-temperature (30°C) fluid ad libitum during exercise (CON), drank ice slurry (–1°C) ad libitum during exercise (ICE), or precooled with iced towels and ice slurry ingestion (15g/kg) before drinking ice slurry ad libitum during exercise (PC+ICE). Power output, rectal temperature, and ratings of thermal comfort were measured. Results: Overall mean power output was possibly higher in ICE (+1.4% ± 1.8% [90% confidence limit]; 0.4 > smallest worthwhile change [SWC]) and likely higher PC+ICE (+2.5% ± 1.9%; 1.5 > SWC) than in CON; however, no substantial differences were shown between PC+ICE and ICE (unclear). Time-trial performance was likely enhanced in ICE compared with CON (+2.4% ± 2.7%; 1.4 > SWC) and PC+ICE (+2.9% ± 3.2%; 1.9 > SWC). Differences in mean rectal temperature during exercise were unclear between trials. Ratings of thermal comfort were likely and very likely lower during exercise in ICE and PC+ICE, respectively, than in CON. Conclusions: While PC+ICE had a stronger effect on mean power output compared with CON than ICE did, the ICE strategy enhanced late-stage time-trial performance the most. Findings suggest that thermal comfort may be as important as thermal state for maximizing performance in the heat. Keywords: thermoregulation, core temperature, pacing, internal cooling, power output Exercise in hot conditions leads to increases in heat storage and core temperature.1 Thermoreceptors located throughout the body detect the thermal change, relaying this information through afferent channels to the brain,2,3 which integrates such information and lowers motor output.4 This response reduces the rate of heat production from the contracting skeletal muscles to minimize possible thermal injury.5 From an applied perspective, it has been found that performance in the heat is most effectively enhanced with aerobic-fitness development and heat acclimation.6 However, 2 acute methods that increase performance in the heat involve a focus on counteracting the bodily reactions to heat development and include preexercise cooling7 and consuming cold fluids during exercise.8,9 Precooling involves the lowering of core body temperature, or the thermal state of the body, before competing in the heat. Cooling can be administered externally, with cold-water immersion or iced garments, or internally, through the ingestion of cold fluids.10 The combined procedure of using both internal and external techniques, such as the cyclic dousing of iced towels on the legs and torso while drinking ice slurry over the 30-minute period before exercise, has emerged as a practical and effective method.7 Ingesting cold fluid during prolonged exercise in the heat can lower core temperature and the perception of the body’s thermal Schulze, Daanen, and Levels are with MOVE Research Inst Amsterdam, VU University, Amsterdam, The Netherlands. Casadio, Plews, Siegel, and Laursen are with High Performance Sport New Zealand, Auckland, New Zealand. Kilding is with Sports Performance Research Inst New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand. Address author correspondence to Emiel Schulze at [email protected].
