Active ascent accelerates the time course but not the overall incidence and severity of acute mountain sickness at 3,600 m

Acute mountain sickness (AMS) typically peaks following the first night at high altitude (HA) and resolves over the next 2-3 days, but the impact of active ascent on AMS is debated. To determine the impact of ascent conditions on AMS, 78 healthy Soldiers (means ± SD; age = 26 ± 5 yr) were tested at baseline residence, transported to Taos, NM (2,845 m), hiked (n = 39) or were driven (n = 39) to HA (3,600 m), and stayed for 4 days. AMS-cerebral (AMS-C) factor score was assessed at HA twice on day 1 (HA1), five times on days 2 and 3 (HA2 and HA3), and once on day 4 (HA4). If AMS-C was ≥0.7 at any assessment, individuals were AMS susceptible (AMS+; n = 33); others were nonsusceptible (AMS-; n = 45). Daily peak AMS-C scores were analyzed. Ascent conditions (active vs. passive) did not impact the overall incidence and severity of AMS at HA1-HA4. The AMS+ group, however, demonstrated a higher (P < 0.05) AMS incidence in the active vs. passive ascent cohort on HA1 (93% vs. 56%), similar incidence on HA2 (60% vs. 78%), lower incidence (P < 0.05) on HA3 (33% vs. 67%), and similar incidence on HA4 (13% vs. 28%). The AMS+ group also demonstrated a higher (P < 0.05) AMS severity in the active vs. passive ascent cohort on HA1 (1.35 ± 0.97 vs. 0.90 ± 0.70), similar score on HA2 (1.00 ± 0.97 vs. 1.34 ± 0.70), and lower (P < 0.05) score on HA3 (0.56 ± 0.55 vs. 1.02 ± 0.75) and HA4 (0.32 ± 0.41 vs. 0.60 ± 0.72). Active compared with passive ascent accelerated the time course of AMS with more individuals sick on HA1 and less individuals sick on HA3 and HA4.NEW & NOTEWORTHY This research demonstrated that active ascent accelerated the time course but not overall incidence and severity of acute mountain sickness (AMS) following rapid ascent to 3,600 m in unacclimatized lowlanders. Active ascenders became sicker faster and recovered quicker than passive ascenders, which may be due to differences in body fluid regulation. Findings from this well-controlled large sample-size study suggest that previously reported discrepancies in the literature regarding the impact of exercise on AMS may be related to differences in the timing of AMS measurements between studies.

Using a Contemporary Portable Metabolic Gas Exchange System for Assessing Energy Expenditure: A Validity and Reliability Study

There are several methods available to assess energy expenditure, all associated with inherent pros and cons that must be adequately considered for use in specific environments and populations. A requirement of all methods is that they must be valid and reliable in their capability to accurately measure oxygen consumption (VO₂) and carbon dioxide production (VCO₂). The purpose of this study was to evaluate the reliability and validity of the mobile CO₂/O₂ Breath and Respiration Analyzer (COBRA) relative to a criterion system (Parvomedics TrueOne 2400^®, PARVO) with additional measurements to compare the COBRA to a portable system (Vyaire Medical, Oxycon Mobile^®, OXY). Fourteen volunteers with a mean of 24 years old, body weight of 76 kg, and a VO_2peak of 3.8 L∙min^-1 performed four repeated trials of progressive exercises. Simultaneous steady-state measurements of VO₂, VCO₂, and minute ventilation (V_E) by the COBRA/PARVO and OXY systems were conducted at rest, while walking (23-36% VO_2peak), jogging (49-67% VO_2peak), and running (60-76% VO_2peak). Data collection was randomized by the order of system tested (COBRA/PARVO and OXY) and was standardized to maintain work intensity (rest to run) progression across study trials and days (two trials/day over two days). Systematic bias was examined to assess the accuracy of the COBRA to PARVO and OXY to PARVO across work intensities. Intra- and inter-unit variability were assessed with interclass correlation coefficients (ICC) and a 95% limit of agreement intervals. The COBRA and PARVO produced similar measures for VO₂ (Bias ± SD, 0.01 ± 0.13 L·min^-1; 95% LoA, (-0.24, 0.27 L·min^-1); R² = 0.982), VCO₂ (0.06 ± 0.13 L·min^-1; (-0.19, 0.31 L·min^-1); R² = 0.982), V_E (2.07 ± 2.76 L·min^-1; (-3.35, 7.49 L·min^-1); R² = 0.991) across work intensities. There was a linear bias across both the COBRA and OXY with increased work intensity. The coefficient of variation for the COBRA ranged from 7 to 9% across measures for VO₂, VCO₂, and V_E. COBRA was reliable across measurements for VO₂ (ICC = 0.825; 0.951), VCO₂ (ICC = 0.785; 0.876), and V_E (ICC = 0.857; 0.945) for intra-unit reliability, respectively. The COBRA is an accurate and reliable mobile system for measuring gas exchange at rest and across a range of work intensities.

Can mHealth Technology Help Mitigate the Effects of the COVID-19 Pandemic?

Goal: The aim of the study herein reported was to review mobile health (mHealth) technologies and explore their use to monitor and mitigate the effects of the COVID-19 pandemic. Methods: A Task Force was assembled by recruiting individuals with expertise in electronic Patient-Reported Outcomes (ePRO), wearable sensors, and digital contact tracing technologies. Its members collected and discussed available information and summarized it in a series of reports. Results: The Task Force identified technologies that could be deployed in response to the COVID-19 pandemic and would likely be suitable for future pandemics. Criteria for their evaluation were agreed upon and applied to these systems. Conclusions: mHealth technologies are viable options to monitor COVID-19 patients and be used to predict symptom escalation for earlier intervention. These technologies could also be utilized to monitor individuals who are presumed non-infected and enable prediction of exposure to SARS-CoV-2, thus facilitating the prioritization of diagnostic testing.

