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Military Clothing and Protective Material: Protection at the Limits of Physiological Regulation
Abstract
Contemporary military environments almost invariably require the use of personal protective clothing and equipment, and the burden accompanying its use can sometimes challenge the integrated regulation of critical physiological variables, pushing some individuals to the limits of regulation. Indeed, it is not uncommon for work to be prematurely terminated due to cardiovascular insufficiency. In such states, operational capability is reduced. In this Chapter, four topics will be addressed, including the impact of battle-dress uniforms, ballistic protection, undergarment moisture management, and chemical and biological protection. The principal emphasis is upon thermal and cardiovascular regulation in the person-clothing-environment system. For battle-dress uniforms, body heat storage is modelled using thermodynamics algorithms, with a three-dimensional summary presented to identify combinations of work rates and thermal exposures that yield positive heat storage. When ballistic protection is considered, one must evaluate both the impact of the added mass and the impediment it presents for dry and evaporative heat exchanges. Various moisture management practices are being marketed to address these matters. However, evidence will be presented that these do not offer measurable thermoregulatory or perceptual benefits when used beneath battle dress and ballistic protection in operational simulations. Finally, the most stressful scenario relates to protecting individuals from chemical, biological and radiological challenges. Indeed, working in such encapsulating ensembles can only be tolerated for short durations without supplementary cooling.
... It has long been recognised that personal protective clothing and equipment, when used during moderate to heavy work, can push individuals to the limits of physiological regulation Goldman, 2001;Fogarty et al., 2004;Caldwell et al., 2011Caldwell et al., , 2012McLellan et al., 2013;Taylor and Patterson, 2015). What is less certain is the physiological strain imposed by such ensembles during light to moderate workloads, such as those encountered during motorcycle riding, and whether or not one need even consider the impact of such levels of strain. ...
... For two individuals known to be much less physically active than the rest of the sample ( insufficiency (Barcroft and Edholm, 1945;Taylor and Patterson, 2015). However, no individual reached that state under any of the experimental conditions, with the highest recorded heart rates from Trial D ranging from 105-165 beats.min ...
... When the air temperature was increased, further chronotropic changes were seen, and these could be assigned to four concurrent factors; sustained oxygen delivery, continued skin heating due to greater heat trapping (reduced dry heat loss), further cutaneous vasodilatation driven now by the rise in deep-body temperature and subserving thermoregulation , and a direct thermal (Q 10 ) effect upon the heart itself. Nevertheless, stable blood pressure regulation was not always achieved, and symptoms of syncope within three individuals in the hottest condition imply they were at the cusp of cardiovascular insufficiency (Barcroft and Edholm, 1945;Bass et al., 1955;Noakes, 2008;Taylor and Patterson, 2015). 2011a; Wishart, 2009;Zwolinska, 2013), has now been supported by physiological evidence. ...
Motorcycle protective clothing can be uncomfortably hot during summer, and this experiment was designed to evaluate the physiological significance of that burden. Twelve males participated in four, 90-min trials (cycling 30 W) across three environments (25, 30, 35 °C [all 40% relative humidity]). Clothing was modified between full and minimal injury protection. Both ensembles were tested at 25 °C, with only the more protective ensemble investigated at 30 and 35 °C. At 35 °C, auditory canal temperature rose at 0.02 °C min⁻¹ (SD 0.005), deviating from all other trials (p < 0.05). The thresholds for moderate (>38.5 °C) and profound hyperthermia (>40.0 °C) were predicted to occur within 105 min (SD 20.6) and 180 min (SD 33.0), respectively. Profound hyperthermia might eventuate in ~10 h at 30 °C, but should not occur at 25 °C. These outcomes demonstrate a need to enhance the heat dissipation capabilities of motorcycle clothing designed for summer use in hot climates, but without compromising impact protection.
Practitioner’s Summary:
Motorcycle protective clothing can be uncomfortably hot during summer. This experiment was designed to evaluate the physiological significance of this burden across climatic states. In the heat, moderate (>38.5 °C) and profound hyperthermia (>40.0 °C) were predicted to occur within 105 and 180 min, respectively.
... Modellers must be cognisant of these principles, and of the extent that air movements (relative or actual) and clothing modify the characteristics of the boundary layer. Vapour-permeable clothing and moisture barriers, contrary to manufacturers' claims, do not enhance evaporative cooling, but they can impede dry-heat transfer (Taylor and Patterson, 2016). ...
... In recent publications (Taylor, 2015;Taylor and Patterson, 2016), we highlighted an apparent hierarchical relationship between two regulatory processes ( Figure 22.10): mean body temperature and mean arterial pressure regulation. Following observations from a combined exposure to metabolic and heat stresses, Rowell (1977) hypothesised that a physiological competition might occur between the vascular beds of the active skeletal muscles and the skin for the available cardiac output, with the outcome being a much lower cutaneous blood flow (Rowell, 1977;Crandall and González-Alonso, 2010;Kenney et al., 2014), even though hyperthermia persisted. ...
The transfer of thermal energy is determined by physical laws that can be modelled mathematically with precision. This chapter emphasises on living organisms containing passive and active thermodynamic systems that collectively participate in heat exchange. The inter‐site variations in the cutaneous thermal sensitivity of the thermolytic effectors within the mild thermal domain appear to have minimal physiological significance. The chapter highlights anatomical and morphological characteristics that may influence the application of the physical laws governing heat exchange within passively heated humans. Modelling the passive system typically relies on the physical characteristics of the average male. The chapter highlights situations where human heat loss might appear to defy the Second Law of Thermodynamics. The chapter focuses on some of the physical characteristics that alter passive heat exchanges, but without the complications of considering heterogeneous tissue compositions.
... Personal protective clothing and equipment support and protect, but may also impair, the wearer. Indeed, such impediments have long been recognised, sometimes driving workers to the precipice of physiological failure (Gonzalez 1988;Goldman 2001;Fogarty et al. 2004;Caldwell et al. 2011;McLellan et al. 2013;Taylor and Patterson 2014). Nevertheless, most contemporary military activities are associated with obligatory ballistic protection, which, when combined with protective clothing, footwear, as-sorted carried loads, and heavy work, will challenge thermal and cardiovascular homeostasis (Goldman 1969;Danielsson and Bergh 2005;Taylor 2006). ...
... These loads represented 22.6%, 25.5%, 29.5%, 30.7%, and 34.5% of the mean body mass of these soldiers, respectively. Readers are directed to previous studies for photographic images of these ensembles (Peoples et al. 2010;Taylor and Patterson 2014). ...
This project was based on the premise that decisions concerning the ballistic protection provided to defence personnel should derive from an evaluation of the balance between protection level and its impact on physiological function, mobility, and operational capability. Civilians and soldiers participated in laboratory- and field-based studies in which ensembles providing five levels of ballistic protection were evaluated, each with progressive increases in protection, mass (3.4–11.0 kg), and surface-area coverage (0.25–0.52 m²). Physiological trials were conducted on volunteers (N = 8) in a laboratory, under hot-dry conditions simulating an urban patrol: walking at 4 km·h⁻¹ (90 min) and 6 km·h⁻¹ (30 min or to fatigue). Field-based trials were used to evaluate tactical battlefield movements (mobility) of soldiers (N = 31) under tropical conditions, and across functional tests of power, speed, agility, endurance, and balance. Finally, trials were conducted at a jungle training centre, with soldiers (N = 32) patrolling under tropical conditions (averaging 5 h). In the laboratory, work tolerance was reduced as protection increased, with deep-body temperature climbing relentlessly. However, the protective ensembles could be grouped into two equally stressful categories, each providing a different level of ballistic protection. This outcome was supported during the mobility trials, with the greatest performance decrement evident during fire and movement simulations, as the ensemble mass was increased (–2.12%·kg⁻¹). The jungle patrol trials similarly supported this outcome. Therefore, although ballistic protection does increase physiological strain, this research has provided a basis on which to determine how that strain can be balanced against the mission-specific level of required personal protection.
