The United States Marine Corps (USMC) has a rigorous field training exercise (FEX) at the end of their basic recruit training course called the Crucible. The Crucible is 54 h in duration and imposes multiple stressors on trainees to simulate the intense physical and cognitive strain of sustained combat operations. During the Crucible, recruits work on a variety of physical and mental challenges for extended time periods, receive little sleep, are on restricted rations, and are subjected to environmental stress and emotional strain. The Crucible also requires that women complete the same physically demanding tasks as men. The ability to characterize men and women engaged in a variety of activities over several days in an outdoor training environment provides a unique opportunity to further understand the interaction of energy expenditure, gender differences, and environmental stress. In previous short-term (≤ 7 d) studies of combat training similar to the Crucible, total energy expenditures of 17-28 MJ·d−1 (~4200-6700 kcal·d−1) (7,12) were observed. However, these studies have only examined the energy expenditure of men. Whether women doing the same work tasks as men have the same absolute and/or normalized energy expenditures is unclear, especially when food intake is limited. Some studies of athletes and climbers report data where men have a higher normalized total energy expenditure than women (1,13,28) when expressed relative to basal metabolism or body mass, although the volunteers appear to be performing the same tasks. These studies used small sample sizes and whether the men and women actually completed the same activities is unclear.
Combined stressors such as exertional fatigue, sleep loss, and negative energy balance increase the risk for hypothermia (30). Based on these findings, the Marine Corps Recruit Depot (MCRD) at Parris Island, SC was concerned that recruits going through the Crucible would have an increased susceptibility to hypothermia during winter training, even though risk management strategies such as rewarming tents, exchanging wet clothes for dry, and providing hot beverages were being used. The MCRD also was concerned that the restriction of rations might cause excessive energy deficits among the recruits, but no quantitative data on energy expenditure of recruits participating in the Crucible existed.
The Crucible, in many respects, is similar to adventure sporting events. These events are multiday, ultraendurance races (e.g., Eco-Challenge, Iditasport, Southern Traverse) where male and female participants compete in a combination of different sports over several days with little rest in varied environmental conditions. Race strategy typically motivates participants to continually exercise with very little sleep, likely resulting in large daily total energy expenditures. The information from the Crucible may provide insight into the physiological consequences of adventure racing in both men and women.
The objectives of this study of the 54-h Crucible field exercise were to: 1) compare the TEE of men and women, and 2) characterize core temperatures. It was hypothesized that TEE would be > 20 MJ·d−1 due to the high physical demands of the FEX, but that normalized TEE would be similar between men and women. We also hypothesized that core temperatures would remain above 35°C throughout the Crucible due to the anticipated work rates, access to clothing, and adherence to sound thermal risk management strategies.
A total of 50 U.S. Marine recruits (30 men, 20 women) volunteered for this study, which was completed during two separate winter FEX iterations. One iteration was completed in January (N = 25) and the other in February (N = 25) at the Marine Corps Recruit Depot, Parris Island, SC. The study was approved by the institutional review boards of the U.S. Army Research Institute of Environmental Medicine and the U.S. Army Medical Research and Materiel Command. The volunteers who participated gave free and informed written consent. Subject characteristics for the combined iterations are presented in Table 1.
Nude weight was measured before and after completing the FEX. The total weight of the fully dressed and equipped volunteers (uniform, weapon, helmet, and backpack) was measured at the start of the FEX. Pre-FEX percent body fat was calculated using the equation of Durnin and Womersley (6) from measurements made at four skinfold sites (biceps, triceps, subscapular, suprailiac).
