Impact of Prolonged Exercise in the Heat and Carbohydrate Supplementation on Performance of a Virtual Environment Task
By Bailey, Stephen P Holt, Craig; Pfluger, Kent C; Budde, Zina La; Afergan, Daniel; Stripling, Roy; Miller, Paul C; Hall, Eric E
ABSTRACT Purpose: The purpose of this investigation was to determine whether performance of a virtual environment (VE) task is influenced by exercise in the heat and carbohydrate supplementation. Methods: Ten males completed four exercise trials to fatigue. During each trial, subjects cycled at a submaximal workload. Subjects exercised in a normal environment (NORM) and in a hot environment on different occasions. During exercise, subjects drank 10 mL x kg^sup -1^ x hour^sup -1^ of body weight of a 6% carbohydrate beverage (CHO) or a placebo. Subjects completed a VE task before, during exercise, and after fatigue. Results: More failures occurred during placebo than CHO during exercise. The NORM CHO trial had the fewest failures at fatigue. More kills occurred during exercise in the NORM CHO. Conclusions: Performance of a VE task was negatively influenced by prolonged exercise and heat stress. CHO supplementation may have a positive impact on performance of the VE task following prolonged exercise. INTRODUCTION
Warfighters on the battlefield are typically exposed to a myriad of physical stressors including, but not limited to, extreme environments, heavy workloads, dehydration, and impaired nutritional status. Anecdotally, these stressors are believed to have a devastating impact on the ability of these individuals to act quickly and appropriately in a combat environment. Cognitive function and mood state have been shown to deteriorate in laboratory and field1,2 experiments designed to simulate these stressful conditions. For example, Lieberman et al.1 found decrements in visual vigilance, choice reaction time, short-term memory, and mood following an 84-hour simulated military sustained operation in a group of soldiers. Lieberman et al.2 had similar finding in soldiers who were part of an elite light infantry unit following 53 hours of simulated combat activities in the heat.
A limitation of these experiments has been that the tools used to evaluate cognitive function typically do not closely approximate the actual psychomotor activities that a warfighter must perform on the batdefield. The use of a virtual environment (VE) can be useful at mimicking these activities and provide a vehicle for quantitative evaluation of performance in a simulated battlefield setting.3 Unfortunately, there is limited empirical information describing how performance of a VE task is influenced by exposure to prolonged physical and environmental stress.
Decrements in cognitive function subsequent to combat training and prolonged military exercises have been associated with symptoms of hypoglycemia.4,5 Owen et al.4 found symptoms of hypoglycemia, precipitated as a result of 48 hours of military activity, were accompanied by reductions in memory recall and vigilance. Similar results have been described following prolonged exercise in a laboratory setting.6-8 This concept is demonstrated by Grego et al.6 who found blood glucose and cognitive processing speed to be reduced in trained cyclists during 3 hours of cycling at 66% of VO^sub 2max^. In contrast, 6% carbohydrate beverage (CHO) supplementation has been shown to attenuate these impairments in cognitive function following prolonged exercise stress.8,9 For example, Welsh et al.9 were able to prevent the decrements in performance of the Stroop Color-Word test precipitated by 1 hour of intermittent high- intensity exercise by providing subjects with regular doses (5 mL x kg^sup -1^ every 30 minutes) of a CHO drink.
Cognitive function is also negatively affected by heat stress.10,11 For example, Radakovic et al.10 found that exposure to a hot environment (40[degrees]C) at rest for 90 minutes resulted in deficits in attention during a computer-based cognitive task. Interestingly, it appears that the negative effects of heat stress on cognitive function are more typically associated with more complex executive tasks12 and these deficits can be attenuated with acclimatization and fluid replacement.13 The purpose of this investigation was to examine the influence of heat and CHO supplementation on VE performance.
METHODS
Subjects
Ten moderately fit males (Table I) served as subjects in this investigation. All subjects were recent (within 5 years) graduates of the U.S. Naval Academy and signed an approved informed consent form before participation. Subjects consisted of temporary assigned duty ensigns and second lieutenants (awaiting departure to flight school, dive school, or The Basic School for the U.S. Marine Corps) and fleet lieutenants assigned to the Naval Academy as instructors. Subjects were excluded from participation if they had an orthopedic or other medical condition that prevented their participation.