... In addition, workers are often exposed to heat stress from prolonged work in hot conditions, which is compounded by the wearing of personal protective clothing and equipment, as these increase heat production and impede heat loss (Sköldström 1987;Nunneley 1989;McLellan, Daanen, and Cheung 2013;Taylor and Patterson 2015). These conditions cause compensatory cardiovascular adjustments to maintain thermal and cardiovascular homoeostasis (Rowell 1974;Sawka and Wenger 1988;Kenney and Johnson 1992;Hayashi et al. 2006), elevating heart rate out of proportion to the metabolic demand (Rowell 1974). ...
... Notwithstanding these between-subject differences, some situations may favour one predictive index over another. For instance, the wearing of thermal protective garments impairs heat loss and elevates body temperatures (Sköldström 1987;Nunneley 1989;McLellan, Daanen, and Cheung 2013;Taylor and Patterson 2015). The corresponding circulatory adjustments to regulate mean body temperature and blood pressure elevate heart rate beyond that associated with oxygen delivery (Rowell 1974), and the heart rate to oxygen consumption relationship becomes flatter, with predictions derived under temperate conditions becoming less precise (e.g. ...
The utility of cardiac and ventilatory predictions of metabolic rate derived under temperate and heated laboratory conditions, was evaluated during three fire-fighting simulations (70-mm hose drag, Hazmat recovery, bushfire hose drag; N = 16 per simulation). The limits of agreement for cardiac (temperate: -0.54 to 1.77; heated: -1.39 to 0.80 L.min(-1)) and ventilatory surrogates (temperate: -0.19 to 1.27; heated: -0.26 to 1.16 L.min(-1)) revealed an over-estimation of oxygen consumption that exceeded the acceptable limits required by occupational physiologists (N = 25; ± 0.24 L.min(-1)). Whilst ventilatory predictions offered superior precision during low-intensity work (P < 0.05), a cardiac prediction was superior during demanding work (P < 0.05). Deriving these equations under heated conditions failed to improve precision, with the exception of the cardiac surrogate during low-intensity work (P < 0.05). These observations imply that individualised prediction curves are necessary for valid estimations of metabolic demand in the field.
... The Soldiers were highly motivated to complete the rucksack march within the required time limits to receive a passing grade for qualification. Rucksack marches were selected because they accentuate physiological heat strain due to clothing and equipment biophysical heat loss impediments [20] and high metabolic demands [21] together impairing performance [22]. Because this event is so strenuous, all rucksack marches are completed in the early morning hours prior to dawn before climatic heat stress peaks, to minimize the risk of serious EHS. ...
We employed wearable multimodal sensing (heart rate and triaxial accelerometry) with machine learning to enable early prediction of impending exertional heat stroke (EHS). US Army Rangers and Combat Engineers (N = 2,102) were instrumented while participating in rigorous 7-mile and 12-mile loaded rucksack timed marches. There were three EHS cases, and data from 478 Rangers were analyzed for model building and controls. The data-driven machine learning approach incorporated estimates of physiological strain (heart rate) and physical stress (estimated metabolic rate) trajectories, followed by reconstruction to obtain compressed representations which then fed into anomaly detection for EHS prediction. Impending EHS was predicted from 33 to 69 min before collapse. These findings demonstrate that low dimensional physiological stress to strain patterns with machine learning anomaly detection enables early prediction of impending EHS which will allow interventions that minimize or avoid pathophysiological sequelae. We describe how our approach can be expanded to other physical activities and enhanced with novel sensors.
... However, our study has implications for military personnel, firefighters, and first responders, who are often required to wear full body clothing ensembles and perform high intensity physical activity in hot and/or humid environments. In these populations, the prevention of heat-related performance decrements, and potentially lethal exertional heat illness, are of high importance; thus, even a slight improvement in the thermal properties of a clothing ensemble (as observed in our study) may be beneficial (Taylor and Patterson, 2014). ...
Purpose: Examine the effect of synthetic fabrics (SYN, 60% polyester: 40% nylon) vs. 100% cotton fabric (CTN) on the 20-km cycling time trial (20 kmCTT) performance of competitive cyclists and triathletes.
Methods: In this randomized controlled crossover study, 15 adults (5 women) aged 29.6 ± 2.7 years (mean ± SE) with a peak rate of O2 consumption of 60.0 ± 2.0 ml/kg/min completed a 20 kmCTT under ambient laboratory conditions (24.3 ± 0.7°C and 17 ± 7% relative humidity) with a simulated wind of ~3 m/s while wearing SYN or CTN clothing ensembles. Both ensembles were of snowflake mesh bi-layer construction and consisted of a loose-fitting long-sleeved shirt with full-length trousers.
Results: Participants maintained a significantly (p < 0.05) higher cycling speed and power output over the last 6-km of the 20 kmCTT while wearing the SYN vs. CTN ensemble (e.g., by 0.98 km/h and 18.4 watts at the 20-km mark). Consequently, 20 kmCTT duration was significantly reduced by 15.7 ± 6.8 sec or 0.8 ± 0.3% during SYN vs. CTN trials (p < 0.05). Improved 20 kmCTT performance with SYN vs. CTN clothing could not be explained by concurrent differences in esophageal temperature, sweat rate, ratings of perceived exertion and/or cardiometabolic responses to exercise. However, it was accompanied by significantly lower mean skin temperatures (~1°C) and more favorable ratings of perceived clothing comfort and thermal sensation during exercise.
Conclusion: Under the experimental conditions of the current study, athletic clothing made of synthetic fabrics significantly improved the 20 kmCTT performance of endurance-trained athletes by optimizing selected thermoregulatory and perceptual responses to exercise.
... Mean skin temperature was obtained based on thermistor (Type EU, Yellow Springs Instruments Co. 170 Ltd., Yellow Springs, OH, U.S.A.) measurements from eight locations (scapula, chest, upper arm, forearm, dorsal hand, anterior thigh, posterior calf) (ISO 9886, 2004). Additional publications from this data has been reported based on the original intent, to include additional physiological data and analyses van den Heuvel et al., 2009;Taylor and Patterson, 2014). ...
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.
... Humans subconsciously endeavour to accommodate those stresses by striving to keep body temperatures stable, but we also regulate other internal physical and chemical states (Bernard, 1879;Cannon, 1929;Werner et al., 2008;Taylor and Patterson, 2016). For example, when working hard, we must deliver oxygen to, and remove metabolic wastes from, the exercising muscles. ...