state and enhance exercise performance.8 A beverage formation with high cooling capacity is ice slurry, a mixture of water, ice, and sugar/electrolytes. Ingesting ice slurry has been shown to lower core temperature before exercise10 and is considered a practical strategy for use in warm conditions,7 as it also provides fluids to replace water lost from sweating.11 To our knowledge, only 2 studies have investigated the effect of ice slurry ingestion during exercise on performance and physiological responses in the heat. Using an Olympic-distance triathlon model, Stevens et al12 showed that the ingestion of ice slurry (10 g/kg body mass) during a race-simulated variable-intensity 60-minute cycle phase led to enhanced performance during a subsequent 10-km running time trial.12 Furthermore, Burdon et al13 found that ice slurry ingestion (260 ± 38 g every 15 min) during a 90-minute cycling phase (62% VO2max) enhanced performance during a subsequent 4-kJ/kg body-mass cycling time trial (~19 min).13 Notably, while no differences in rectal temperature between trials were found, rinsing of the mouth with ice slurry in 1 of these trials was also found to result in a faster time trial compared with thermoneutral (32°C) fluid ingestion. The latter finding suggests that even the presence of a cold substance in the mouth, which solely altered the perception of temperature, may be beneficial. That is, the perceived comfort of one’s body temperature, as much as one’s actual body temperature per se, may itself influence power output during exercise in the heat.14 Perceived thermal comfort is therefore an interesting variable to measure, since it indicates how the body’s thermal state is interpreted and whether exercising in a specific environment with a particular bodily condition is tolerable, not merely whether one recognizes a warm or cold sensation.14 Such perception may be more important for regulating exercise in the heat than the actual thermal state. For example, Schlader et al15 showed that exercise in 655
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the heat commencing with a low skin temperature reduced thermal perception and subsequently improved cycling performance. This was in contrast to an alternate trial where participants finished exercise with a low skin temperature; mean skin and core temperatures were similar in both conditions. This highlights the importance of investigating novel ways of altering thermal comfort (compared with thermal state) to maximize performance in the heat. In the previously described studies by Stevens et al12 and Burdon et al,13 participants had to cycle at a fixed intensity during the preload phase and drank fluids at set rates and were without forward-facing convective wind, all factors that do not take place in the field setting. In light of these considerations, our aim was to use 2 practical cooling strategies that had the potential to alter thermal comfort, thermal state, and performance in the heat—namely, practical precooling and ice slurry ingestion during exercise. As endurance-performance outcomes in field settings can have different “critical periods,” we compared effects over a long (~90-min) cycling trial under self-paced conditions, including a finishing 20-km time trial, using ecologically valid wind speeds and ad libitum drinking conditions. The different cooling strategies allowed us to compare the effects of thermal state and thermal comfort on cycling performance in the heat.
Methods
highest power output reached during the test and in a pro rata manner when participants ended the test partway through a step. VO2peak was defined as the highest VO2 measured during a 30-second period. Approximately 20 minutes after termination of the progressive exercise test, the first familiarization trial was performed.
Familiarization The experimental trials consisted of self-paced exercise, divided into 2 phases. The first phase consisted of exercise at a fixed rating of perceived exertion (RPE),17 while the second phase consisted of a 20-km time trial. As consistent fixed-RPE performance can be achieved within 3 trials,18 2 familiarization trials were performed before the first experimental trial. The first familiarization trial involved 30 minutes of cycling in thermoneutral conditions on a cycle ergometer (IndoorTrainer, SRM, Jülich, Germany) at a fixed RPE corresponding to a score of 14 on a 6-to-20 Borg scale, between somewhat hard (13) and hard (15).19 The second familiarization trial identically replicated the experimental trials (see next section). For the familiarization trial, participants drank 15 g/kg body mass of cool sports drink (4°C) during the 30-minute preexercise sitting period and ad libitum during exercise. Participants were briefed on the use of the RPE scale by the same investigator and in a similar fashion, to ensure familiarity with the scale throughout the study.
Participants
Experimental Trials
Seven experienced (>1 y competitive) and well-trained (>200 km cycling/week) male triathletes (mean ± SD age 33 ± 8 y, body mass 73.1 ± 3.3 kg, height 179.5 ± 4.9 cm, sum of 8 skin folds 63.7 ± 13.3 mm, peak oxygen uptake [VO2peak] 61.7 ± 3.0 mL · kg–1 · min–1, maximal aerobic power [MAP] 399 ± 38 W) volunteered for this study. All participants provided written informed consent before their first trial, and the study was approved by the Auckland University of Technology Ethics Committee.