Experimental Design
To participate in this investigation, subjects were required to complete a maximal exercise test and four experimental trials. Experimental trials were separated by at least 7 days and the order of experimental treatments for each subject was varied using a Latin square design. All experimental trials were initiated between 7:00 and 7:30 a.m. Before each experimental trial, subjects were required to refrain from consumption of caffeine, alcohol, over-the-counter medications, and any nutritional supplements for 24 hours and from eating for 12 hours.
Maximal Exercise Test
Subjects completed the maximal exercise test on a Lode Excalibur Sport (Lode Medical Technology, Groningen, The Netherlands). Before beginning the test, subjects warmed up for 10 minutes at 50 W. Upon initiation of the test, workload (W) was increased to 100 W and increased 25 W every minute until the subject was unable to maintain revolutions per minute greater than 65. During the test, expired gases were continuously collected and analyzed using a True One 2400 Metabolic Measurement system (ParvoMedics, Inc., Sandy, Utah). Although data were collected continuously, measures were averaged over 15-second intervals to determine peak oxygen consumption (VO^sub 2^ peak) and other physiologic measures used in subsequent analyses. To calculate VO^sub 2^ peak, the four greatest adjacent VO^sub 2^ values were averaged.
After completion of the test, the level of oxygen consumption and heart rate associated with the ventilatory threshold (T^sub vent^) were determined by computer analysis (True One Software; ParvoMedics, Inc., Sandy, Utah). This was done by plotting ventilation (V^sub E^) versus oxygen consumption (VO^sub 2^) and the point where V^sub E^ increased disproportionately to an increase in VO^sub 2^ was identified as T^sub vent^. The heart rate associated with T^sub vent^ was then identified by plotting heart rate versus VO^sub 2^ and determining the heart rate associated with the VO^sub 2^ at which T^sub vent^ occurred. Visual inspection of the data by an experienced researcher (completed more than 500 maximal exercise tests used to determine T^sub vent^) was used to confirm the physiologic measures associated with T^sub vent^.
Experimental Trials
During each experimental trial, subjects exercised to volitional fatigue at a workload equivalent to 80% of ventilatory threshold (T^sub vent^) as determined during the maximal exercise test. Exercise during experimental trials was completed on a Monark 818e cycle ergometer (Monark Exercise AB, Vansbro, Sweden). The desired workload (W) was created by having the subjects cycle at 80 revolutions per minute and manually manipulating the resistance on the flywheel. During two of the experimental trials, subjects exercised in a normal environment (NORM; ~22[degrees]C and 50% humidity). During the other two experimental trials, subjects exercised in a hot environment (hot; ~35[degrees]C and 70% humidity). Every 30 minutes during exercise, subjects drank 5 mL x kg^sup -1^ of body weight of either a CHO (18 mEq x L^sup -1^ of sodium and 3 mEq x L^sup -1^ of potassium) or a water placebo (PLAC) indistinguishable in color and taste. Drinks were administered to subjects in a double-blind fashion (i.e., both the subject and all researchers were blinded to the contents of the drink). Drinks were provided by the Gatorade Sports Science Institute (Barrington, Illinois). Weights of the subjects were recorded immediately before and after completion of each experimental trial.
Physiologic Measures
Heart rate was recorded at rest, every 30 minutes during exercise, and at fatigue using a Polar Heart Rate monitor (Polar Electro, Inc., Lake Success, New York). Core temperature (T^sub core^) was monitored continuously and recorded at rest, every 30 minutes during exercise, and at fatigue using a CorTemp Ingestible Core Body Temperature Sensor and a CorTemp Data Recorder (HQ, Inc., Palmetto, Florida). To measure Tcore during experimental trials, subjects were required to ingest the CorTemp Ingestible Core Body Temperature Sensor just before sleeping the night before the experimental trial. Each individual sensor is calibrated by the manufacturer to +-0.01[degrees]C in temperatures ranging from 20 to 45[degrees]C before shipment. This system has been shown to be highly correlated (r = 0.98) to rectal temperature during exercise in the heat.14
VE Task
Subjects completed a VE task at rest, after 60 minutes of exercise, and immediately after fatigue. The VE task was completed on a desktop personal computer in a room where the environment was consistent with the NORM environmental condition. The only visual and auditory stimuli available to the subjects during the VE task was that provided by the VE task. Subjects interacted with the VE task using a standard computer keyboard and a mouse. The VE task used in this experiment was an adapted version of Virtual Battlefield Systems (Coalescent Technologies Corporation, Orlando, Florida). Before completion of all experimental trials, subjects received 1 hour of training and practice with the VE task. During this training, subjects were exposed to several scenarios similar to that completed during the experimental trials. The VE task was 10 minutes in length and was set in an urban warfare setting. Performance in the VE task was evaluated by four blinded evaluators. The number of failures (being killed by combatant forces) and number of kills (elimination of enemy combatants) during the VE task were quantitatively recorded. The ability of subjects to complete their mission, effectively use their resources, and move safely and effectively were qualitatively evaluated by each evaluator using a 1 (extremely poor) to 10 (excellent) scale.
