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Background: Maximal oxygen uptake (VO 2max ) is an important determinant of endurance performance. Heat acclimation/acclimatization (HA/HAz) elicits improvements in endurance performance. Upon heat exposure reduction, intermittent heat training (IHT) may alleviate HA/HAz adaptation decay; however, corresponding VO 2max responses are unknown., Hypothesis: VO 2max is maintained after HAz/HA; IHT mitigates decrements in aerobic power after HAz/HA., Study Design: Interventional study., Level of Evidence: Level 3., Methods: A total of 27 male endurance runners (mean ± SD; age, 36 ± 12 years; body mass, 73.03 ± 8.97 kg; height, 178.81 ± 6.39 cm) completed VO 2max testing at 5 timepoints; baseline, post-HAz, post-HA, and weeks 4 and 8 of IHT (IHT 4 , IHT 8 ). After baseline testing, participants completed HAz, preceded by 5 days of HA involving exercise to induce hyperthermia for 60 minutes in the heat (ambient temperature, 39.13 ± 1.37°C; relative humidity, 51.08 ± 8.42%). Participants were assigned randomly to 1 of 3 IHT groups: once-weekly, twice-weekly, or no IHT. Differences in VO 2max , velocity at VO 2max (vVO 2 ), and maximal heart rate (HR max ) at all 5 timepoints were analyzed using repeated-measure analyses of variance with Bonferroni corrections post hoc., Results: No significant VO 2max or vVO 2 differences were observed between baseline, post-HAz, or post-HA ( P = 0.36 and P = 0.09, respectively). No significant group or time effects were identified for VO 2max or vVO 2 at post-HA, IHT 4 , and IHT 8 ( P = 0.67 and P = 0.21, respectively). Significant HR max differences were observed between baseline and post-HA tests ( P < 0.01). No significant group or time HR max differences shown for post-HA, IHT 4 , and IHT 8 ( P = 0.59)., Conclusion: VO 2max was not reduced among endurance runners after HA/HAz and IHT potentially due to participants' similar aerobic training status and high aerobic fitness levels., Clinical Relevance: HAz/HA and IHT maintain aerobic power in endurance runners, with HAz/HA procuring reductions in HR max ., Competing Interests: The authors report no potential conflicts of interest in the development or publication of this article.

Introduction: Personal protective equipment (PPE) inhibits heat dissipation and elevates heat strain. Impaired cooling with PPE warrants investigation into practical strategies to improve work capacity and mitigate exertional heat illness., Purpose: Examine physiological and subjective effects of forearm immersion (FC), fan mist (MC), and passive cooling (PC) following three intermittent treadmill bouts while wearing PPE., Methods: Twelve males (27 ± 6 years; 57.6 ± 6.2 ml/kg/min; 78.3 ± 8.1 kg; 183.1 ± 7.2 cm) performed three 50-min (10 min of 40%, 70%, 40%, 60%, 50% vVO 2 max) treadmill bouts in the heat (36 °C, 30% relative humidity). Thirty minutes of cooling followed each bout, using one of the three strategies per trial. Rectal temperature (T core ), skin temperature (T sk ), heart rate (HR), heart rate recovery (HRR), rating of perceived exertion (RPE), thirst, thermal sensation (TS), and fatigue were obtained. Repeated-measures analysis of variance (condition x time) detected differences between interventions., Results: Final T core  was similar between trials (P > .05). Cooling rates were larger in FC and MC vs PC following bout one (P < .05). HRR was greatest in FC following bouts two (P = .013) and three (P < .001). T sk , fluid consumption, and sweat rate were similar between all trials (P > .05). TS and fatigue during bout three were lower in MC, despite similar T core  and HR., Conclusion: Utilizing FC and MC during intermittent work in the heat with PPE yields some thermoregulatory and cardiovascular benefit, but military health and safety personnel should explore new and novel strategies to mitigate risk and maximize performance under hot conditions