Exercise and Heat Stress: Inflammation and the Iron Regulatory Response.

This study determined the impact of heat stress on postexercise inflammation and hepcidin levels. Twelve moderately trained males completed three, 60-min treadmill running sessions under different conditions: (a) COOL, 18 °C with speed maintained at 80% maximum heart rate; (b) HOTHR, 35 °C with speed maintained at 80% maximum heart rate; and (c) HOTPACE, 35 °C completed at the average running speed from the COOL trial. Venous blood samples were collected pre-, post-, and 3-hr postexercise and analyzed for serum ferritin, interleukin-6 (IL-6), and hepcidin concentrations. Average HR was highest during HOTPACE compared with HOTHR and COOL (p < .001). Running speed was slowest in HOTHR compared with COOL and HOTPACE (p < .001). The postexercise increase in IL-6 was greatest during HOTPACE (295%; p = .003). No differences in the IL-6 response immediately postexercise between COOL (115%) and HOTHR (116%) were evident (p = .992). No differences in hepcidin concentrations between the three trials were evident at 3 hr postexercise (p = .407). Findings from this study suggest the IL-6 response to exercise is greatest in hot compared with cool conditions when the absolute running speed was matched. No differences in IL-6 between hot and cool conditions were evident when HR was matched, suggesting the increased physiological strain induced from training at higher intensities in hot environments, rather than the heat per se, is likely responsible for this elevated response. Environmental temperature had no impact on hepcidin levels, indicating that exercising in hot conditions is unlikely to further impact transient alterations in iron regulation, beyond that expected in temperate conditions.

Heat acclimation does not affect maximal aerobic power in thermoneutral normoxic or hypoxic conditions.

NEW FINDINGS: What is the central question of this study? Controlled-hyperthermia heat-acclimation protocols induce an array of thermoregulatory and cardiovascular adaptations that facilitate exercise in hot conditions. We investigated whether this ergogenic potential can be transferred to thermoneutral normoxic or hypoxic exercise conditions. What is the main finding and its importance? We showed that heat acclimation did not affect maximal cardiac output or maximal aerobic power in thermoneutral normoxic or hypoxic conditions. Heat acclimation augmented the sweating response in thermoneutral normoxic conditions. The cross-adaptation theory, according to which heat acclimation could facilitate hypoxic exercise capacity, is not supported by our data. ABSTRACT: Heat acclimation (HA) mitigates heat-induced decrements in maximal aerobic power ( V ̇ O 2 peak ) and augments exercise thermoregulatory responses in the heat. Whether this beneficial effect of HA is observed in hypoxic or thermoneutral conditions remains unresolved. We explored the effects of HA on cardiorespiratory and thermoregulatory responses to exercise in normoxic, hypoxic and hot conditions. Twelve men [ V ̇ O 2 peak 54.7(standard deviation 5.7) ml kg-1 min-1 ] participated in a HA protocol consisting of 10 daily 90-min controlled-hyperthermia (target rectal temperature, Tre  = 38.5°C) exercise sessions. Before and after HA, we determined V ̇ O 2 peak in thermoneutral normoxic (NOR), thermoneutral hypoxic (fractional inspired O2  = 13.5%; HYP) and hot (35°C, 50% relative humidity; HE) conditions in a randomized and counterbalanced order. Preceding each maximal cycling test, a 30-min steady-state exercise bout at 40% of the NOR peak power output was used to evaluate thermoregulatory responses. Heat acclimation induced the expected adaptations in HE: reduced Tre and submaximal heart rate, enhanced sweating response and expanded plasma volume. However, HA did not affect V ̇ O 2 peak or maximal cardiac output (P = 0.61). The peak power output was increased post-HA in NOR (P < 0.001) and HE (P < 0.001) by 41 ± 21 and 26 ± 22 W, respectively, but not in HYP (P = 0.14). Gross mechanical efficiency was higher (P = 0.004), whereas resting Tre and sweating thresholds were lower (P < 0.01) post-HA across environments. Nevertheless, the gain of the sweating response decreased (P = 0.05) in HYP. In conclusion, our data do not support a beneficial cross-over effect of HA on V ̇ O 2 peak in normoxic or hypoxic conditions.

Effect of two-weeks endurance training wearing additional clothing in a temperate outdoor environment on performance and physiology in the heat.