The Asia-Pacific contains over half of the world's population, 21 countries have a Gross Domestic Product <25% of the world's largest economy, many countries have tropical climates and all suffer the impact of global warming. That ‘perfect storm’ exacerbates the risk of occupational heat illness, yet first responders must perform physically demanding work wearing personal-protective clothing and equipment. Unfortunately, the Eurocentric emphasis of past research has sometimes reduced its applicability to other ethnic groups. To redress that imbalance, relevant contemporary research has been reviewed, to which has been added information applicable to people of Asian, Melanesian and Polynesian ancestry. An epidemiological triad is used to identify the causal agents and host factors of work intolerance within hot-humid climates, commencing with the size dependency of resting metabolism and heat production accompanying load carriage, followed by a progression from the impact of single-layered clothing through to encapsulating ensembles. A morphological hypothesis is presented to account for inter-individual differences in heat production and heat loss, which seems to explain apparent ethnic- and gender-related differences in thermoregulation, at least within thermally compensable states. The mechanisms underlying work intolerance, cardiovascular insufficiency and heat illness are reviewed, along with epidemiological data from the Asia-Pacific. Finally, evidence-based preventative and treatment strategies are presented and updated concerning moisture-management fabrics and barriers, dehydration, pre- and post-exercise cooling, and heat adaptation. An extensive reference list is provided, with >25 recommendations enabling physiologists, occupational health specialists, policy makers, purchasing officers and manufacturers to rapidly extract interpretative outcomes pertinent to the Asia-Pacific.
... [18]) measured minute-by-minute Tc. Full reporting specific to the original intent of this data has been previously published [19,20]. ...
Purpose:
We examined the accuracy of the Heat Strain Decision Aid (HSDA) as a predictor of core body temperature in healthy individuals wearing chemical protective clothing during laboratory and field exercises in hot and humid conditions.
Methods:
The laboratory experiment examined three chemical protective clothing ensembles in 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). The field experiment examined four different chemical protective clothing ensembles in twenty activity military volunteers (26 ± 5 years; 175 ± 8 cm; 80.2 ± 12.1 kg) during a prolonged road march (26.0 ± 0.5 °C; 55 ± 3%; 4.3 ± 0.7 m s-1). Predictive accuracy and precision were evaluated by the bias, mean absolute error (MAE), and root mean square error (RMSE). Additionally, accuracy was evaluated using a prediction bias of ±0.27 °C as an acceptable limit and by comparing predictions to observations within the standard deviation (SD) of the observed data.
Results:
Core body temperature predictions were accurate for each chemical protective clothing ensemble in laboratory (Bias -0.10 ± 0.36 °C; MAE 0.28 ± 0.24 °C; RMSE 0.37 ± 0.24 °C) and field experiments (Bias 0.23 ± 0.32 °C; MAE 0.30 ± 0.25 °C; RMSE 0.40 ± 0.25 °C). From all modeled data, 72% of all predictions were within one standard deviation of the observed data including 92% of predictions for the laboratory experiment (SD ± 0.64 °C) and 67% for the field experiment (SD ± 0.38 °C). Individual-based predictions showed modest errors outside the SD range with 98% of predictions falling <1 °C; while, 81% of all errors were within 0.5 °C of observed data.
Conclusion:
The HSDA acceptably predicts core body temperature when wearing chemical protective clothing during laboratory and field exercises in hot and humid conditions.
... An equally important objective for the more physically demanding jobs is reducing the risk of workplace injuries (duty of care or due diligence). Many such injuries are preventable, including those accompanying the use of overly protective clothing and equipment (Goldman 2001;McLellan and Havenith 2016;Taylor and Patterson 2016), arduous materials handling (Knapik et al. 2004;Taylor et al. 2015a, and ineffective screening procedures and standards that result in recruiting and retaining higher-risk individuals. Employers cannot abdicate their responsibilities to ensure a safe and healthy working environment. ...
... An equally important objective for the more physically demanding jobs is reducing the risk of workplace injuries (duty of care or due diligence). Many such injuries are preventable, including those accompanying the use of overly protective clothing and equipment (Goldman 2001;McLellan and Havenith 2016;Taylor and Patterson 2016), arduous materials handling (Knapik et al. 2004;Taylor et al. 2015a, and ineffective screening procedures and standards that result in recruiting and retaining higher-risk individuals. Employers cannot abdicate their responsibilities to ensure a safe and healthy working environment. ...
While the scope of the term physical employment standards is wide, the principal focus of this paper is on standards related to physiological evaluation of readiness for work. Common applications of such employment standards for work are in public safety and emergency response occupations (e.g., police, firefighting, military), and there is an ever-present need to maximize the scientific quality of this research. Historically, most of these occupations are male-dominated, which leads to potential sex bias during physical demands analysis and determining performance thresholds. It is often assumed that older workers advance to positions with lower physical demand. However, this is not always true, which raises concerns about the long-term maintenance of physiological readiness. Traditionally, little attention has been paid to the inevitable margin of uncertainty that exists around cut-scores. Establishing confidence intervals around the cut-score can reduce for this uncertainty. It may also be necessary to consider the effects of practise and biological variability on test scores. Most tests of readiness for work are conducted under near perfect conditions, while many emergency responses take place under far more demanding and unpredictable conditions. The potential impact of protective clothing, respiratory protection, load carriage, environmental conditions, nutrition, fatigue, sensory deprivation, and stress should also be considered when evaluating readiness for work. In this paper, we seek to establish uniformity in terminology in this field, identify key areas of concern, provide recommendations to improve both scientific and professional practice, and identify priorities for future research.
This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.
The purpose of this study was to investigate smart features required for the next generation of personal protective equipment (PPE) for firefighters in Australia, Korea, Japan, and the U.S.A. Questionnaire responses were obtained from 167 Australian, 351 Japanese, 413 Korean, and 763 U.S. firefighters (1,611 males and 61 females). Preferences concerning smart features varied among countries, with 27% of Korean and 30% of U.S. firefighters identifying 'a location monitoring system' as the most important element. On the other hand, 43% of Japanese firefighters preferred 'an automatic body cooling system' while 21% of the Australian firefighters selected equally 'an automatic body cooling system' and 'a wireless communication system'. When asked to rank these elements in descending priority, responses across these countries were very similar with the following items ranked highest: 'a location monitoring system', 'an automatic body cooling system', 'a wireless communication system', and 'a vision support system'. The least preferred elements were 'an automatic body warming system' and 'a voice recording system'. No preferential relationship was apparent for age, work experience, gender or anthropometric characteristics. These results have implications for the development of the next generation of PPE along with the international standardisation of the smart PPE.
In this study, ventilation of clothing microenvironment, thermal insulation and vapor resistance of two jackets made of different materials were measured locally at front, back, side and arm by using an articulated thermal manikin in a controlled climate chamber (29±1°C, 40±10%RH). The various conditions of microenvironment ventilation were created by making the manikin stand and walk, combined with three wind speeds of <0.15, 0.4 and 2.0m/s respectively. The analysis of the measurement results showed that clothing ventilation affected vapor resistance more than it did thermal insulation. The effects of ventilation also varied because of different ways of ventilation arising: penetration through the fabric was proven to be the most effective way in vapor diffusion although it does not seem helpful for heat diffusion.