For both study iterations, the FEX began at 0200 on Thursday morning and lasted until 0800 on Saturday morning (54 h). During the FEX, recruits completed numerous military tasks for 20 h each day. Tasks included physically demanding activities such as marching, often with 30-kg loads, to different points (10-km hike out to site in 2 h; 10-km night hike for 2 h; 15-km hike in 3.5 h to finish training exercise), a medical litter carry, infiltration of an enemy position during the day and night (1 h, 200 m of low crawl, sprinting, and wall climbing), and traversing a resupply course (1 h, low crawl, wall climbing, dragging two 23-kg ammunition boxes while under enemy fire). Additionally, somewhat less strenuous tasks were also completed including a leadership course, group problem solving, discussion of Marine Corps values, and weapons cleaning. If recruits became wet during a task, they followed standard procedures to change into dry clothes immediately after task completion. Recruits were allowed to sleep for about 4 h each night in unheated shelters. At the start of the FEX, the recruits were grouped into pairs, with each pair receiving five meals of ready-to-eat (MRE) rations (~27.7 MJ total) to divide and consume ad libitum throughout the FEX. Water was freely available and recruits were encouraged to drink. Warm carbohydrate-electrolyte beverages were also freely available for consumption. Recruits always wore a battle dress uniform with boots (insulation ≈ 1.3 clo). Other clothing items worn included Gore-Tex® jackets and pants, knit hats, and leather gloves with wool inserts. Recruits were free to choose which of these extra clothing items they wore.
Core temperature measurement.
Core temperature (Tpill) was measured by an FDA-certified ingestible temperature telemetry sensor (CorTemp™, Human Technologies Inc., St. Petersburg, FL) that transmitted a 260-kHz signal to a body core temperature monitor (BCTM, Personal Electronic Devices, Inc., Wellesley, MA) worn by each volunteer. The validity of the pill/BCTM system for core temperature measurement has been confirmed in both resting and exercising subjects in hot (15,16) and cold (16) environments.
Energy expenditure/energy intake measurements.
Total energy expenditure (TEE) was assessed by the doubly labeled water technique (5). Between 1300 and 1600 h on the Wednesday before the FEX, subjects provided a baseline urine sample and then ingested standard doses of 99.9% atom percent excess (APE) deuterium oxide (2H2O; men, 9 g; women, 8 g) and 10% APE H218O (men, 115 g; women, 90 g), weighed to a precision of 0.01 g. Standard doses were used, rather than doses weighed to individual body weight, due to logistical constraints. Urine was then collected at 0200 h on day 1 (Thursday, 10-13 h postdose), 2300 h on day 1 (31-34 h postdose), 2300 h on day 2 (Friday, 55-58 h postdose), and at 0830 h on day 3 (Saturday, 64.5-67.5 h postdose) following completion of the 54-h training period. Total energy expenditure was measured over the 54-h time period from 0200 h on day 1 until 0830 h on day 3. Physical activity level (PAL) was calculated as TEE/basal metabolic rate (BMR) where BMR was estimated from Harris and Benedict (9). PAL quantifies physical activity by expressing the average daily metabolic rate as a multiple of basal metabolism (27).
The 18O isotope abundances were measured on a Finnigan DeltaS gas-inlet Isotope Ratio Mass Spectrometer with a CO2-water equilibration device (5). Urine and saliva samples were equilibrated with CO2 at 18°C in a shaking water bath for at least 8 h. The CO2 was then cyrogenically purified under vacuum before introduction into the mass spectrometer. The hydrogen isotope abundances were measured on a Finnigan MAT 252 gas-inlet Isotope Ratio Mass Spectrometer, as previously described (5). Urine and saliva samples were distilled under vacuum into Vycor tubes containing zinc reagent (Friends of Biogeochemistry, Bloomington, Indiana). The reduction tube was sealed with a flame and placed in a 500°C oven for 30 min to reduce the water to hydrogen gas, which was then introduced into the mass spectrometer.
The 2H and 18O isotope elimination rates (kH and kO) were calculated by linear regression using the isotopic enrichments in the urine samples collected during the study. The rate of CO2 production is calculated using the equations of Schoeller et al. (20) as recently modified (17) as follows:
where N is the total body water calculated from the 18O enrichment, and rH2Of is the rate of fractionated evaporative water loss, which is estimated to be 1.05N(1.007kO − 1.041 kH). Energy expenditure was calculated by multiplying rCO2 by the energy equivalent of CO2 calculated from the food quotient of the food consumed and estimated changes in body energy stores during thestudy, as previously described (5). Because total body water was not measured at the end of training, N was estimated by taking the initial body weight and subtracting the energy deficit (estimated from TEE and measured energy intake) to yield an estimated total body water loss. The estimated total body water loss was then halved, and this value was subtracted from the initial measured total body water to give the value of N that was utilized to estimate TEE throughout the 54-h period.