Divers risk accelerated fatigue and core temperature rise during fully-immersed exercise in warmer water temperature extremes

Physiological responses to work in cold water have been well studied but little is known about the effects of exercise in warm water; an overlooked but critical issue for certain military, scientific, recreational, and professional diving operations. This investigation examined core temperature responses to fatiguing, fully-immersed exercise in extremely warm waters. Twenty-one male U.S. Navy divers (body mass, 87.3 ± 12.3 kg) were monitored during rest and fatiguing exercise while fully-immersed in four different water temperatures (Tw): 34.4, 35.8, 37.2, and 38.6°C (Tw_34.4, Tw_35.8, Tw_37.2, and Tw_38.6 respectively). Participants exercised on an underwater cycle ergometer until volitional fatigue or core temperature limits were reached. Core body temperature and heart rate were monitored continuously. Trial performance time decreased significantly as water temperature increased (Tw_34.4, 174 ± 12 min; Tw_35.8, 115 ± 13 min; Tw_37.2, 50 ± 13 min; Tw_38.6, 34 ± 14 min). Peak core body temperature during work was significantly lower in Tw_34.4 water (38.31 ± 0.49°C) than in warmer temperatures (Tw_35.8, 38.60 ± 0.55°C; Tw_37.2, 38.82 ± 0.76°C; Tw_38.6, 38.97 ± 0.65°C). Core body temperature rate of change increased significantly with warmer water temperature (Tw_34.4, 0.39 ± 0.28°C·h⁻¹; Tw_35.8, 0.80 ± 0.19°C·h⁻¹; Tw_37.2, 2.02 ± 0.31°C·h⁻¹; Tw_38.6, 3.54 ± 0.41°C·h⁻¹). Physically active divers risk severe hyperthermia in warmer waters. Increases in water temperature drastically increase the rate of core body temperature rise during work in warm water. New predictive models for core temperature based on workload and duration of warm water exposure are needed to ensure warm water diving safety.

Core temperature responses of military working dogs during training activities and exercise walks

Heat strain is common in military working dogs (MWDs), but can be mitigated by limiting duration of activity to avoid overheating and allowing sufficient time for recovery. To determine work/rest times for MWDs, temperature responses during training must be characterized. This study measured body core temperature of 48 MWDs at Lackland Air Force Base, San Antonio, TX. Twenty-four MWDs in training for patrol and detection activities participated under a range of ambient temperatures in August (27°C-32°C), October (22°C-26°C) and March (approximately 13°C). These MWDs swallowed a telemetric thermometer pill to measure continuous gastrointestinal tract temperature (Tgi). Twenty-four kennel MWDs participated in July (25°C-29°C). In these dogs rectal temperature (Tre) was measured manually during a standard exercise walk. For the MWDs in training, Tgi before the first activity was 38.5±0.5°C (mean±SD) and final Tgi was 39.8±0.6°C after sessions that lasted 13.1±4.9 minutes (5.4 to 26.3 minutes). Peak Tgi, 0.4±0.4°C above final Tgi, occurred 8 to 12 minutes into recovery. Before beginning a second activity 40 to 165 minutes later, Tgi was within 0.5°C of initial values for 80% of dogs. For the kennel MWDs, Tre was 39.0±0.8°C (37.7°C to 40.7°C) at the start and 40.1±0.6°C at the end of the 21.3±2.8 minute walk. The continuous increase in core temperature during activity of both groups of MWDs indicates that limiting exercise duration is important for minimizing risk of overheating in MWDs. The observation of continued increase in Tgi to a peak after exercise ends suggests that for MWDs suspected of overheating temperature should be monitored for at least 15 minutes postexercise to ensure recovery.

Heat strain imposed by personal protective ensembles: quantitative analysis using a thermoregulation model

The objective of this paper is to study the effects of personal protective equipment (PPE) and specific PPE layers, defined as thermal/evaporative resistances and the mass, on heat strain during physical activity. A stepwise thermal manikin testing and modeling approach was used to analyze a PPE ensemble with four layers: uniform, ballistic protection, chemical protective clothing, and mask and gloves. The PPE was tested on a thermal manikin, starting with the uniform, then adding an additional layer in each step. Wearing PPE increases the metabolic rates [Formula: see text], thus [Formula: see text] were adjusted according to the mass of each of four configurations. A human thermoregulatory model was used to predict endurance time for each configuration at fixed [Formula: see text] and at its mass adjusted [Formula: see text]. Reductions in endurance time due to resistances, and due to mass, were separately determined using predicted results. Fractional contributions of PPE's thermal/evaporative resistances by layer show that the ballistic protection and the chemical protective clothing layers contribute about 20 %, respectively. Wearing the ballistic protection over the uniform reduced endurance time from 146 to 75 min, with 31 min of the decrement due to the additional resistances of the ballistic protection, and 40 min due to increased [Formula: see text] associated with the additional mass. Effects of mass on heat strain are of a similar magnitude relative to effects of increased resistances. Reducing resistances and mass can both significantly alleviate heat strain.

