Heat Adaptation for Today’s Battlefi eld

On today’s battlefield, soldiers frequently endure intense heat typical of the desert environments in the Middle East where temperatures routinely exceed 120oFahr-enheit. This extreme environment requires soldiers to endure intense daily heat while conducting dismounted patrols of 6 to 12 hours, covering 10 to 25 kilometers in urban areas, deserts, mountains, or broken and wooded terrain. In addition to the challenges posed by the regional climate, soldiers wear interceptor body armor (IBA) and the advanced combat helmet (ACH) to protect them from enemy hostility. Further, soldiers carry a weapon with a combat load of ammunition (usually 210 rounds for an M4 carbine), a Camelback or multiple canteens for proper hydration, a secondary weapon, first aid bags, casualty litters, and other miscellaneous equipment dictated by the mission. This carried equipment places an incredible amount of stress on the body by adding 50 to 75 pounds of weight and insulating heat around the core area. To mitigate the risks associated with severe environmental conditions and the weight that soldiers must bear, it is imperative that leaders and soldiers alike understand the importance of heat adaptation in surviving the current operating environment. Heat Adaptation Most Army units today routinely deploy to the Middle East in support of the ongoing combat operations in Iraq or Afghanistan. In preparation for deployment, these units make an effort to replicate every aspect of combat by using the newest training techniques and facilities available. The growing training resources essentially simulate every imaginable complex combat scenario to include hostile villages, detainee operations, and forward operating base procedures. As advanced as these training resources are, there is still one aspect of the Middle East that cannot be duplicated in the United States — the extreme heat. However, soldiers can overcome this challenge through successful heat adaptation preparation. Heat adaptation occurs in response to repeated stress application factors such as solar radiation, temperature, humidity, work and exercise intensity, clothing, fitness, and so on.1 Generally, heat adaptation works as a response to naturally occurring climatic changes in the environment (acclimatization), heat exposure in an artificial climate (acclimation), and training induced elevations in body temperatures.2 Understanding this process is extremely important as heat adaptation is essentially the sum of acclimation and acclimatization, where the “former is induced experimentally in an artificial environment; whereas, the latter is induced by exposure to natural environments.”3 Thus, the objective of heat adaptation, as an outcome of acclimation and acclimatization, is to achieve three primary physiological changes: a heightened sweat response with an increased sweat output; a lowered heart rate; and a lowered core temperature. The body’s sweat response is critical to support the body’s cooling mechanism by maximizing evaporative by Captain Russ Nowels, Captain Coley Tyler, and Dr. Phil Henson cooling, which lowers the temperature of the peripheral blood prior to its return to the deeper tissues or the body’score. Likewise, a lower heart rate results from a more powerful stroke volume, which enables the heart to regulate more efficiently the body’s plasma levels. As an outcome, the system is more stable when blood is pumped to the skin and muscles, or during a significant loss of fluids. These two physiological adaptations drive down the core temperature, which is the final objective of heat adaptation. Therefore, to achieve the physiological responses necessary to complete heat adaptation, an analysis of acclimation and acclimatization must be conducted. Heat Acclimation As previously stated, heat acclimation refers to the component of adaptation that can be induced experimentally while its purpose is to exercise the physiological mechanisms that facilitate adaptation. Physically fit subjects or highly trained individuals exhibit many of the characteristics of heat acclimation.4 Researchers commonly refer to this as “partial acclimation,” and credit the result to repetitive bouts of exercise.5 Repeated exercise applications yield an elevation in the internal body temperature causing an increase in the sweat drive and the subsequent boost in evaporative cooling. The desired results of acclimation include: increased stroke volume, increased blood flow to the working muscles and skin, and increased sweat response during exercise or heat exposure. These results mirror and directly relate to those