Evaluators were trained to evaluate and score the VE task by reviewing five “example” trials. During this training period, inter- rater reliability for the quantitative measures (failures and kills) was very high (r^sup 2^ >/= 0.96). In comparison, inter-rater reliability for the qualitative measures was also strong (r^sup 2^ >/ = 0.74). Intrarater reliability was also established by having reviewers re-evaluate a subset of trials (10 each). Intrarater reliability for the quantitative measures (failures and kills) was very high (r^sup 2^ >/= 0.99). Intrarater reliability for the qualitative measures was also strong (r^sup 2^ >/= 0.86).
Volitional Fatigue
For this investigation, volitional fatigue was defined as the point in time subjects were unable to maintain a workload within 10% of desired workload continuously for 1 minute. Subjects were verbally informed when they were not exercising at the desired workload. If they were not able to return to the desired workload after 30 seconds, they were verbally informed that the trial would be terminated in 30 seconds if they did not return to the desired workload. If subjects did not return to the desired workload within another 30 seconds, the trial was terminated and total exercise time was recorded. Subjects were not allowed any extraneous visual (television) or auditory (music) stimuli at any point during the experimental trials. Furthermore, verbal encouragement was not provided by researchers during any of the experimental trials. Subjects were not aware of the elapsed time during experimental trials.
Data Analyses
All data presented in tables and figures are presented as mean +- SE. Differences in dependent measures across treatments and time were analyzed using a three-way (environment x drink x time) multiple analysis of variance. Significance was set a priori at the p < 0.05 level. When appropriate, the Newman-Keuls post-hoc procedure was used to identify specific differences between cell means.
RESULTS
Time to Fatigue
Exercise time to fatigue was impacted by both CHO supplementation p = 0.043) and heat stress (p = 0.013) (Fig. 1); however, there was not an interaction (environment x drink) effect observed (p = 0.27). Exercise time to fatigue was greatest during the NORM CHO (169 +- 23 minutes) condition, followed by the NORM PLAC (128 +- 19 minutes), HOT CHO (110 +- 9 minutes), and HOT PLAC (101 +- 10 minutes) conditions, respectively.
Physiologic Measures
Heart rate was greater during the HOT conditions as compared to NORM conditions (p < 0.0001); however, a drink x environment interaction was not observed (p = 0.81). Specifically, heart rate was greater (p < 0.05) during exercise in HOT conditions as compared to NORM conditions after 60 minutes of exercise (NORM PLAC = 136 +- 4 beats per minute (bpm); NORM CHO = 129 +- 4 bpm; HOT PLAC = 154 +- 4 bpm; HOT CHO = 157 +- 4 bpm) (p = 0.0001) and at fatigue (NORM PLAC = 145 +- 4 bpm; NORM CHO = 143 +- 3 bpm; HOT PLAC = 157 +- 6 bpm; HOT CHO = 159 +- 4 bpm) (p = 0.010). No differences were observed between drink conditions at any time point.
Core temperature was greater during the HOT conditions as compared to NORM conditions (p < 0.0001); however, a drink x environment interaction was not observed (p = 0.07). Specifically, core temperature was greater during exercise in HOT conditions as compared to NORM conditions after 60 minutes of exercise (NORM PLAC = 37.91 +- 0.14[degrees]C; NORM CHO = 37.50 +- 0.14[degrees]C; HOT PLAC = 38.18 +- 0.07[degrees]C; HOT CHO = 38.33 +- 0.17[degrees]C) (p = 0.003) and at fatigue (NORM PLAC = 37.55 +- 0.22[degrees]C; NORM CHO = 37.95 +- 0.16[degrees]C; HOT PLAC = 38.52 +- 0.22[degrees]C; HOT CHO = 38.59 +- 0.29[degrees]C) (p = 0.032). No differences were observed between drink conditions at any time point.