The purpose of this study was to investigate the physiological and subjective responses of the European, Japanese (JPN) and US firefighters' helmet, gloves and boots for international standardisation. Three experimental conditions were evaluated (clothing mass: 9.4, 8.2 and 10.1 kg for the three conditions, respectively) at the air temperature of 32°C and 60% relative humidity. The results showed that there was no significant difference among the three conditions in oxygen consumption, heart rate, total sweat rate, rectal temperature and mean skin temperature, whereas peripheral temperatures and subjective perceptions were lower in the JPN condition than in the other conditions (P < 0.05). These results indicate that a 0.5-kg reduction in helmet mass and a 1.1-kg reduction in boot mass during exercise resulted in a significant decrease in head and leg temperatures and subjective perceptions, while a 1.9-kg reduction in total clothing mass had insignificant influences on the metabolic burden and overall body temperature.
Syncope which occurs suddenly in the setting of recovery from exercise, known as post-exercise syncope, represents a failure of integrative physiology during recovery from exercise. We estimate that between 50 and 80 % of healthy individuals will develop pre-syncopal signs and symptoms if subjected to a 15-min head-up tilt following exercise. Post-exercise syncope is most often neurally mediated syncope during recovery from exercise, with a combination of factors associated with post-exercise hypotension and loss of the muscle pump contributing to the onset of the event. One can consider the initiating reduction in blood pressure as the tip of the proverbial iceberg. What is needed is a clear model of what lies under the surface; a model that puts the observational variations in context and provides a rational framework for developing strategic physical or pharmacological countermeasures to ultimately protect cerebral perfusion and avert loss of consciousness. This review summarizes the current mechanistic understanding of post-exercise syncope and attempts to categorize the variation of the physiological processes that arise in multiple exercise settings. Newer investigations into the basic integrative physiology of recovery from exercise provide insight into the mechanisms and potential interventions that could be developed as countermeasures against post-exercise syncope. While physical counter maneuvers designed to engage the muscle pump and augment venous return are often found to be beneficial in preventing a significant drop in blood pressure after exercise, countermeasures that target the respiratory pump and pharmacological countermeasures based on the involvement of histamine receptors show promise.
It is important to understand the process of evaporation and steam transfer through firefighter protective clothing in order to be able to prevent steam burns. As humidity sensors are too slow to measure fast changes of humidity inside the clothing layers, temperature changes were used to analyze the evaporation of moisture. Temperature measurements turned out to be useful to predict the evaporation speed within the clothing layers, as temperatures remain constant during the evaporation. The measurements showed that the temperatures within the clothing layers containing a wet layer never rose higher than the temperatures within dry clothing. As soon as all moisture had evaporated, temperature increase followed exactly the curves of the measurements of dry samples.
A model of combined heat and water vapor transport in multi-layered clothing is presented. Transport of heat by conduction and radiation and vapor transport by diffusion are included; heat and mass transport by forced convection are not. The calculations are performed in a time-dependent mode and compared to experiments performed on a sweating hot plate in a nonsteady state mode. Illustrative applications of the model and experiments display effects due to condensation and evaporation of water within the clothing and absorption and desorption by hygroscopic materials. Generally, it appears that all the observed heat loss from the hot plate can be explained in terms of the three mechanisms and the interactions between them. For the most part, the time-dependent treatment is necessary to understand the interactions. Water from sweat may accumulate within clothing because of condensation or absorption by hygroscopic materials and subsequently evaporate after the sweating has ceased. In both phases, the condensation, absorption, or evaporation substantially influences the heat loss. In the case of clothing that has been previously wetted by external means, the heat loss continuously changes as the clothing dries.
Literature from the past 168 years has been filtered to provide a unified summary of the regional distribution of cutaneous water and electrolyte losses. The former occurs via transepidermal water vapour diffusion and secretion from the eccrine sweat glands. Daily insensible water losses for a standardised individual (surface area 1.8 m2) will be 0.6-2.3 L, with the hands (80-160 g.h-1) and feet (50-150 g.h-1) losing the most, the head and neck losing intermediate amounts (40-75 g.h-1) and all remaining sites losing 15-60 g.h-1. Whilst sweat gland densities vary widely across the skin surface, this same individual would possess some 2.03 million functional glands, with the highest density on the volar surfaces of the fingers (530 glands.cm-2) and the lowest on the upper lip (16 glands.cm-2). During passive heating that results in a resting whole-body sweat rate of approximately 0.4 L.min-1, the forehead (0.99 mg.cm-2.min-1), dorsal fingers (0.62 mg.cm-2.min-1) and upper back (0.59 mg.cm-2.min-1) would display the highest sweat flows, whilst the medial thighs and anterior legs will secrete the least (both 0.12 mg.cm-2.min-1). Since sweat glands selectively reabsorb electrolytes, the sodium and chloride composition of discharged sweat varies with secretion rate. Across whole-body sweat rates from 0.72 to 3.65 mg.cm-2.min-1, sodium losses of 26.5-49.7 mmol.L-1 could be expected, with the corresponding chloride loss being 26.8-36.7 mmol.L-1. Nevertheless, there can be threefold differences in electrolyte losses across skin regions. When exercising in the heat, local sweat rates increase dramatically, with regional glandular flows becoming more homogeneous. However, intra-regional evaporative potential remains proportional to each local surface area. Thus, there is little evidence that regional sudomotor variations reflect an hierarchical distribution of sweating either at rest or during exercise.
The energetic cost of generating muscular force was studied by measuring the energetic cost of carrying loads in rats, dogs, humans, and horses for loads ranging between 7 and 27 % of body mass.
Oxygen consumption increased in direct proportion to mass supported by the muscles, i.e. where is the oxygen consumption of the animal running with a load, is the oxygen consumption at the same speed without a load, mL is the mass of the animal plus the load, and m is the mass of the animal.
Stride frequency, average number of feet on the ground over an integral number of strides, the time of contact of each foot relative to the other feet, and the average vertical acceleration during the contact phase were not measurably changed by the loads used in our experiments. From these observations we conclude that the average accelerations of the centre of mass of the animal are not changed by carrying the loads, and that muscular force developed by the animal increases in direct proportion to the load.
It follows that the rate of energy utilization by muscles of an animal as it runs along the ground at any particular speed is nearly directly proportional to the force exerted by its muscles.
The energetic cost of generating force over an interval of time (∫Fdt) increases markedly with running speed.
An important consequence of the direct proportionality between increased oxygen consumption and mass of the load is that small animals expend much more energy to generate a given force at a given speed than large animals.
When prolonged intense exercise is performed at high ambient temperatures, cardiac output must meet dual demands for increased blood flow to contracting muscle and to the skin. The literature has commonly painted this scenario as a fierce competition, wherein one circulation preserves perfusion at the expense of the other, with the regulated maintenance of blood pressure as the ultimate goal. This review redefines this scenario as commensalism, an integrated balance of regulatory control where one circulation benefits with little functional effect on the other. In young, healthy subjects, arterial pressure rarely falls to any great extent during either extreme passive heating or prolonged dynamic exercise in the heat, nor does body temperature rise disproportionately due to a compromised skin blood flow. Rather, it often takes the superimposition of additional stressors-e.g., dehydration or simulated hemorrhage-upon heat stress to substantially impact blood pressure regulation.
Objectives:
The high values of thermal resistance (Rct) and/or vapor resistance (Ret) of chemical protective clothing (CPC) induce a considerable thermal stress. The present study compared the physiological strain induced by CPCs and evaluates the relative importance of the fabrics' Rct, Ret, and air permeability in determining heat strain.