while wearing PPE., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Background: Athletes training in heat experience physiological and perceptual symptoms that risk their safety and performance without adaptation., Purpose: We examined the changes in environmental symptoms, assessed with the Environmental Symptoms Questionnaire (ESQ), during heat acclimatization (HAz), heat acclimation (HA), and intermittent heat training (HT)., Methods: Twenty-seven participants (mean ± standard deviation [M ± SD], age of 35 ± 12 y, VO 2max of 57.7 ± 6.8 mL·kg -1 ·min -1 ) completed five trials involving 60 mins of running (60% vVO 2max ) followed by a 4 km time trial in heat (M ± SD, temperature of 35.5 ± 0.7 °C, humidity of 46.4 ± 1.5%). The trials occurred at baseline, post-HAz, post-HA, at week 4 of HT (post-HT4), and at week 8 of HT (post-HT8). The participants completed HT once/week (HT MIN ), completed HT twice/week (HT MAX ), or did not complete HT (HT CON ). ESQ symptoms, thermal sensation (TS), and heart rate (HR) were measured pre- and post-trial., Results: Post-ESQ symptoms improved post-HA (3[0.40, 4.72], p = 0.02) and post-HAz (3[0.35, 5.05], p = 0.03) from baseline. During HT, symptoms improved in the HT MAX group and worsened in the HT MIN and HT CON groups. Symptoms improved in the HT MAX group versus the HT CON group at post-HT8 (4[1.02, 7.23], p = 0.012). Higher TS and HR values were weakly associated with ESQ symptoms during HT ( r = 0.20, p = 0.04), only explaining 20% of variance., Conclusions: ESQ symptoms improved during HAz, HA, and HT 2x/week. ESQ symptoms were not statistically correlated with HR during exercise heat stress. TS was not sensitive to detecting adaptation and did not subjectively change. The ESQ may be valuable in monitoring adaptation and may contribute to performance post-acclimation.

Recruiting companies recommend elite female soccer players be ≥165 cm (5'5″) in stature. This study investigated if stature limits match-playing time and performance in elite World Cup soccer among players, positions, and countries. We hypothesized stature would not affect match-playing time or performance. Descriptive data were collected on 552 players from 2019 FIFA Women's World Cup. Odds ratios determined likelihood of starting for players <165 cm and ≥165 cm. ANOVAs compared playing time between stature groups, among positions, and between countries. Performance factors including assists, goals, attempts, corners, shots blocked, and defending blocks were reported. Independent t-tests compared differences between players (≥165 cm, < 165 cm). Data are reported, mean difference [95% confidence interval] [MD (95%CI)] and effect sizes (ES). On average, 32.3% of players were <165 cm. Of total players, no differences existed in total minutes ( F =  0.98, p  = 0.32) , matches ( F  = 0.27 p  = 0.59), or average minutes per match ( F =  0.48, p  = 0.49) between stature groups, regardless of position. No differences existed in playing time between players <165 cm and ≥165 cm among any positions ( p  > 0.05), or between countries ( p  > 0.05). Taller mid-fielders exhibited greater performance in goals, assists, attempts, shots blocked, and defending blocks (MD [95%CI] ES; assists, -0.44[-0.76,-0.11]0.59, p  = 0.009; goals, -0.35[-0.69,-0.01]0.44, p  = 0.047); attempts, 3.14[1.38, 4.90]0.80, p  = 0.001; corners, 2.04[0.12, 3.95]0.48, p  = 0.037; shots blocked, 0.96[0.40, 1.51]0.75, p  = 0.001; defending blocks, 0.43[0.32,0.82]0.48, p  = 0.035), however, actual differences were minimal. Our findings indicate stature does not inhibit playing and performing elite women's soccer, as nearly one-third of players were <165 cm., Competing Interests: Author GC was employed by the United States Soccer Federation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2022 Manning, Sekiguchi, Benjamin, Spaulding, Dierickx, Spaulding, Davenport, Picard-Busky, Chiampas, Nassis and Casa.)