Body weight decreased more during the HOT (HOT PLAC = 1.4 +- 0.3%, HOT CHO = 1.7 +- 0.4%) trials as compared to the NORM (NORM PLAC = 1.0 +- 0.2%, NORM CHO = 0.9 +- 0.2%) trials (p = 0.015); however, no differences were observed between the drink conditions.
Performance in the VE
Performance in the VE was evaluated quantitatively by recording the number of “kills” and the number of “failures” while completing the task over a 10-minute period. Performance was also evaluated qualitatively, for their ability to complete their mission, effectively use their resources, and move safely and effectively.
No differences were observed across drink or environmental conditions in the number of failures during the VE task; however, there was a significant drink x environment interaction (p = 0.023). More failures were observed during the PLAC condition than the CHO condition after 60 minutes of exercise (p = 0.041) (Fig. 2). Furthermore, the NORM CHO (2.1 +- 0.5) trial had the fewest number of failures at fatigue as compared to the other three conditions (NORM PLAC = 4.2 +- 0.5; HOT PLAC = 3.9 +- 0.3; HOT CHO = 4.2 +- 0.4) (Fig. 2).
Similarly, no differences were observed across drink or environmental conditions in the number of kills during the VE task. There was a significant drink x environment interaction (p = 0.034). Specifically, more kills (p = 0.015) occurred after 60 minutes of exercise in the NORM CHO (10.3 +- 1.2) trial as compared to the other three conditions (NORM PLAC = 5.3 +- 1.2; HOT PLAC = 6.2 +- 0.7; HOT CHO = 3.7 +- 0.8) (Fig. 3). No differences between groups were observed at any other time point.
No differences were observed between any of the conditions at any time point for completion of mission (Fig. 4), strategic movement (Fig. 5), or use of resources (Fig. 6).
DISCUSSION
The results of this investigation found that exercise time to fatigue was increased by CHO supplementation and reduced by exposure to heat stress. Furthermore, quantitative performance of the VE task, during and after cessation of exercise, in an urban warfare setting was positively impacted by CHO supplementation.
Exercise time to fatigue at 80% T^sub vent^ was increased 32% in the normal environment. These findings are consistent with findings of other investigations.15,16 By providing the body with a steady flow of exogenous CHO during prolonged exercise, CHO supplementation has been theorized to improve exercise performance by maintaining blood glucose, sparing endogenous glycogen, and influencing central nervous system function. The evidence that the use of CHO supplementation strategies similar to that used in this investigation are effective at maintaining euglycemia during prolonged exercise is strong.
CHO supplementation during prolonged exercise has been shown to “spare” Uver glycogen”; however, the impact of CHO supplementation on the breakdown of muscle glycogen remains unclear.15 CHO supplementation during moderate intensity exercise has repeatedly been shown to prevent the drop in blood glucose that is typically seen after 1 hour or more of continuous exercise when a water placebo is consumed.18,19
CHO supplementation may also improve physical performance through central mechanisms. Evidence suggesting that CHO supplementation influences physical performance through central mechanisms is provided by three different types of experiments. CHO feeding during relatively short (60 minutes or less) high-intensity exercise (>75% of VO^sub 2max^) consistently has been shown to have an ergogenic effect despite the fact that it is extraordinarily unlikely that an exogenous CHO source could influence energy production within the muscle.20 Furthermore, a unique experiment by Carter et al.21 found that exercise performance of a task approximately 60 minutes in length was slightly (2.8%), but significantly improved when subjects simply rinsed their mouth with a CHO solution. CHO supplementation during prolonged exercise has also been shown to indirectly influence the availability of various amino acids to brain, ultimately impacting production of various neurotransmitters that could influence exercise performance.22,23
Improvement of exercise performance in the heat subsequent to CHO feeding is theorized to result from attenuation of water loss and improved availability of glucose for energy production. Fluid loss equivalent to 2% of body weight has consistently been shown to negatively influence exercise performance.24 In this investigation, changes in body weight were greater during the hot trials than during the normal environment trials; however, there were no differences between drink conditions. Furthermore, the decreases in body weight were consistently less than 2% during all conditions. Similar findings were found for heart rate and core temperature. Both of these variables were negatively affected by exercise in the heat; however, heart rate never exceeded 80% of maximal heart rate and core temperature never approached 39[degrees]C. All of these physiologic findings suggest that it is unlikely that dehydration played a significant role in the changes in performance observed here. Quantitative performance during the VE task was best when subjects consumed CHO during the normal environment. This is demonstrated by the greater number for kills and lower number of failures seen under these conditions as compared to the other conditions (Figs. 2 and 3). CHO supplementation during exercise in the hot environment also resulted in a fewer number of failures than when the subject consumed the water placebo (Fig. 2). Considering the lack of differences observed in the physiologic variables (body weight, heart rate, and core temperature) it is not likely that the positive influence of CHO feedings was the result of differences in hydration status. Rather, it is much more likely that these differences are subsequent to differences in glucose availability.
