A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

The effectiveness of heat preparation and alleviation strategies for cognitive performance: A systematic review

A range of occupational and performance contexts (e.g. military personnel operations, emergency services, sport) require the critical maintenance of cognitive performance in environmentally challenging environments. Several reviews exist which evaluate the effectiveness of heat preparation strategies to facilitate physical performance. To date, no review has explored the usefulness of heat preparation strategies for cognitive performance. Therefore, this systematic review aimed to evaluate a range of interventions for the maintenance of cognitive performance, during or following active or passive heat exposure. Studies to be included were assessed by two authors reviewing title, abstract, and full-text. Forty articles were identified which met the inclusion criteria. Interventions were categorised into chronic (i.e. acclimation/acclimatisation) and acute strategies (i.e. hydration, cooling, supplementation, psychological). The results indicate that medium-term consecutive heat acclimation may mitigate some cognitive deficits under heat stress, although heat acclimation effectiveness could be influenced by age. Further, pre-cooling appears the most effective cooling method for maintaining cognitive performance under heat stress, although results were somewhat ambiguous. The hydration literature showed that the most effective hydration strategies were those which individualised electrolyte fortified fluid volumes to match for sweat loss. Limited research exploring psychological interventions indicates that motivational self-talk could be facilitative for maintaining cognitive skills following exercise in hot conditions. These findings can be used to help inform strategies for maintaining critical cognitive and decision-making skills in hot environments.

Personal Cooling Garments: A Review

Thermal comfort is of critical importance to people during hot weather or harsh working conditions to reduce heat stress. Therefore, personal cooling garments (PCGs) is a promising technology that provides a sustainable solution to provide direct thermal regulation on the human body, while at the same time, effectively reduces energy consumption on whole-building cooling. This paper summarizes the current status of PCGs, and depending on the requirement of electric power supply, we divide the PCGs into two categories with systematic instruction on the cooling materials, working principles, and state-of-the-art research progress. Additionally, the application fields of different cooling strategies are presented. Current problems hindering the improvement of PCGs, and further development recommendations are highlighted, in the hope of fostering and widening the prospect of PCGs.

The Impact of Thermal Stress on Cognition and the Use of Cooling Wear for Alleviation - an Explorative Study

Increased ambient temperatures during the care of severely burned patients are essential for mitigating hypothermia and minimizing the impact of the consecutive hypermetabolism. For involved medical staff, those conditions may lead to impaired concentration, possibly negatively affecting an optimal patient care. Yet, alleviation strategies are lacking. As a novel coping strategy, cooling wear may be an effective means. This explorative study aimed to investigate the effect of high ambient temperatures on the concentration capacity and cooling wear to alleviate thermal stress. The effects of high ambient temperatures and the additional use of cooling wear on the concentration capacity of medical staff were investigated in six subjects during two simulated burn surgeries. Each individual served as his/her own control undergoing one simulation with and one without cooling wear. Concentration capacity was measured before and after each simulation with a standardized test. The results suggested that high ambient temperatures, as used in burn medicine, negatively affect human concentration capacity. The initial assessment of concentration capacity yielded homogenous values. After heat exposure, subjects wearing cooling wear showed a higher concentration capacity and a lower error rate compared to subjects without cooling wear. Summing up, temperature-related decrements in vigilance and performance among medical personnel may impair the patients' outcome. As an opportunity to withstand thermal stress and improve medical care and safety, cooling wear showed promising results and may be used as heat alleviator. Burn medicine may particularly benefit from further development and rigorous investigation of cooling strategies.

Personal protective equipment in the COVID-19 pandemic and the use of cooling-wear as alleviator of thermal stress : A pilot study in plastic surgery staff members

Background

High temperatures at workplaces lead to health-related risks and premature exhaustion. The coronavirus disease 2019 (COVID-19) pandemic requires many health professionals to perform under unfavorable conditions. Personal protective equipment (PPE) causes thermal stress and negatively affects performance.

Patients, materials and methods

This pilot project investigated the effects of PPE and additional cooling wear on physiological parameters and concentration of six healthy staff members of the Plastic Surgery Department of the Medical University of Graz, Austria during simulated patient care. In this study two 1‑hour cycles with patient care-related tasks with PPE and PPE + cooling-wear, respectively, were conducted. A third cycle with scrubs exclusively served as baseline/negative control. The assessment occurred immediately pre-cycles and post-cycles.

Results

Pre-cycle assessments showed no significant differences between the cycles. After PPE cycle, increased physical stress levels and decrements in concentration capacity were observed. Physiological parameters were significantly less affected in the cooling cycle, while concentration capacity slightly increased.

