The athlete (Age = 43 years, body mass = 96.9 kg, stature = 190 cm) was a professional soccer match official with 11 years’ experience at international level (Weston et al. 2011). During the 2013 Confederations Cup tournament in Brazil 2013, the referee experienced symptoms associated with heat illness which he perceived as negative for health and performance. The athlete requested support to minimise this risk during the 2014 World Cup. The athlete was informed of the risks and discomforts of the proposed sessions and provided written informed consent. The study was approved by the local ethics board and data collection was conducted in accordance with the Declaration of Helsinki (2013).
Experimental design
The athlete visited the laboratory on eight occasions between 13th May 2014 and 30th May 2014. Prior to each training session the athlete was advised to follow his prescribed diet by his professional organisation and asked to consume at least 500 ml of non-caffeinated fluid 2 h before each session to promote euhydration. During each visit, pre-session assessments of body mass (kg), stature (cm), haematocrit (%) (Sodium heparinised MicroHaematocrit tubes, Hawksley, UK) and haemoglobin (g dL) (HemoCue Hb 201, Radiometer Ltd, UK) (fingertip capillary samples) and urine osmolality (mOsmol kgH2O) (Osmocheck, Vitech Scientific Ltd, UK) were conducted. Changes in plasma volume were calculated using the method of Dill and Costill (1974). Towel dried nude body mass was recorded pre and post exercise and used in the assessment of whole body sweat rate.
Figure 1 depicts the time-line of support, to investigate heat acclimation state the athlete performed a heat stress test (HST) and a repeated sprint test, in an environmental chamber set at 30 °C 80 % RH. All tests were performed at the same time of day to minimise the effects of circadian rhythms. To quantify the thermoregulatory and perceptual responses to heat stress, the athlete completed 10 min of exercise at three different intensities (6, 11 and 13.5 km h−1) and 5 min at 16 km h−1 on a motorised treadmill. Core temperature was assessed at the tympanic (Tytemp) membrane (Thermoscan 5, Braun GmbH, Germany) (test re-test typical error = 0.24 °C, coefficient of variation = 0.64 %, assessed in our laboratory). Skin temperature was recorded using U-type thermistors (Grant Instruments, Cambridge, UK), at four sites (chest, bicep, thigh and calf). Expired air was sampled and assessed (Ultima, CardiO2, Medgraphics, USA) for 3 min at the end of each stage. Thermal perception was assessed using a 9 point scale (1 very cold–9 very hot) (Nielsen et al. 1989
). The repeated high-intensity running test required the athlete to perform as many efforts as possible, up to a maximum of 20 repetitions of 30 s with 30 s passive recovery on a non-motorised treadmill test (Woodway Curve, WI, USA). The aim of the test was to complete all 20 repetitions and cover as much distance as possible. This protocol was implemented due to its similarity to a standardised FIFA refereeing test (Weston et al. 2009).
Athlete availability necessitated the use of a novel medium-term HA regime (MTHA 7–10 days) consisting of both isothermic and passive-heat exposures. During the isothermic sessions, tropical environmental conditions (40 °C 80 % RH) were used to limit external heat transfer, increase heat storage and stimulate sudomotor activity. Intermittent high-intensity exercise was performed on a non-motorised treadmill (Woodway Curve, WI, USA) after a 5 min warm-up at a self-selected pace, the athlete performed six, 20-second high-intensity (RPE = 17) efforts followed by 40 s low intensity running (RPE = 11). This protocol was used to simulate refereeing match demands and rapidly increase body temperature providing ecological validity to training. In line with previous work describing the attainment and maintenance of a core temperature ≈38.5 °C, a modified isothermic target core temperature of 38.0 °C measured at the tympanic membrane (Tytemp) was maintained, with the 0.5 °C difference reflecting a tendency for tympanic temperature to under-read, relative to rectal (Easton et al. 2007). Upon attaining the target temperature, the number of high-intensity efforts was adjusted to maintain a Tytemp at 38 °C for 60–80 min. A typical session consisted of four to five sets of six, 20 s high-intensity efforts with 40 s passive recovery. The athlete performed 10–15 min of jogging and walking between the sets. When Tytemp exceeded 38 °C the athlete rested (Fig. 2).
To increase physiological strain, we used permissive dehydration and restricted fluid intake to 500 ml per session, to provide a stimulus for plasma volume expansion (Garrett et al. 2014). Passive HA sessions required the athlete to undertake 30 min of intermittent submersion in a hot water bath (≈48 °C) after his typical (temperate condition) high-intensity interval and resistance training sessions. The passive HA sessions involved 30 min whole-body immersion in a hot water bath (≈48 °C) to induce heat strain, similar to the methods of (Scoon et al. 2007; Stanley et al. 2015; Zurawlew et al. 2015). The water bath rapidly increased Tytemp to 38 °C; when Tytemp exceeded 38 °C the athlete exited the bath and rested until Tytemp fell below 38 °C and then re-entered the bath. This intermittent water-bathing protocol lasted 30 min.
Statistical analysis
Data were assessed for normality and homogeneity of variance. Pre-and post-heat acclimation data were assessed with freely available spreadsheets (http://sportsci.org/resource/stats/xcontrial.xls and http://sportsci.org/resource/stats/xprecisionsubject.xls) using Cohen’s d as a measure of effect size and 90 % confidence intervals. In the initial repeated high-intensity running test the athlete reached volitional exhaustion and terminated exercise after 15 efforts, citing a thermal strain of 9 and RPE of 20. Thus, comparison was conducted on the first 15 efforts despite completing 20 sprints post-HA. When the assumption of normality was violated the Wilcoxon Signed Rank test was used. Statistical significance was set at P < 0.05.