Heat acclimation as a training tool for highly trained athletes
Creators: name: Philp, Calvin P.
Publisher: name: University of Tasmania
Year: 2024

description: Heat acclimation to improve exercise performance in hot conditions has been extensively researched. However, the thermoregulatory and cardiovascular adaptations associated with heat acclimation may also enhance exercise performance in cool and temperate conditions, with current research demonstrating effects ranging from large improvements to trivial changes. Further clouding the efficacy of heat acclimation on performance are the populations used in previous heat acclimation studies. As habitual aerobic exercise elicits many similar adaptations to that of heat acclimation, highly trained and elite athletes are likely to elicit smaller magnitudes of response to heat acclimation than untrained individuals. Therefore, heat acclimation undertaken on clearly defined highly trained athlete cohorts are important for the successful implementation of heat acclimation in applied settings.<br>Both team and endurance sport athletes compete in cool-temperate conditions with each facing its own unique physical, tactical, and logistical challenges, with practitioners having several heat acclimation modalities available to best fit their training program and objectives. These include both active and passive heat acclimation interventions, with the most appropriate method dependent on factors including: the sport they are working with, stage of competitive season, time available to allocate to interventions, overall training load prescription, training intensity required, and level of adaptation desired. Once these parameters are established, practitioners can determine the most appropriate heat acclimation strategy for their given training environment.<br>Due to high training volumes during an Australian rules football preseason, increasing intermittent running performance while minimising additional training stress is of interest to practitioners, with the common use of “off-legs” training sessions. Despite these modalities reducing the impact load, off-legs training sessions still impose mechanical load on the athlete. Heat acclimation, prescribed via the maintenance of a cardiovascular strain (relative intensity) throughout the intervention may allow similar physiological and performance adaptations with reduced mechanical load (power output), ultimately assisting in load management. Consequently, the aim of the first study in this thesis was to investigate the effect of a short-term (5 x 50 min) cycling-based heat acclimation intervention on intermittent running performance in semi-professional Australian rules footballers, whereby exercise intensity was prescribed via relative intensity (70% of heart rate reserve). The results of Study 1 demonstrate that when heat acclimation (35°C) is prescribed at a similar cardiovascular load to cool temperature exercise (14°C), highly trained Australian rules footballers can achieve similar improvements in intermittent running performance with ~30% less power output during the 50 min cycling intervention. These findings are important for athletes and practitioners aiming to improve exercise performance whilst minimising increases in mechanical load, which may have applications for both intense training and rehabilitation periods.<br>Rowing performance in temperate conditions is highly correlated with aerobic energy system measurements such as maximum oxygen consumption (<i>V̇</i>O<sub>2</sub>max) and power at lactate threshold, therefore ways to improve aerobic capacity, and ultimately rowing performance is important for practitioners. Heat acclimation prescribed using absolute intensity has increased <i>V̇</i>O<sub>2</sub>max in highly trained cyclists in hot and cool conditions, as well as improved temperate-condition time-trial performance in well-trained runners. However, it is unclear whether similar benefits can be seen in highly trained rowing athletes. Given that rowing athletes regularly utilise absolute intensity (power output) during their habitual cross-training sessions, the aim of the second study in this thesis was to investigate whether a moderate-term heat acclimation intervention in national-level rowing athletes, with exercise intensity prescribed via absolute intensity (45 – 55% of 4-min power output) could elicit heat acclimation, impact physiological measures during submaximal rowing, and improve 4-min TT rowing performance in temperate conditions (20°C). The results of Study 2 demonstrated that 10 x 60 min sessions (2 x 5-day blocks interspersed by two days) of heat acclimation (34°C) prescribed as a replacement for habitual evening cross-training sessions in ambient conditions (14°C) can produce physiological adaptions consistent with heat acclimation and may improve 4-min TT rowing performance.<br>Although active heat acclimation strategies have been shown to improve performance in highly trained athletes, the inclusion of additional mechanical load through exercise isn’t always conducive to performance, especially during competitive in-season team sport settings. Therefore, Study 3 aimed to investigate the effect of an in-season 6-week hot water immersion (HWI) protocol on both intermittent running performance and self-belief in game running ability in highly trained Australian rules football (ARF) players. The results of Study 3 showed that a 6-week graded exposure HWI protocol (39.5°C) consisting of 2 – 3 sessions of up to 30 min per week could elicit heat acclimation, improve perception of in-game running ability, with potential to improve intermittent running performance. The highly trained ARF players who completed HWI in the evening following team training commitments demonstrated significantly greater plasma volume expansion, improved belief that their in-game running ability, and a large non-significantly greater increase in intermittent running performance when compared to a control group who completed normal training requirements. Blood parameters and belief of in-game running ability were compared at both the midpoint (week 3) and end of the intervention (week 6), with belief of in-game running ability and PV showing large increases in the HWI group at midpoint, while only belief showed large increases at the end of the intervention compared to the control group. Physiological and perceptual markers of heat acclimation increased throughout the intervention, with thermal sensation the only marker to show large decreases by the midpoint, however, large decreases in heart rate, tympanic temperature and thermal sensation were seen by the end of the intervention. Thus, partial heat acclimation may be achieved following as little as five HWI sessions across a 3-week period, however, a longer and more intensive period appears to elicit more complete adaptation. Furthermore, HWI could benefit highly trained (ARF) players through increasing self-belief in in-game running ability, with potential to increase in-season intermittent running performance.<br>The findings of this thesis demonstrate heat acclimation to be a potentially useful strategy to improve exercise performance in highly trained athletes competing in cool-temperate conditions. Furthermore, the heat acclimation strategies in this thesis have been specifically designed to fit within the highly trained athletes daily training program and represent a range of strategies that may be implemented in the applied environment.<br>