Heat Acclimation Training
Training for a spring race requires some extra caution. In many situations, you will have been training through the winter, so you will have no adaptation to the heat. A spring race may be cool, but there is also a risk of conditions that are warm enough (above 50f) to impair performance. Heat adaptation training can prepare you for these warmer conditions. This type of training is also valuable if you are traveling to a warmer climate for a race, or if you are training in the cool part of the day for a race in the warmer times.
Exercise becomes harder as the temperature rises, with 50 degrees Fahrenheit being close to optimal. Exercise in the heat causes blood vessels in the skin to expand to help with cooling. The demands of the extra blood for cooling creates added stress on the cardiovascular system. The athlete’s body will also sweat to produce cooling; in dry conditions evaporation of sweat provides 98% of cooling and in humid conditions 80%. The loss of fluids due to sweating can lead to dehydration that also impairs performance. The impact of dehydration is in addition to the impact of the heat.
3 Changes with heat acclimation
Heat acclimation will produce a number of benefits
- Sweating occurs at lower temperatures
- Sweat contains less electrolytes
- Sweating is more profuse
- Increased cardiac output in hot conditions
- Reduced core temperature for given exercise time and intensity
- The athlete becomes psychologically prepared for heat stress. 
- The ability to consume and absorb more fluids (anecdotal evidence only)
4 How To Acclimate to Heat
- Exercise at 50% VO2max or above
- Use gradually increasing periods from 30 to 100 minutes over 10 to 14 days
- Acclimation fully developed after 7 to 14 days
- Up to 75% of acclimation after 5 days 
- Reduce your training load to compensate for the added stress of the heat
- Consider alternating heat acclimation training and cooler training to preserve intensity
- Training in a warmer environment is ideal, but creating a microclimate by overdressing also works
- Exercise in heat produces better acclimation than passive heat
5 Notes on Heat Acclimatization
- Younger runners do better in the heat than older runners but training can negate this
- Acclimation is faster in fitter athletes
- On return to a cool climate, acclimation lasts for about a week, then decays
- People who have always lived in hot climates are believed to have superior adaptation
Generally, an athlete reaches ‘voluntary exhaustion’ when their core temperature reaches about 39c/102f. A dedicated athlete can push themselves hard enough to raise their core temperature to dangerous levels (>40c), leading to heatstroke, which can be fatal. Heatstroke can be the result of prolonged exercise in hot conditions, but it can also be the result of shorter periods of very high intensity exercise, especially in the untrained or overweight.
- http://journals.lww.com/acsm-msse/Abstract/1997/09000/Effects_of_ambient_temperature_on_the_capacity_to.18.aspx Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man
- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1322918/pdf/jathtrain00007-0030.pdf Exercise in the Heat. I. Fundamentals of Thermal Physiology, Performance Implications, and Dehydration
- The Lore of Running, Tim Noakes, pp 188
- The Lore of Running, Tim Noakes, pp 214
- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1175224/ Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment.
- http://www.ncbi.nlm.nih.gov/pubmed/1763248 The induction and decay of heat acclimatisation in trained athletes.
- https://www.thieme-connect.com/ejournals/abstract/sportsmed/doi/10.1055/s-2007-971986 Acclimatization Strategies - Preparing for Exercise in the Heat
- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC164365/ National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses