Short-term heat acclimation training: effects on performance and inflammation

Guy, Joshua Hayward (2016) Short-term heat acclimation training: effects on performance and inflammation. PhD thesis, James Cook University.

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Background: Extreme environmental conditions pose diverse challenges to the event preparation and competitive practices of athletes. In particular, uncompensable heat stress, whether experienced either passively or in response to exercise in the heat, influences a complex network of thermoregulatory, immune and inflammatory processes. However, there are complexities over the accurate assessment of responses to hot conditions due to factors such as the variability of blood biomarker concentrations associated with heat stress and inflammation. These may present difficulties when trying to characterise physiological responses to exercise and training in the heat. Furthermore, while the intention of endurance athletes may be to rapidly gain meaningful physical adaptation via short-term heat training prior to competition, it is currently unclear whether or not this condensed, intense process also presents an overt, acute challenge to the immune system. In addition, factors such as residual, accumulative fatigue as a consequence of intense, short duration heat training, have yet to be addressed. Therefore, interventions that may facilitate both rapid adaptation to the heat, coupled with reduced fatigue sensations and rapid recovery could be attractive. Consequently, post-exercise recovery interventions such as rapid-cooling following exercise in the heat could be a useful stratagem to minimise potential adverse residual effects of heat training if they do not compromise the adaptation process to a hot environment. These problems have been addressed through four separate research projects.

Common Methods: Study One employed a two group design (exercise and non-exercise control) with participants being sampled at different time points. Studies Two, Three and Four employed a randomised control trial design with multiple groups in each study (e.g. intervention and control). The participants recruited for this thesis were recreationally active males aged 18-30 years and all exercise was performed on a cycle ergometer. The heat stress test (HST) utilised was the same for all studies and comprised three x sub-maximal intervals (50%, 60%, and 70% of power output associated with VO₂ max) followed by a 5 km time trial (TT) on a cycle ergometer. The HST lasted for ~60 min and was performed in an environmental chamber at a temperature of 35 °C and 70% relative humidity. Venous blood samples were drawn at rest (10 min) in a seated position from a prominent superficial forearm vein and serum biomarker concentrations analysed in duplicate with commercial immunoassay kits.

Study One – Biomarker Reliability

Aims: To examine the biological variation and reliability of blood biomarkers that are associated with heat stress and inflammation at rest and in response to a strenuous cycling task in a hot and humid environment.

Method: The short-term reliability (at rest, seven days apart) and the acute responsiveness of each biomarker to a single HST was evaluated (Subject n=32). Serum was analysed for the concentration of C-reactive protein (CRP), interleukin-6 (IL-6), extracellular heat shock protein 72 (eHSP72), immunoglobulin M (IgM) and lipopolysaccharide (LPS). Test-retest reliability was determined as the coefficient of variation (CV).

Results: Biomarkers with the least short-term within-subject variation were IL-6 (19%, ±20%; CV, ±95% confidence limits) and LPS (23%, ±13%). Greater variability was observed for IgM, eHSP72 and CRP (CV range 28-38%). IL-6 concentration exhibited the largest increase in response to acute exercise (550%, ±158%, p = <0.001, percent change, ±95% confidence limits) and, although CRP concentration had a modest CV (12%, ±7%), it increased significantly post-exercise by 21% ±16% (p = 0.02). In contrast, eHSP72 and LPS exhibited trivial changes post-exercise.

Conclusions: Variation of common inflammatory markers after exercise in the heat is not always discernible from short-term (weekly) variation.

Study Two – Reponses to Exercise in the Heat between Tropical and Temperate Residents

Aims: To compare the physiological and inflammatory responses of tropical and temperate residents to repeated bouts of exercise in a hot and humid environment.

Method: Tropical and Temperate participants (n=24) were recruited based on their location of residency (Cairns, Australia or Plymouth, UK, respectively) Participants undertook three HSTs seven days apart. Venous blood samples were drawn before and after each HST and serum analysed for concentrations of IL-6, LPS, and IgM. Data are presented as between-group effects.

Results: Tropical residents reported significantly lower rating of perceived exertion than Temperate (-2, ±1 units, mean, ±95% confidence limits, RPE scale 6-20) in each of the three HSTs (p = 0.03, large difference). Tropical residents exhibited a ~1.5-fold (p = 0.05) greater concentration in post-exercise concentrations of IL-6 at HST1, a ~3-fold greater pre-exercise concentration of LPS at HST2 (p = 0.02), and a ~2-fold greater pre-exercise concentration of IgM at HST2 (p = 0.04) and HST3 (p = 0.02) than Temperate.

Conclusions: Tropical residents reported lower levels of exertion following strenuous exercise in the heat compared with temperate residents, however, these perceptions do not appear to influence performance. Background heat acclimatisation status may influence resting concentrations of IgM and LPS following exercise in the heat; however, both populations (tropical and temperate) appear to regulate these biomarkers within safe homeostatic limits.

Study Three – Inflammation during Short-Term Heat Acclimation Training

Aims: To examine the inflammatory and immune effects of heat acclimation (HA) training, as well as the performance benefits associated with short-term HA (STHA), and to determine the effectiveness of periodic "top-up" sessions following the STHA.