Methods:
Twelve young (20-30 years) healthy, heat-acclimated male subjects were exposed fully encapsulated for 3h daily to an exercise-heat stress (35°C and 30% relative humidity, walking on a motor-driven treadmill at a pace of 5 km h(1) and a 4% inclination, in a work-rest cycle of 45 min work and 15 min rest). Two bipack CPCs (PC1 and PC2) were tested and the results were compared with those attained by two control suits-a standard cotton military BDU (CO1) and an impermeable material suit (CO2).
Results:
The physiological burden imposed by the two bilayer garments was within the boundaries set by the control conditions. Overall, PC2 induced a lower strain, which was closer to CO1, whereas PC1 was closer to CO2. Air permeability of the PC2 cloth was almost three times higher than that of PC1, enabling a better heat dissipation and consequently a lower physiological strain. Furthermore, air permeability characteristic of the fabrics, which is associated with its construction and weave, significantly correlated with the physiological strain, whereas the correlation with Rct, Ret, and weight was poor.
Conclusions:
The results emphasize the importance of air permeability in reducing the physiological strain induced by CPCs.
Personal protective equipment (PPE) refers to clothing and equipment designed to protect individuals from chemical, biological, radiological, nuclear, and explosive hazards. The materials used to provide this protection may exacerbate thermal strain by limiting heat and water vapor transfer. Any new PPE must therefore be evaluated to ensure that it poses no greater thermal strain than the current standard for the same level of hazard protection. This review describes how such evaluations are typically conducted. Comprehensive evaluation of PPE begins with a biophysical assessment of materials using a guarded hot plate to determine the thermal characteristics (thermal resistance and water vapor permeability). These characteristics are then evaluated on a thermal manikin wearing the PPE, since thermal properties may change once the materials have been constructed into a garment. These data may be used in biomedical models to predict thermal strain under a variety of environmental and work conditions. When the biophysical data indicate that the evaporative resistance (ratio of permeability to insulation) is significantly better than the current standard, the PPE is evaluated through human testing in controlled laboratory conditions appropriate for the conditions under which the PPE would be used if fielded. Data from each phase of PPE evaluation are used in predictive models to determine user guidelines, such as maximal work time, work/rest cycles, and fluid intake requirements. By considering thermal stress early in the development process, health hazards related to temperature extremes can be mitigated while maintaining or improving the effectiveness of the PPE for protection from external hazards.
Clothing has two primary affects upon workers. First, it modifies the ease with which thermal energy (heat) is transferred between the body and the environment by providing the body with a layer of insulation. This can be advantageous in a thermally dangerous environment (e.g. fire fighting, cold-water immersion), but disadvantageous during strenuous exercise where a significant amount of metabolic heat is produced (Gonzales, 1988). Second, it affects moisture evaporation from the skin surface, and this has a critical impact upon both thermal comfort and body temperature regulation (Candas, 2002). When clothing is worn, evaporation at the skin surface will be reduced, and the extent of this reduction is a function of the properties of the fabric used to manufacture the garment. Thus, less permeable fabrics allow less water vapour to pass through a garment. Some fabrics are designed to allow water vapour, but not water droplets to pass, while others are designed to protect the wearer from chemical, biological and radiological agents, and are almost impermeable. Ensembles made from minimally permeable fabrics are the focus of this project.
Sumario: The purpose of the present study is to extend the database of the effects of body posture, movement and wind on clothing insulation with data obtained on one group of subjects, with the same clothing ensembles, using a number of combinations of posture and body movements with wind. Furthermore, the effect of clothing fit is investigated, and an attempt is made to combine the present results with those from the literature, resulting in a generalized quantitative description of the various effects on clothing insulation
Based on a physical model, in which a human is depicted as a collection of appropriately sized cylinders, clothing insulation and vapour resistance are calculated for standing persons in still air, when the clothing ensemble thickness, total fabric thickness, number of clothing layers and number of trapped air layers are specified for each cylinder. Specific knowledge of the clothing material is not required, except when coatings of films are involved. The resulting reference values for clothing insulation and vapour resistance are accurate to a standard deviation of 0.011 m2K/W and 1.8 mm of air equivalent, respectively, compared to thermal manikin measurements. The reference values are modified for sitting, walking, and cycling at various rates, and for the combined effect with wind. The formulas are regression equations on a database of literature. The resulting total insulation and vapour resistance are accurate to 0.022 m2K/W and 3.6 mm of air equivalent, respectively. The physical model, which is available as software, is a challenge to existing methods for the determination of insulation and vapour resistance with respect to simpleness and accuracy.
Protective clothing (PPC) can have negative effects on worker performance. Currently little is known about the metabolic effects of PPC and previous work has been limited to a few garments and simple walking or stepping. This study investigated the effects of a wide range of PPC on energy consumption during different activities. It is hypothesized that wearing PPC would significantly increase metabolic rate, disproportionally to its weight, during walking, stepping and an obstacle course. Measuring a person's oxygen consumption during work can give an indirect, but accurate estimate of energy expenditure (metabolic rate). Oxygen consumption was measured during the performance of continuous walking and stepping, and an obstacle course in 14 different PPC ensembles. Increases in perceived exertion and in metabolic rate (2.4-20.9%) when wearing a range of PPC garments compared to a control condition were seen, with increases above 10% being significant (P < 0.05). More than half of the increase could not be attributed to ensemble weight.
Typical heavy, protective outdoor winter clothing ensembles may only allow heat loss of 2. 5 kcal/hr per degree C difference between the skin of the wearer and the ambient environment. While some additional heat would be lost by respiration, since heat production for sustainable high activity is about 500 Watts about half of the total heat production would require sweat evaporative cooling for it to be eliminated, but such clothing stringently limits the wearer's maximum evaporative cooling. In this chapter, the question of human heat balance is analyzed by a well-defined heat balance equation.
An empirical formula was developed from which mean values for energy expenditure per kilogram of body weight (Ew) can be calculated for groups of normal adult males, walking on a motor-driven treadmill with speeds in the range of 35–115 m/min. and gradients in the range of 0–12 degrees. The formula was derived from experiments with two subjects. From the results it appears that Ew increases with the square of speed both in level and grade walking and that log Ew increases linearly with the geometric angle (in degrees). The validity of the formula was tested by experiments with 4 other subjects and experimental data from the literature. A close agreement between actual and predicted values was found. Factors influencing individual values for Ew are discussed. A diagram is presented from which Ew may be read off for various combinations of gradient and speed.
Submitted on September 17, 1959
Unlabelled:
Protective clothing with high insulation properties helps to keep the wearer safe from flames and other types of hazards. Such protection presents some drawbacks since it hinders movement and decreases comfort, in particular due to heat stress. In fact, sweating causes the accumulation of moisture which directly influences firefighters' performance, decreasing protection due to the increase in radiant heat flux. Vaporisation and condensation of hot moisture also induces skin burn. To evaluate the heat protection of protective clothing, Henrique's equation is used to predict the time leading to second-degree burn. The influence of moisture on protection is complex, i.e., at low radiant heat flux, an increase in moisture content increases protection, and also changes thermal properties. Better understanding of heat and mass transfer in protective clothing is required to develop enhanced protection and to prevent burn injuries.
Practitioner summary:
This paper aims to contribute to a better understanding of heat and mass transfer inside firefighters' protective clothing to enhance safety. The focus is on the influence of moisture content and the prevention of steam burn.
