jeudi 10 juin 2021

Effets selon l'heure de la journée de l'exposition au rayonnement solaire sur la thermorégulation lors d'exercices en plein air par temps chaud

Aperçu: G.M.

Un rayonnement solaire élevé a été reconnu comme un facteur contribuant aux maladies liées à la chaleur à l'effort chez les personnes faisant de l'exercice à l'extérieur par temps chaud. 

Bien que l'intensité du rayonnement solaire soit connue pour avoir une variation horaire similaire à celle de la température corporelle, la relation entre les fluctuations du rayonnement solaire associées au changement diurne de l'angle de la lumière solaire et les réponses thermorégulatrices chez les personnes faisant de l'exercice à l'extérieur dans un environnement chaud reste largement inconnue.
La présente étude a donc examiné les effets de l'heure de la journée des variations du rayonnement solaire associées au changement de l'angle d'élévation solaire sur les réponses thermorégulatrices lors d'exercices extérieurs d'intensité modérée dans la chaleur de l'été. 

Huit joueurs de baseball du secondaire en bonne santé, des volontaires masculins acclimatés à la chaleur ont suivi un entraînement de baseball en plein air de 3 heures sous le ciel clair et dans la chaleur. Les entraînements ont commencé à 09h00 dans l'essai du matin et à 16h00 dans l'essai de l'après-midi chacun un jour différent. Le rayonnement solaire et l'angle d'élévation solaire pendant l'exercice ont continué d'augmenter en AM (672-1107 W/m2 et 44-69°) et de diminuer en PM (717-0 W/m2 et 34-0°) et étaient plus élevés le matin que l 'après midi (tous deux P < 0,001).
Bien que la température ambiante (AM 32-36°C, PM 36-30°C) et la température du globe humide (AM 31-33°C, PM 34-27°C) aient également continué d'augmenter en AM et de diminuer en PM, il n'y avait aucune différence entre les essais dans ceux-ci (les deux P > 0,05).
La température tympanique mesurée par un thermomètre tympanique infrarouge et la température cutanée moyenne étaient plus élevées le matin que le soir à 120 et 180 min (P < 0,05).
La température de la peau était plus élevée le matin que le soir au niveau du bras et de la cuisse à 120 min (P < 0,05) et au mollet à 120 et 180 min (tous deux P < 0,05).
Le gain de chaleur corporelle provenant du soleil était plus important pendant l'exercice le matin que l'après-midi (P < 0,0001), à 0-60 min en après-midi qu'en matin (P < 0,0001) et à 120-180 min en matin qu'en après-midi (P < 0,0001).
La perte de chaleur sèche pendant l'exercice était plus importante à 0-60 min (P < 0,0001) et plus faible à 60-120 min (P < 0,05) et 120-180 min (P < 0,0001) le matin que le soir. La perte de chaleur par évaporation pendant l'exercice était plus importante en PM qu'en AM à 120-180 min (P < 0,0001).
La perte de chaleur totale (sèche + évaporation) au niveau de la peau était plus importante pendant l'exercice en PM qu'en AM (P < 0,0001), à 0-60 min en AM qu'en PM (P < 0,0001) et à 60-120 et 120-180 min en PM que AM (P < 0,05 et 0,0001).
La fréquence cardiaque à 120-150 min était également plus élevée le matin que le soir (P < 0,05).
Ni la sensation thermique perçue ni l'évaluation de l'effort perçu n'étaient différentes entre les essais (tous deux P > 0,05). 

La présente étude démontre une contrainte thermorégulatrice plus importante le matin que l'après-midi résultant d'une température corporelle et d'une fréquence cardiaque plus élevées par rapport à une augmentation du stress thermique environnemental avec une augmentation du rayonnement solaire et de l'angle d'élévation solaire lors d'exercices extérieurs d'intensité modérée par temps chaud.

Cette réponse est associée à une moindre perte nette de chaleur au niveau de la peau et à un plus grand gain de chaleur corporelle provenant du soleil le matin par rapport à l'après-midi. 

. 2017;34(9):1224-1238. doi: 10.1080/07420528.2017.1358735. Epub 2017 Sep 14.

