Thermal stress during preparation for team sport performance.

  • Fergus O'Connor

Student thesis: Doctoral Thesis


It is well known that exposure to high levels of the environmental heat stress variables of ambient temperature, relative humidity and solar radiation elicits adverse acute effects on human physiology and performance during prolonged duration, endurance-type exercise. However, the physiological and performance responses to high intensity intermittent exercise in the heat are not as clearly defined. Specifically, intermittent or repeated sprints may be impaired but maximal sprint efforts might be improved during exercise performance in hot environments. Moreover, the physiological, performance and recovery responses of team sport athletes undertaking training and competition in hot environments is also unclear. The limited number of studies on team sport athletes training or competing in hot environments shows substantial variation in physiological and performance outcomes. Indeed, while some studies have shown total distance and high-speed running to be negatively affected by high ambient temperatures, others have shown no effect. Similarly, some studies have shown maximal sprint efforts to be maintained in hot environments, while others have reported declines in sprint performance. There is a paucity of data on the effect of the environment on physiological responses to the demands of team sport training, where the interactions between the environment and core and skin temperature is poorly characterised. Accordingly, the primary aims of the series of studies that constitute this thesis were to determine (1) the relationships between individual environmental heat stress variables and physiological and psychometric responses during team sport training in the heat, (2) the efficacy of different strategies to mitigate the impact of heat stress on team sport training performance and (3) new methods for quantifying external load during team sport training in hot environments.

In order to establish relationships between environmental conditions and effects on physiology and performance during professional team sport training, study one (Chapter Two) assessed eight weeks of training (579 observations) in a hot environment during the pre-season preparation phase for the Australian Football League (AFL) competitive season. Professional Australian Rules football (ARF) players (n=45) completed their ‘normal’ training schedule and ambient temperature (Ta), relative humidity (RH) and solar radiation (SR) were recorded at every training session. External load data (distance covered, m.min-1; percent high speed running >14.4 km.h-1; %HSR) was collected via a global positioning system, while internal load data (ratings of perceived exertion [RPE], heart rate [HR]), and markers of recovery (subjective ratings of wellbeing and heart rate variability (root mean sum of squared differences [rMSSD]) were also monitored throughout the training period. Relationships between the training environment, internal load, external load and recovery variables were analysed using mixed effect linear models and reported as standardised regression coefficients (β). The mean environmental conditions during the training period were 30.9 ± 2.1 °C Ta, 61.7 ± 6.2 % RH and 718 ± 227 W/m2SR. Increasing solar radiation exposure was associated with reduced distance covered (-19.7 m.min-1, β=-0.909, p<0.001) and %HSR (-10%, β=-0.953, p<0.001) during training, and rMSSD 48 h post-training (-16.9ms, β=-0.277, p=0.019). Greater relative humidity was associated with decreased %HSR (-3.4%, β=-0.319, p=0.010), but increased % duration >85% HRmax(3.9%, β=0.260, p<0.001), RPE (1.8AU, β=0.968, p<0.001) and self-reported stress 24 h post-training (-0.11AU, β=-0.24, p=0.002). In contrast, higher ambient temperature was associated with increased distance covered (19.7 m.min-1, β=0.911, p<0.001) and %HSR (3.5%, β=0.338, p=0.005). These results highlight the importance of considering the individual factors contributing to thermal load in isolation for team sport athletes, and that SR and RH reduce work capacity during team sport training and have potential to slow recovery between sessions.

Once relationships between the training environment and physiological, perceptual and performance measures were established in study one (Chapter Two) they were then tested in study two (Chapter Three). Specifically, the study determined the effect of solar radiation exposure on physiological, perceptual and performance responses to high-intensity exercise in a hot environment. Two separate, but related studies, were completed whereby outdoor exercise, with exposure to high solar radiation was undertaken followed by indoor exercise without solar radiation exposure. In Study 2A well-trained cyclists completed 5 × 4 min intervals (~80% peak power output (PPO) with 2 min recovery ~40% PPO) before a 20 km self-paced ride. In Study 2B professional Australian Rules footballers completed a standardised 20 min warm-up (~65% mean 4-min power) then 5 × 2 min maximal-effort intervals. Heart rate (HR), power output (PO), ratings of perceived exertion (RPE), thermal comfort (TC), and thermal sensation (TS) were recorded during both sub-studies but restrictions with the professional Australian Rules footballers dictated that core (Tc) and skin temperature (Tsk) were monitored only in cyclists. The ambient temperature, relative humidity and solar radiation were monitored outdoors and during indoor trials the ambient temperature and relative humidity was matched for cyclists, generating a significantly different wet-bulb globe temperature (WBGT) between outdoor (29.6°C WBGT) and indoor (26.0°C WBGT) environments in the absence of solar radiation, but for AFL players in study 2B humidity was adjusted to generate similar WBGT (outdoors 26.7°C; indoors 26.4°C) despite the absence of solar radiation. Cyclists HR (p = 0.05), Tc(p = 0.03) and Tsk(p = 0.03) were higher outdoors with variable effects for increased RPE, TS and TC (d= 0.2-1.3). PO during intervals was not different between trials but there were small-moderate improvements in cyclists PO and 20 km time indoors (d =0.3-0.6). Generally, there were small-moderate effects for lower RPE and TS indoors (d= 0.2-0.5). In contrast, there were no differences in perceptual variables between trials in AFL players. The results from study two show that indoor training in hot conditions without solar radiation that results in a lower WBGT may promote modest reductions in physiological strain and improve performance capacity in well-trained athletes.