physiological changes of heat adaptation. The most important consideration of acclimation for military professionals, though, is that it can be developed in any environment, even cool climates.6 This implies that acclimation can be controlled and achieved at any installation, or in any environment, across the Army prior to deployment. As a result, soldiers can acclimate through targeted physical training at home stations. Specifically, morning physical training and frequent road marches (foot marches) provide sufficient opportunities to exercise the acclimation responses. A more complete discussion of the desired acclimation outcomes and some physical training recommendations are offered below. These recommendations are given to ensure soldiers comprehend the simple training they can implement to achieve acclimation. Increased stroke volume. An increased stroke volume is achieved through interval training. Conducted properly, interval training stresses the heart through exertion, but ultimately strengthens it during recovery. A stronger heart enables a more powerful heart stroke volume, which increases its efficiency with a slowed heart rate. Eight repetitions of 400 meters (at or below their established 2-mile run pace) on a track using a 2- or 3-minute cycle is an entirely realistic option during the morning physical training period. This means that an entire platoon starts together every 3 minutes, but slower runners will get less recovery. A platoon conducting interval training at least once a week will quickly obtain an increased stroke volume. Increased blood flow to working muscles and skin. An increased blood flow to the working muscles and skin is accomplished through longer durations of physical activity, which stress the aerobic system and cause the heart to push more blood to the working muscles and skin. In the muscles, more capillaries will open to generate greater blood flow within the muscle fibers. Likewise, larger quantities of blood pump to the skin and cool through evaporation. This process returns cooled blood to the inner organs and allows the core temperature to remain more stable. Most physiologists and researchers agree that 60 to 90 minutes of continuous physical activity during the warmest hours of the day sufficiently familiarizes the body to such a redistribution of blood.7 Long, steady distance running throughout a physical training session or constant physical activity (a foot march at apace greater than 15 minutes/ mile) for 60 minutes or longer satisfactorily achieves this result. In addition, some investigations indicate that the same ef- “In addition to the challenges posed by the regional climate, soldiers wear interceptor body armor (IBA) and the advanced combat helmet (ACH) to protect them from enemy hostility. Further, soldiers carry a weapon with a combat load of ammunition (usually 210 rounds for an M4 carbine), a Camelback or multiple canteens for proper hydration, a secondary weapon, first aid bags, casualty litters, and other miscellaneous equipment dictated by the mission.” 12 — fect can be achieved through shorter duration, moderately intense, continuous running for 30 to 35 minutes.8 This method adjusts to the time constraint of morning physical training in the Army. There is no prescribed frequency or limit on long, steady distance, so platoons should incorporate this activity into physical training as frequently as possible. Increased sweat response. Increasing the sweat rate during exercise allows for greater evaporation on the skin. Through evaporation, the subcutaneous blood cools and returns to the inner organs. This process cools and regulates the core temperature. Achieving the sweat response is the easiest of the acclimation responses because it is practiced during any activity that results in sweating. Therefore, most physical training sessions, or any of the recommended workouts listed above, will yield an increased sweat response. The body can attain an increased sweat response by training or working in the heat of the day or through exposure to a climate-controlled environment. Although more controversial, another method is wearing extra layers of clothes to create a microclimate, inducing a greater sweat response.9 The concern over this method is the additional stress from thermal strain placed on a subject wearing additional layers. However, elite athletes, such as Meb Keflezighi, the 2004 Athens Olympic Games silver medalist in the marathon, advocate this method. In preparation for the 2004 Olympic Games, Ke-flezighi trained by wearing additional layers to prepare his body for the humidity of Athens, Greece.10 This