Energy intake was indirectly estimated by collecting the wrappers from the individually wrapped ration items consumed by each individual and summing the published kilocalorie content of each food item, under the assumption that the entire item had been consumed. That value was added to the energy intake attributable to the carbohydrate sports beverages as estimated from individual written records of water and sport beverages consumed throughout the FEX.
Weather data collection.
Ambient temperature, relative humidity, and wind speed during the FEX were measured every 15 min, using two portable weather stations (21X Datalogger, Campbell Scientific, Inc., Logan UT).
Total energy expenditure (TEE) was successfully measured in 49 out of 50 subjects, and energy intake in 44 of 50 subjects. Energy deficits were calculated by subtracting TEE from energy intake. Differences in TEE between genders were determined with independent t-tests. Pearson product correlations were determined between TEE and different body composition factors suggested to be related to TEE. A forward stepwise regression was performed on these variables. Linear regression was used to determine best-fit prediction lines between TEE and body mass and TEE and fat-free mass. An adjusted fat-free mass and body mass was determined by extending the regression line observed for TEE/fat-free mass and TEE/body mass until the x-intercept for TEE was equal to 0 (18). The value of the fat-free mass and body mass (negative value) where TEE = 0 was then added to each individual's actual fat-free mass and body mass value and then divided into TEE to determine a TEE/corrected fat-free mass and TEE/corrected body mass value. As pointed out by Ravussin and Bogardus (21), this correction is needed because a mathematical bias exists when dividing energy expenditure by fat-free mass. That is, metabolic activity would be underestimated in people with low fat-free mass and overestimated in those with high fat-free mass (21). Independent t-tests were used to compare between genders for the corrected TEE value. Tpill was obtained in 14 men and 12 women over both FEX iterations and compared using t-tests. Data are presented as mean ± SD and the level to achieve statistical significance was P < 0.05.
The average environmental conditions for the January test were: ambient temperature (Tamb), 10.8 ± 3.8°C (range 3.6-18.8°C); relative humidity (RH), 70 ± 16%; wind speed, 1.6 ± 0.7 m·s−1; and no measurable rain. For the February test, the average environmental conditions were: Tamb, 13.9 ± 4.1°C (range 6.3-21.4°C); RH, 72 ± 21%; wind speed, 1.6 ± 0.7 m·s−1; and no rain. These conditions were similar to the typical low temperatures in the Beaufort, SC area for January (3.8°C) and February (5.6°C). The high temperatures during both FEX iterations were 4-5°C higher than the average seasonal highs in January (14.4°C) and February (16.7°C).
Total energy expenditure (TEE), PAL, and daily energy intake are shown in Table 2. The women expended less energy than the men, but when TEE was expressed relative to body mass and fat-free mass, there were no differences between women and men. Likewise, PAL was not different between genders. Dietary energy intake was 1.2 MJ·d−1 lower (P < 0.05) in women compared with the men. Carbohydrates, fats, and proteins, respectively, accounted for 50 ± 6, 37 ± 6, and 14 ± 3% of energy intake for the women and 53 ± 7, 36 ± 5, and 13 ± 3% for the men (P > 0.05). Total mean energy deficits over 54 h for men and women were −43.2 ± 10.4 and −34.0 ± 6.0 MJ, respectively (P < 0.002). Figure 1 depicts the relationship between TEE and energy intake, with the line of identity representing energy balance. Clearly, the subjects were in an energy deficit. Body mass loss over 54 h was −3.1 ± 0.8 and −1.6 ± 0.5 kg for the men and women, espectively (P < 0.05).