Thermal-work strain during Marine rifle squad operations in Afghanistan

The physiological burden created by heat strain and physical exercise, also called thermal-work strain, was quantified for 10 male Marines (age 21.9 ± 2.3 years, height 180.3 ± 5.2 cm, and weight 85.2 ± 10.8 kg) during three dismounted missions in Helmand Province, Afghanistan. Heart rate (HR) and core body temperature (T core) were recorded every 15 seconds (Equivital EQ-01; Hidalgo, Cambridge, United Kingdom) during periods of light, moderate, and heavy work and used to estimate metabolic rate. Meteorological measures, clothing characteristics, anthropometrics, and estimated metabolic rates were used to predict T core for the same missions during March (spring) and July (summer) conditions. Thermal-work strain was quantified from HR and T core values using the Physiological Strain Index (PSI) developed by Moran et al. July PSI and T core values were predicted and not observed due to lack of access to in-theater warfighters at that time. Our methods quantify and compare the predicted and observed thermal-work strain resulting from environment and worn or carried equipment and illustrate that a small increase in ambient temperature and solar load might result in increased thermal-work strain.

Estimation of human core temperature from sequential heart rate observations

Core temperature (CT) in combination with heart rate (HR) can be a good indicator of impending heat exhaustion for occupations involving exposure to heat, heavy workloads, and wearing protective clothing. However, continuously measuring CT in an ambulatory environment is difficult. To address this problem we developed a model to estimate the time course of CT using a series of HR measurements as a leading indicator using a Kalman filter. The model was trained using data from 17 volunteers engaged in a 24 h military field exercise (air temperatures 24-36 °C, and 42%-97% relative humidity and CTs ranging from 36.0-40.0 °C). Validation data from laboratory and field studies (N = 83) encompassing various combinations of temperature, hydration, clothing, and acclimation state were examined using the Bland-Altman limits of agreement (LoA) method. We found our model had an overall bias of -0.03 ± 0.32 °C and that 95% of all CT estimates fall within ±0.63 °C (>52 000 total observations). While the model for estimating CT is not a replacement for direct measurement of CT (literature comparisons of esophageal and rectal methods average LoAs of ±0.58 °C) our results suggest it is accurate enough to provide practical indication of thermal work strain for use in the work place.

Human thermoregulatory system state estimation using non-invasive physiological sensors

Small teams of emergency workers/military can often find themselves engaged in critical, high exertion work conducted under challenging environmental conditions. These types of conditions present thermal work strain challenges which unmitigated can lead to collapse (heat exhaustion) or even death from heat stroke. Physiological measurement of these teams provides a mechanism that could be an effective tool in preventing thermal injury. While indices of thermal work strain have been proposed they suffer from ignoring thermoregulatory context and rely on measuring internal temperature (IT). Measurement of IT in free ranging ambulatory environments is problematic. In this paper we propose a physiology based Dynamic Bayesian Network (DBN) model that estimates internal temperature, heat production and heat transfer from observations of heart rate, accelerometry, and skin heat flux. We learn the model's conditional probability distributions from seven volunteers engaged in a 48 hour military field training exercise. We demonstrate that sum of our minute to minute heat production estimates correlate well with total daily energy expenditure (TDEE) measured using the doubly labeled water technique (r(2) = 0.73). We also demonstrate that the DBN is able to infer IT in new datasets to within ±0.5 °C over 85% of the time. Importantly, the additional thermoregulatory context allows critical high IT temperature to be estimated better than previous approaches. We conclude that the DBN approach shows promise in enabling practical real time thermal work strain monitoring applications from physiological monitoring systems that exist today.

United States Army Research Institute of Environmental Medicine: Warfighter research focusing on the past 25 years

The United States Army Research Institute of Environmental Medicine (USARIEM) celebrated its 50th anniversary on July 1, 2011. This article reviews its history, evolution, and transition of its research programs as well as its scientific and military accomplishments, emphasizing the past 25 yr. During the 1990s, USARIEM published a series of pocket guides providing guidance for sustaining Warfighter health and performance in Southwest Asia, Somalia, the former Republic of Yugoslavia, Rwanda, and Haiti. Issues identified during Operation Desert Storm elicited research that improved nutritional guidelines for protracted desert operations; safer use of nuclear, chemical, and biological protective clothing; equipment, development, and fielding of efficient microclimate cooling systems; and effective evaluation of pharmaceuticals to protect soldiers from chemical and biological threats. During the first decade of the 21st century, USARIEM and the Department of the Army published official medical/performance doctrines for operations in the heat and cold and at high altitude. The current Global War on Terrorism focused research to improve doctrines for hot, cold, and high-altitude operations, reduce musculoskeletal training injuries, provide improved field nutrition, more efficient planning for operational water requirements, and improve both military clothing and materiel. This article also describes the critically important interactions and communications between USARIEM and deployed units and the benefits to Warfighters from this association. This report presents USARIEM's unique and world-class facilities, organizational changes, scientific and support personnel, and major research accomplishments, including the publication of 2,200 scientific papers over the past 25 yr.

Methods of evaluating protective clothing relative to heat and cold stress: thermal manikin, biomedical modeling, and human testing

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.

Field studies of exercise and food deprivation

PURPOSE OF REVIEW

The increase in obesity in developed societies drives interest in the interplay of energy intake, metabolic energy expenditure, and body energy stores. A better understanding of energy management in physically active and undernourished humans should help guide strategies to manage obesity safely and effectively. This review focuses on field studies of men and women engaged in prolonged strenuous activities, ranging from ranger training to extreme expeditions.

RECENT FINDINGS

Although scientifically unconventional and limited, field studies of exercise and food deprivation have yielded interesting findings: 4-5% body fat is the normal lower limit to fat reserves in physically active underfed young adult men, and in response to exercise and underfeeding, women used more fat mass and less fat-free mass to meet metabolic fuel requirements.