Assessing the adaptation of rectal temperature (T rec ) is critical following heat acclimatization (HAz) and heat acclimation (HA) because it is associated with exercise performance and safety; however, more feasible and valid methods need to be identified. The purpose of this study was to predict adaptations in T rec from heart rate (HR), sweat rate (SR), and thermal sensation (TS) using predictive modeling techniques. Twenty-five male endurance athletes (age, 36 ± 12 y; VO 2max , 57.5 ± 7.0 mL⋅kg -1 ⋅min -1 ) completed three trials consisting of 60 min running at 59.3 ± 1.7% vVO 2max in a hot environment. During trials, the highest HR and TS, SR, and T rec at the end of trials were recorded. Following a baseline trial, participants performed HAz followed by a post-HAz trial and then completed five days HA, followed by a post-HA trial. A decision tree indicated cut-points of HR (<-13 bpm), SR (>0.3 L·h -1 ), and TS (≤-0.5) to predict lower T rec . When two or three variables met cut-points, the probability of accuracy of showing lower T rec was 95.7%. Greater adaptations in T rec were observed when two or three variables met cut-points (-0.71 ± 0.50 °C) compared to one (-0.13 ± 0.36 °C, p < 0.001) or zero (0.0 3 ± 0.38 °C, p < 0.001). Specificity was 0.96 when two or three variables met cut-points to predict lower T rec . These results suggest using heart rate, sweat rate, and thermal sensation adaptations to indicate that the adaptations in T rec is beneficial following heat adaptations, especially in field settings, as a practical and noninvasive method.

Background: The purpose of this study was to investigate effects of heat acclimatization (HAz) followed by heat acclimation (HA), and intermittent heat training (IHT) on time-trial performance., Hypothesis: Time-trial performance will improve after HA and will further improve with twice a week of IHT., Study Design: Interventional study., Level of Evidence: Level 3., Methods: A total of 26 male athletes (mean ± SD; age, 35 ± 12 years; body mass, 72.8 ± 8.9 kg; peak oxygen consumption [VO 2peak ], 57.3 ± 6.7 mL·kg -1 ·min -1 ) completed five 4-km time trials (baseline, post-HAz, post-HA, post-IHT4, post-IHT8) in the heat (ambient temperature, 35.4°C ± 0.3°C; relative humidity, 46.7% ± 1.2%) on a motorized treadmill. After baseline time trial, participants performed HAz (109 ± 10 days) followed by post-HAz time trial. Then, participants completed 5 days of HA, which involved exercising to induce hyperthermia (38.50°C-39.75°C) for 60 minutes. Participants were then divided into 3 groups and completed IHT either twice per week (IHT MAX ), once per week (IHT MIN ), or not at all (IHT CON ) over an 8-week period. The exercise used for the IHT matched the HA. Four-kilometer time trials were performed after 4 weeks (post-IHT4) and 8 weeks of IHT (post-IHT8)., Results: Time trial was faster in post-HA (17.98 ± 2.51 minutes) compared with baseline (18.61 ± 3.06 minutes; P = 0.037) and post-HAz (18.66 ± 3.12 minutes; P = 0.023). Percentage change in time trial was faster in IHT MAX (-3.9% ± 5.2%) compared with IHT CON (11.5% ± 16.9%) ( P = 0.020) and approached statistical significance with large effect (effect size = 0.96) compared with IHT MIN (1.6% ± 6.2%; P = 0.059) at post-IHT8. Additionally, IHT MAX (-2.2% ± 4.2%) was faster than IHT CON (3.6% ± 6.9%) ( P = 0.05) at post-IHT4., Conclusion: These results indicate that HA after HAz induces additional improvement in time-trial performance. IHT twice per week shows improvement after 8 weeks, while once per week maintains performance for 8 weeks. No IHT results in a loss of adaptations after 4 weeks and even greater losses after 8 weeks., Clinical Relevance: HA after HAz improves time-trial performance, twice a week of IHT improves performance further, and once a week of IHT maintains performance for at least 8 weeks.

