Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives

Figures & data

Figure 1. Heat acclimation adaptations that support exercise performance and safety in the heat.

Table 1. Exercise intensity to raise T_c within various time limits in 40°C and 39% relative humidity.

Tc rise	+1.0°C			+1.5°C			+2.0°C
Time (min)	20	30	40	20	30	40	20	30	40
Relative power (W kg^−1)*	2.7	1.9	1.5	4.0	2.7	2.1	5.2	3.6	2.7
Absolute power (% max)	64	48	40	88	64	52	112	80	64
RPE (AU)	17	15	12	>20	17	14	>20	>20	17
V̇O2 utilization (% V̇O2peak)	68	55	48	89	68	58	>100	82	68
HR (% HRpeak)	95	79	70	>100	95	83	>100	>100	95

* Strongest predictor of T_c change during exercise in the heat. Resting T_c ranges 36.5–37.5°C. RPE = ratings of perceived exertion on Borg’s 6–20 point scale. HR = heart rate. Table adapted from Gibson et al. [35]. Once target T_c is reached, exercise intensity should decrease so T_c does not continue to rise.

Table 2. Various heat acclimation induction methods with advantages and disadvantages.

Protocol	Duration	Intensity	Length	Advantages	Disadvantages
Variable workload
Controlled hyperthermia	60–90 min	Initially high to achieve Tc = 38.5°C within ~30 min, then lower intensity to sustain Tc thereafter	5–14 days	Sustained thermal adaptive impulse	Requires valid Tc measurement [60,61]
				Potential optimization of adaptations [18]*	Ideal (usually high) exercise intensity to achieve Tc ≥ 38.5°C within 30 min unknown and changes as adaptations occur; may be challenging for novice athletes
				Plasma volume, HR, and the emergence of thermoregulatory and metabolic adaptations improve thermal tolerance and performance after only 5 d in trained athletes [75]	Overall mean relative work rate and HR ↓ over protocol [75]
Self-paced	60–90 min or duration of practice	Varies	7–14 days	Very practical for team-based sports	Less control of exercise variables and adaptive stimuli, day-to-day improvements not easily measured
				Induce HAwhile training sports skills during practices or pre-season activities [91–93]	Requires motivated athletes
				Trained athletes can sustain intensities within narrow range [94]	May not induce all HA adaptations
Controlled heart rate	60–90 min	~70% heart rate reserve or heart rate at lactate threshold	5–14 days	Sustained relative exercise intensity and cardiovascular strain [32,78]	Thermal adaptive impulse difficult to quantify
				May use sport-relevant intensities	Cardiac drift may lower absolute exercise intensity
Constant workload
Average fitness athletes	100 min	40–50% V̇O2max	10–14 days	Easy to implement	Thermal strain/adaptation stimuli ↓ as HA progresses
				Day-to-day improvements easily measured	May ↓ magnitude of adaptation*
High fitness athletes	30–45 min	75% V̇O2max	5–9 days	Easy to implement	Thermal strain/adaptation stimuli ↓ as HA progresses
				Day-to-day improvements easily measured	May ↓ magnitude of adaptation
				May supplement with passive-heat exposures
Passive-heat exposure
Sauna bathing	20–30 min	50–100°C, 10–20% RH	5–21 days	↓ exercise-heat training volume	Requires sauna room
				May use every other day	Time constraint due to exercise immediately before or after
Hot water immersion	30–60 min	40–44°C	14–21 days	↓exercise-heat training volume	Time constraint due to exercise immediately before or after
				May use every other day

T_c = core temperature. V̇O₂max = maximal oxygen consumption. Table modified from [95]. * One studied showed controlled hyperthermia and constant workload protocols exhibited similar physiological and cellular adaptations [76].

Figure 2. Integration of heat acclimation into a periodization scheme for an elite triathlete.

Table 3. Pre-heat acclimation coaches checklist.

Training Facility	Is there a cold-water immersion treatment area closely available to all athletes participating in the exercise-heat exposure?
	Are the signs and symptoms of exertional heat illness posted for all athletes so they are aware when they should immediately stop training?
	Is there a person besides the athlete designated to monitor the temperature and humidity of the training facility?
Individual Athlete	Has the athlete’s bodyweight changed by <1% from the previous day?
	Has the athlete had an adequate night of sleep >6 hours in a cool environment?
	Has the athlete avoided supplements that may affect thermoregulation within the past 6 hours (e.g., “pre-workout”)?
	Is the athlete free of any fever, flu, cold-like, or digestive problem symptoms in the previous 24 hours?

In both individual and team sports, coaches should review the following checklist regarding their heat acclimation environment and each individual athlete prior to every exercise-heat exposure session. If the answer to any question is “no”, exercise heat acclimation for the day should be suspended for the team or individual. The next exercise-heat bout should occur when all responses to these questions are “yes”, beginning with the missed training bout. See Casa et al. [20] for additional information on exertional heat illness prevention strategies.

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