Exercise-heat tolerance (EHT) in children is influenced by many physiological factors, including sweat gland activity, cardiac output, exercise economy, ability to acclimate to heat, and maturation of organ systems. It is generally believed that children cannot tolerate hot environments as well as adults, although some children exhibit EHT that is superior to that of adults. There has been no research showing large exercise-induced differences between the core body temperatures of children versus adults, but differences in the time to onset of syncope and fatigue have been observed. This suggests that the greatest risk of heat illness for children is heat exhaustion (i.e., cardiovascular instability) and not heat stroke (i.e., hyperthermia). Therefore this review (a) examines the conclusions of previous studies to clarify misinterpretations of data, and (b) identifies research questions that require future study.
Lawrence E. Armstrong and Carl M. Maresh
Margaret C. Morrissey, Michael R. Szymanski, Andrew J. Grundstein, and Douglas J. Casa
condition. Many EHS-prevention strategies have been adopted to enhance exercise heat tolerance, but not necessarily to decrease the incidence of EHS ( Alhadad, Tan, & Lee, 2019 ). Some examples of common strategies to enhance exercise heat tolerance include heat acclimatization (HA), hydration, work
Daniel S. Moran, Tomer Erlich, and Yoram Epstein
Individuals in the population who are not able to sustain heat and whose body temperature will start rising earlier and at a higher rate than that of others, under the same conditions, are defined as “heat intolerant.”
The applicability of the heat tolerance test (HTT) in identifying individuals’ tolerance/intolerance to heat is presented.
HTT is performed according to the following protocol: 120 minutes exposure to 40°C and 40% relative humidity in a climatic chamber while walking on a treadmill, dressed in shorts and T-shirt, at a pace of 5 km/h and 2% elevation. Rectal temperature and heart rate are continuously monitored, and sweat rate is calculated.
Results and Conclusion:
The HTT that is based on controlled exposure to an exercise-heat stress is an applicable and an efficient tool in differentiating between a temporary and permanent state of heat susceptibility.
Francis G. O’Connor, Aaron D. Williams, Steve Blivin, Yuval Heled, Patricia Deuster, and Scott D. Flinn
Since Biblical times, heat injuries have been a major focus of military medical personnel. Heat illness accounts for considerable morbidity during recruit training and remains a common cause of preventable nontraumatic exertional death in the United States military. This brief report describes current regulations used by Army, Air Force, and Navy medical personnel to return active duty warfighters who are affected by a heat illness back to full duty. In addition, a description of the profile system used in evaluating the different body systems, and how it relates to military return to duty, are detailed. Current guidelines require clinical resolution, as well as a profile that that protects a soldier through repeated heat cycles, prior to returning to full duty. The Israeli Defense Force, in contrast, incorporates a heat tolerance test to return to duty those soldiers afflicted by heat stroke, which is briefly described. Future directions for U.S. military medicine are discussed.
João C. Dias, Melissa W. Roti, Amy C. Pumerantz, Greig Watson, Daniel A. Judelson, Douglas J. Casa, and Lawrence E. Armstrong
Dieticians, physiologists, athletic trainers, and physicians have recommended refraining from caffeine intake when exercising because of possible fluid-electrolyte imbalances and dehydration.
To assess how 16-hour rehydration is affected by caffeine ingestion.
59 college-age men.
Subjects consumed a chronic caffeine dose of 0 (placebo), 3, or 6 mg · kg−1 · day−1 and performed an exercise heat-tolerance test (EHT) consisting of 90 minutes of walking on a treadmill (5.6 km/h) in the heat (37.7 °C).
There were no between-group differences immediately after and 16 hours after EHT in total plasma protein, hematocrit, urine osmolality, specific gravity, color, and volume. Body weights after EHT and the following day (16 hours) were not different between groups (P > .05).
Hydration status 16 hours after EHT did not change with chronic caffeine ingestion.
Brendon P. McDermott, Douglas J. Casa, Susan W. Yeargin, Matthew S. Ganio, Lawrence E. Armstrong, and Carl M. Maresh
To describe the current scientific evidence of recovery and return to activity following exertional heat stroke (EHS).
Information was collected using MEDLINE and SPORTDiscus databases in English using combinations of key words, exertional heat stroke, recovery, rehabilitation, residual symptoms, heat tolerance, return to activity, and heat illness.
Relevant peer-reviewed, military, and published text materials were reviewed.
Inclusion criteria were based on the article’s coverage of return to activity, residual symptoms, or testing for long-term treatment. Fifty-two out of the original 554 sources met these criteria and were included in data synthesis.