As mentioned previously, the CHO feeding regime used in this investigation has been shown to maintain blood glucose levels during exercise in NORM and HOT environments.19,25 Decrements in cognitive function following strenuous military field exercises and prolonged physical activity in a laboratory setting have previously been found to be associated with decreases in glucose availability. Furthermore, CHO feeding during prolonged exercise has been shown to be an effective strategy for attenuating these negative changes in cognitive function. Although assessment of cognitive function during these military exercises has used procedures that approximate function during combat (complex decision making, marksmanship, etc.)1,10 this is the first investigation where changes in performance in a virtual warfare environment are negatively affected by prolonged exercise and heat stress. Furthermore, the use of CHO feedings during these stressors was effective at attenuating these negative changes.
A significant limitation of this investigation, is thie inability to more completely assess performance in the VE. We attempted to more fully understand this phenomenon by using blinded evaluators to qualitatively assess performance for three important areas (completion of mission, strategic movement, and use of resources). Unfortunately, these analyses provided no useful insight and do not appear useful for future analyses. Current advances in VE technology already allow for more sophisticated quantitative analyses of these types of missions and should be applied in future experiments that use a similar design. Furthermore, it seems prudent that evaluating cognitive function separately, but nearly simultaneously to completion of the VE task would provide valuable insight as to “cognitive” mechanisms that underlie changes in VE performance.
In summary, the results of this investigation demonstrate that the performance of a VE urban warfare task are negatively affected by prolonged exercise and heat stress. These negative changes appear to be attenuated by regimented CHO feedings, suggesting that availability of a CHO/electtolyte beverage during environmentally stressful combat environments may be a useful strategy for optimizing performance.
ACKNOWLEDGMENT
This work was supported by the Office of Naval Research Sabbatical Leave Program and the Gatorade Sports Science Institute.
REFERENCES
1. Lieberman HR, Niro P, Tharion WJ, Nindl BC, Castellani JW, Montato S: Cognition during sustained operations: comparison of a laboratory simulation to field studies. Aviat Space Environ Med 2006; 77: 929-35.
2. Lieberman HR, Bathalon GP, Falco CM, Kramer M, Morgan CA, Niro P: Severe decrements in cognition function and mood induced by sleep loss, heat, dehydration, and undernutrition during simulated combat. Biol Psychiatry 2005; 57: 422-9.
3. Morrison P, Barlow M, Bethel G, Clothier S: Proficient soldier to skilled gamer: training for COTS success. Proceedings of SimTecT 2005, May 9-12, 2005. Sydney, Australia.
4. Owen G, Turley H, Casey A: The role of blood glucose availability and fatigue in the development of cognitive impairment during combat training. Aviat Space Environ Med 2004; 75: 240-6.
5. Lieberman HR, Falco CM, Slade SS: Carbohydrate administration during a day of sustained aerobic activity improves vigilance, assessed with a novel ambulatory monitoring device, and mood. Am J Clin Nutr 2002; 76: 120-7.
6. Grego F, Vallier JM, Collardeau M, et al: Effects of long duration exercise on cognitive function, blood glucose, and counterregulatory hormones in male cyclists. Neurosci Lett 2004; 364: 76-80.
7. Grego F, Vallier JM, Collardeau M, Rousseu C, Cremieux J, Brisswalter J: Influence of exercise duration and hydration status on cognitive function during prolonged cycling exercise. Int J Sports Med 2005; 26: 27-33.
8. Winnick JJ, Davis JM, Welsh RS, Carmichael MD, Murphy EA, Blackmon JA: Carbohydrate feedings during team sport exercise preserve physical and CNS function. Med Sci Sports Exerc 2005; 37: 306-15.