In this overview, human morphological and functional adaptations during naturally and artificially induced heat adaptation are explored. Through discussions of adaptation theory and practice, a theoretical basis is constructed for evaluating heat adaptation. It will be argued that some adaptations are specific to the treatment used, while others are generalized. Regarding ethnic differences in heat tolerance, the case is put that reported differences in heat tolerance are not due to natural selection, but can be explained on the basis of variations in adaptation opportunity. These concepts are expanded to illustrate how traditional heat adaptation and acclimatization represent forms of habituation, and thermal clamping (controlled hyperthermia) is proposed as a superior model for mechanistic research. Indeed, this technique has led to questioning the perceived wisdom of body-fluid changes, such as the expansion and subsequent decay of plasma volume, and sudomotor function, including sweat habituation and redistribution. Throughout, this contribution was aimed at taking another step toward understanding the phenomenon of heat adaptation and stimulating future research. In this regard, research questions are posed concerning the influence that variations in morphological configuration may exert upon adaptation, the determinants of postexercise plasma volume recovery, and the physiological mechanisms that modify the cholinergic sensitivity of sweat glands, and changes in basal metabolic rate and body core temperature following adaptation. © 2014 American Physiological Society. Compr Physiol 4:325-365, 2014.
In many occupational settings, clothing must be worn to protect individuals from hazards in their work environment. However, personal protective clothing (PPC) restricts heat exchange with the environment due to high thermal resistance and low water vapor permeability. As a consequence, individuals who wear PPC often work in uncompensable heat stress conditions where body heat storage continues to rise and the risk of heat injury is greatly enhanced. Tolerance time while wearing PPC is influenced by three factors: (i) initial core temperature (Tc), affected by heat acclimation, precooling, hydration, aerobic fitness, circadian rhythm, and menstrual cycle (ii) Tc tolerated at exhaustion, influenced by state of encapsulation, hydration, and aerobic fitness; and (iii) the rate of increase in Tc from beginning to end of the heat-stress exposure, which is dependent on the clothing characteristics, thermal environment, work rate, and individual factors like body composition and economy of movement. Methods to reduce heat strain in PPC include increasing clothing permeability for air, adjusting pacing strategy, including work/rest schedules, physical training, and cooling interventions, although the additional weight and bulk of some personal cooling systems offset their intended advantage. Individuals with low body fatness who perform regular aerobic exercise have tolerance times in PPC that exceed those of their sedentary counterparts by as much as 100% due to lower resting Tc, the higher Tc tolerated at exhaustion and a slower increase in Tc during exercise. However, questions remain about the importance of activity levels, exercise intensity, cold water ingestion, and plasma volume expansion for thermotolerance. Published 2013. Compr Physiol 3:1363-1391, 2013.
In Parts I and II, we reported that during nonsteady-state transport of water vapor through layered fabrics, the temperature between two fabrics rose. In this study, we measure the surface temperature of the first-layer fabrics in similar experiments. The mass of the first layer is changed by tightly stacking the same fabrics, and the surface temperature rise is measured as a function of the mass and the kind of polymers. The rise of surface temperature is proportional to the mass of fabrics and the water ab sorption characteristics of the polymers. We can confirm that the temperature rise that occurs in the space between layered fabrics is mainly due to the heat of absorption of water vapor by the polymers (fabrics).
During high-intensity dynamic exercise, O2 delivery to active skeletal muscles is enhanced through marked increases in both cardiac output and skeletal muscle blood flow. When the musculature is vigorously engaged in exercise, the human heart lacks the pumping capacity to meet the blood flow demands of both the skeletal muscles and other organs such as the brain. Vasoconstriction must therefore be induced through activation of sympathetic nervous activity to maintain blood flow to the brain and to produce the added driving pressure needed to increase flow to the skeletal muscles. In this review, we first briefly summarize the local vascular and neural control mechanisms operating during high-intensity exercise. This is followed by a review of the major neural mechanisms regulating blood pressure during high-intensity exercise, focusing mainly on the integrated activities of the arterial baroreflex and muscle metaboreflex. In high cardiac output situations, such as during high-intensity dynamic exercise, small changes in total peripheral resistance can induce large changes in blood pressure, which means that rapid and fine regulation is necessary to avoid unacceptable drops in blood pressure. To accomplish this rapid regulation, arterial baroreflex function may be modulated in various ways through activation of the muscle metaboreflex and/or other neural mechanisms. Moreover, this modulation of the arterial baroreflex may change over the time course of an exercise bout, or to accommodate changes in exercise intensity. Within this model, integration of arterial baroreflex modulation with other neural mechanisms plays an important role in cardiovascular control during high-intensity exercise.
The effect of wearing two widely used body armours (BA) weighing 9.0 kg and 11.0 kg by the security personnel in India was evaluated. Six male soldiers underwent treadmill exercise in the laboratory (26–28°C) at a fixed speed of 2.2 m · sec−1 for 10 minutes with and without wearing the 11.0 kg BA over their regular uniform. Six other soldiers were exposed to a hot humid climatic chamber (34°C Wet Bulb Globe Temperature/60% relative humidity) with and without 11.0 kg BA for one hour with light physical exercise. Pulmonary function test was conducted on 16 soldiers with 9.0 kg, 11.0 kg BA and without it. The heart rate (HR), minute ventilation (V̇E) and oxygen uptake (VO2) on wearing the 11.0 kg BA increased significantly (p < 0.01) as compared to the values of without wearing it in treadmill exercise. The difference in magnitude being 15 beats · min−1, 9.4 1 · min−1 and 6.0 ml · kg−1 min−1 respectively, for HR, V̇E, and VO2. In hot humid exposure the HR and mean skin temperature (TS) with 11.0 kg BA also increased significantly (p < 0.05) as compared to without wearing it during exercise. Pulmonary functions deteriorated significantly with wearing BA and recorded further decrease with increase in weight of the armour. The significant increase in energy cost of physical task, increase cardiovascular strain in hot humid exposure and increase restrictive ventilatory effect which have been found when BA is worn have important practical implications. It is expected that the BA wearer would develop early onset of fatigue if they are to carry out the same task at the same rate as before without wearing BA. These factors need due consideration when planning work/rest cycles of BA wearer placed in demanding and/or endurance type of tasks.
From theoretical considerations a new clothing parameter, moisture permeability index, has been developed. The existing "clo" formula relating dry clothing insulation and ambient temperature to man's heat loss has been extended to include evaporative heat transfer. This extension indicates a range over which the clothed man may maintain thermal equilibrium. This concept of range applies in all types of environments and thus one theory is applicable to hot, temperate, and cold environments. The theory also indicates the limitations of sweat evaporation as a cooling mechanism. A method is described for measuring moisture permeability index.
Human tolerance to heat exposure is limited by body heat storage, as the body is unable to eliminate all the heat it produces and/or receives from the environment, and by the physiologic consequencies of such storage. Heat storage of about 80 kcal represents the 'voluntary heat tolerance' limit at which an average, fit, 170 kg man usually decides he is not willing to work much longer in the heat; and increase of 160 kcal in his heat content is associated with a 50% risk of heat exhaustion collapse. As the difference between skin and air temperatures decreases, a demand for evaporative cooling in the heat is imposed by the interplay of 3 factors: (a) the metabolic heat production; (b) the 'effective' solar heat load; and (c) the radiative and convective heat exchange through the clothing insulation. This demand may be greater than the maximum evaporative cooling allowed by 3 other factors: (a) the body's maximum sustainable sweat production (about 1L/hr approximately = to 675 Watts of cooling power); (b) the limit to sweat evaporation imposed by clothing moisture permeability and thickness; and (c) the difference between the vapor pressure of sweat at the skin surface and the ambient vapor pressure.