method supports continuous training in IBA since it creates a microclimate around the body’score area similar to wearing extra layers of clothing. Again, it is important to remember that acclimation is primarily achieved through work or exercise. It is obvious, then, that physical training is paramount to acclimation. Still, it only represents apart of the adaptation process since repeated bouts of physical training are conducted to exercise the mechanisms for adaptation, not to allow physiological changes. More simply put, acclimation is analogous to a runner who trains for the 800- meter run, but then decides to run a marathon. The athlete worked the mechanisms to run, but is not fully prepared for the length of a marathon. To prepare for the longer duration of the marathon, the athlete requires considerably more endurance training. Likewise, soldiers exercise the mechanisms for heat adaptation through acclimation training; however, soldiers still require exposure to the real elements of the regional environment over a longer, more consistent period of time to complete adaptation. This final phase in adaptation is known as “acclimatization.” Heat Acclimatization Exposure to the natural environment induces heat acclimatization and results in improved heat tolerance and decreased physiological strain. The purpose of heat acclimatization is to efficiently transfer heat from the body’score to the skin and ultimately the external environment, and “An increased blood flow to the working muscles and skin is accomplished through longer durations of physical activity, which stress the aerobic system and cause the heart to push more blood to the working muscles and skin. In the muscles, more capillaries will open to generate greater blood flow within the muscle fibers. Likewise, larger quantities of blood pump to the skin and cool through evaporation. This process returns cooled blood to the inner organs and allows the core temperature to remain more stable.” “Exposure to the natural environment induces heat acclimatization and results in improved heat tolerance and decreased physiological strain. The purpose of heat acclimatization is to efficiently transfer heat from the body’score to the skin and ultimately the external environment, and improve cardiovascular functioning to deal with the stressors of dehydration and a decreased blood volume from an increased skin blood flow. The primary difference from heat acclimation is that acclimatization requires continuous, long-term exposure to heat.” — 13 improve cardiovascular functioning to deal with the stressors of dehydration and a decreased blood volume from an increased skin blood flow.11 The primary difference from heat acclimation is that acclimatization requires continuous, long-term exposure to heat. Subsequently, the desired results of acclimatization are some what similar to acclimation, yet even more critical (and more effective) for the adaptation process. Therefore, the desired results of heat acclimatization include an improved skin blood flow, a decreased heart rate, a decreased perception of work exertion, and an increased sweat output and more effective distribution of sweat. Similar to acclimation, these results reflect the physiological changes necessary for adaptation. While heat acclimation can occur in any region or environment, given certain training conditions, heat acclimatization must take place in the region of interest. Therefore, the only way to truly achieve heat acclimatization is to live in the environment. Specifically, soldiers must experience the discomfort of the heat by training, exercising, and feeling the physiological strain. Physiologists and researchers recommend a minimum of 10 to 14 days of living, training, and exercising in the environment to acclimatize. The number of days is based on physiological adaptations during heat acclimatization (the point at which approximately 95 percent of adaptation occurs) for variables such as a decreased heart rate, expansion of plasma volume, a decreased rectal temperature, a decreased perceived exertion, and an increased sweat rate.12 Knowing this information explains why it is prudent for Army units to train in Kuwait for a few weeks prior to moving into Iraq. Successful acclimation allows for more efficient acclimatization.13 First, a higher level of fitness (acclimation) allows subjects to function at a lower heart rate while carrying a greater relative workload compared to unfit subjects. This enables fit subjects to use less energy to complete a greater amount of work while showing less cardiovascular strain during work in the heat.14 Second, acclimation