Training in Heat: The Free Performance Gain Most Runners Avoid
Heat acclimatization produces cardiovascular adaptations equivalent to altitude training. It costs nothing, takes 10-14 days, and most runners treat hot weather as a reason to stay inside.
Running in hot weather feels bad. Heart rate climbs faster, pace drops, perceived exertion spikes, and the whole experience is uncomfortable enough that most runners either skip training days or dramatically reduce intensity when the temperature rises above 28°C. This is a rational response to an unpleasant stimulus. It is also a massive missed opportunity.
Deliberate heat acclimatization — 10-14 days of running in warm conditions at reduced pace to maintain relative effort — produces measurable cardiovascular adaptations that persist for 2-3 weeks after returning to cooler conditions. The primary adaptation is plasma volume expansion: after 5-10 heat exposure sessions, circulating blood plasma volume increases by 4-12%. More plasma means more blood available simultaneously for cardiovascular delivery to working muscles and thermoregulatory delivery to the skin for cooling. The cardiovascular system becomes more capable without any structural increase in cardiac muscle.
Additional adaptations: earlier and more efficient sweat onset (the body learns to cool itself sooner), lower core temperature at any given workload, reduced cardiovascular strain at fixed pace, and improved perceived exertion at intensities that previously felt hard. Multiple studies have compared the performance benefits of altitude training versus heat acclimatization over equivalent time periods and found broadly comparable gains — at zero altitude camp cost.
The protocol is straightforward. For 10-14 consecutive days, run at easy to moderate effort in air temperatures above 28°C, or in warm clothing if the ambient temperature is cooler. Reduce pace by whatever amount is necessary to maintain the same relative effort level — typically 30-60 seconds per kilometre. Do not attempt quality sessions during the acclimatization window. Hydrate aggressively before, during, and after. The discomfort is highest in days 1-5; most runners report noticeable improvement by day 8-10.
For indoor runners: sauna exposure produces a subset of the heat adaptations without the running component. 20-30 minutes in a sauna at 80-100°C after running sessions, performed 3-4 times per week for 3 weeks, has measurable effects on plasma volume and heat tolerance. Not a replacement for proper outdoor heat training, but an accessible option for runners in cold climates or without outdoor access.
The performance window after heat acclimatization peaks at 2-3 weeks post-exposure. Plan accordingly: 10-14 days of heat training concluding 14-21 days before a target race places you exactly in the adaptation window when it matters most.