Heat Strain Decision Aid (HSDA) accurately predicts individual-based core body temperature rise while wearing chemical protective clothing

'Super boots' for soldiers: theoretical ergogenic and thermoprotective benefits of energetically optimised military combat boots

Soldiers typically perform physically demanding tasks while wearing military uniforms and tactical footwear. New research has revealed a substantial increase of ~10% in energetic cost of walking when wearing modern combat boots versus running shoes. One approach to mitigating these costs is to follow in the footsteps of recent innovations in athletic footwear that led to the development of 'super shoes', that is, running shoes designed to lower the energetic cost of locomotion and maximise performance. We modelled the theoretical effects of optimised combat boot construction on physical performance and heat strain with the intent of spurring similarly innovative research and development of 'super boots' for soldiers. We first assessed the theoretical benefits of super boots on 2-mile run performance in a typical US Army soldier using the model developed by Kipp and colleagues. We then used the Heat Strain Decision Aid thermoregulatory model to determine the metabolic savings required for a physiologically meaningful decrease in heat strain in various scenarios. Combat boots that impart a 10% improvement in running economy would result in 7.9%-15.1% improvement in 2-mile run time, for faster to slower runners, respectively. Our thermal modelling revealed that a 10% metabolic savings would more than suffice for a 0.25°C reduction in heat strain for the vast majority of work intensities and durations in both hot-dry and hot-humid environments. These findings highlight the impact that innovative military super boots would have on physical performance and heat strain in soldiers, which could potentially maximise the likelihood of mission success in real-world scenarios.

Heat Stress Management in the Military: Wet-Bulb Globe Temperature Offsets for Modern Body Armor Systems

OBJECTIVE

The aim of this study was to model the effect of body armor coverage on body core temperature elevation and wet-bulb globe temperature (WBGT) offset.

BACKGROUND

Heat stress is a critical factor influencing the health and safety of military populations. Work duration limits can be imposed to mitigate the risk of exertional heat illness and are derived based on the environmental conditions (WBGT). Traditionally a 3°C offset to WBGT is recommended when wearing body armor; however, modern body armor systems provide a range of coverage options, which may influence thermal strain imposed on the wearer.

METHOD

The biophysical properties of four military clothing ensembles of increasing ballistic protection coverage were measured on a heated sweating manikin in accordance with standard international criteria. Body core temperature elevation during light, moderate, and heavy work was modeled in environmental conditions from 16°C to 34°C WBGT using the heat strain decision aid.

RESULTS

Increasing ballistic protection resulted in shorter work durations to reach a critical core temperature limit of 38.5°C. Environmental conditions, armor coverage, and work intensity had a significant influence on WBGT offset.

CONCLUSION

Contrary to the traditional recommendation, the required WBGT offset was >3°C in temperate conditions (<27°C WBGT), particularly for moderate and heavy work. In contrast, a lower WBGT offset could be applied during light work and moderate work in low levels of coverage.

APPLICATION

Correct WBGT offsets are important for enabling adequate risk management strategies for mitigating risks of exertional heat illness.

Scoping review on the state of the integration of human physiological responses to evaluating heat-stress

OBJECTIVES

To determine the state of the literature on assessing heat-stress using physiological parameters. To provide recommendations to the nuclear industry regarding worker heat-stress management practices.

METHODS

A scoping review identified relevant articles. A search strategy was developed based on a research question concepts. Identified records were screened with inclusion-exclusion criteria. Included articles underwent data extraction using a qualitative data charting method. A thematic analysis and frequency counts were performed.

RESULTS

1687 articles were identified through four databases. The final inclusion consisted of 34 studies. Articles were classified by determinants of heat exposure risks: core body temperature (direct and indirect), scoring scale including core body temperature, scoring scale including human perception, and others. Heart rate and rectal temperature were the two most utilized physiological measurements.

CONCLUSION

A significant amount of literature examined estimation of core temperature using non-invasive methods, sometimes integrated into wearables. Heat-stress management practices could include perceptual measures to better evaluate heat-strain.

Field validation of The Heat Strain Decision Aid during military load carriage

OBJECTIVES

We aimed to determine the agreement between actual and predicted core body temperature, using the Heat Strain Decision Aid (HSDA), in non-Ground Close Combat (GCC) personnel wearing multi terrain pattern clothing during two stages of load carriage in temperate conditions.

DESIGN

Cross-sectional.

METHODS

Sixty participants (men = 49, women = 11, age 31 ± 8 years; height 171.1 ± 9.0 cm; body mass 78.1 ± 11.5 kg) completed two stages of load carriage, of increasing metabolic rate, as part of the development of new British Army physical employment standards (PES). An ingestible gastrointestinal sensor was used to measure core temperature. Testing was completed in wet bulb globe temperature conditions; 1.2-12.6 °C. Predictive accuracy and precision were analysed using individual and group mean inputs. Assessments were evaluated by bias, limits of agreement (LoA), mean absolute error (MAE), and root mean square error (RMSE). Accuracy was evaluated using a prediction bias of ±0.27 °C and by comparing predictions to the standard deviation of the actual core temperature.