Time-of-day effects of exposure to solar radiation on thermoregulation during outdoor exercise in the heat

Hidenori Otani  1 , Takayuki Goto  2 , Heita Goto  3 , Minayuki Shirato  4
Affiliations

Erratum in

Abstract

High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672-1107 W/m2 and 44-69°) and decrease in PM (717-0 W/m2 and 34-0°) and were higher on AM than on PM (both P < 0.001). Although ambient temperature (AM 32-36°C, PM 36-30°C) and wet-bulb globe temperature (AM 31-33°C, PM 34-27°C) also continued to increase in AM and decrease in PM, there were no differences between trials in these (both P > 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P < 0.05). Skin temperature was higher in AM than PM at the upper arm and thigh at 120 min (P < 0.05) and at the calf at 120 and 180 min (both P < 0.05). Body heat gain from the sun was greater during exercise in AM than PM (P < 0.0001), at 0-60 min in PM than AM (P < 0.0001) and at 120-180 min in AM than PM (P < 0.0001). Dry heat loss during exercise was greater at 0-60 min (P < 0.0001), and lower at 60-120 min (P < 0.05) and 120-180 min (P < 0.0001) in AM than PM. Evaporative heat loss during exercise was greater in PM than AM at 120-180 min (P < 0.0001). Total (dry + evaporation) heat loss at the skin was greater during exercise in PM than AM (P < 0.0001), at 0-60 min in AM than PM (P < 0.0001) and at 60-120 and 120-180 min in PM than AM (P < 0.05 and 0.0001). Heart rate at 120-150 min was also higher in AM than PM (P < 0.05). Neither perceived thermal sensation nor rating of perceived exertion was different between trials (both P > 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.

Keywords: core temperature; diurnal variation; heat stress; skin temperature; sunlight.

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dimanche 6 juin 2021

L'acclimatation à la chaleur ne protège pas les hommes entraînés contre les déficiences induites par l'hyperthermie dans l'exécution de tâches complexes

Aperçu: G.M.

Cette étude a évalué si l'adaptation au stress thermique environnemental peut contrecarrer les effets négatifs de l'hyperthermie sur les performances motrices complexes. 

Treize hommes en bonne santé et entraînés ont effectué 28 jours d'acclimatation à la chaleur avec 1 h d'exercice quotidien d'exposition à la chaleur ambiante (39,4 ± 0,3 °C et 27,0 ± 1,0 % d'humidité relative). Après une familiarisation complète, les participants ont effectué des tests moteurs et cognitifs avant l'acclimatation, ainsi qu'après 14 et 28 jours d'entraînement sous la chaleur. Aux trois occasions, les participants ont été testés, au départ (après environ 15 minutes d'exposition à la chaleur passive) et à la suite d'une hyperthermie induite par l'exercice qui a provoqué une augmentation de la température centrale de 2,8 ± 0,1 °C (similaire d'un jour à l'autre). 

Les résultats aux tests dominés par la cognition et les performances motrices ont été maintenus pendant l'exposition passive à la chaleur (aucune réduction ni différence entre les jours 0, 14 et 28 par rapport aux conditions fraîches). En revanche, les performances des tâches motrices complexes étaient significativement réduites dans des conditions hyperthermiques de 9,4 ± 3,4 % au jour 0 ; 15,1 ± 5,0 % au jour 14 et 13,0 ± 4,8 % au jour 28 (tous p < 0,05 par rapport à la ligne de base mais pas différents d'un jour à l'autre). 

Ces résultats nous permettent de conclure que l'acclimatation à la chaleur ne peut pas protéger les hommes entraînés contre les effets négatifs de l'hyperthermie lorsqu'ils effectuent des tâches complexes reposant sur une combinaison de performances cognitives et de fonctions motrices.

. 2019 Feb 28;16(5):716.  doi: 10.3390/ijerph16050716.