To begin all trials, body mass was measured with the participant wearing only cycling shorts after voiding the bladder. Afterward, participants were equipped with a heart-rate monitor; skin-temperature thermistors (DS-1922L, Maxim Integrated, San Jose, CA) on the chest, arm, thigh, and calf; and a disposable rectal thermistor (Monatherm Thermistor, 400 Series, Mallinckrodt Medical, St Louis, MO), which was self-inserted ~12 cm past the anal sphincter. Participants entered an environmental chamber (Design Environmental Ltd, Gwent, UK) set to 30°C and 80% relative humidity, equal to a wet bulb globe temperature of 34°C. In a randomized order, the 3 experimental trials involved (1) a control trial where participants drank an ambient-temperature sports drink (30°C) during the 30-minute preexercise sitting period and during exercise (CON), (2) a trial where participants drank an ambient-temperature sports drink (30°C) during the preexercise sitting period and ice slurry (–1°C) during exercise (ICE), and (3) a trial in which participants were precooled during the 30-minute preexercise sitting period16 and drank ice slurry (–1°C) during exercise (PC+ICE). The participants were seated on a chair in the heat for a 40-minute period. The first 10-minute period was used to achieve stabilization, while during the remaining 30 minutes participants ingested 7.5 g/kg body mass of fluid during two 15-minute periods, so that 15 g/kg was consumed during the preexercise period.16 The drink contained electrolytes and carbohydrates (Gatorade 02 Perform Instant Powder Mix) and was identical in content for all conditions. The practical precooling routine consisted of ingesting ice slurry and being doused with iced towels on the legs and torso.16 Each 30 seconds, a towel was replaced with one from a container with ice and water and the warm towel was returned to the cool container. This format allowed a towel to remain on the torso or legs for 60 seconds before being replaced. After this preliminary phase, participants performed a 10-minute warm-up on a cycle ergometer (IndoorTrainer, SRM, Jülich, Germany), which included
Preparation Before the experimental trials, participants visited the laboratory on 2 separate occasions. The first visit involved a progressive maximal exercise test and a short familiarization trial, while the second visit involved a complete familiarization trial. After these preliminary tests, 3 experimental trials were performed, making a total of 5 separate visits to the laboratory separated by 3 to 7 days. Participants were asked to maintain their normal training regimen and to replicate this each week throughout the study. They were asked to replicate their typical daily dietary consumption for each trial, to avoid long (>2 h) or strenuous physical activity (>70% HRmax) the day before trials, and to avoid consumption of alcohol and caffeine.
Progressive Test The progressive maximal exercise test was performed on a cycle ergometer (Lode Excalibur Sport, Groningen, The Netherlands) in thermoneutral conditions (22°C, 50% relative humidity). Participants were equipped with a heart-rate monitor (Suunto M5, Suunto Ltd, Finland), and oxygen uptake was measured with a calibrated diagnostic system (TrueOne2400, ParvoMedics, East Sandy, UT). The test started with a warm-up of 5 minutes at a power output of 100 W, and thereafter intensity increased in a stepwise fashion by 25 W every 60 seconds until the participant reached volitional exhaustion.16 The MAP of each participant was determined as the
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3 minutes at 25% MAP followed by 5 minutes at 60% MAP and 2 minutes at 80% MAP.16 Frontal-facing convective wind movement was simulated with 2 industrial fans (FS-75, FWL, Auckland, New Zealand) generating a wind speed of ~33 km/h, an appropriate wind speed for reducing heat storage and comparable to outdoor cycling.20 Wind speed was measured using an anemometer (3000 Wind Meter, Kestrel, Sylvan Lake, MI). Four minutes after warming up, participants began the 60-minute steady-state phase at the RPE of 14. RPE was reaffirmed every 5 minutes, and participants self-adjusted power output to maintain the required RPE. Ingested drink volume was measured every 10 minutes by recording the weight of the drink bottle (Floe Bottle, Teknicool Ltd, Auckland, New Zealand). The bottle was specifically designed to keep ice slurry cold over a longer period of time. As the density of ice slurry is different than that of water, change in bottle mass was used to report fluid ingestion as opposed to volume. On completion of the fixed-RPE phase, participants were given 2 minutes of rest before they commenced the 20-km time trial. They were asked to perform their best and finish the trial in the shortest time possible. During the time trial, power output, heart rate, elapsed time, and thermoregulatory measures were monitored and recorded but blinded to participants. Participants were informed of their progress every 1 km of the trial elapsed, but no direct encouragement was given.