9. Welsh RS, Davis JM, Burke JR, Williams HG: Carbohydrates and physical/mental performance during intermittent exercise to fatigue. Med Sci Sports Exerc 2002; 34: 723-31.
10. Radakovic SS, Marie J, Surbatovic M, et al: Effects of acclimation on cognitive performance in soldiers during exertional heat stress. Milit Med 2007; 172: 133-6.
11. McMorris T, Swain J, Smith M, et al: Heat stress, plasma concentration of adrenaline, noradrenaline, 5-hydroxytryptamine and Cortisol, mood state and cognitive performance. Int J Psychophysiol 2006; 61: 204-15.
12. Cian C, Barraud PA, Melin B, Raphel C: Effects of fluid ingestion on cognitive function after heat stress or exercise- induced dehydration. Int J Psychophysiol 2001; 42: 243-51.
13. Cheuvront SN, Carter R, Kolka MA, Lieberman HR, Kellog MD, Swaka MN: Branched-chain amino acid supplementation and human performance when hypohydrated in heat. J Appi Physiol 2004; 97: 1275- 82.
14. Easton C, Fudge BW, Pitsiladis YP: Rectal, telemetry pill and tympanic membrane thermometry during exercise heat stress. J Therm Biol 2007; 32: 78-86.
15. Jeukendrup AE: Carbohydrate intake during exercise and performance. Nutrition 2004; 20: 669-77.
16. Coyle EF: Fluid and fuel intake during exercise. J Sports Sci 2004; 22: 39-55.
17. Jeukendrup AE, Raben A, Gijsen A, et al: Glucose kinetics during prolonged exercise in highly trained human subjects: effects of glucose ingestion. J Physiol (Lond) 1999; 51: 579.
18. Coyle EF, Coggan AR, Hemmert MK, Ivy JL: Muscle glycogen utilization during prolonged exercise when fed carbohydrate. J Appi Physiol 1986; 61: 165.
19. Davis JM, Bailey SP, Woods JA, Galiano FJ, Hamilton MT, Bartoli WP: Effects of carbohydrate feedings on plasma free tryptophan and branched-chain amino acids during prolonged cycling. Eur J Appl Physiol 1992; 65: 513-9.
20. Carter JM, Jeukendrup AE, Mundel T, Jones DA: Carbohydrate supplementation improves moderate and high-intensity exercise in the heat. Pflugers Arch 2003; 446: 211-9.
21. Carter JM, Jeukendrup AE, Jones DA: The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med Sci Sports Exerc 2004; 36:2107-11.
22. Meeusen R, Watson P, Dvorak J: The brain and fatigue: new opportunities for nutritional interventions. J Sports Sci 2006; 24: 773-82.
23. Meeusen R, Watson P, Hasegawa H, Roelands B, Piacentini MF: Central fatigue: the serotonin hypothesis and beyond. Sports Med 2006; 36: 881-909.
24. Maughan RJ: Impact of mild dehydration on wellness and on exercise performance. Eur J Clin Nutr 2003; 57(Suppl 2): S19-23.
25. Davis JM, Burgess WA, Slentz CA, Bartoli WP, Pate RR: Effects of ingestion 6% and 12% glucose/electrolyte beverage during prolonged cycling in the heat. Eur J Appi Physiol Occup Physiol 1988; 57: 563-9.
Stephen P. Bailey, PhD*; Craig Holt, MS[dagger]; Maj Kent C. Pfluger, USMCR[double dagger] Zina La Budde, BS[section]; Daniel Afergan, BS[para]; Roy Stripling, PhD[section]; Paul C. Miller, PhD*; Eric E. Hall, PhD*
* Elon University, Elon, NC 27244.
[dagger] U.S. Naval Academy, Annapolis, MD 21402.
[double dagger] Trestles, Inc., Woodbury, MN 55129.
[section] U.S. Naval Research Laboratory, Washington, DC 20375.
[para] Strategic Analysis, Inc., Arlington, VA 22201.
This manuscript was received for review in May 2007. The revised manuscript was accepted for publication in November 2007.
Reprint & Copyright (c) by Association of Military Surgeons of U.S., 2008.
Copyright Association of Military Surgeons of the United States Feb 2008
(c) 2008 Military Medicine. Provided by ProQuest Information and Learning. All rights Reserved.