The thermophysiology of self-paced exercise in trained male cyclists

  • Felicity Bright

    Student thesis: Doctoral Thesis

    Abstract

    The four environmental parameters that contribute to environmental heat stress during prolonged outdoor exercise are ambient temperature, humidity, airflow and radiant (i.e., solar) heat. In three separate laboratory-based studies, ambient temperature, humidity and air velocity were manipulated to investigate the graded impact of these parameters on human heat exchange and the concomitant effect on thermal, cardiovascular and perceptual responses, as well as performance, during prolonged self-paced exercise. The first study (Chapter 3) investigated the effect of ambient (i.e., dry-bulb) temperature (13, 20, 28 and 36ºC) on self-paced exercise performance by minimising differences in the skin-to-air water vapour pressure gradient (i.e., evaporative potential) between conditions to isolate the effect of temperature. During exercise in the 13 and 20ºC conditions when the skin-to-air water vapour pressure gradient was matched (~2.02 kPa), exercise performance (i.e., power output) was similar (~274 W; P=1.00). However, mean power output was lower in 28ºC (~262 W) and was further declined in 36ºC (~228 W). Peak core temperature was higher in 36ºC (39.6 ± 0.4ºC) than all other conditions (P<0.001) and higher in 28ºC (39.1 ± 0.4ºC) than 13ºC (38.7 ± 0.3ºC; P<0.001) and 20ºC (38.8 ± 0.3ºC; P<0.01). Mean heart rate was higher in 36ºC (~163 ± 14 beats·min-1) than all other conditions (P<0.001), and in 20ºC (~156 ± 11 beats·min-1; P=0.009) and 28ºC (~159 ± 11 beats·min-1; P<0.001) than 13ºC (~153 ± 11 beats·min-1). These findings indicate that performance is reduced at 28ºC and further impaired at 36ºC in association with a large decrease in dry heat loss and a moderate reduction in evaporative potential. The second study (Chapter 4) investigated the impact of relative humidity (RH; 33%, 50%, 70% and 88%) on heat exchange and the associated thermal, cardiovascular and perceptual responses during a prolonged self-paced cycling time trial. Cycling performance was similar in the drier environments of 33% and 50% RH (~259 W; P=1.000) but decreased at 70% RH (~246 W) and further reduced at 88% RH (~222 W). Peak core temperature was higher in 88% RH (39.49 ± 0.56ºC) than 33% (38.97 ± 0.44ºC; P<0.001), 50% (39.04 ± 0.39ºC; P=0.002) and 70% RH (39.12 ± 0.47ºC; P=0.010). Heart rate was similar between all conditions (P>0.056) and thermal discomfort was greater in 88% RH than all conditions. Elevations in humidity progressively narrowed the skin-to-air water vapour pressure gradient, reducing the evaporative potential. Consequently, reductions in evaporative potential exacerbated thermal and perceptual strain, especially in the 88% RH condition, despite the maintenance of a lower work rate, and by extension, a lower metabolic heat production. The third study (Chapter 5) investigated the effect of different air velocities (still air, 16, 30 and 44 km·h-1) on heat exchange and performance during prolonged self-paced exercise in 32ºC and 40% RH. Cycling time trial performance was similar between 16 (274 ± 30 W), 30 (250 ± 32 W) and 44 km·h-1 (248 ± 32 W) relative to no airflow (232 ± 42 W; all P<0.001). Peak core temperature was higher in still air (39.36 ± 0.68ºC) than 16 (38.99 ± 0.49ºC), 30 (38.84 ± 0.34ºC) and 44 km·h-1 (38.83 ± 0.47ºC; all P<0.002). Mean skin temperature was lower in the higher air velocity conditions (P<0.001), but similar in 30 and 40 km·h-1 (P=1.00), and mean heart rate was ~2 beats·min-1 higher in still air than 44 km·h-1 (P=0.035). In the still air condition, the lower evaporative potential increased thermal strain, and led to a similar or greater circulatory strain than in conditions with airflow, despite the maintenance of a lower work rate. Moreover, the comparable exercise performance in the airflow conditions suggests additional airflow ≥16 km·h-1 provides no further benefit to self-paced exercise in the heat. The relatively small improvements in evaporative efficiency (i.e., the proportion of sweat that contributed to evaporative cooling) in conditions ≥16 km·h-1 fail to provide meaningful cooling power and do not further attenuate the development of thermal and cardiovascular strain or lead to improvements in exercise performance. The results of this thesis demonstrate reductions in work rate (i.e., power output), and therefore metabolic heat extension do not attenuate the development of thermal and physiological strain during exercise at high ambient temperatures, high humidity levels and when airflow is withheld. Furthermore, the greatest decrements to performance were observed when evaporative potential reduced, and evaporative efficiency declined leading to elevations in thermal, cardiovascular and perceptual strain. Specifically, our results show that exercise performance is affected once evaporative potential is reduced <200 W·m-2 and the heat stress index is greater than 2.0. Our findings highlight the need to consider all environmental parameters when discussing heat stress because a more comprehensive understanding of all environmental parameters can help better prepare athletes and coaches with training and competition strategies in a warming world.
    Date of Award2023
    Original languageEnglish
    SupervisorJulien Periard (Supervisor) & Brad Clark (Supervisor)

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