Measurements Every 10 minutes during a trial, participants were asked to rate their thermal comfort on a 1-to-4 scale and thermal sensation on a 1-to-7 scale.21 Before and after a phase, participants were asked to rate their gastrointestinal comfort, measured on a 4-point scale, varying from very comfortable to very uncomfortable22; thermal comfort and sensation21; and thirst sensation, measured on a 9-point scale, varying from not thirsty at all to severely thirsty.23 Body mass was measured after exiting the environmental chamber and toweling dry, while only wearing cycling shorts. Rectal temperature (Tre) was recorded with a data logger (Grant Instruments, Shepreth, UK), skin temperature (Tsk) was recorded on the thermistor (DS-1922L, Maxim Integrated, San Jose, CA), heart rate was measured using a heart-rate monitor (Suunto M5, Suunto Ltd, Finland), and power output was measured using the SRM power meter linked wirelessly to the PowerControl 7 unit (SRM, Jülich, Germany). All data were recorded at a sampling frequency of 1 Hz. Mean Tsk was calculated using the formula24 Tsk = [0.3 × (Tchest + Tarm)] + [0.2 × (Tthigh + Tcalf)], and body-mass loss was determined as body mass loss = Δ body mass + total fluid ingestion.
Statistical Analyses Data are presented as mean ± SD or with 90% confidence limits (CL) as appropriate. The differences in mean cycling power output during the steady-state phase, time-trial performance time, and power output, as well as physiological and subjective measurements, were analyzed using a magnitude-based inference approach.25 This method is used to indicate the possible benefit, in terms of beneficial or harmful effects, of each condition. The magnitude of differences in the changes between trials is expressed as standardized differences (Cohen effect sizes, ES). The criteria to interpret the magnitude of the ES were 0.8, large.25 ESs, with uncertainty of the estimates shown as 90% CL, were determined using a custom-made spreadsheet (Post Only,
Sportscience).26 Data were log-transformed and, after calculations, back-transformed to obtain differences between trials. The smallest worthwhile change (SWC) in performance measures was set at 1% for mean power output.27 For physiological measures, these changes were set at 0.2 multiplied by the between-subjects SD.28 To calculate this, the between-subjects SD from CON was used for analyses between CON and ICE or PC+ICE, while the between-subjects SD from ICE was used for analyses between ICE and PC+ICE. Quantitative chances of being beneficial or harmful were assessed qualitatively as 99.5%, most likely or almost certainly.25 When an effect was both beneficial and harmful for >5%, the true difference was described as unclear.26 Furthermore, correlations (r) between measures were determined and the magnitude of correlation (r [90% CL]) was assessed with the following thresholds: SWC). Notably, this enhanced performance occurred despite a lower fluid (and therefore carbohydrate; –18 ± 19 g, –20% ± 26%) ingestion in ICE than in CON and with only a trivial effect on Tre (Figure 4). While ice slurry ingestion independently of precooling had little effect on overall thermal state during exercise, it improved thermal comfort in both experimental conditions compared with CON (ICE ES = –0.67 ± 0.68, PC+ICE ES = –0.43 ± 0.42) and likely contributed toward the differences in power output shown between these trials. A decoupling of thermal state and thermal comfort, as shown between the ICE and CON trials in the current study, has been documented previously. Burdon et al13 showed that ice slurry ingestion during exercise elicited a higher power output during a finishing cycling time trial. While ratings of thermal comfort were lower with ice slurry ingestion than in the control trial (37°C fluids), core temperature was similar.13 As with the current study, the comparable levels of thermal state between ICE and CON trials, despite the
