The thermophysiological and ergogenic response to heat stress intervention strategies

Abstract

Endurance exercise in hot environments puts a great strain on both the physiological and cellular mechanisms of the body to maintain efficient heat dissipation and thermal homeostasis. Once the body is unable to dissipate more heat than is gained thermal stress increases core (TCore) and skin temperature (TSkin) impairing performance. Athletes and military personnel train and compete/work in many extreme environments; the utilisation of intervention strategies prior to exercise will delay the onset of fatigue and reduce thermal strain at a physiological and cellular level. The purpose of the first experiment was to investigate the combined effect of hyperhydration and pre-cooling methods on endurance cycling performance in the heat. Five healthy males completed a mile (16.1 km) self-paced time trial (TT) in a hot and humid environment (30°C & 50% RH) on 4 occasions: Glycerol hyperhydration (HH), pre-cooling (PC), glycerol hyperhydration and pre-cooling (HH+PC) and control (C). The cellular stress response was assessed via Heat Shock 70 kDa Protein 2 (HSP72) mRNA expression within leukocytes. There was a significant difference in completion time between the conditions (p = 0.025). On average, completion time during the PC trial was 6% faster than C (p = 0.03, 95% CI = -15 to - 210 s) and 4% faster than HH (p = 0.02, 95% CI = - 21 to -132 s). There was no significant difference in HSP72 mRNA expression between conditions (p = 0.26). PC via CWI alone or in combination with HH, enhanced endurance performance in hot and humid environments with no further ergogenic effect seen when HH was used in combination with PC. In light of the findings from the first experimental chapter, experiment 2 looked at the kinetics and mechanisms of G-HH compared to hyperhydration with water (W-HH) at rest. 16 resting males’ on 2 occasions: ingested one of two solutions evenly over a 90 min period. Glycerol solution (G-HH) or a water solution (W-HH). It was revealed that peak change in 2 plasma volume (%ΔPV) was significantly higher after G-HH (19.1 ± 6.3%) than W-HH (10.2 ± 4.5%) (F1, 9.3 = 14.37, p = 0.004). G-HH effectively expanded PV more than water hyperhydration for the full 120 min observation period (p = 0.02). It is recommended that exercise and extreme environment occupational pursuits (such as military and bush firefighters), commences immediately post the 90 min ingestion period when PV expansion is highest, to delay the onset of dehydration. The third experimental chapter investigated the potential pre-cooling action of an acute dose of acetaminophen and its comparison to established pre-cooling methods: cold water immersion and ice slurry ingestion on exercise in extreme heat. Evaluated from physiological and cellular perspective. 8 recreationally active males completed a 40 min sub-maximal run in extreme heat (40°C & 30% RH) on 4 occasions: cold water immersion (CWI), ice slurry ingestion (ICE), acetaminophen ingestion (ACT) and control (CON). There was significant reduction in TRectal (-0.48°C) and Tskin (4°C lower than all other conditions) after CWI compared to ICE, CON and ACT. A significant down regulation of HSP72 expression post exercise after ACT compared to CWI. ACT did not elicit a thermoregulatory reduction but did however reduce strain on a cellular level during exercise in extreme heat. CWI proved to be the most effective form of pre-cooling through the reduction of TRectal and Tskin prior to exercise. These findings confirm previous research that cold water immersion alters the robust PV expansion produced by glycerol hyperhydration (Gordon, Fogarty, Greenleaf et al., 2003). Cold water immersion is the most effective pre-cooling method to reduce thermal strain and improve performance it does however lack practical application. Acetaminophen did not prove to effectively reduce thermoregulatory strain but did however reduce strain on a cellular level. These results suggest that individuals participating in prolonged exercise in hot conditions due to its practicality for use in the field further research needs to be conducted in 3 to acetaminophen’s mechanisms of action and potential to reduce thermal strain at a cellular and possibly in the correct settings physiological level.