Heat Acclimation Does Not Protect Trained Males from Hyperthermia-Induced Impairments in Complex Task Performance

Jacob F Piil  1 , Chris J Mikkelsen  2 , Nicklas Junge  3 , Nathan B Morris  4 , Lars Nybo  5
Affiliations
Free PMC article

Abstract

This study evaluated if adaptation to environmental heat stress can counteract the negative effects of hyperthermia on complex motor performance. Thirteen healthy, trained males completed 28 days of heat acclimation with 1 h daily exercise exposure to environmental heat (39.4 ± 0.3 °C and 27.0 ± 1.0% relative humidity). Following comprehensive familiarization, the participants completed motor-cognitive testing before acclimation, as well as after 14 and 28 days of training in the heat. On all three occasions, the participants were tested, at baseline (after ~15 min passive heat exposure) and following exercise-induced hyperthermia which provoked an increase in core temperature of 2.8 ± 0.1 °C (similar across days). Both cognitively dominated test scores and motor performance were maintained during passive heat exposure (no reduction or difference between day 0, 14, and 28 compared to cool conditions). In contrast, complex motor task performance was significantly reduced in hyperthermic conditions by 9.4 ± 3.4% at day 0; 15.1 ± 5.0% at day 14, and 13.0 ± 4.8% at day 28 (all p < 0.05 compared to baseline but not different across days). These results let us conclude that heat acclimation cannot protect trained males from being negatively affected by hyperthermia when they perform complex tasks relying on a combination of cognitive performance and motor function.

Keywords: core temperature; heat stress; hyperthermia; mathematics; motor performance; task complexity; visuo-motor tracking.

Conflict of interest statement

The authors declare no conflict of interest.

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L'exposition directe de la tête au rayonnement thermique solaire altère les performances motrices et cognitives

Aperçu: G.M.

Les problèmes de santé et de performance provoqués par le stress thermique sont des défis sociétaux qui s'étendent géographiquement et s'intensifient avec le réchauffement climatique. Pourtant, la science sous-estime peut-être le véritable impact, car aucune étude n'a évalué les effets de l'exposition au soleil sur la température et le  fonctionnement du cerveau humain. 

En conséquence, les performances dans les tâches cognitives dominées et combinées motrices-cognitives et les marqueurs de l'augmentation de la température du tronc cérébral ont été évalués lors d'une exposition à la lumière solaire simulée (égale à ~ 1000 watts/m2). 

L'exposition aiguë n'a affecté aucune mesure de performance, tandis qu'une exposition prolongée de la tête et du cou a provoqué une élévation de la température centrale de 1 °C et des altérations significatives des performances cognitives et motrices. Il est important de noter que les déficiences sont apparues à des niveaux d'hyperthermie considérablement inférieurs par rapport aux expériences précédentes et aux essais de la présente étude sans chauffage radiant de la tête. 

Ces résultats soulignent l'importance d'inclure l'effet du rayonnement solaire sur la tête et le cou dans les futures évaluations scientifiques des impacts du stress thermique environnemental et de la protection spécifique de la tête afin de minimiser les effets néfastes.

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. 2020 May 8;10(1):7812.  doi: 10.1038/s41598-020-64768-w.

Direct exposure of the head to solar heat radiation impairs motor-cognitive performance

Jacob F Piil  1 , Lasse Christiansen  2   3 , Nathan B Morris  2 , C Jacob Mikkelsen  2 , Leonidas G Ioannou  2   4 , Andreas D Flouris  4 , Jesper Lundbye-Jensen  2 , Lars Nybo  2
Affiliations
Free PMC article

Abstract

Health and performance impairments provoked by thermal stress are societal challenges geographically spreading and intensifying with global warming. Yet, science may be underestimating the true impact, since no study has evaluated effects of sunlight exposure on human brain temperature and function. Accordingly, performance in cognitively dominated and combined motor-cognitive tasks and markers of rising brainstem temperature were evaluated during exposure to simulated sunlight (equal to ~1000 watt/m2). Acute exposure did not affect any performance measures, whereas prolonged exposure of the head and neck provoked an elevation of the core temperature by 1 °C and significant impairments of cognitively dominated and motor task performances. Importantly, impairments emerged at considerably lower hyperthermia levels compared to previous experiments and to the trials in the presents study without radiant heating of the head. These findings highlight the importance of including the effect of sunlight radiative heating of the head and neck in future scientific evaluations of environmental heat stress impacts and specific protection of the head to minimize detrimental effects.

Conflict of interest statement

The authors declare no competing interests.

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