To test the efficacy of an alternate strategy to training indoors for mitigating the detrimental effect of solar radiation exposure on team sport training performance the third study included the use of a specifically designed commercial ‘skin-cooling’ garment (Chapter Four). Study three employed statistical modelling techniques to determine the effect of the garment, designed to limit SR to the skins surface, on work capacity during training, and recovery after training. Professional Australian Rules footballers (n=44) completed five months of pre-season training (645 observations) in a hot environment. Participants were permitted the choice to wear either standard training attire (singlet and shorts) or standard training attire with the addition of the commercial garment to limit sun exposure to the upper body during each training session. When individuals chose to wear the garment, they were assigned to the intervention condition for the session, and if they chose not to wear the garment, they were assigned to the control condition. External load data (distance covered, m.min-1; percent high speed running >14.4 km.h-1; %HSR, distance covered above 75% individual maximum velocity; %>75%) was collected via a global positioning system, internal load from RPE, and recovery through subjective ratings of wellbeing throughout the training period. The mean environmental conditions during the training period were 28.9 ± 1.8 °C Ta, 63.1 ± 10.5% RH and 720 ± 155 W/m2SR. Statistical analysis showed no effect of the training garment on any of the variables quantified within training sessions (m.min-1; -0.3 m.min-1, p = 0.56, %HSR; -0.05%, p = 0.86, %>75%; 0.01%, p= 0.09), RPE (0.06 AU, p= 0.28) or physical wellbeing (-0.26 AU, p= 0.77), mental wellbeing (-0.007 AU, p= 0.94), emotional wellbeing (-0.007 AU, p= 0.77) nor overall recovery (-0.003 AU, p= 0.95) during the 24 h post-training period. When statistical modelling was applied to an individuals’ control data to predict their responses with the garment there was a small decrease in observed m.min-1from predicted m.min-1 (-3.1 m.min-1, p= 0.001). Prediction models failed to identify any other significant relationships. Therefore, the garment was not associated with improved training performance nor reduced rating of perceived exertion, but it also did not impair the quality of training or promote higher perceptions of effort.

The final study of this thesis (study four, Chapter Five) utilised mathematical modelling to establish a novel method for quantifying the effect of hot environments on training load during team sport training, termed the daily environment index. Professional Australian Rules footballers (n=67) completed 22 weeks of team sport training spanning two pre-season preparation periods in a hot environment (1178 observations). Total distance, total duration, %HSR, RPE and the % duration >85% HRmax were quantified during training. Ambient temperature, relative humidity and solar radiation were monitored throughout all training sessions. The mean environmental conditions during the training period were 29.8 ± 2.3 °C Ta, 62.2 ± 9.4 % RH and 729 ± 176 W/m2SR.Structural relationships between the coefficients of the atmospheric conditions in relation to internal training load were identified and were statistically appropriate to subsequently construct a daily environment index using weightings for average relative humidity, temperature and solar radiation, and the included dependent variables interactions. Applying the daily environment index calculation to the completed external load on a given training day to predict the equivalent distance completed per session when accounting for the impact of the environmental conditions, resulted in a mean increase of ~7%in total distance per training session (p<0.0001) and a total increase of ~21 km across the experimental period (p<0.0001). Undertaking team sport training when environmental heat stress is high, particularly when demanding ambient temperature and humidity coincides with high SR exposure, is strongly associated with increased internal load that can be used to estimate an equivalent external load effect. Employing the daily environment index to monitor training load provides greater resolution and better understanding of the training stress incurred by team sport athletes training in a hot environment. Consequently, practitioners are provided with a new tool with which to effectively plan, prescribe, monitor and adjust team sport training and recovery programs.

In summary, the series of studies that comprise this thesis provide novel information regarding the effect that the environmental variables of ambient temperature, relative humidity and solar radiation individually impart on physiological and performance parameters during team sport training in hot environments. In particular, a greater understanding of the influence of solar radiation on physiological and performance parameters in hot environments is identified. With greater understanding of the individual components of environmental heat stress, and the influence that each component of heat stress has on team sport physiology and performance, comes an increased ability to optimise training and competition outcomes.
Date of Award13 Oct 2022
Original languageEnglish
SupervisorVernon Coffey (Supervisor), Steven Stern (Supervisor), Peter Reaburn (Supervisor) & Jonathan Bartlett (Supervisor)

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