Hot and humid environments challenge an unaccustomed individual’s physical performance placing a significant strain on the thermoregulatory processes, thereby increasing the risk of heat illness. When exposed to these extreme environmental conditions, unadapted individuals have decreased performance and reduced workload capacity. Heat stress results in heat illness, including fatal heat stroke, if the heat stress exceeds the body’s ability to dissipate heat. Cold water immersion is the first-line treatment for heat stroke to make use of the high specific capacity of water to transfer heat through conduction and convection (1–3).
Heat mitigation strategies are valuable to athletes and workers in high-risk occupations that are exposed to abrupt changes in environmental or industrial conditions. In these populations, cold water immersion therapies are utilized to compensate and manage heat stress before exposure and decrease the risk of heat illness through heat adaptation (HA) (1–3). In contrast to cold water immersion in the treatment and mitigation of heat illness, hot water immersion (HWI) has been shown to induce HA that improve athletic performance in hot conditions (4).
This article will
- briefly review the general principles of HA, including a discussion of the distinction between acclimation and acclimatization;
- discuss general guidelines and considerations in using heat mitigation and HA strategies; and
- introduce two methods that use water immersion to mitigate hyperthermic stress or induce HA
UNDERSTANDING HA STRATEGIES
Understanding the difference between heat acclimatization and heat acclimation is essential to understand available HA strategies better. Heat acclimatization is the biological adaptation of the body to the physiological stressors of a naturally occurring hot and humid environment. Heat acclimation is similar to acclimatization, but the hot or humid environment to which the individual is exposed is induced or man-made (5). The methods discussed in this article are examples primarily of acclimation strategies. The physiological response is the same, whether acclimatization or acclimation methods are used. Thus, this article will refer to heat acclimatization or acclimation as HA.
HA strategies reduce the risk of exertional heat illness (EHI) while improving physical performance in a hot and humid environment. Physiological adaptations to multiday sessions of active or passive HA include decreased exercise core temperature, decreased heart rate, increased plasma volume, earlier onset of sweating, and increased total sweat volume with lower sodium concentration (6–9).
Active HA exposes an individual to heat and humidity conditions similar to those anticipated at an athletic event or in occupational environmental conditions. Over the multiday course of HA, the intensity and duration of physical exertion and heat stress exposure are increased (controlled hyperthermia), that is, not isothermal throughout HA. Active HA protocols vary in duration between short-term protocols of up to 7 d to more extended protocols of greater than 14 d. Active HA protocols generally fall into one of three categories:
- A controlled hyperthermia protocol that adjusts the workload and environmental conditions to achieve and maintain a target core temperature through the duration of an activity
- A controlled workload and environmental protocol that seeks to maintain a constant workload and environment
- A self-regulated protocol that exposes an individual to an anticipated environment while maintaining their own pace of exertion (6)
Active methods of HA require significant adjustments of training or workload to fit the parameters of the adaptation protocol. Passive methods of HA expose an individual to varying degrees of heat and humidity but do not require physical exertion during heat stress exposure.
HA strategies can place a logistical burden and alter training or work patterns for athletes and personnel in high-risk industries. Consensus statements recommend athletes from temperate environments perform 1 to 2 wk of HA before athletic events in hot and humid environments (2,10–12). If the team or athlete chooses to perform active HA, they may conduct HA in a laboratory environment at home station, or by arriving on-site 1 to 2 wk before the event or an area with a similar climate. Thus, active HA could result in a high cost, and a significant time and training burden on the team or athlete. Although the total effect on preevent training regimens and the athlete’s overall performance is unknown, athletes will need to alter their training beyond conventional periodization to accommodate an active HA protocol. The training load has to be reduced while physiological adaptation to a hot environment occurs. Recent evidence from the 2015 International Association of Athletics Federations World Championships in Beijing, China, suggests that <15% of athletes and teams performed HA strategies before the international competition without specific reasons reported (13).