The sections in this article are:
Body Heat Balance Equations
Independent Variables in Human Thermal Environment
Ambient Temperature
Dew‐point Temperature and Ambient Vapor Pressure
Air (and Fluid) Movement
Mean Radiant Temperature or Effective Radiant Field
Clothing Insulation
Barometric Pressure
Time of Exposure
Dependent Physiological Variables in Body Heat Balance Equation
Mean Skin Temperature
Skin Wettedness
Body Heat Storage and Rate Change of Mean Body Temperature
Metabolic Energy
Sensible Heat Exchange by Radiation and Convection
Operative Temperature
Clothing in Sensible Heat Exchange
Radiation Exchange
Mean Radiant Temperature and Effective Radiant Field
Direct Evaluation of Effective Radiant Field
Solar Radiation
Measurement of Radiation Exchange
Convective Heat Exchange
Heat Transfer Theory
Free and Forced Convection
Measurement of Convective Heat Transfer Coefficient
Effect of Barometric Pressure
Evaporative Heat Exchange
Direct Measurement of Evaporative Heat Loss
Maximum Evaporative Heat Loss from Skin Surface
The Lewis Relation Between Heat and Mass Transfer Coefficients
Skin Wettedness vs. Efficiency of Evaporative Regulation
Special Environments
Water Immersion
Hyperbaric Helium‐Oxygen Atmospheres
Rational Temperature Indices of Thermal Environment
Operative Temperature
Humid Operative Temperature
Standard Operative Temperature
Standard Humid Operative Temperature
Standard Effective Temperature
A New Effective Temperature Index
Summary
Standard No. EN 15831:2004 provides 2 methods of calculating insulation: parallel and serial. The parallel method is similar to the global one defined in Standard No. ISO 9920:2007. Standards No. EN 342:2004, EN 14058:2004 and EN 13537:2002 refer to the methods defined in Standard No. EN ISO 15831:2004 for testing cold protective clothing or equipment. However, it is necessary to consider several issues, e.g., referring to measuring human subjects, when using the serial method. With one zone, there is no serial-parallel issue as the results are the same, while more zones increase the difference in insulation value between the methods. If insulation is evenly distributed, differences between the serial and parallel method are relatively small and proportional. However, with more insulation layers overlapping in heavy cold protective ensembles, the serial method produces higher insulation values than the parallel one and human studies. Therefore, the parallel method is recommended for standard testing.
The thermal resistances of clothing assemblies comprising up to eight dry layers, in series, of a single fabric (either woven cotton, polyester or nylon) in air at atmospheric pressure have been measured and compared with other materials. Application of a compressive loading led typically to the resistance of a stack of eight polyester cloth layers falling from 46 x 10-3 Km2 W-1 under zero loading to 32 x 10-3 Km2 W-1 under 31 Nm-2. The percentage reduction in resistance for an additional equal increment of loading would be considerably smaller. Under load, the thermal behaviour of multiple layer assemblies is identical to that of a single layer of the same material and overall thickness and so it was concluded that contact resistances between successive layers are usually of secondary importance for clothing fabrics in air at atmospheric pressure and that the insulation provided by such an assembly is primarily dependent upon the volume of air held stagnant within the matrix, provided that no thermal radiation [`]windows' are present.
Individuals exposed to extreme heat may experience reduced physiological and cognitive performance, even during very light work. This can have disastrous effects on the operational capability of aircrew, but such impairment could be prevented by auxiliary cooling devices. This hypothesis was tested under very hot-dry conditions, in which eight males performed 2 h of low-intensity exercise (~30 W) in three trials, whilst wearing biological and chemical protective clothing: temperate (control: 20°C, 30% relative humidity) and two hot-dry trials (48°C, 20% relative humidity), one without (experimental) and one with liquid cooling (water at 15°C). Physiological strain and six cognitive functions were evaluated (MiniCog Rapid Assessment Battery), and participants drank to sustain hydration state. Maximal core temperatures averaged 37.0°C (±0.1) in the control trial, and were significantly elevated in the experimental trial (38.9°C ± 0.3; P < 0.05). Similarly, heart rates peaked at 92 beats min(-1) (±7) and 133 beats min(-1) (±4; P < 0.05), respectively. Liquid cooling reduced maximal core temperatures (37.3°C ± 0.1; P < 0.05) and heart rates 87 beats min(-1) (±3; P < 0.05) in the heat, such that neither now differed significantly from the control trial (P > 0.05). However, despite inducing profound hyperthermia and volitional fatigue, no cognitive degradation was evident in the heat (P > 0.05). Since extensive dehydration was prevented, it appears that thermal strain in the absence of dehydration may have minimal impact upon cognitive function, at least as evaluated within this experiment.
Load carriage increases physiological strain, reduces work capacity and elevates the risk of work-related injury. In this project, the separate and combined physiological consequences of wearing the personal protective equipment used by firefighters were evaluated. The overall impact upon performance was first measured in 20 subjects during a maximal, job-related obstacle course trial and an incremental treadmill test to exhaustion (with and without protective equipment). The fractional contributions of the thermal protective clothing, helmet, breathing apparatus and boots were then separately determined during steady-state walking (4.8 km h(-1), 0% gradient) and bench stepping (20 cm at 40 steps min(-1)). The protective equipment reduced exercise tolerance by 56% on a treadmill, with the ambulatory oxygen consumption reserve (peak minus steady-state walking) being 31% lower. For the obstacle course, performance declined by 27%. Under steady-state conditions, the footwear exerted the greatest relative metabolic impact during walking and bench stepping, being 8.7 and 6.4 times greater per unit mass than the breathing apparatus. Indeed, the relative influence of the clothing on oxygen cost was at least three times that of the breathing apparatus. Therefore, the most efficient way to reduce the physiological burden of firefighters' protective equipment, and thereby increase safety, would be to reduce the mass of the boots and thermal protective clothing.
This project was aimed at evaluating the impact of combat armor on physiological and cognitive functions during low-intensity exercise in hot-humid conditions (36 degrees C and 60% relative humidity).
Nine males participated in three trials (2.5 hours), walking at two speeds and wearing different protective equipment: control (combat uniform and cloth hat); torso armor with uniform and cloth hat; and full armor (uniform, torso armor, and helmet).
As time progressed, core temperatures increased and deviated significantly among trials, rising at 0.37 degrees C h(-1) (control), 0.41 degrees C h(-1) (torso armor), and 0.51 degrees C h(-1) (full armor). Heart rates also progressively diverged, and subjects lost significantly more sweat during the two armored trials. However, cognitive-function tests revealed neither significant main effects nor time by treatment interactions.
The combat armor and helmet significantly increased thermal and cardiovascular strain, but these were unlikely to lead to either exertional heat illness or impaired cognitive function during uneventful urban, military patrols in hot-humid conditions.
The aim of this study was to evaluate how the textile composition of torso undergarment fabrics may impact upon thermal strain, moisture transfer, and the thermal and clothing comfort of fully clothed, armored individuals working in a hot-dry environment (41.2 degrees C and 29.8% relative humidity).