improves acclimatization efficiency because a fit subject arrives in the new environment with an already improved skin blood flow and decreased heart rate due to his exercise regime.15 This suggests that the most important response during acclimatization is the increased sweat output and the distribution of sweat. It is true that the sweat mechanisms are exercised during acclimation, but only for short durations. Once introduced into the new environment, the body now requires the sweat system (as part of the cooling system) to work continuously — day after day, week after week. This is a critical consideration because few locations in the Army can replicate this process. To acclimatize the sweat system, physiologists initially recommend light exercise during the coolest hours of the day, followed by subsequent daily increases in the intensity of the physical training (and Army training). In no more than 14 days, soldiers should be ready to conduct training at near normal levels.16 One decisive facet of heat acclimatization involves the importance of hydration and its relationship to sweat output. Although hydration is important during acclimation, it is not vital since repeated bouts of exercise rarely last longer than 90 minutes. Thus, once a subject cools down and the sweat system shuts off, lost fluids can be quickly replaced. Contrary to acclimation, acclimatization does not offer the opportunity to quickly restore lost fluids since the sweat system works continuously during the heat. The sweat response may be stimulated as a result of work, yet it does not stop once the work is complete due to intense heat and the body’s efforts to cool itself through the evaporation process. In fact, sweat losses in extreme heat often exceed rehydration rates.17 The effect is constant sweat output, which challenges the body’s ability to maintain healthy plasma levels. The body’s plasma level (fluid levels) declines without adequate hydration (dehydration). This drop in the plasma level yields a less powerful heart stroke, which decreases the heart’s ability to pump an ample quantity of blood to the skin. Subsequently, this cycle impairs the body’s cooling system. The heart attempts to push more blood (from a lower total volume) to the skin by pumping more rapidly. In doing so, the heart rate increases, which consequently elevates the body’score temperature. Ultimately, this negates any previous advantages of acclimatization and can potentially lead to severe heat injury. Yet, even through continuous fluid intake, dehydration may be unavoidable. Recent studies conducted in Iraq indicate that “a threshold may exist for water consumption above which additional consumption may not help in preventing dehydration.”18 Still, the impacts of dehydration are so severe that constant hydration is imperative to remain functional in the new environment and also a necessity to complete acclimatization. There may be no substitute for living, training, and fighting under hot conditions in the regional environment to improve performance in the heat. However, through heat acclimation, soldiers reap “While heat acclimation can occur in any region or environment, given certain training conditions, heat acclimatization must take place in the region of interest. Therefore, the only way to truly achieve heat acclimatization is to live in the environment. Specifically, soldiers must experience the discomfort of the heat by training, exercising, and feeling the physiological strain. Physiologists and researchers recommend a minimum of 10 to 14 days of living, training, and exercising in the environment to acclimatize.” 14 — the benefits of intense home station physical training — especially endurance exercises — to develop the critical response mechanisms needed to improve heat tolerance and advance acclimation. Acclimatization then promotes a reduction in soldiers’ physiological strain when they live in the environment; yet, it is critical to recognize that all soldiers acclimatize (and thus adapt) at different rates. Finally, soldiers gain confidence by embracing the discomfort while working in the “The body’s plasma level (fluid levels) declines without adequate hydration (dehydration). This drop in the plasma level yields a less powerful heart stroke, which decreases the heart’s ability to pump an ample quantity of blood to the skin. Subsequently, this cycle impairs the body’s cooling system. The heart attempts to push more blood (from a lower total volume) to the skin by pumping more rapidly. In doing so, the heart rate increases, which consequently elevates the body’score temperature. Ultimately, this negates any previous advantages of acclimatization and can potentially lead to severe heat injury.” heat and learning the value of hydration to complete the adaptation process. Properly conducted, these two steps complete heat adaptation, which ultimately enables soldiers to execute more safely their missions on the contemporary battlefield. Notes 1Nigel A. Taylor and James D. Cotter, “Heat Adaptation: Guidelines for the Optimization of Human Performance,” International SportMed Journal, Vol. 7, No.1, 2006, p. 36. 