SUMMARY

Field studies have shown that fat energy reserves in young adult men can be estimated as percentage body fat minus 5%, and initial body fat mass has a significant positive influence on fat oxidation rates per kilogram of fat-free mass during rapid weight loss associated with underfeeding and exercise. Data logging pedometers, activity monitors, global positioning systems, and wireless body and personal-area networks promise to make it easier to study and care for free-living humans.

Negative energy balance in male and female rangers: effects of 7 d of sustained exercise and food deprivation

BACKGROUND

A challenging 7-d ranger field exercise (FEX) by cadets in the Norwegian Military Academy provided a venue in which to study the effects of negative energy balance.

OBJECTIVE

We quantified total energy expenditure (TEE), food intake, and changes in body composition in male and female cadets.

DESIGN

TEE (measured by doubly labeled water), food intake, activity patterns (measured by accelerometry), and body composition (measured by dual-energy X-ray absorptiometry) were measured in 16 cadets (10 men and 6 women aged 21-27 y).

RESULTS

The physically active (approximately 23 h/d) and semistarved (0.2-2.2 MJ/d) cadets lost weight (x +/- SD: men, -7.7 +/- 1.1 kg; women, -5.9 +/- 1.1 kg; P < 0.05). Absolute TEE differed by sex (men, 26.6 +/- 2.0 MJ/d; women, 21.9 +/- 2.0 MJ/d; P < 0.05) but body weight-specific TEE did not (men, 343 +/- 26 kJ . kg(-1) . d(-1); women, 354 +/- 18 kJ . kg(-1) . d(-1); NS). Fat-free mass (FFM) loss differed significantly by sex (men, -4.0 +/- 1.2 kg; women, -2.5 +/- 1.1 kg; P < 0.05), but percentage FFM loss did not (men, -6.3 +/- 1.9%; women, -5.6 +/- 2.4%). In contrast, absolute FM loss did not differ significantly by sex (men, -3.45 +/- 0.72 kg; women, -3.42 +/- 0.22 kg), but fat oxidation (men, 5.2 +/- 1.0 mg . min(-1) . kg FFM(-1); women, 7.3 +/- 0.5 mg . min(-1) . kg FFM(-1)) and the relative contribution of FM to TEE (men, 74 +/- 14%; women, 89 +/- 6%) were significantly greater in women than in men (P < 0.05).

CONCLUSION

Female cadets maintained a significantly more fat-predominant fuel metabolism than did male cadets in response to sustained exercise and semistarvation.

Energy Expenditure in Men and Women during 54 h of Exercise and Caloric Deprivation

Fifty U.S. Marine recruits (30 men, 20 women) were studied during a physically intense, energy intake-restricted, winter-time 54-h field training exercise (FEX) at Parris Island Marine Corps Recruit Depot. Men and women completed the same physical tasks.

PURPOSE

To characterize and compare the total energy expenditure (TEE) and core temperature responses in men and women working almost continuously for 2.25 d in an outdoor environment while developing a substantial energy deficit.

METHODS

TEE was measured using doubly labeled water (D(2)O(18)). Energy intake was estimated using beverage diaries and collecting ration wrappers saved by each volunteer and adding the known caloric value of each food item consumed. Core temperature was measured using an ingested thermometer pill. Physical activity level (PAL) was calculated by dividing TEE by the calculated basal metabolic rate.

RESULTS

TEE was higher (P < 0.001) for the men (25.7 MJ.d(-1)) than women (19.8 MJ.d(-1)), but there were no differences between men and women in TEE normalized to body mass (men, 0.35 +/- 0.05 MJ.d(-1).kg(-1); women, 0.34 +/- 0.06 MJ.d(-).kg(-1)), corrected body mass (men, 0.29 +/- 0.04 MJ.d(-1).kg(-1) corrected body mass; women, 0.27 +/- 0.04 MJ.d(-1).kg(-1) corrected body mass), fat-free mass (men, 0.41 +/- 0.07 MJ.d(-1).kg(-1) FFM; women, 0.46 +/- 0.07 MJ.d(-1).kg(-1) FFM), or corrected fat-free mass (men, 0.30 +/- 0.05 MJ.d(-1).kg corrected body mass; women, 0.30 +/- 0.04 0.30 +/- 0.05 MJ.d(-1).kg(-1) corrected body mass). PAL was the same for men (3.4 +/- 0.5) and women (3.3 +/- 0.4). Energy intakes were higher (P < 0.05) in men (6.0 +/- 2.0 MJ.d(-1)) than women (4.8 +/- 1.8 MJ.d(-1)). The average minimum core temperature was 36.0 +/- 0.4 degrees C, and the mean maximum core temperature was 38.5 +/- 0.3 degrees C.

CONCLUSIONS

For both men and women, total energy expenditures were among the highest observed for a military FEX. TEE, when normalized or corrected to body mass and fat-free mass, and PAL were the same for men and women.

Body fluid regulation in a simulated disabled submarine: effects of cold, reduced O2, and elevated CO2

INTRODUCTION

Survivors awaiting rescue aboard a disabled submarine (DISSUB) lacking power and/or environmental control would potentially be exposed to cold and reduced O2 and elevated CO2 levels. We hypothesized that elevated CO2 levels would lead to hormone changes that would mitigate cold and hypoxia-induced body fluid losses.

METHODS

Blood was drawn from seven men on three mornings: baseline (21% O2, 0.05% CO2), after 4 d of low O2, cold, and high CO2 (T101; 4 degrees C, 16.75% O2, 2.5% CO2), and following acute withdrawal of cold and high CO2 (T173, 16.75% O2, 0.05% CO2). Total body water (TBW) was measured using deuterium oxide dilution at baseline and at T148. Hormone analyses included atrial natriuretic peptide (ANP), aldosterone (ALDO), and plasma renin activity (PRA).