Correlation coefficients were computed between TEE and body composition variables. Data were pooled between men and women because there were no differences between genders. The following relationships were found: fat-free mass (r = 0.71, P < 0.001), body mass (r = 0.66, P < 0.001), height (r = 0.64, P < 0.001), % body fat (r = −0.44, P < 0.003), body mass index (r = 0.43, P < 0.003), and fat mass (r = −0.09, P = 0.60). A forward-stepwise regression analysis among TEE and the selected variables was done. Only fat-free mass significantly contributed to predicting TEE over the 54-h period. Figure 2 presents the regression equation computed for the relationships between daily TEE, body mass, and fat-free mass. For the TEE/body mass and TEE/FFM relationships, the x-intercept for TEE = 0 was determined to be −15.6 and −22.5 kg, respectively. A mass-specific TEE was then determined by adding 15.6 and 22.5 kg to each subject's body mass and fat-free mass, and a value for TEE/corrected body mass and TEE/corrected fat free mass was computed to allow comparisons between genders (19). This analysis yielded no differences between men and women (Table 2).
The average low and high Tpill for the men and women are presented in Table 3. Tpill values did not differ between genders. There were no cases of clinically significant hypothermia (Tpill < 35.0°C) during either FEX. The lowest individual Tpill recorded was 35.28°C. The lowest Tpill occurred during sleep in 19 out of 26 individuals (73%) with 4 more individuals experiencing their low Tpill at night just before or after sleep. Figure 3 presents the overall group average Tpill during the two 4-h sleep periods for the two study iterations in January and February. There were no significant correlations between the low Tpill and either TEE or the calculated energy deficit.
The highest Tpill values recorded were 38.8 and 39.1°C for the January and February FEX, respectively. In 50% of the volunteers, the highest Tpill values occurred during a physically demanding infiltration course event. This event required the recruits to sprint, crawl, and climb over obstacles. There were no correlations between the extreme Tpill values and either TEE or calculated energy deficits.
Our primary findings were that despite sustaining substantial energy deficits (15-19 MJ·d−1), both men and women could maintain high energy expenditure and physical activity levels (3.4 × BMR) for 54 h. Furthermore, no differences existed between men and women when total energy expenditure was expressed relative to body weight or fat-free mass. Finally, there was no evidence of hypothermia in Marine Corps recruits in the winter months during the 54 h of exertional fatigue, sleep loss, and a negative energy balance.
The daily energy expenditures in the Crucible are some of the highest reported for military activities (25), although one limitation to our reported TEE results is that we did not obtain a second total body water measurement and estimated body water losses for use in the TEE calculations. We estimate that the volunteers may have lost approximately 0.5-1 L of total body water; this was accounted for in our TEE measure and by the fact that the potential error in estimated TEE was < 2%. In large part, the high TEE was caused by recruits remaining active for approximately 20 h·d−1. In addition, many of the activities were strenuous. Both the men and the women were able to sustain physical activity level (PAL) values of 3.4-3.5, well above what is considered the upper limit of sustainable metabolic rate for the general (nonmilitary) population (26). The PAL during the Crucible was 42% higher than that observed (2.5-2.8) in other military field training exercises (2,5,7,11,12,14). Hoyt et al. did find an average PAL of 4.0 during the initial 4 d of training in a cold, mountainous environment (11), but these values declined during subsequent days. The TEE of the Marines was also near the upper limit recorded (PAL = 4.0-5.0) of well-trained, elite athletes who remain in energy balance during the Tour de France and cross-country skiing training (22,29) and slightly lower compared with data from expeditions where (8,23,24) TEE was as high as 28-35 MJ·d−1 over a period of 2-7 wk.
Westerterp (27) suggested athletes can maintain high PAL because they remain in energy balance by consuming large amounts of food and using carbohydrate/energy rich drinks to supplement food intake. However, many military personnel often fail to match energy intake to energy expenditure during field training. First, there is little time to eat when soldiers work upwards of 20 h·d−1. Second, many training scenarios, such as the Crucible, intentionally restrict energy intake to simulate conditions likely to be encountered during combat. This might be expected to reduce the PAL level that could be sustained over several days. However, the Marines in this study were able to maintain high PAL, even though their energy intake was only 24% of that required to remain in energy balance.
