Training in heat and at altitude

What a heat block changes

Repeated exercise in the heat produces measurable physiological change rather than only a higher tolerance for discomfort. Plasma volume expands, sweating begins at a lower core temperature and delivers more fluid, sweat sodium concentration falls, and heart rate at a fixed workload comes down ¹.

• The cardiovascular adjustments arrive first, over roughly the first week of daily exposure ¹.
• Sweating and skin blood flow keep adapting through the second week, which is why a full block runs one to two weeks rather than a few days ¹.
• Adaptation decays once exposure stops, so the block is scheduled to finish near the event rather than months out ¹.

The usual prescription is around 60 to 90 minutes of exercise in the heat per day for the length of the block. Shorter or intermittent exposure gives partial adaptation, and the cardiovascular part of it is what shows up first ¹.

The same target costs more when it is hot

Heat raises cardiovascular and thermal strain at any given workload, and the effect compounds as the session runs longer. Prolonged performance drops in hot conditions regardless of how disciplined the pacing is ¹.

Fluid loss makes it worse. Once sweat losses reach about 2% of body mass, aerobic performance degrades, and the penalty is larger in the heat than in cool conditions ². Sweat rates vary widely between individuals, so intake is built from your own measured losses rather than a schedule borrowed from a training partner ².

Drift, and what it does to heart-rate pacing

During prolonged exercise at a fixed workload, heart rate climbs steadily while stroke volume falls. Heat and progressive dehydration both amplify the effect ³.

That breaks heart-rate targets in a specific way. Capping heart rate at a zone boundary late in a long hot session means shedding power or pace to stay under the cap, so the session gets easier while the number looks unchanged ³.

Living high, training low

The altitude approach with the strongest controlled evidence separates where you sleep from where you train. Runners who lived at about 2,500 m and did their hard training near 1,250 m for four weeks improved 5,000 m time-trial performance; a group that both lived and trained high did not, despite comparable gains in red cell mass and VO2max ⁴.

The mechanism is unglamorous. Training at altitude costs you absolute intensity, because the same session cannot be run as fast, so quality erodes quietly across a camp. Sleeping high supplies the haematological stimulus while descending to train preserves the speeds you actually race at ⁴.

Responses varied within the live-high/train-low group, so a camp is worth measuring rather than assuming. The group-level result does not guarantee an individual one ⁴.

Coming back down

Returning to sea level is a transition rather than a switch. Ventilatory and blood-chemistry adjustments made for thin air take time to settle, and performance in the days immediately after a camp is a poor read on what the camp produced ⁴.

Plan the descent with room in it, and expect the first sessions back to look worse than the fitness behind them ⁴.

References

Numbered in order of first appearance. Every link goes to the original work.

1. Périard, Racinais & Sawka. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports. 2015.
2. Sawka, Burke, Eichner et al.. American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007.
3. Coyle & González-Alonso. Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews. 2001.
4. Levine & Stray-Gundersen. 'Living high-training low': effect of moderate-altitude acclimatization with low-altitude training on performance. Journal of Applied Physiology. 1997.