RESULTS

TBW decreased by 0.4 +/- 0.4 L. Water turnover was 3.0 +/- 0.5 L x d(-1). ANP (pg x ml(-1)) was lower (p < 0.05) at T101 (3.46 +/- 1.17) and T173 (4.97 +/- 2.28) vs. baseline (8.19 +/- 3.40). PRA (pg x ml(-1)) was higher (p < 0.05) at T101 (10.43 +/- 4.90) and T173 (14.23 +/- 4.48) vs. baseline (6.81 +/- 3.43). ALDO, serum osmolality, and electrolytes were not different across time. Urine flow was lower at T101 and T173 vs. baseline, and urine osmotic clearance was lower at T173 vs. baseline. Free water clearance did not change across time.

DISCUSSION

These data indicate that the combination of cold, low O2, and high CO2 for 5-7 d did not change total body water and hormone changes and urinary measures across the DISSUB were consistent with fluid retention.

Modeling physiological responses to military scenarios: initial core temperature and downhill work

INTRODUCTION

Previous field studies suggested that a thermoregulatory model developed by the U.S. Army Research Institute of Environmental Medicine (USARIEM) needed an adjustment of initial core temperature (Tcr) for individual variation and a metabolic (M) correction during downhill movements. This study evaluated the updated version of the model incorporating these new features using a dataset collected during U.S. Marine Corps marksmanship training at Quantico, VA.

METHODS

Individual anthropometrics, physiological, and environmental time series data were obtained from five Marine men. The study focused on the marksmanship training for approximately 2 h, then 30-min marching including uphill and downhill movements in a moderately hot environment (air temperature: approximately 30 degrees C; dew point: approximately 21 degrees C). The predicted and observed heart rate (HR) and Tcr measurements were compared by root mean square deviations (RMSD).

RESULTS

Overall, the current model improved predictions of physiological measures (HR RMSD = 23 bpm, Tcr RMSD = 0.46 degrees C), particularly for marching in the heat (HR RMSD = 21 bpm, Tcr RMSD = 0.32 degrees C). The model under-predicted both HR and Tcr during marksmanship training, indicating that a greater solar effect or non-thermal factors may have required higher M rates during these periods.

CONCLUSIONS

Updated features of the model significantly improved physiological predictions. However, accurate M estimates are required for slow movements of subjects under heat stress, such as movements on the firing range. Such improvement should result in more accurate simulations of physiological status and better risk assessment, thereby reducing heat injuries and improving performance of deployed military personnel.

Energy requirements of military personnel

Energy requirements of military personnel (Soldiers, Sailors, Airmen, and Marines) have been measured in garrison and in field training under a variety of climatic conditions. Group mean total energy expenditures for 424 male military personnel from various units engaged in diverse missions ranged from 13.0 to 29.8 MJ (3109-7131 kcal) per day. The overall mean was 19.3+/-2.7 MJ (mean+/-SD) (4610+/-650 kcal) per day measured over an average of 12.2 days (range 2.25-69 days). For the 77 female military personnel studied, mean total energy expenditures for individual experimental groups ranged from 9.8 to 23.4 MJ (2332-5597 kcal) per day, with an overall mean of 11.9+/-2.6 MJ (2850+/-620 kcal) per day, measured over an average of 8.8 days (range 2.25-14 days). Women, presumably due to their lower lean body mass, resting metabolic rate, and absolute work rates, had lower total energy expenditures. Combat training produced higher energy requirements than non-combat training or support activities. Compared to temperate conditions, total energy expenditures did not appear to be influenced by hot weather, but tended to be higher in the cold or high altitude conditions.

Total energy expenditure estimated using a foot-contact pedometer

BACKGROUND

Total energy expenditure (TEE) assessment using pedometers provide an easy and less expensive method than doubly labeled water (DLW). This study assessed TEE by a new pedometry method (TEEpedo) compared to the doubly labeled water method (TEEdlw).

MATERIAL/METHODS

Shipboard sailors (7 men, age: 23.0+/-3.9 yrs; ht: 180.2+/-6.5 cm; wt: 83.8+/-11.8 kg, and 10 women, age: 24.7+/-4.4 yrs; ht: 165.2+/-8.0 cm; wt: 63.5+/-14.0 kg) (Mean +/-SD) were studied for 8 days. The energy cost of activity was estimated using (a) total body weight, (b) foot-ground contact times [Tc] during running, walking, and non-exercise activity [NEAT], and (c) the known proportion of time spent in each activity category. Resting metabolic rate (RMR) was estimated from lean body mass.

RESULTS

TEEPEDO was calculated as: TEEpedo (MJ) = (1440 x [%Run Time x ((0.0761 x [Total Body Weight/TcRun]) - 7.598) +%Walk Time x ((0.056 x [Total Body Weight/TcWalk]) - 2.938) + (%NEAT Time x 0.1 x [RMR/Minute])] + RMR)/239. This method, explained 79% of the variance of TEEpedo with a 95% confidence interval of +/-0.81 MJ/day, relative to TEEdlw (12.55+/-3.3MJ/day). Mean TEEpedo (12.65+/-3.1 MJ/day) did not differ from mean TEEdlw (p=0.95).

CONCLUSIONS

At TEEs >14 MJ/day, the TEEpedo method underestimated actual TEE, possibly due to unaccounted for upper body exercise. At more moderate TEEs of 9 to 14 MJ/day, the Tc pedometry method provided accurate estimates of TEE.