RESULTS

Modelling individual predictions provided an acceptable level of accuracy based on bias criterion; where the total of all trials bias ± LoA was 0.08 ± 0.82 °C. Predicted values were in close agreement with the actual data: MAE 0.37 °C and RMSE 0.46 °C for the collective data. Modelling using group mean inputs were less accurate than using individual inputs, but within the mean observed.

CONCLUSION

The HSDA acceptably predicts core temperature during load carriage to the new British Army non-GCC PES, in temperate conditions.

Comparison of two mathematical models for predicted human thermal responses to hot and humid environments

PURPOSE

We compared the accuracy and design of two thermoregulatory models, the US Army's empirically designed Heat Strain Decision Aid (HSDA) and the rationally based Health Risk Prediction (HRP) for predicting human thermal responses during exercise in hot and humid conditions and wearing chemical protective clothing.

METHODS

Accuracy of the HSDA and HRP model predictions of core body and skin temperature (Tc, Ts) were compared to each other and relative to measured outcomes from eight male volunteers (age 24 ± 6 years; height 178 ± 5 cm; body mass 76.6 ± 8.4 kg) during intermittent treadmill marching in an environmental chamber (air temperature 29.3 ± 0.1 °C; relative humidity 56 ± 1%; wind speed 0.4 ± 0.1 m∙s^-1) wearing three separate chemical protective ensembles. Model accuracies and precisions were evaluated by the bias, mean absolute error (MAE), and root mean square error (RMSE) compared to observed data mean ± SD and the calculated limits of agreement (LoA).

RESULTS

Average predictions of Tc were comparable and acceptable for each method, HSDA (Bias 0.02 °C; MAE 0.18 °C; RMSE 0.21 °C) and HRP (Bias 0.10 °C; MAE 0.25 °C; RMSE 0.34 °C). The HRP averaged predictions for Ts were within an acceptable agreement to observed values (Bias 1.01 °C; MAE 1.01 °C; RMSE 1.11 °C).

CONCLUSION

Both HSDA and HRP acceptably predict Tc and HRP acceptably predicts Ts when wearing chemical protective clothing during exercise in hot and humid conditions.

Heat strain in chemical protective clothing in hot-humid environment: Effects of clothing thermal properties

Heat strain experienced by individuals wearing chemical protective clothing (CPC) is severe and dangerous especially in hot-humid environment. The development of material science and interdisciplinary studies including ergonomics, physiology and heat transfer is urgently required for the reduction of heat strain. The aim of this paper was to study the relationship among clothing thermal properties, physiological responses and environmental conditions. Three kinds of CPC were selected. Eight participants wore CPC and walked (4 km/h, two slopes with 5% and 10%) on a treadmill in an environment with (35±0.5) °C and RH of (60±5)%. Core temperature, mean skin temperature, heart rate, heat storage and tolerance time were recorded and analyzed. Physiological responses were significantly affected by the clothing thermal properties and activity intensity in hot-humid environment. The obtained results can help further development of heat strain model. New materials with lower evaporative resistance and less weight are necessary to release the heat strain in hot-humid environments.

A canine thermal model for simulating temperature responses of military working dogs

Military working dogs (MWDs) are often required to operate in dangerous or extreme environments, to include hot and humid climate conditions. These scenarios can put MWD at significant risk of heat injury. To address this concern, a two-compartment (core, skin) rational thermophysiological model was developed to predict the temperature of a MWD during rest, exercise, and recovery. The Canine Thermal Model (CTM) uses inputs of MWD mass and length to determine a basal metabolic rate and body surface area. These calculations are used along with time series inputs of environmental conditions (air temperature, relative humidity, solar radiation and wind velocity) and level of metabolic intensity (MET) to predict MWD thermoregulatory responses. Default initial values of core and skin temperatures are set at neutral values representative of an average MWD; however, these can be adjusted to match known or expected individual temperatures. The rational principles of the CTM describe the heat exchange from the metabolic energy of the core compartment to the skin compartment by passive conduction as well as the application of an active control for skin blood flow and to tongue and lingual tissues. The CTM also mathematically describes heat loss directly to the environment via respiration, including panting. Thermal insulation properties of MWD fur are also used to influence heat loss from skin and gain from the environment. This paper describes the CTM in detail, outlining the equations used to calculate avenues of heat transfer (convective, conductive, radiative and evaporative), overall heat storage, and predicted responses of the MWD. Additionally, this paper outlines examples of how the CTM can be used to predict recovery from exertional heat strain, plan work/rest cycles, and estimate work duration to avoid overheating.