This investigation assessed performance, physiological and perceptual responses to wearing additional clothing during endurance training for two-weeks in temperate environments, to determine if this approach could be used as a practical, alternative, heat acclimation strategy for athletes. Fifteen trained male triathletes assigned to performance-matched groups completed a two-week unsupervised endurance cycling and running program in either (i) shorts and a short sleeve top (CON; n = 8) or (ii) additional clothing of full-length pants, a "winter" jacket and gloves made from nylon, polyurethane and polyester (AC; n = 7). Participants completed three separate (i.e. familiarisation, pre-program and post-program), identical, pre-loaded cycling time-trials (20 min at 180 W followed by a 40 min self-paced time trial) in 32.5 ± 0.1°C and 55 ± 6% RH. Core and skin temperatures, heart rate, sweat rate, perceived exertion, thermal sensation and thermal comfort were measured across the pre-loaded time trials, and heart rate and thermal sensation were measured across the training program. All of the participants recorded in their diaries that they completed all of the programmed training sessions in the required attire. Mean thermal sensation was most likely hotter in AC (5.5 ± 0.4 AU) compared to CON (4.4 ± 0.4 AU; ES = 1.61, ± 0.68) during the training sessions. However, follow up tests revealed no physiological or perceptual signs of heat acclimation, and the change in time-trial performance from pre-post between groups was trivial (CON: -3.5 ± 12.0 W, AC: -4.1 ± 9.6 W; difference = -0.7%, ± 5.4%). Training in additional clothing for two-weeks in a temperate environment was not an effective heat acclimation strategy for triathletes.

Acute physiological and perceptual responses to wearing additional clothing while cycling outdoors in a temperate environment:A practical method to increase the heat load.

This investigation assessed the acute physiological and perceptual responses to wearing additional clothing during outdoor cycling to determine if this strategy could increase the heat load while training in temperate environments. Seven male cyclists (age: 32 ± 13 y, height: 179 ± 10 cm, body mass: 74 ± 10 kg, body fat percentage: 10.3 ± 1.0%) completed 2 randomized outdoor (∼17°C and ∼82% RH), 80 min cycling sessions at moderate-hard intensities (CR10 RPE = 3-5). They wore spandex shorts and a short sleeve top (CON) or additional clothing including full-length spandex pants and a 'winter' cycling jacket and gloves (AC). Core temperature, heart rate, sweat rate, thermal sensation and thermal comfort were measured across the trials. Moderate increases were observed in AC vs. CON for the change in mean core temperature (0.4 ± 0.3°C, effect size, ES = 1.16 ± 0.55), change in maximum core temperature (0.5 ± 0.3°C, ES = 1.07 ± 0.48) and sweat rate (0.24 ± 0.16 L . h-1, ES = 1.04 ± 0.59). A small increase in mean heart rate (3 ± 3 bpm, ES = 0.32 ± 0.28) was observed as well as a 'very likely' (percentage difference = 22.4 ± 7.1) and 'most likely' (percentage difference = 42.9 ± 11.9) increase in thermal sensation and thermal comfort, respectively, in AC vs. CON. Dressing in additional clothing while cycling outdoors in a temperate environment increased physiological strain and sensations of warmth and discomfort. Training in additional clothing during outdoor cycling represents a practical alternative to increasing the heat load of a training session.

The effect of high versus low intensity heat acclimation on performance and neuromuscular responses.

This study examined the effect of exercise intensity and duration during 5-day heat acclimation (HA) on cycling performance and neuromuscular responses. 20 recreationally trained males completed a 'baseline' trial followed by 5 consecutive days HA, and a 'post-acclimation' trial. Baseline and post-acclimation trials consisted of maximal voluntary contractions (MVC), a single and repeated countermovement jump protocol, 20km cycling time trial (TT) and 5×6s maximal sprints (SPR). Cycling trials were undertaken in 33.0 ± 0.8°C and 60 ± 3% relative humidity. Core (Tcore), and skin temperatures (Tskin), heart rate (HR), rating of perceived exertion (RPE) and thermal sensation were recorded throughout cycling trials. Participants were assigned to either 30min high-intensity (30HI) or 90min low-intensity (90LI) cohorts for HA, conducted in environmental conditions of 32.0 ± 1.6°C. Percentage change time to complete the 20km TT for the 90LI cohort was significantly improved post-acclimation (-5.9 ± 7.0%; P=0.04) compared to the 30HI cohort (-0.18 ± 3.9%; P<0.05). The 30HI cohort showed greatest improvements in power output (PO) during post-acclimation SPR 1 and 2 compared to 90LI (546 ± 128W and 517 ± 87W, respectively; P<0.02). No differences were evident for MVC within 30HI cohort, however, a reduced performance indicated by % change within the 90LI (P=0.04). Compared to baseline, mean Tcore was reduced post-acclimation within the 30HI cohort (P=0.05) while mean Tcore and HR were significantly reduced within the 90LI cohort (P=0.01 and 0.04, respectively). Greater physiological adaptations and performance improvements were noted within the 90LI cohort compared to the 30HI. However, 30HI did provide some benefit to anaerobic performance including sprint PO and MVC. These findings suggest specifying training duration and intensity during heat acclimation may be useful for specific post-acclimation performance.