High-speed running performance: a new approach to assessment and prediction

We hypothesized that all-out running speeds for efforts lasting from a few seconds to several minutes could be accurately predicted from two measurements: the maximum respective speeds supported by the anaerobic and aerobic powers of the runner. To evaluate our hypothesis, we recruited seven competitive runners of different event specialties and tested them during treadmill and overground running on level surfaces. The maximum speed supported by anaerobic power was determined from the fastest speed that subjects could attain for a burst of eight steps (approximately 3 s or less). The maximum speed supported by aerobic power, or the velocity at maximal oxygen uptake, was determined from a progressive, discontinuous treadmill test to failure. All-out running speeds for trials of 3-240 s were measured during 10-13 constant-speed treadmill runs to failure and 4 track runs at specified distances. Measured values of the maximum speeds supported by anaerobic and aerobic power, in conjunction with an exponential constant, allowed us to predict the speeds of all-out treadmill trials to within an average of 2.5% (R2 = 0.94; n = 84) and track trials to within 3.4% (R2 = 0.86; n = 28). An algorithm using this exponent and only two of the all-out treadmill runs to predict the remaining treadmill trials was nearly as accurate (average = 3.7%; R2 = 0.93; n = 77). We conclude that our technique 1) provides accurate predictions of high-speed running performance in trained runners and 2) offers a performance assessment alternative to existing tests of anaerobic power and capacity.

Adequacy of Garrison Feeding for Special Forces Soldiers during Training

This study evaluated whether Special Forces (SF) soldiers training in garrison would meet nutrient intake recommendations using the available garrison dining facility. Dietary intakes were obtained by a visual estimation method and self-reported food records from 32 SF and 13 support soldiers for 9 days. Total energy expenditure (TEE) was measured in nine soldiers from each group using doubly labeled water. Mean (+/- SD) total energy expenditure of SF (4,099 +/- 740 kcal/day) was higher than support soldiers (3,361 +/- 939 kcal/day, p < 0.01). Energy intake did not differ between groups. Median energy intake for all soldiers was 3,204 kcal/day. The nutrient intake goals of SF soldiers were not fully met by eating in the dining facility. Extending meal times and providing additional meals or "take out" foods may allow energy needs of SF soldiers (approximately 4,200 kcal/day) to be met, while reducing the reliance on potentially less nutritious outside foods.

Total energy expenditure estimated using foot-ground contact pedometry

Routine walking and running, by increasing daily total energy expenditure (TEE), can play a significant role in reducing the likelihood of obesity. The objective of this field study was to compare TEE estimated using foot-ground contact time (Tc)-pedometry (TEE(PEDO)) with that measured by the criterion doubly labeled water (DLW) method. Eight male U.S. Marine test volunteers [27 +/- 4 years of age (mean +/- SD); weight = 83.2 +/- 10.7 kg; height = 182.2 +/- 4.5 cm; body fat = 17.0 +/- 2.9%] engaged in a field training exercise were studied over 2 days. TEE(PEDO) was defined as (calculated resting energy expenditure + estimated thermic effect of food + metabolic cost of physical activity), where physical activity was estimated by Tc-pedometry. Tc-pedometry was used to differentiate inactivity, activity other than exercise (i.e., non-exercise activity thermogenesis, or NEAT), and the metabolic cost of locomotion (M(LOCO)), where M(LOCO) was derived from total weight (body weight + load weight) and accelerometric measurements of Tc. TEE(PEDO) data were compared with TEEs measured by the DLW (2H2(18)O) method (TEE(DLW)): TEE(DLW) = 15.27 +/- 1.65 MJ/day and TEE(PEDO) = 15.29 +/- 0.83 MJ/day. Mean bias (i.e., TEE(PEDO) - TEE(DLW)) was 0.02 MJ, and mean error (SD of individual differences between TEE(PEDO) and TEE(DLW)) was 1.83 MJ. The Tc-pedometry method provided a valid estimate of the average TEE of a small group of physically active subjects where walking was the dominant activity.

Life sign decision support algorithms

There is a pressing need in the military for a system that interprets data from a suite of wearable physiological sensors to infer a soldier's current clinical state on the battlefield. The Warfighter Physiological Status Monitoring (WPSM) concept is envisioned by the US Army to address this need. Life sign detection is a key component. The future WPSM system will consist of a body-worn network of biosensors with a central processing control unit containing firmware for assessing the soldier's physiological status. In the present application, the system will diagnose a Dead, Alive, or Unknown (DAU) physiological state, and this information will be made available to field medics and others over separate communication channels. This paper describes the various modules in the DAU determination algorithms and how they interact with each other, as well as a simulator system built for parametric studies of the overall system.

Effects of a 3-Month Endurance Event on Physical Performance and Body Composition: The G2 Trans-Greenland Expedition☆

OBJECTIVE

Prolonged physical exertion with inadequate time for recuperation may result in an overtraining phenomenon characterized by reduced physical strength and endurance capacity. We tested the hypothesis that highly motivated men pushed to the limits of their endurance capacity for 3 months would suffer physical breakdown characterized by loss of lean mass and reduced physical performance capacity.

METHODS

Two well-trained men (VO2max > 60 mL/kg/min), aged 25 years, completed an unsupported, 2928-km, south-to-north ski trek across Greenland in 86 days. The trek involved ski marching, typically for 9 h/d, pulling sleds initially containing 150 kg and a high-fat (60%) energy-dense diet of 25.1 MJ/d. Body composition and physical performance data were collected 14 days before and 4 days after the trek.

