Environmental heat stress and motor-cognitive performance

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Introduction: Environmental heat stress affects a range of physiological factors of importance for human health and performance. While there is good agreement between laboratory and field studies in relation to the negative effects of heat on endurance performance, as well as on cardiovascular factors of importance for aerobic capacity, the findings in relation to cognitively dominated performances and tasks relying on motorcognitive function are less clear. Elevated prevalence of work site accidents and reduced productivity across a range of occupations during heat waves indicate that heat has a marked effect in ecological settings. Yet, several laboratory-based studies report no impact or some slightly faster/better cognitive performance when exposed to low to moderate levels of hyperthermia, whereas others report that hyperthermia does impair cognitive performance. Therefore, the present thesis aimed to develop a novel test battery, sufficiently sensitive to detect how graded levels of hyperthermia affects motorcognitive performance, as well as to elucidate potential confounding effects of dehydration, solar radiation and potential protective effects following heat acclimation. Methods: During a series of pilot testing, we developed a test battery comprised of four tasks appearing in a semi-randomized order: A complex motor task (visuomotor tracking [VMT]), combined motor-cognitive task (MATH_pinch), a simple cognitive task (MATH_type) and a simple motor task (TARGET_pinch). First, we verified the comprehensive test battery both as a separate (only the VMT task) and multipart protocol (all tasks), and explored the effect of: Study I) moderate [1°C] and severe hyperthermia [≥2°C] on both the separate and multipart protocol; Study II) dehydration [2% loss in body mass] with moderate and severe hyperthermia compared to a euhydrated control with similar levels of hyperthermia; Study II) solar heat radiation and its effect on motorcognitive performance and brain temperature evaluated by applied simulated sunlight directly to the head or lower-body; Study IV) moderate (14 days) and prolonged (28 days) heat acclimation to possibly protect against hyperthermia-induced motor-cognitive performance decrements. Results: Study I revealed that severe hyperthermia negatively affected VMT performance (4% reduction compared to baseline) when independently tested and the performance effect was exacerbated (~10% decrease compared to baseline) when the VMT testing was integrated in the multipart protocol. Severe hyperthermia also provoked a minor reduction in simple motor task performance, whereas none of the tasks were affected by a moderate elevation in core temperature. In Study II, moderate dehydration (i.e. 2% body mass loss) combined with severe hyperthermia markedly affected VMT performance (16% reduction). Furthermore, the combination of hyperthermia and dehydration also impaired simple motor task performance and the cognitively dominated tasks, and the effect on the MATH_type task corresponded to a doubling of errors. Study III demonstrated that direct exposure of solar radiation to the head profoundly reduced motor-cognitive performance at a moderate level of hyperthermia (~1°C above resting levels), compared to both rest and to radiation directed to the body at equivalent core temperatures. Finally, study IV revealed that heat acclimation was unable to protect against hyperthermia-induced decrements in motor-cognitive performance at a fixed level of hyperthermia, i.e. >2°C increases above resting core temperature. See graphical overview of performance effects across studies. Graphical overview of performance effects across studies (I-IV). The overview highlight the decrements in task performance during each study.In summary, the present series of studies demonstrated that sev ere hyperthermia impairs complex motor peformance, with the largest effect emerging when the motor-cognitive task was alternated with other tasks (multitasking prototocol). Dehydration without highheat stress did not impair motor-cognitive performance; however, if dehydration is not prevented during high heat stress, all tasks in the test-battery ranging from simple motor tasks to combined motor-cognitive and cognitively dominated performances become impaired. The study with solar radiation, revealed that impairments in motor-cognitive 5performance emmerged at lower body core temperature levels, when the head was directly exposed to simulated sunlight; both compared to the within-study trial with matched heat-radiation exposure of the lower body and any of the other heat-exposure scenarios investigated in this project. Heat acclimation may postpone the development of hyperthermia and therefore “indirectly” prevent heat effects on motor-cognitive performance, however, when matched for core temperature, heat acclimation cannot prevent hyperthermia-induced impaiments. Collectively, the experimental work covered in this thesis demonstrated that hyperthermia per se can influence complex motorcognitive task performance, and especially in heat-exposure scenarios with added heat radiation on the head or superimposition of dehydration can create conditions with marked impairments in performance accross a range of both cognitively dominated tasks and tests relying on simple motor control. Contextualized to the hydration observations from various industries and considering the exposure experienced in occupational settings, this thesis provides a framework for translating findings from laboratory settings to real-life imp
OriginalsprogEngelsk
ForlagDepartment of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen
StatusUdgivet - 2019

ID: 240141035