The purpose of this study was to examine the changes in metabolic heat production (Hprod), evaporative heat loss (Hevap), and dry heat loss (Hdry), following heat acclimatization (HAz) and heat acclimation (HA). Twenty-two male endurance athletes (mean ± standard deviation; age, 37 ± 12 y; body mass, 73.4 ± 8.7 kg; height, 178.7 ± 6.8 cm; and VO2max, 57.1 ± 7.2 mL·kg−1·min−1) completed three trials (baseline; post-HAz; and post-HA), which consisted of 60 min steady state exercise at 59 ± 2% velocityVO2max in the heat (ambient temperature [Tamb], 35.2 ± 0.6 °C; relative humidity [%rh] 47.5 ± 0.4%). During the trial, VO2 and RER were collected to calculate Hprod, Hevap, and Hdry. Following the baseline trial, participants completed self-directed outdoor summer training followed by a post-HAz trial. Then, five days of HA were completed over eight days in the heat (Tamb, 38.7 ± 1.1 °C; %rh, 51.2 ± 2.3%). During the HA sessions, participants exercised to maintain hyperthermia (38.50 °C and 39.75 °C) for 60 min. Then, a post-HA trial was performed. There were no differences in Hprod between the baseline (459 ± 59 W·m−2), post-HAz (460 ± 61 W·m−2), and post-HA (464 ± 55 W·m−2, p = 0.866). However, Hevap was significantly increased post-HA (385 ± 84 W·m−2) compared to post-HAz (342 ± 86 W·m−2, p = 0.043) and the baseline (332 ± 77 W·m−2, p = 0.037). Additionally, Hdry was significantly lower at post-HAz (125 ± 8 W·m−2, p = 0.013) and post-HA (121 ± 10 W·m−2, p < 0.001) compared to the baseline (128 ± 7 W·m−2). Hdry at post-HA was also lower than post-HAz (p = 0.049). Hprod did not change following HAz and HA. While Hdry was decreased following HA, the decrease in Hdry was smaller than the increases in Hevap. Adaptations in body heat exchange can occur by HA following HAz.

Objectives: To examine the efficacy of weekly and bi-weekly heat training to maintain heat acclimatization (HAz) and heat acclimation (HA) for 8 weeks in aerobically trained athletes., Design: Randomized, between-group., Methods: Twenty-four males (mean [m ± standard deviation [sd]; (age, 34 ± 12 y; body mass, 72.6 ± 8.8 kg, VO 2peak , 57.7 ± 6.8 mL·kg -1 ·min -1 ) completed five trials (baseline, following HAz, following HA (HAz + HA), four weeks into heat training [HT WK4 ], and eight weeks into HT [HT WK8 ] that involved 60 min of steady-state exercise (59.1 ± 1.8% vVO 2peak ) in an environmental laboratory (wet bulb globe temperature [WBGT], 29.6 ± 1.4 °C) on a motorized treadmill. Throughout exercise, heart rate (HR) and rectal temperature (T rec ) were recorded. Following HAz + HA, participants were assigned to three groups: control group (HT 0 ), once per week heat training (HT 1 ), and twice per week heat training (HT 2 ). HT involved heated exercise (WBGT, 33.3 ± 1.3 °C) to achieve hyperthermia (38.5-39.75 °C) for 60 min. Repeated measures ANOVAs were used to determine differences., Results: HAz + HA resulted in significant improvements in HR (p < 0.001) and T rec (p < 0.001). At HT WK8 , HR was significantly higher in HT 0 (174 ± 22 beats⋅min -1 ) compared to HT 2 (151 ± 17 beats⋅min -1 , p < 0.023), but was not different than HT 1 (159 ± 17 beats⋅min -1 , p = 0.112). There was no difference in % change of T rec from post-HAz + HA to HT WK4 (0.6 ± 1.3%; p = 0.218), however, HT WK8 (1.8 ± 1.4%) was significantly greater than post-HAz + HA in HT 0 (p = 0.009)., Conclusions: Bi-weekly HT provided clear evidence for the ability to maintain physiological adaptions for 8 weeks following HA., (Copyright © 2021 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.)