RESULTS

Energy expenditure based on doubly labeled water during three 2-week periods ranged from 28.3 and 34.6 MJ/d in rugged terrain to 14.6 and 16.1 MJ/d during travel on flat terrain for subjects 1 and 2, respectively. Both men lost weight, completing the trek with low-normal fat stores (approximately 13% body fat). The lighter man gained 0.6 kg lean mass, while the heavier man lost 1.4 kg lean mass and a larger amount of fat weight (7.0 kg). Most performance measures showed trivial changes within the errors of measurement and test reproducibility, indicating no loss of endurance capacity, but anaerobic tests (Wingate and vertical jump) were markedly reduced. Markers of metabolic status, including oral glucose tolerance tests, indicated no functional impairments.

CONCLUSIONS

Although the number of subjects was limited, this observational study demonstrated that well-trained and experienced long-distance ski trekkers who eat an adequate high-calorie diet can perform endurance treks in severe cold, with little or no loss of lean mass and physical capability.

Combat medical informatics: present and future

In this paper, we describe a U.S. Army concept to monitor soldier physiologic status and provide computer-based medical support to increase the likelihood of soldier survival on the battlefield. Supported by an underlying platform of complex wearable computerized systems, the "Warfighter Physiological Status Monitoring" (WPSM) concept consists of an array of biosensors embedded in the soldier's uniform integrated with a database management system and a decision support system that will provide assistance in casualty prevention and casualty management. We discuss the main components of the WPSM, its present status, key requirements and outstanding challenges, and near- and far-term research directions.

Ambulatory estimates of maximal aerobic power from foot -ground contact times and heart rates in running humans

Seeking to develop a simple ambulatory test of maximal aerobic power (VO(2 max)), we hypothesized that the ratio of inverse foot-ground contact time (1/t(c)) to heart rate (HR) during steady-speed running would accurately predict VO(2 max). Given the direct relationship between 1/t(c) and mass-specific O(2) uptake during running, the ratio 1/t(c). HR should reflect mass-specific O(2) pulse and, in turn, aerobic power. We divided 36 volunteers into matched experimental and validation groups. VO(2 max) was determined by a treadmill test to volitional fatigue. Ambulatory monitors on the shoe and chest recorded foot-ground contact time (t(c)) and steady-state HR, respectively, at a series of submaximal running speeds. In the experimental group, aerobic fitness index (1/t(c). HR) was nearly constant across running speed and correlated with VO(2 max) (r = 0.90). The regression equation derived from data from the experimental group predicted VO(2 max) from the 1/t(c). HR values in the validation group within 8.3% and 4.7 ml O(2) x kg(-1) x min(-1) (r = 0.84) of measured values. We conclude that simultaneous measurements of foot-ground constant times and heart rates during level running at a freely chosen constant speed can provide accurate estimates of maximal aerobic power.

Ambulatory physiological status monitoring during a mountaineering expedition

OBJECTIVE

To evaluate an ambulatory physiological monitoring system during a mountaineering expedition. We hypothesized that the Environmental Symptoms Questionnaire, combined with frequent measurement of oxygen saturation and core temperature, would accurately identify cases of environmental illness.

METHODS

Twelve military mountaineers took a daily Environmental Symptoms Questionnaire, monitored fingertip oxygen saturations, and recorded core temperatures while climbing a 4,949-m peak. Illnesses identified by the system were compared with those identified by spontaneous reports.

RESULTS

The system correctly identified one case of high-altitude pulmonary edema and two illnesses that were not reported to the physician (one case of acute mountain sickness and one of self-limited symptomatic desaturation). However, it did not identify two illnesses that were severe enough to preclude further climbing (one case of sinus headache and one of generalized fatigue).

CONCLUSIONS

Our monitoring system may complement, but cannot replace, on-site medical personnel during mountaineering expeditions.

Endocrine markers of semistarvation in healthy lean men in a multistressor environment

We tested the hypothesis that key endocrine responses to semistarvation would be attenuated by changing only the food intake in a multistressor environment that also included sustained workload, inadequate sleep, and thermal strain. Serum hormones were compared within and between two groups of healthy young male volunteers participating in the 8-wk US Army Ranger course, with four repeated cycles of restricted energy intakes and refeeding: group 1 (n = 49) and group 2 (n = 48); energy deficits averaged 1,200 and 1,000 kcal/day, respectively. After 8 wk, most of group 1 achieved a minimum body fat, serum 3,5,3'-triiodothyronine (T(3)) was below normal (78 +/- 20 ng/dl), testosterone (T) approached castrate levels (4.5 +/- 3.9 nmol/l), insulin-like growth factor I (IGF-I) declined by one-half (75 +/- 25 microg/l), and cholesterol rose from 158 +/- 31 to 217 +/- 39 mg/dl. Bioavailable T(3) and T were further reduced by increases in their specific binding proteins in response to declining insulin. Refeeding, even with continuation of the other stressors, produced prompt recovery of T(3), T, and IGF-I. Higher energy intakes in group 2 attenuated the subclinical hypothyroidism and hypercholesterolemia, whereas consistent luteinizing hormone suppression indicated centrally mediated threshold effects on gonadal hormone suppression. We conclude that low T, T(3), and IGF-I remained reliable markers of acute energy deficits in the presence of other stressors; elevated cholesterol and cortisol provided information about chronic status, corresponding to diminishing body fat stores.