HA strategies also pose unique challenges for defense professions and commercial industries. Military personnel are called to perform their duties in hostile areas on short notice in environments where they are not acclimatized. The ability to actively acclimatize military personnel over a 14-d protocol might not be practical before they are required to perform a mission. Various commercial and industrial occupations require physical exertion in extreme heat, for which it may not be practical to train consistently in these environments, for example, firefighters, paramedics, police officers, hazardous goods handlers, and similar occupations. Protective clothing and equipment also add to occupational heat stress.
Active HA has many challenges for nonacclimatized personnel typically performing in temperate climates; therefore, passive HA (laboratory) and other heat mitigation strategies are likely more feasible solutions. Multiple passive HA protocols in the literature elicit physiological adaptations similar to active HA protocols, including dry and wet saunas, resting in heat chambers, and hot baths (8). Heat acclimation protocols vary from 4 d to greater than 21 d performed on consecutive days or intermittently. The duration of heat exposure in these protocols ranges anywhere from a few minutes up to 4 h. The outcome measures of these studies include physiologic adaptations and performance measures.
However, HA cannot wholly remove the physiologic stresses induced by physical requirements in hot and humid environments. Therefore, it is desirable to have methods to mitigate heat stresses during physical exertion, especially for those whose physical exertion requires additional protective equipment that increases heat stress. Methods such as preexercise cooling, ingestion of cold fluids or ice slush during exertion, donning of a cooling apparatus, or extremity immersion in hypothermic water have been described and studied (9). Immersion of the hands and forearms in cold water has shown promise as a method that is effective and can be implemented with comparatively minimal effort to reduce core temperature during exertion and prolong performance under heat stress conditions (14,15).
HEAT MITIGATION: HAND AND FOREARM IMMERSION
Protective equipment and clothing often increase physiologic stress and interfere with the body’s evaporative cooling even in those who are HA. Therefore, active cooling or heat mitigation strategies are sought to help reduce the risk and severity of EHI. Most sports require some form of protective equipment, but this equipment can be removed without placing the athlete at increased risk of other environmental dangers. Physical exertion under conditions that require constant wear of protective equipment and clothing because of other risks, such as firefighting and the military, poses a unique challenge for heat mitigation strategies because removing their protective gear is impractical and dangerous (16).
Cold water immersion therapy of the extremities is used in occupations that limit protective equipment removal because of the physical constraints of the occupation. Various studies of cold water immersion therapy of the extremities include hand, hand and forearm, and foot immersion. Most studies concentrate on hand immersion alone and immersion of the hand and forearm because the practical application of foot immersion is difficult in most work and industrial settings, but similar cooling rates are seen in all extremities (15,17). When an extremity is immersed in cold water in normothermic environmental conditions, it results in vasoconstriction to prevent heat loss. However, in a person with an elevated core temperature, the extremity remains vasodilated in the cold water resulting in the rapid removal of heat through conduction (18). Heat transfer is quicker and more efficient in water than with ambient air cooling because of a much higher coefficient of heat transfer in water than air (15).
The high thermal exchange of water and the peripheral vasodilation of individuals with elevated core temperatures led to further research into the implementation of cold water immersion therapy of the extremities to aid in reducing core temperature. Extremity immersion therapy has been studied across a range of water temperatures from 10°C to 30°C (15,16,19). In 1989, Livingstone et al. (20) evaluated the effectiveness of core temperature reduction from 38.5°C when firefighters’ hands were immersed in 10°C, 20°C, or 30°C water. Water at each temperature reduced core temperatures when compared with a noncooling control group, but 10°C and 20°C water was shown to lower core temperature more effectively than 30°C water. Core temperature declined nearly 1°C in the first 10 min of immersion in 10°C water and 0.75°C in 20°C water.
The effectiveness of hand and forearm immersion to improve athletic performance is unknown. However, in occupational settings, cold water immersion of the extremities does demonstrate a decrease in thermal strain and an increase in work tolerance in hot conditions. As mentioned previously, Selkirk et al. (19) demonstrated that core temperature decreases with cold water immersion therapy of the extremities. A study of firefighters working on a 50-min work and a 30-min rest cycle showed a decrease in core temperature during intermittent rest periods and overall slower rise of core temperature over a total work period in the experimental group with cold water immersion therapy of the arms in 17°C water versus the control group without immersion therapy. Using cold water immersion, the experimental group demonstrated a 66% increase in total heat tolerance and a 62% increase in total work time.