Conclusion

COVID-19 PPE causes considerable thermal stress, ultimately affecting human performance. As opportunity to withstand thermal stress, and improve patients’ and professionals’ safety, cooling-wear can be considered relevant. Medical personnel performing in exceptional situations may particularly benefit from further development and investigation of cooling strategies.

Thermoregulatory responses to ice slurry ingestion during low and moderate intensity exercises with restrictive heat loss

OBJECTIVES

We investigated the thermoregulatory responses to ice slurry ingestion during low- and moderate-intensity exercises with restrictive heat loss.

DESIGN

Randomised, counterbalanced, cross-over design.

METHODS

Following a familiarisation trial, ten physically active males exercised on a motorised treadmill at low-intensity (L; 40% VO_2max) or moderate-intensity (M; 70% VO_2max) for 75-min, in four randomised, counterbalanced trials. Throughout the exercise bout, participants donned a raincoat to restrict heat loss. Participants ingested 2gkg^-1 body mass of ambient water (L+AMB and M+AMB trials) or ice slurry (L+ICE and M+ICE trials) at 15-min intervals during exercise in environmental conditions of T_db, 25.1±0.6°C and RH, 63±5%. Heart rate (HR), gastrointestinal temperature (T_gi), mean weighted skin temperature (T_sk), estimated sweat loss, ratings of perceived exertion (RPE) and thermal sensation (RTS) were recorded.

RESULTS

Compared to L+AMB, participants completed L+ICE trials with lower ΔT_gi (0.8±0.3°C vs 0.6±0.2°C; p=0.03), mean RPE (10±1 vs 9±1; p=0.03) and estimated sweat loss (0.91±0.2L vs 0.78±0.27L; p=0.04). Contrastingly, T_gi (p=0.22), T_sk (p=0.37), HR (p=0.31), RPE (p=0.38) and sweat loss (p=0.17) were similar between M+AMB and M+ICE trials. RTS was similar during both low-intensity (4.9±0.5 vs 4.7±0.3; p=0.10) and moderate-intensity exercise (5.3±0.47 vs 5.0±0.4; p=0.09).

CONCLUSIONS

Per-cooling using ice slurry ingestion marginally reduced thermal strain during low-intensity but not during moderate-intensity exercise. Ice slurry may be an effective and practical heat mitigation strategy during low-intensity exercise such as in occupational and military settings, but a greater volume should be considered to ensure its efficacy.

Recycled Cellulose Aerogels from Paper Waste for a Heat Insulation Design of Canteen Bottles

Exercising in a tropical climate with constant high temperatures and high humidity increases the risk of heatstroke for active people who frequently train outdoors. For these active persons, a cooling source of water nearby can be essential, and this is usually carried in canteen bottles. However, commercially available water canteen bottles have limited thermal insulation capability to keep the liquid content cooled for the required period. This work proposed an engineering solution to enhance the heat insulation performance of water canteen bottles, using recycled cellulose aerogels made from paper waste for the first time as an insulating layer. Recycled cellulose aerogels wrapped around the water canteen bottle provides excellent thermal insulation performance, while not adding significant weight to the bottle. The temperature of the ice slurry in the canteen bottle was measured periodically over four hours with a mercury thermometer. The effects of the static and dynamic conditions on the temperature rate were also quantified. A 1.5 cm thickness of 1.0 wt.% recycled cellulose aerogel wrapped around the canteen bottle can provide an excellent thermal insulation performance with the lowest rise in temperature, achieving a low final temperature of the ice slurry content of 3.5 °C after 4 h. This result is much better than that provided by available commercial bottles under the same conditions.

Physiological and perceptual effects of a cooling garment during simulated industrial work in the heat

OBJECTIVE

Evaluate physiological and perceptual responses using a phase change cooling (PCC) garment during simulated work in the heat.

METHODS

Twenty males wearing compression undergarments, coverall suit, gloves, and hard-hat, completed two randomly assigned trials (with PCC inserts or control, CON) of simulated industrial tasks in the heat (34.2 ± 0.05 °C, 54.7 ± 0.3%RH). Trials consisted of two 20 min work bouts, a maximum performance bout, and 10 min of recovery.

RESULTS

Physiological strain index (PSI) was lower during PCC after the second work bout and during recovery (all P < 0.05). PCC reduced heat storage (27.0 ± 7.6 W m^-2) compared to CON (42.7 ± 9.9 W m^-2, P < 0.001). Perceptual strain index (PeSI) was reduced with PCC compared to CON (P < 0.001), however performance outcomes were not different between trials (P = 0.10).

CONCLUSIONS

PCC during work in the heat attenuated thermal, physiological, and perceptual strain. This PCC garment could increase safety and reduce occupational heat illness risk.