Study overview
On separate days following heat acclimation and an incremental exercise test to exhaustion, participants performed a total of three hilly 46.4-km experimental cycling time trials (described below) in hot environmental conditions (33.3 ± 1.1°C; 50 ± 6% r.h.). Three trials were conducted in a randomized counterbalanced order. Prior to the commencement of all performance trials (t=−180 min), subjects were required to ingest 25 g.kg
-1 BM of a cold (4°C) beverage containing 6% carbohydrate (CHO; Gatorade, Pepsico, Australia, NSW, Australia). Additionally, on two occasions, subjects were also exposed to an established combined external and internal precooling technique, whereby iced towels were applied to the subject’s skin while ingesting additional fluid in the form of an ice slurry (slushie) made from sports drink (PC). The precooling method used in this study, as previously described [
11], commenced 60 min prior to the start of the trial (t=−60 min) and was applied for a period of 30 min. During one of the precooling trials, the recommended dose [
25] of 1.2 g.kg
-1 BM glycerol (PC+G) was added to the large fluid bolus in a double blind fashion. PC and PC+G trials were compared to a control trial, which consisted of the large beverage ingestion without glycerol and received no precooling (CON). Experimental trials were separated by 3–7 d with a consistent recovery time between trials for each subject.
Experimental time trials
Subjects followed a standardized pre-packaged diet and training schedule for 24 h prior to each experimental trial. The standardized diet was supplied in the form of pre-packaged meals and snacks, providing 9 g.kg
-1 BM CHO; 1.5 g.kg
-1 BM protein; 1.5 g.kg
-1 BM fat, with a total energy goal of 230 kJ.kg
-1 BM. Subjects refrained from any intake of caffeine and alcohol over this period. Individualized menus were prepared accounting for food preferences using FoodWorks Professional Edition (Version 6.0, Xyris Software, Brisbane, Australia), as described previously [
26]. Subjects were provided with all foods and drinks in portion controlled packages for the first 20 h of the standardized period and were given verbal and written instructions on how to follow the diet. Subjects were allowed to undertake light exercise on the day prior to each trial and were asked to repeat this for subsequent trials. Compliance to the diet and exercise protocol was determined from a checklist kept by each subject and presented on arrival to the laboratory prior to each trial. Subjects’ ‘first-waking’ urine sample was also analyzed for the determination of specific gravity to ensure the cyclist attended the laboratory for each trial in a similar hydration state.
For each experimental trial subjects were required to cycle a 46.4-km time trial on a Velotron cycle ergometer, (Velotron 3D Software, RacerMate Inc., Seattle, WA, USA) which was fitted with a calibrated [
27] SRM cycling power meter (scientific version, 8 strain gauge, Schoberer Rad Meβtechnik; Jülich, Germany), which was set to sample at 1 s intervals. The measurement error for cycling time trials during laboratory protocols such as this has been established as 1.7%, as described previously [
11]. The course profile for this time trial was a simulation of the 2008 Beijing Olympic Games time trial course, as described previously [
11]. All experimental trials were carried out in the afternoon, to mimic the schedule of the 2008 Olympic Games cycling time trial. On arrival to the laboratory, three hours prior each trial (t=−180 min), subjects voided their bladder (not for collection) and inserted a single use thermal probe (Mon-a-therm General Purpose Temperature Probe, Mallinckrodt Medical Inc., St Louis, MO, USA) 12 cm beyond the anal sphincter for determination of rectal temperature (T
re). Changes in rectal temperature at the end of the precooling phase (t=−30 min) and at the end of the warm-up phase (t=0 min) were used to reflect the effectiveness of the precooling treatment and the potential differential for heat storage at the commencement of the time trial. Reduction in rectal temperature as a result of precooling were categorized as either small (<0.3°C), moderate (0.3-0.6°C), large (0.6-0.8°C) or very large (>0.8°C) based on our previous work [
11].
On arrival at the laboratory, subjects were immediately given a large cold beverage (given as two boluses of 12.5 g.kg-1 BM at t =−180 and −165 min) to consume within 30 min. At t=−150 min and every 30 min leading up to the commencement of the time trial, and immediately afterwards, subjects were required to void their bladder. Urine was weighed and analyzed for specific gravity. At this time, subjects consumed the last of their standardized diet as a “pre-race meal” which provided 2 g.kg-1 BM CHO.
Rating of thermal comfort, T
re and HR (Polar S810i HR monitor; Polar Electro OY, Kempele, Finland) were recorded before entering the heat chamber, and every 5 min during 60 min of passive rest in the heat chamber (heat stabilization; t=−120 to −60 min). The environmental conditions inside the chamber were measured and corrected every 5 min throughout the duration of the trial. On two occasions (PC and PC+G trials), following the completion of the stabilization phase, subjects consumed 1,024 ± 122 g slushie containing 6% CHO, which was equivalent to 13.6 g.kg
-1 BM, providing a CHO intake of 61 g (0.8 g.kg
-1 BM). The slushie was given in two ~7 g.kg
-1 BM boluses and subjects were given 15 min to consume each bolus while wearing iced towels, as previously described [
11]. During the control trial subjects received no cooling intervention (CON). During this time subjects were also asked to provide ratings of stomach fullness.
