What Factors Should I Consider for HAz and HA Sessions?
Internal Body Temperature. To ensure that each HAz and HA session is effective, internal body temperature should be monitored throughout the exercise session, with each session eliciting sufficient hyperthermia over an adequate duration of time to elicit desired physiological and psychological adaptations.7 Although a minimum temperature of 38.5 °C and duration of at least 60 minutes has been recommended for this response, HAz and HA should be based on individual responses to exercise in the heat because several variations of protocols have been successful.7,18 Rectal thermometer and ingestible thermometer are the two most common methods of collecting internal body temperature during exercise. Tympanic, aural, oral, skin, temporal, and axillary temperature are not valid assessments of this measurement and should not be used to assess internal temperature.23 All of the following variables can be adjusted based on this internal temperature criterion. Optimal internal temperature is critical to achieve performance benefits. Similar to other aspects of training, optimal adaptations require the application of an appropriate load.7 Similar to aerobic or strength training, HAz and HA can be thought of as a supercompensation response. Adaptations obtained by HA have been shown to be maximum 3 days following HA induction.24 Too little load, in this case rise in internal temperature, will result in suboptimal adaptations. Meanwhile, too much load can put an individual at risk for exertional heat illnesses.25
Environmental Conditions. When using the natural environment to achieve HAz and HA, environmental conditions should be appropriately monitored with valid measures of ambient temperature and relative humidity, or with a wet-bulb globe temperature device that incorporates ambient temperature, humidity, and radiant heat load.7 When using an artificial setting (heat laboratory, indoor facility, etc) environmental conditions can be altered, based on the desired exercise intensity, to achieve sufficient hyperthermia. The primary environmental conditions that can be altered are ambient temperature and relative humidity, unless in a facility that can allow for precise radiant heat exposure. Coaches should consider the environmental conditions of the target competition to guide selection of the environmental conditions and optimize the benefits of HAz and HA.7
Exercise Frequency, Intensity, Time, Type, Volume, Progression Model for HAz and HA
Frequency. The frequency of the HAz and HA protocol should be based on the needs analysis of the team or individual. Daily heat exposure is the fastest way to obtain adaptations, although two sessions in 1 day does not appear to enhance this process.26 When daily heat exposure is not feasible, intermittent heat exposures can also be conducive for achieving adapatations.27
Intensity. The intensity of the sessions can be based on several variables (eg, VO2max, heart rate, and the internal body temperature).17 If internal temperature is measured during these sessions, the intensity can be adjusted throughout the sessions to obtain adaptations. If internal temperature is not measured, the session intensity should be above 50% VO2max.9 Although laboratory-grade VO2max values may not be achievable in a practical setting, there are several validated tests that can be used to estimate an athlete's VO2max, including the Yo-Yo Intermittent Recovery Level 2 test, 1.5-mile run, and 12-minute run test.28,29 Additionally, sports training can be used to induce HAz.30
Time: Circadian Rhythm. Based on the nature of circadian rhythm, attainability of HAz and HA is more conducive to exercise in the afternoon.31 This can decrease the overall training intensity and volume on the athlete when considering time to reach the hyperthermia and training time.
Time: Before Competition. To ensure maximum benefits, coaches should consider completing HAz or HA at least 2 weeks prior to the competition of interest and begin implementation of the maintenance protocol to reduce training stress and peak for the target competition.24 If HAz or HA is desired for a longer period, such as throughout an entire season, the maintenance protocol discussed above should be implemented to prevent decay.
Environmental Variation Throughout Competitive Season. Coaches should also consider the environmental conditions of each sport and how that affects an athlete's HAz and HA status. For example, fall sport athletes (eg, football, soccer, and cross-country) typically train in warm environmental conditions throughout the summer and during pre-season to prepare for their competitive season, and therefore, start the season heat acclimatized and may benefit from a maintenance protocol.32–34 Spring athletes (eg, track and field and field hockey), who typically begin their season in cooler environmental conditions and are not heat acclimatized, would benefit from HA training to prepare for warmer competitions later in the season.
Type. HAz and HA can use sport-specific exercise modalities to achieve the adaptations as long as athletes are monitored appropriately to ensure the safety of the athlete's exercise in the heat.35 Alternatively, exercise modalities that reduce the training stress of the athlete can also be used to achieve adaptations (biking, incline walking, etc). Alternative methods, such as sauna and wearing extra clothing, can be used to induce adaptations; however, exercise is needed to achieve optimal adaptations.35,36 When alternative methods are used, coaches and athletes should strongly consider combining those alternative methods with exercise to achieve maximum performance enhancement.35
When an environmental chamber is not available, a small enclosed room can be used to induce or maintain HA. To create this room, select a room that is large enough to add exercise equipment but small enough to sufficiently heat with space heaters. Use a wet-bulb globe temperature monitor to determine the environmental conditions in the room throughout all exercise sessions, because temperature and humidity can rise substantially with exercise to maintain appropriate environmental conditions. Internal body temperature should also be continuously monitored in this setting to ensure safety.25 Additionally, extreme fatigue and exhaustion during exercise should be avoided to reduce the risk of heat illnesses, such as exertional heat stroke and exertional heat exhaustion, and to perform sessions safely.25
Volume. As with any additional training stress, coaches should consider the total physical and mental demands placed on the athlete.35 If HAz or HA is desired for improved sport performance, other training volume, intensity, or duration may need to be modified to ensure the athlete does not overtrain or develop maladaptations.35 It is important to note that HAz can be achieved in the scope of the athlete's normal training, as long as sufficient hyperthermia is met during those sessions.
Progression. Intensity, time, environmental conditions, and volume should be increased gradually over the course of the protocol.5 To achieve the adaptations, intensity and environmental conditions are the two primary modifiable variables.
Fluid Intake. Coaches should ensure that athletes are consuming adequate fluid to maintain euhydration to minimize fluid loss and reduce the risk of heat illness during exercise.37 Fluid intake can be prescribed based on losses (before to after body mass measures) and sweat rate (Table A, available in the online version of this article).38
Heat Acclimation Testing Data Sheet
Safety. Gradual implementation of heat exposures is important to maintain the safety of all players.39 Special considerations are needed for at-risk populations, such as those taking medications that could affect thermoregulation and fluid balance and individuals with a past history of heat illness or sickle cell trait. These individuals may need careful monitoring of internal temperature.