The US Army Research Institute of Environmental Medicine is studying an arm immersion cooling (AIC) system in hot and humid environments at Army training sites within the United States that have a higher incidence of EHI (21). The AIC system consists of portable water troughs that can accommodate hand and forearm immersion for up to six people at one time. The system is set up in training areas accessible to trainees during rest cycles to allow immersion. DeGroot et al. (14) retrospectively evaluated the incidence and morbidity associated with EHI in 2010–2012, after the implementation of the AIC system, compared with EHI from 2007 to 2009, before the system was used. The study found no change in the incidence (4.06 vs 4.00/1000 person-days in controls vs AIC, respectively) of EHI among trainees. However, there was a noticeable decrease in the severity of EHI in the study as measured by more patients who were treated and released (4.84 vs 18.43/1000 person-days in controls vs AIC, respectively) versus patients who were evacuated or admitted to the hospital during the period of AIC use. Further studies need to be performed to evaluate the effectiveness of this treatment. However, randomized controlled trials will likely be limited because of the known benefit to decrease the severity of EHIs.
HWI is a passive HA modality. HWI baths are most commonly used immediately after exercise but also as stand-alone heat exposures. The literature using HWI to induce physiologic heat response passively is inconsistent about the immersion protocols used to include a variation in the extent of immersion (e.g., immersion to neck, chest, or waist level), the number of exposures, and the time per exposure to HWI (4,22,23).
Despite the difficulty in determining which HWI measures to use, results from various studies have demonstrated positive outcomes after HA. As early as 1971, Allan and Wilson (22) showed that HWI could induce a decrease in sweat sodium concentration and increased sweat volume. Although the intention of the study was not acclimation to heat, the low-powered study of three participants introduced to 40°C bath for 1 h daily for 3 wk (excluding Saturday and Sunday) demonstrated a mean 70% increase in sweat volume after the HWI acclimation period with decreased sweat sodium concentration.
A recent series of studies by Zurawlew et al. (4) evaluated the physiological adaptations and changes in athletic performance due to postexercise HWI and the short-term maintenance of adaptations after exposure. The 2016 study exposed 17 recreational athletes accustomed to exercise in temperate conditions to a 6-d postexercise HWI therapy. Participants performed pretreatment and posttreatment fitness tests, which included a 40-min treadmill run at 65% V̇O2max followed by a 5-km treadmill time trial in temperate (18°C, 40% relative humidity) and hot (33°C, 40% relative humidity) conditions. The 6-d study included an experimental group of 10 participants who performed 40 min of HWI in 40°C water after a 40-min treadmill run at 65% V̇O2max. The control group of seven participants received 40 min of immersion in 34°C after the same submaximal exercise. This study demonstrated HA in the HWI group in both temperate and hot conditions by lowering resting and postinterventional rectal temperature, lowering skin temperature, lowering rectal temperature at the onset of sweating, and decreasing perceived exertion during submaximal exercise. The experimental group had a 4.9% improvement in their time trial performance in the hot conditions but not in temperate conditions. The control group did not experience changes in thermoregulatory measures or performance in either environmental condition. This study is the first to demonstrate that HWI can elicit HA and improves performance in hot conditions within a short (<7 d) HA period. However, this study’s physical exertion parameters may not represent the physiologic stressors experienced in athletic competition or work conditions. The study was also performed in recreational male athletes. Therefore, it may not represent adaptations seen in more trained athletes or be directly extrapolated to female athletes.