The recovery time following EHS is dependent on numerous factors, and recovery length is individually based and largely dependent on the initial care provided.
Future research should focus on developing a structured return-to-activity strategy following EHS.
Tal Marom, David Itskoviz, Haim Lavon, and Ishay Ostfeld
Exertional heat stroke (EHS) is a major concern in military trainees performing intense physical exercise, with substantial morbidity rates. Prehospital diagnosis of EHS is essentially clinical. Thus, soldiers, command personnel, and medical staff are taught to recognize this injury and immediately begin aggressive treatment to prevent further deterioration.
Patients and Methods:
During 2007, 5 otherwise healthy Israeli Defense Forces (IDF) soldiers were diagnosed with EHS while performing strenuous exercise. They were treated vigorously according to the IDF EHS-treatment protocol and were referred to the emergency department.
On arrival at the emergency department, physical examination including rectal temperature was unremarkable in all soldiers. Blood and urine workup showed near-normal values. No other medical conditions that could have explained the clinical presentation were found. All soldiers were discharged shortly afterward, with no further consequences. A heat-tolerance test was performed several weeks after the event and was interpreted as normal. All soldiers returned to active service.
Because the initial clinical findings were very suggestive of EHS and because no other condition could have explained the prehospital transient hyperthermia, we suggest that these soldiers were correctly diagnosed with EHS, and we propose that rapid vigorous cooling prevented further deterioration and complications. We suggest calling this condition aborted heat stroke.
Erin L. McCleave, Katie M. Slattery, Rob Duffield, Philo U. Saunders, Avish P. Sharma, Stephen Crowcroft, and Aaron J. Coutts
Purpose: To determine whether combining training in heat with “Live High, Train Low” hypoxia (LHTL) further improves thermoregulatory and cardiovascular responses to a heat-tolerance test compared with independent heat training. Methods: A total of 25 trained runners (peak oxygen uptake = 64.1 [8.0] mL·min−1·kg−1) completed 3-wk training in 1 of 3 conditions: (1) heat training combined with “LHTL” hypoxia (H+H; FiO2 = 14.4% [3000 m], 13 h·d−1; train at <600 m, 33°C, 55% relative humidity [RH]), (2) heat training (HOT; live and train <600 m, 33°C, 55% RH), and (3) temperate training (CONT; live and train <600 m, 13°C, 55% RH). Heat adaptations were determined from a 45-min heat-response test (33°C, 55% RH, 65% velocity corresponding to the peak oxygen uptake) at baseline and immediately and 1 and 3 wk postexposure (baseline, post, 1 wkP, and 3 wkP, respectively). Core temperature, heart rate, sweat rate, sodium concentration, plasma volume, and perceptual responses were analyzed using magnitude-based inferences. Results: Submaximal heart rate (effect size [ES] = −0.60 [−0.89; −0.32]) and core temperature (ES = −0.55 [−0.99; −0.10]) were reduced in HOT until 1 wkP. Sweat rate (ES = 0.36 [0.12; 0.59]) and sweat sodium concentration (ES = −0.82 [−1.48; −0.16]) were, respectively, increased and decreased until 3 wkP in HOT. Submaximal heart rate (ES = −0.38 [−0.85; 0.08]) was likely reduced in H+H at 3 wkP, whereas CONT had unclear physiological changes. Perceived exertion and thermal sensation were reduced across all groups. Conclusions: Despite greater physiological stress from combined heat training and “LHTL” hypoxia, thermoregulatory adaptations are limited in comparison with independent heat training. The combined stimuli provide no additional physiological benefit during exercise in hot environments.
Dawn M. Emerson, Toni M. Torres-McGehee, Susan W. Yeargin, Kyle Dolan, and Kelcey K. deWeber
. Caffeine, fluid-electrolyte balance, temperature regulation, and exercise-heat tolerance . Exerc Sport Sci Rev . 2007 ; 35 ( 3 ): 135 – 140 . PubMed ID: 17620932 doi:10.1097/jes.0b013e3180a02cc1 10.1097/jes.0b013e3180a02cc1 17620932 12. Ganio MS , Klau JF , Casa DJ , Armstrong LE , Maresh
Bareket Falk, Panagiota Klentrou, Neil Armstrong, Thomas Rowland, and Han C.G. Kemper
children’s thermoregulatory capability, especially during exercise in the heat and their allegedly low heat tolerance ( 13 ). At the time, in view of the observed differences between children and adults (particularly, children’s lower sweating rate), it was widely assumed that children were at a relative