2E. R. Nadel, et al., “Mechanisms of Thermal Acclimation to Exercise and Heat,” Journal of Applied Physiology, Vol. 37, No. 4, October 1974, pp. 517-519; and Carl Gisolfi and Sid Robinson, “Relations between Physical Training, Acclimatization, and Heat Tolerance,” Journal of Applied Physiology, Vol. 26, No. 5, 1969, pp. 533-534. 3Lawrence E. Armstrong, Performing in Extreme Environments, Human Kinetics, Champaign, IL, 2000, p. 3. 4Gisolfi and Robinson, pp. 533-534. 5Nadel, et al., pp. 517-519. 6Carl V. Gisolfi, “Work-Heat Tolerance Derived from Interval Training,” Journal of Applied Physiology, Vol. 35, No. 3, 1973, p. 349. 7Ron Maughan and Susan Shirreffs, “Exercise in the Heat: Challenges and Opportunities,” Journal of Sports Sciences, Vol. 22, 2004, p. 919. 8Joseph A. Houmard, et al., “The Influence of Exercise Intensity on Heat Acclimation in Trained Subjects,” Medicine and Science in Sports and Exercise, Vol. 22, No. 5, 1990, pp. 618-620. 9Brian Dawson, “Exercise Training in Sweat Clothing in Cool Conditions to Improve Heat Tolerance,” Sports Medicine, Vol. 17, No. 4, 1994, pp. 233-243. 10Peter Pfitzinger, “Simulating Race Conditions for Optimal Performance,” Running Times, January-February 2006. 11Armstrong, p. 28. 12Lawrence E. Armstrong and Carl M. Maresh, “The Induction and Decay of Heat Acclimatization in Trained Athletes,” Sports Medicine, Vol. 12, No. 5, 1991, p. 304. 13Kent B. Pandolf, “Effects of Physical Training and Cardio-respiratory Physical Fitness on Exercise Heat Tolerance: Recent Observations,” Medicine and Science in Sports, Vol. 11, No. 1, 1979, p. 65. 14Christine L. Wells, et al., “Training and Acclimatization: Effects on Responses to Exercise in a Desert Environment,” Aviation, Space, and Environmental Medicine, Vol. 51, No. 2, 1980, p. 106. 15Kent B. Pandolf, et al., “Role of Physical Fitness in Heat Acclimatization, Decay and Reinduction,” Ergonomics, Vol. 20, No. 4, 1977, pp. 403-406. 16Armstrong, Performing, pp. 28-29. 17Taylor and Cotter, p. 40. 18Edward Manning and Bradley Wilson, “Dehydration in Extreme Temperatures While Conducting Stability and Support Operations in a Combat Zone,” Military Medicine, Vol. 172, 2007, p. 975. Captain Russ Nowels is currently working on his M. S. in Kinesiology at Indiana University. He received a B. S. from the United States Military Academy. His military education includes the Armor Officer Basic Course, Armor Captains Career Course, Airborne School, Scout Leaders Course, and Master Fitness Trainer Course. He has served in various command and staff positions, to include commander, A Troop, 1st Squadron, 10th Cavalry, Fort Carson, CO; assistant operations officer, 3d Squadron, 3d Armored Cavalry Regiment, South Baghdad, Iraq; and adjutant, Regimental Headquarters and Headquarters Troop, 3d Armored Cavalry Regiment, Tall Afar, Iraq. Captain Coley Tyler is currently working on his M. S. in Kinesiology at Indiana University. He received his B. S. from the United States Military Academy. His military education includes the Field Artillery Officer Basic Course, Field Artillery Captains Career Course, Ranger School, Airborne School, and Master Fitness Trainer Course. He has served in various command and staff positions, to include commander, Headquarters and Headquarters Company, 3d Brigade Special Troops Battalion, 3d Brigade, 1st Cavalry Division (CD), Fort Hood, TX; battalion intelligence officer and assistant operations officer, 2d Battalion, 82d Field Artillery, Baghdad and Balad, Iraq, and Fort Hood; and battalion fire support officer, 2d Battalion, 7th Cavalry, 3d Brigade, 1st CD, Taji and Fallujah, Iraq. Dr. Phil Henson is an assistant professor in the Department of Kinesiology at Indiana University. He has a Ph. D. in Human Performance from Indiana University. His activities and research focus on understanding and improving human athletic performance, especially through the sport of track and field. Most notably, he served as the competition director for the 1996 Olympic Games in Atlanta, GA, where he focused on heat issues indigenous to track and field events, to include the marathons and the 50K walk. — 15