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enhanced performance in ICE, suggest that thermal comfort is at least as important as thermal state for maximizing performance.14 It has been proposed that thermal receptors in the mouth29 and gastrointestinal region10 are important for the regulation of body temperature during exercise in the heat.11 Ingesting a –1°C ice slurry likely lowers the temperature of these regions,30 potentially influencing thermal comfort. Evidence for this in the current investigation was shown through a substantially lower thermal comfort during the first 60 minutes of cycling in the ICE trial, despite a similar Tre and power output compared with CON. This suggests that ingested ice slurry cooled regions near internal thermoreceptors and likely lowered heat sensation, despite no detectable change in the global thermal state of the body measured through Tre and Tsk. Indeed, Levels et al31 recently showed that reduced body-heat content and sensation of coolness is beneficial for pacing during later exercise stages. In the current study, the improved thermal comfort with ICE likely allowed for the higher power outputs achieved in the time trial,14 with the additional metabolic heat produced from the higher power outputs in ICE likely offsetting any potential cooling effect, resulting in the similar Tre shown.10 The combination of practical precooling and ice slurry ingestion during exercise led to the highest power output during the fixed-RPE phase compared with both CON (4.3% ± 4.3%, 3.3 > SWC) and ICE (3.3% ± 6.2%, 2.3 > SWC) conditions. While higher power outputs were achieved during this initial fixed-RPE phase in the PC+ICE trial, this aggressive pace resulted in a reduced time-trial performance compared with ICE (–2.8% ± 3.0%, 1.8 < SWC; Figure 1). Indeed, it has been shown that starting Tsk has a large influence on anticipatory pacing in the heat compared with thermal perception.32 Quod et al33 showed that when well-trained cyclists were precooled using an ice jacket, they tended to start aggressively during a performance trial, irrespective of Tre, and resulting overall performance was similar to a control condition. In the current study, the lower Tsk and Tre measured near the start of the fixed-RPE phase in the PC+ICE trial likely enabled the higher work output and associated heat production, resulting in a greater rise of core temperature and fatigue accumulation, leading to equivalent time-trial performances (Figure 1). Notably, when Tre in PC+ICE became similar to, and above, that of CON and ICE, after ~30 minutes during the fixed-RPE phase, the power output remained higher until the end of the phase (Figure 3[C]). This occurrence again suggests that one’s thermal state is not the only regulator of power output during exercise in the heat.11,13 Since ratings of thermal comfort remained lower throughout the fixed-RPE phase of the trial, these perceptions likely influenced power output and performance, as shown previously.14 Two final observations worthy of mention are the decreased thirst shown in PC+ICE compared with the CON trial and the strong correlation between overall power output and thirst sensation. Compared with CON, both volume of fluid ingestion and weight loss were similar in PC+ICE during the trial. The lower sensation of thirst may have been a result of decreased skin blood flow due to the (external) precooling, thereby increasing water content to central circulation and lowering plasma osmolality and ultimately thirst.23,33,34 This in turn may have contributed to the enhanced overall performance effect shown.35
Practical Implications Precooling and cooling during exercise may be useful strategies to employ during prolonged cycling performance in the heat,36 but caution with administering such strategies should be taken, as the
perceived cooler state can elicit inappropriate pacing, especially when an end spurt is tactically key to the overall performance outcome. While increased heat-storage capacity (thermal state) can enhance performance in the heat, methods of improving thermal comfort during exercise are also effective.
Conclusions While PC+ICE had a stronger effect on mean power output compared with CON than ICE did, the ICE strategy enhanced late-stage time-trial performance the most. Despite equivalent thermal states between ICE and CON, performance was enhanced in the ICE condition. These findings suggest that thermal comfort may be as important as thermal state for maximizing performance in the heat. Acknowledgments No sources of funding were used to assist in the preparation of this article. Paul Laursen is a minority shareholder in Teknicool Ltd, the company that produced the ice slurry bottle used in the current study. The authors express their gratitude to all participants in this study. We would like to thank Professor Will Hopkins for his expert opinion and advice on statistical analysis.
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