This study provided a unique opportunity to examine the energy requirements and relationship between TEE and fat-free mass (FFM) of women and men who performed the same activities. Other studies of swimmers, mountain climbers, and other athletes have reported PAL that are 13-22% higher in men than in women when the subjects are apparently doing the same activities (1,13,28). These studies are difficult to interpret, however, because it is not known whether the men and women actually completed the same tasks or the same volume and intensity of training. Furthermore, the sample sizes are small (5-8 total subjects) and may not be representative of the population. In contrast, on an absolute basis, the men expended about 5.9 MJ more per day than the women, but TEE expressed relative to either body mass or fat-free mass was the same for men and women, as was the PAL. We found that fat-free mass had the highest correlation with TEE, with FFM accounting for 50% of the variance in TEE. Compared with other studies that had similar energy expenditures as our study, the correlation between TEE and FFM was higher during the Crucible compared with cold-weather training (r2 = 0.35, (12)), but less compared with work in a cold, hypoxic (r2 = 0.89, (11)) or hot, humid (r2 = 0.85, (7)) environment. Studies in very active women indicate that FFM accounts for 27-96% of the variance in groups as varied as wildland firefighters (r2 = 0.27, (19)), cross-country skiers (r2 = 0.44, (22)), elite female runners (r2 = 0.52, (21)), and trained cyclists (r2 = 0.96, (10)).
This study was conducted in January and February in anticipation of cold, rainy conditions that could increase the risk of hypothermia in physically fatigued, sleep-restricted subjects who are in a negative energy balance. However, the conditions turned out to be warmer and dryer than anticipated, reducing the likelihood of hypothermia. The majority of low core temperatures occurred just before, during, or after the 4-h scheduled period of inactivity (it was assumed that subjects slept during this time) reflecting the normal circadian variation in human core temperatures. However, Tpill values observed in some subjects (< 35.3°C) during sleep suggests the risk of hypothermia may be greater in the early morning just after the recruits awake, especially if more severe cold or rain conditions exist. Severe weather could also affect the core temperature during night activities when core temperatures normally decrease. Previous studies report a reduction in core temperature after strenuous training (30), an impaired shivering thermogenesis after 3.5 d of exertional fatigue, sleep deprivation, and negative energy balance (3), and greater heat losses after exertional fatigue (4). All of these factors could also contribute to an increased susceptibility to low core temperatures.
In conclusion, both men and women Marine Corps recruits sustained work for 54 h at physical activity levels of 3.3- to 3.5-fold basal levels while incurring large energy deficits and body weight losses of 3-4%. Men and women demonstrated similar total energy expenditures when corrected for body mass or fat-free mass. Despite the combination of fatigue and a negative energy balance for 2 d, hypothermia did not occur in the Marine test volunteers participating in the Crucible FEX at Parris Island, SC, where relatively mild ambient temperatures were prevalent. The lack of hypothermia was also attributable to the Marine Corps' standard operating procedures that were in place to mitigate the effects of the wet/cold weather conditions during the Crucible FEX (e.g., sleeping in sheltered areas, issuing winter clothing including gloves and hats, exchanging wet for dry clothing as needed, and providing a warming tent). Nevertheless, hypothermia remains a possibility during cold weather Crucible field exercises.
Portions of this study were funded by Defense Women's Health Research Program Grant (DAMD17-96-2-6025) awarded to Dr. DeLany. Dr. DeLany's current affiliation is the University of Pittsburgh Medical Center, Department of Endocrinology and Metabolism.
This study would not have been possible without the support of the chain of command at the Marine Corps Recruit Depot in Parris Island, SC. We are particularly indebted to Brigadier General James R. Battaglini and Colonel Michael Becker for their support to this research. Special thanks go to the U.S. Marines who participated in this study. The expert technical assistance of SSG James Moulton is gratefully acknowledged.
The views, opinions and/or findings in this report are those of the authors and should not be construed as official Department of the Army or Navy position, policy, or decision unless so designated by other official designation. Human subjects participated in these studies after giving their free and informed voluntary consent. Investigators adhered to AR 70-25 and USMRMC Regulation 70-25 on Use of Volunteers in Research.
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Keywords:©2006The American College of Sports Medicine
DEUTERIUM OXIDE; FATIGUE; MILITARY PERSONNEL; TELEMETRY