Zurawlew et al (24) replicated their previous study to better understand the effects of their postexercise HWI therapy not only on recreational athletes but also on well-trained endurance athletes. The 2018 study evaluated the same parameters and exercise components as their 2016 study. However, the cohort changed to an experimental group of eight endurance-trained male athletes and a control group of eight recreational male athletes. After the intervention, the endurance-trained and recreational athletes had similar HA. The two groups did not have a significant difference in baseline or magnitude of change for postexercise core temperature, reduction in resting rectal temperature, reduction in rectal temperature at the onset of sweating, or reduction in skin temperature (P < 0.05 for the magnitude of change in all parameters before to after the intervention). Also, the study demonstrated no difference in perceived exertion or thermal sensation. Recreational athletes did have a statistically significant reduction in end-exercise heart rate (P < 0.01) compared with endurance-trained athletes who showed no change before to after the intervention. This study demonstrates that highly trained athletes can achieve HA through HWI. Because there was no difference in baseline parameters between the groups, there is still a question of whether an HA athlete can benefit further by HWI. More research is needed to relate these changes to the general population.
Zurawlew et al. (25) evaluated whether HWI has a temporal relationship with the expected time of day at which an anticipated heat stress event will occur an experimental group of 10 participants was introduced to the 6-d HA protocol of a daily 40-min treadmill run at 65% V̇O2max followed by ≤40 min HWI at 40°C between the time of 0630 and 1100 h. The group completed a treadmill run at 65%V̇O2max before and after the intervention with a morning session at 0945 h and an afternoon session at 1445 h. Physiological adaptations were equal after the acclimation, whether testing was done in the morning or afternoon. The study found significantly reduced resting rectal temperature, reduced rectal temperature at the onset of sweating, and end-exercise rectal temperature, heart rate, and perceived exertion. No significant difference was found between the morning and afternoon testing sessions.
A 2019 study by Zurawlew et al. (26), using similar postexercise HA protocols in recreational athletes, demonstrated that the induced physiological adaptations persist for at least 2 wk after completion of the intervention without additional sessions of HWI. This study demonstrated that at 2 wk after the intervention, there were decreases in the following parameters compared with pre-intervention: resting core temperature, the rectal temperature at the onset of sweating, and the postexercise rectal temperature. Skin temperature, heart rate, perceived exertion, and thermal sensation remained decreased 2 wk after the intervention (P < 0.05). Reductions in total hemoglobin, blood volume, plasma volume, sweating onset, and the rate of whole-body sweating were not statistically different at 2 wk after the intervention (P > 0.05). Demonstrating that the physiological adaptations to HA with postexercise HWI can last for at least 2 wk, it is still unknown whether the performance adaptation of these athletes lasts for an extended period. Other studies have found that HA degrades over time but can be maintained with intermittent active exposures to heat (27,28). However, further research could elucidate whether intermittent passive exposures can also maintain HA. This information would be valuable for multiple professionals, industrial workers, and athletes when planning for an event in an environment in which they are not acclimatized but may compete or work intermittently. Intermittent HWI would provide a practical means to maintain HA if it was proven effective.
Heat mitigation strategies are a crucial topic not only for athletes but also for professions that are less amenable to work–rest cycles, the reduction or removal of protective equipment, or the time to actively acclimate in extreme environments due to intermittent exposures or abrupt travel. Cold water immersion therapy of the extremities demonstrates a reduction in core temperature, a decrease in thermal strain, and an increase in work tolerance in hot conditions. Medical personnel managing or advising individuals at high risk for heat illness need to be aware of immersion therapies as an effective preventative strategy for heat stress illness. Additional research into risk mitigation strategies should build on these concepts with the development of personal protective equipment that can directly cool the extremity or enable cold water immersion. The development of equipment and protocols should continue to give direct attention to mitigating risks of heat stress.
HWI therapy offers a practical means of HA with minimal disruption to training programs and periodization cycles while inducing an effective adaptive response that can last for up to 2 wk after completion of the HA protocol. Despite these initial outcomes, more data are needed to evaluate HA strategies for physiological adaptations, including the time that adaptations and performance changes endure. Comparison studies need to evaluate HWI acclimation strategies versus other established active acclimatization strategies to demonstrate if a significant difference exists. The relationship of passive HA and the effects it has on EHI will benefit medical providers in advising their patients on the risks and benefits of each strategy.
The authors declare no conflicts of interest and do not have any financial disclosures. Disclaimer: The views expressed in this article are those of the authors and do not reflect the official policy, position, or endorsement of the US Army, the US Air Force, the Department of Defense, the US Government, or American College of Sports Medicine.
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