Five undergarment configurations were assessed using eight men who walked for 120 min (4 km x h(-1)), then alternated running (2 min at 10 km x h(-1)) and walking (2 min at 4 km x h(-1)) for 20 min. Trials differed only in the torso undergarments worn: no t-shirt (Ensemble A); 100% cotton t-shirt (Ensemble B); 100% woolen t-shirt (Ensemble C); synthetic t-shirt (Ensemble D: nylon, polyethylene, elastane); hybrid shirt (Ensemble E).
Thermal and cardiovascular strain progressively increased throughout each trial, with the average terminal core temperature being 38.5 degrees C and heart rate peaking at 170 bpm across all trials. However, no significant between-trial separations were evident for core or mean skin temperatures, or for heart rate, sweat production, evaporation, the within-ensemble water vapor pressures, or for thermal or clothing discomfort.
Thus, under these conditions, neither the t-shirt textile compositions, nor the presence or absence of an undergarment, offered any significant thermal, central cardiac, or comfort advantages. Furthermore, there was no evidence that any of these fabrics created a significantly drier microclimate next to the skin.
It has been proposed that self-paced exercise in the heat is regulated by an anticipatory reduction in work rate based on the rate of heat storage. However, performance may be impaired by the development of hyperthermia and concomitant rise in cardiovascular strain increasing relative exercise intensity. This study evaluated the influence of thermal strain on cardiovascular function and power output during self-paced exercise in the heat. Eight endurance-trained cyclists performed a 40 km simulated time trial in hot (35°C) and thermoneutral conditions (20°C), while power output, mean arterial pressure, heart rate, oxygen uptake and cardiac output were measured. Time trial duration was 64.3 ± 2.8 min (242.1 W) in the hot condition and 59.8 ± 2.6 min (279.4 W) in the thermoneutral condition (P < 0.01). Power output in the heat was depressed from 20 min onwards compared with exercise in the thermoneutral condition (P < 0.05). Rectal temperature reached 39.8 ± 0.3 (hot) and 38.9 ± 0.2°C (thermoneutral; P < 0.01). From 10 min onwards, mean skin temperature was ~7.5°C higher in the heat, and skin blood flow was significantly elevated (P < 0.01). Heart rate was ~8 beats min(-1) higher throughout hot exercise, while stroke volume, cardiac output and mean arterial pressure were significantly depressed compared with the thermoneutral condition (P < 0.05). Peak oxygen uptake measured during the final kilometre of exercise at maximal effort reached 77 (hot) and 95% (thermoneutral) of pre-experimental control values (P < 0.01). We conclude that a thermoregulatory-mediated rise in cardiovascular strain is associated with reductions in sustainable power output, peak oxygen uptake and maximal power output during prolonged, intense self-paced exercise in the heat.
Traité par Marc Augier (CERAM Business School) et Georges Vignaux (CNRS - Directeur du programme) dans le cadre des travaux du programme CoLiSciences (http ://colisciences.in2p3.fr) de la Maison des Sciences de l’Homme Paris Nord
Criteria of a rectal temperature (T(re)) ranging from 39.0-40.0°C and/or 180 bts/min heart rate (HR) are used as tolerance limits for work in heat. Two studies, S1 and S2, suggest convergence of skin (T(sk)) and rectal (T(re)) temperatures is a better indicator of tolerance when evaporative cooling is minimized. In S1, seven heat acclimatized subjects performed mild exercise in impermeable clothing in hot-dry (46°C, 10% rh) and hot-wet (35°C, 75% rh) 1.1 m/s wind, 0.11 W/cm2 radiant load environments. In the hot-dry phase, convergence of T(sk) and T(re) accompanied termination (mean=37 min) from subjective distress, although mean T(re) of 38.3°C and HR of 142 bts/min were well below 'tolerance levels'; convergence of T(sk) and T(re) was also associated with early termination in the hot-wet phase at 65 min with 38.7°C mean T(re), 166 bts/min HR. In S2, six semipermeable systems were evaluated (49°C, 20% rh) while six subjects attempted a 50-min walk at 1.34 m/s; exposures terminated at 33 min, with convergence of T(sk) on T(re), at near-collapse levels despite 38.4°C mean T(re) and 166 bts/min HR. Convergence of predicted T(re) rise (J. Appl. Physiol. 32:812, 1972) and projected T(sk) rise (linear regression of initial T(sk) values) matched (±5 min) actual convergence. Convergence, under extreme stress, appears more practical as a limit for tolerance than any T(re) or HR absolute value.
1. The mechanical power spent to accelerate the limbs relative to the trunk in level walking and running, Ẇ int , has been measured at various ‘constant’ speeds (3‐33 km/hr) with the cinematographic procedure used by Fenn (1930 a ) at high speeds of running.
2. Ẇ int increases approximately as the square of the speed of walking and running. For a given speed Ẇ int is greater in walking than in running.
3. In walking above 3 km/hr, Ẇ int is greater than the power spent to accelerate and lift the centre of mass of the body at each step, Ẇ ext (measured by Cavagna, Thys & Zamboni, 1976 b ). In running Ẇ int < Ẇ ext up to about 20 km/hr, whereas at higher speeds Ẇ int > Ẇ ext .
4. The total work done by the muscles was calculated as W tot = ǀ W int ǀ + ǀ W ext ǀ. Except that at the highest speeds of walking, the total work done per unit distance W tot /km is greater in running than in walking.
5. The efficiency of positive work was measured from the ratio W tot /Net energy expenditure: this is greater than 0·25 indicating that both in walking and in running the muscles utilize, during shortening, some energy stored during a previous phase of negative work (stretching).
6. In walking the efficiency reaches a maximum (0·35‐0·40) at intermediate speeds, as may be expected from the properties of the contractile component of muscle. In running the efficiency increases steadily with speed (from 0·45 to 0·70‐0·80) suggesting that positive work derives mainly from the passive recoil of muscle elastic elements and to a lesser extent from the active shortening of the contractile machinery. These findings are consistent with the different mechanics of the two exercises.
The use of bubble canopies to improve vision in fighter aircraft exposes the cockpit to a high radiant heat load. Incoming sunlight increases the heat stress on crewmembers, both by raising air temperature and by directly heating exposed skin and clothing. An F-15 aircraft at Edwards AFB was modified to permit cockpit ventilation by external ground carts. Eight volunteers from the Test Pilot School were studied during 1-h periods in the closed cockpit, in sun and in shade. Mean cockpit air temperatures were 35.2 degrees C in shade and 51.9 degrees C in sun with PH2O less than 10 torr. The corresponding WBGT's were 22.6 and 36.4 degrees C. Sunlight added significantly to overall heat stress, as indicated by a rising heart rate and evaporative weight loss of 284 g/m2 - h (shade value was 109 g/m2 - hr). Mean skin temperatures were 34.3 degrees C in shade and 35.8 degrees C in sun. Particularly high skin temperatures were observed on the chest, the forehead and the top of the head under the helmet. The legs remained cool due to the flow of conditioned air, and this may explain why rectal temperature showed no meaningful change. Heat stress, which alone poses no physiological hazard, may cause crew performance decrements as well as diminishing acceleration tolerance. Possible means of eliminating or ameliorating these effects are discussed.