Energy expenditure and balance during spaceflight on the space shuttle

The objectives of this study were as follows: 1) to measure human energy expenditure (EE) during spaceflight on a shuttle mission by using the doubly labeled water (DLW) method; 2) to determine whether the astronauts were in negative energy balance during spaceflight; 3) to use the comparison of change in body fat as measured by the intake DLW EE, 18O dilution, and dual energy X-ray absorptiometry (DEXA) to validate the DLW method for spaceflight; and 4) to compare EE during spaceflight against that found with bed rest. Two experiments were conducted: a flight experiment (n = 4) on the 16-day 1996 life and microgravity sciences shuttle mission and a 6 degrees head-down tilt bed rest study with controlled dietary intake (n = 8). The bed rest study was designed to simulate the flight experiment and included exercise. Two EE determinations were done before flight (bed rest), during flight (bed rest), and after flight (recovery). Energy intake and N balance were monitored for the entire period. Results were that body weight, water, fat, and energy balance were unchanged with bed rest. For the flight experiment, decreases in weight (2.6 +/- 0.4 kg, P < 0.05) and N retention (-2. 37 +/- 0.45 g N/day, P < 0.05) were found. Dietary intake for the four astronauts was reduced in flight (3,025 +/- 180 vs. 1,943 +/- 179 kcal/day, P < 0.05). EE in flight was 3,320 +/- 155 kcal/day, resulting in a negative energy balance of 1,355 +/- 80 kcal/day (-15. 7 +/- 1.0 kcal. kg-1. day-1, P < 0.05). This corresponded to a loss of 2.1 +/- 0.4 kg body fat, which was within experimental error of the fat loss determined by 18O dilution (-1.4 +/- 0.5 kg) and DEXA (-2.4 +/- 0.4 kg). All three methods showed no change in body fat with bed rest. In conclusion, 1) the DLW method for measuring EE during spaceflight is valid, 2) the astronauts were in severe negative energy balance and oxidized body fat, and 3) in-flight energy (E) requirements can be predicted from the equation: E = 1.40 x resting metabolic rate + exercise.

Telemetry pill measurement of core temperature in humans during active heating and cooling

PURPOSE

This study compared the agreement between core temperature measurements obtained using an ingestible temperature pill telemetry system (Tpill) with those obtained from rectal (Tre) and esophageal (Tes) thermocouples under conditions of increasing and decreasing body temperature.

METHODS

Four men and five women (age 25+/-2 yr, BSA 1.81+/-0.05 m2, VO2 peak 3.1+/-0.4 L x min[-1]) participated in four 3-h trials: cold (18 degrees C) water rest (CWR), cold water exercise (CWE), warm (36 degrees C) water rest (WWR), and warm water exercise (WWE). Subjects were immersed to the neck for each trial. During resting trials, subjects sat quietly. During exercise trials, subjects completed three bouts of 15 min of rest, followed by 45 min of exercise on a cycle ergometer at 50% of peak oxygen uptake. The temperature pill was taken 10-12 h before testing, after which the subjects fasted.

RESULTS

The trials created conditions of constantly decreasing (CWR) or increasing (WWR) core temperature, as well as periods of oscillating core temperature (CWE and WWE). Root mean squared deviation (RMSD) was calculated for each pair of measurements (Tpill vs Tre, Tpill vs Tes, Tre vs Tes) for each trial. An RMSD of "0" indicates perfect agreement; as RMSD increases, agreement worsens. On CWR, the RMSD for Tpill-Tes (0.23+/-0.04) was lower (P < 0.05) than for Tpill-Tre (0.43+/-0.10) or Tre-Tes (0.46+/-0.09). There were no significant differences in RMSD between measurement pairs on any other trial (average RMSD = 0.26 degrees C). Telemetry pill temperature and response time tended to be intermediate between Tre and Tes.

CONCLUSION

These results suggest the telemetry pill system provides a valid measurement of core temperature during conditions of decreasing as well as increasing body temperature and during steady state.

Exercise responses after altitude acclimatization are retained during reintroduction to altitude

Following 2 to 3 wk of altitude acclimatization, ventilation is increased and heart rate (HR), plasma volume (PV), and lactate accumulation ([La]) are decreased during submaximal exercise. The objective of this study was to determine whether some degree of these exercise responses associated with acclimatization would be retained upon reintroduction to altitude (RA) after 8 d at sea level (SL). Six male lowlanders (X +/- SE; 31 +/- 2 yr, 82.4 +/- 4.6 kg) exercised to exhaustion at the same relative percentages of peak oxygen uptake (VO2peak) at SL, on acute altitude (AA) exposure, after a 16-d chronic altitude (CA) exposure on Pikes Peak (4,300 m), and during a 3- to 4-h RA in a hypobaric chamber (4,300 m; 446 mm Hg) after 8 d at SL. The submaximal exercise to exhaustion time (min) was the same at SL (66.0 +/- 1.6), AA (67.7 +/- 7.3), CA (79.9 +/- 6.2), and RA (67.9 +/- 1.9). At 75% VO2peak: (1) arterial oxygen saturation (SaO2) increased from AA to CA (67.0 +/- 1.5 vs 78.5 +/- 1.8%; P < 0.05) and remained increased at RA (77.0 +/- 2.0%); (2) HR decreased from SL to CA (171 +/- 6 vs 152 +/- 9 beats x min-1; P < 0.05) and remained decreased at RA (157 +/- 5 beats x min-1); (3) calculated PV decreased 6.9 +/- 10.0% at AA, 21.3 +/- 11.1% at CA, and 16.7 +/- 5.4% at RA from SL baseline values, and (4) [La] decreased from AA to CA (5.1 +/- 0.9 vs 1.9 +/- 0.4 mmol x L-1; P < 0.05) and remained decreased at RA (2.6 +/- 0.6 mmol x L-1). Upon RA after 8 d at SL, the acclimatization responses were retained 92 +/- 9% for SaO2, 74 +/- 8% for PV, and 58 +/- 3% for [La] at 75% VO2peak. In conclusion, although submaximal exercise to exhaustion time is not improved upon reintroduction to altitude after 8 d at sea level, retention of beneficial exercise responses associated with altitude acclimatization is likely in individuals whose work, athletic competition, or recreation schedules involve intermittent sojourns to high elevations.