Following stabilization and precooling, subjects completed a standardized 20-min warm-up on the Velotron ergometer. The warm-up consisted of two bouts of 3 min at 25% MAP, 5 min at 60% MAP and 2 min at 80% MAP, which is a protocol used by some elite time trial cyclists prior to competition. The final 10 min before the start of the time trial allowed subjects to complete their own preparations. During this time subjects were provided with standard pre-race instructions and the zero offset of the SRM crank was set according to manufacturer’s instructions.
Feedback provided to the subject was limited to distance covered (km), cycling gear-ratio (12-27/42-54), road gradient (%) and instantaneous velocity (km.h-1). Subjects were provided with 314 ± 207 g fluid containing 6% carbohydrate (Gatorade, Pepsico Australia, Chatswood, Australia), which provided a further CHO intake of 19 g (0.25 g.kg-1 BM) at the “top of each climb” (12.5 and 37.5 km), which simulated the ideal time to consume fluid on the Beijing time trial course based on the experience of professional cyclists during training and racing on the actual course. On the first trial, subjects were given a total of 325 ml at each of these points and were permitted to drink ad libitum for the next kilometer on the first trial. The volume that was consumed was measured and repeated for subsequent trials. Drinks were removed from ice storage at the commencement of the time trial and left in the heat chamber to simulate drink temperatures that would be experienced in race conditions. To further replicate competition, the cyclist was positioned in front of a large industrial fan (750 mm, 240 V, 50 Hz, 380 W, model Number: N11736, TQ Professional), which was adjusted to simulate uphill or downhill wind speeds. Specifically, the fan was fixed on low speed to simulate 12 km.h-1 wind speed for 0–12.5; 23.2 - 35.7 km and switched to high speed to simulate 32 km.h-1 wind speed for 12.5 -23.2 and 35.7 - 46.4 km.
Split times, velocity and power output data were collected for each trial, with the periods of interest being time to top of first climb (12.5 km), end of first lap (23.2 km), time to top of second climb (35.7 km) and finish (46.4 km). Throughout the trials, HR and T
re were recorded every 2 min, while self-reports of perception of effort [
28], thermal sensation [
29], and gastrointestinal comfort (5-point Likert scale), were recorded at approximately 5-km intervals. On the completion of each time trial, subjects were asked a series of questions related to their effort, motivation, sensation and comfort, as reported previously [
11].
Statistical analysis
Pre-trial body mass, percentage dehydration, and post-trial subjective ratings were compared between trials (i.e., CON, PC, PC+G) using a one-way analysis of variance (ANOVA). A two-way (trial × time) repeated measures ANOVA was used to examine differences in dependant variables (i.e., rectal temperature, heart rate, urine specific gravity and volume, thermal comfort, stomach fullness and RPE) between trial means at each time point. If a significant main effect was observed, pairwise comparisons were conducted using Newman-Keuls post hoc analysis. These statistical tests were conducted using Statistica for Microsoft Windows (Version 10; StatSoft, Tulsa, OK) and the data are presented as means and standard deviations (SD). For these analyses, significance was accepted at P<0.05.
The performance data from the three trials were analysed using the magnitude-based inference approach recommended for studies in sports medicine and exercise sciences [
30]. A spreadsheet (Microsoft Excel), designed to examine post-only crossover trials, was used to determine the clinical significance of each treatment (available at newstats.org/xPostOnlyCrossover.xls), as based on guidelines outlined by Hopkins [
31]. Performance data are represented by time trial time and power output during the various segments of the course, and are presented as means ± SD. The magnitude of the percentage change in time was interpreted by using values of 0.3, 0.9, 1.6, 2.5 and 4.0 of the within-athlete variation (coefficient of variation) as thresholds for small, moderate, large, very large and extremely large differences in the change in performance time between the trials [
30]. These threshold values were also multiplied by an established factor of −2.5 for cycling [
32], in order to interpret magnitudes for changes in mean power output. The typical variation (coefficient of variation) for road cycling time trials has been previously established as 1.3% by Paton and Hopkins [
33], with the smallest worthwhile change in performance time established at 0.4% [
34], which is equivalent to 1.0% in power output. These data are presented with inference about the true value of a precooling treatment effect on simulated cycling time trial performance. In circumstances where the chance (%) of the true value of the statistic being >25% likely to be beneficial (i.e., faster performance time, greater power output), a practical interpretation of risk (benefit:harm) is given. An odds ratio (OR) of >66 was used to establish that the benefit to performance time gained by using one strategy outweighed any potential harm (in performance time) that could result.