Method: Moderately-active males (n=24) were allocated randomly to either HOT (35 °C and 70% RH); NEUTRAL (20 °C and 45% RH); or a non-exercising control group, (CON). Over the 18 day study HOT and NEUTRAL performed seven training sessions (40 min cycling at 55% of VO₂ max) and all participants completed three HSTs. HOT and NEUTRAL undertook an initial HST followed by four training sessions on consecutive days. Participants then rested for 48 h and performed their second HST. HOT and NEUTRAL then undertook three additional "top up" training sessions every third day, rested for 48 h and completed their final HST. CON completed three HSTs at the same time points as HOT and NEUTRAL but did not undertake any training. Venous blood samples were collected before and after each HST and serum analysed for IL-6, IgM and LPS.

Results: Both HOT and NEUTRAL groups experienced substantial improvement in 5 km TT performance (HOT -33, ±20 s, p = 0.02, NEUTRAL -39, ±18 s, p = 0.01, mean, ±95% confidence limits) but only HOT were faster (-45, ±25 s and -12, ±7 s, p = 0.01) in HST3 compared with baseline and HST2. IL-6 was elevated ~4 fold after exercise for all groups, however, there were no significant changes for IgM or LPS.

Conclusions: Short- and medium-term heat acclimation training consisting of ~60 min of heat exposure exercising at ~55% of VO₂ max does not appear to pose a substantial threat to the immune system or invoke endotoxemia in healthy, recreationally active males. Additional "top up" training every three days further improves cycling time trial performance in hot conditions compared to short-term heat training only.

Study Four – Post-Exercise Cooling following Heat Acclimation Training

Aims: To examine the effect of rapid whole-body cooling as a means of recovery during STHA training.

Method: Twenty four moderately trained males were allocated to either whole-body cooling (WBC) or passive recovery control (PRC) training groups. Both WBC and PRC undertook a VO₂ max and time-to-exhaustion (TTE) tests on a cycle ergometer in a thermo-neutral condition (20°C, 50% relative humidity) and a HST, before and after four days HA training on a cycle ergometer. Participants in WBC received a 20 min post-exercise rapid cooling intervention that comprised of whole-body fanning (~3.6 m.s⁻¹) and ingestion of a 500 mL ice-slushy immediately following each exercise in the heat session.

Results: Following the HA training program WBC had a 4.0%, ±5.8% (mean, ±95% confidence limits) greater improvement in 5 km TT performance in hot conditions (p = 0.04), and a 30%, ±45% greater improvement in TTE performance (p = 0.03) compared with PRC . WBC also reported lower levels of fatigue compared with PRC following the HA training (6.5 ± 0.5 vs 8.5 ± 1.0 units, p <0.001, mean ± SD).

Conclusions: Recreational athletes can benefit from short-term heat acclimation training at a fixed intensity at ~55% of VO₂ max for 60⁻¹ to improve exercise performance in the heat, although progressive increases in work intensity of ~5% each day are recommended to elicit greater performance and physiological adaptations. Short-term heat acclimation training is enhanced with immediate post-exercise cooling utilising an ice slushy (7⁻¹) and whole body fanning (3.6 m.s⁻¹) to improve performance, enhance physiological adaptations and ameliorate accumulated fatigue that can occur from a high frequency heat acclimation program.

Item ID: 49459
Item Type: Thesis (PhD)
Keywords: athletic performance, cycling, cytokine, endurance performance, exercise physiology, exercise, heat acclimation, heat adaptation, heat shock proteins, heat tolerance, inflammation, inflammatory cytokines, lipopolysaccharide, physiological response to heat
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Guy, Joshua H., Deakin, Glen B., Edwards, Andrew M., Miller, Catherine M., and Pyne, David B. (2014) Adaptation to hot environmental conditions: an exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes. Sports Medicine, 45 (3). pp. 303-311.

Chapter 4: Guy, Joshua H., Edwards, Andrew M., Miller, Catherine M., Deakin, Glen B., and Pyne, David B. (2017) Short-term reliability of inflammatory mediators and response to exercise in the heat. Journal of Sports Sciences. pp. 1-7. (In Press)

Chapter 6: Guy, Joshua H., Pyne, David B., Deakin, Glen B., Miller, Catherine, and Edwards, Andrew M. (2016) Acclimation training improves endurance cycling performance in the heat without inducing Endotoxemia. Frontiers in Physiology, 7. pp. 1-9.

Date Deposited: 23 Jun 2017 04:07
FoR Codes: 11 MEDICAL AND HEALTH SCIENCES > 1106 Human Movement and Sports Science > 110699 Human Movement and Sports Science not elsewhere classified @ 50%
11 MEDICAL AND HEALTH SCIENCES > 1106 Human Movement and Sports Science > 110602 Exercise Physiology @ 50%
SEO Codes: 92 HEALTH > 9201 Clinical Health (Organs, Diseases and Abnormal Conditions) > 920108 Immune System and Allergy @ 50%
92 HEALTH > 9299 Other Health > 929999 Health not elsewhere classified @ 50%
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