For twelve summers, the logic felt unassailable: train through the heat, suffer through it, come out tougher on the other side. Plenty of athletes share this belief. The problem is that it confuses discomfort with adaptation, and the two are very different things.
When a coach finally sat down with the data, paces, heart rate trends, HRV scores mapped across twelve consecutive summers — the picture was uncomfortable. Not because the training was pointless, but because so much of it had been applied in entirely the wrong way. The body can adapt to heat. Brilliantly, in fact. But only when you understand what it is actually doing under thermal stress, and work with those mechanisms rather than blindly ignoring them.
Key takeaways
- A coach’s data analysis exposed twelve years of heat training applied in entirely the wrong way
- Your body can adapt brilliantly to heat—but cardiac drift and suppressed HRV aren’t signs of progress, they’re signs of thermal siege
- Seven to fourteen days of structured heat exposure creates real adaptations; twelve chaotic summers created nothing that lasted past autumn
What heat actually does to your physiology
The combination of environmental heat stress and exercise increases physiological strain, producing elevated heart rate and core temperature, increased oxygen uptake, and a general sense that every step requires more energy. This is not toughness. This is your cardiovascular system being stretched in two directions at once.
When temperature and humidity rise, the body must move more blood to the skin to cool itself down, leaving less blood volume available for the working muscles unless you compensate. The result is what exercise physiologists call cardiac drift: a gradual increase in heart rate response over time, occurring even when the pace, the gradient, and the effort stay the same. Your watch tells you that you are working harder. You feel it too. But you are not necessarily getting faster or stronger, you are simply hotter.
Athletes training in high heat may experience reductions in cardiac output, increased carbohydrate metabolism, glycogen depletion, dehydration, an accelerated decline in performance, and increased risk of exertional heat illnesses, including heat exhaustion and heat stroke. None of those are markers of progress. They are markers of an organism under siege.
The HRV data made this visible. Heat raises heart rate and lowers HRV, and in hot weather, a lower HRV and higher resting heart rate can be normal and temporary. The key word there is temporary. Twelve years of summer sessions had regularly produced suppressed HRV numbers, which had been interpreted as evidence of hard work done. In reality, they were often evidence of inadequate recovery compounded by thermal stress, two things the body cannot easily distinguish.
The real science of heat adaptation
Here is where the story gets genuinely interesting, because the body’s response to structured heat exposure is one of the more elegant pieces of physiology in sport. The problem was never training in the heat. The problem was training in the heat badly.
Heat acclimation is associated with plasma volume expansion that occurs within the first week of exposure. That expansion matters enormously. Hypervolemia found in acclimatised individuals is associated with decreased heart rate, increased stroke volume during both rest and exercise, and reduced haematocrit. More blood volume means the heart can deliver more oxygen per beat, skin cooling improves, and the same absolute pace suddenly feels more manageable. This is a genuine physiological upgrade, but it requires a deliberate protocol to achieve it, not simply grinding through August.
Usually, about seven to fourteen days of heat exposure are needed to induce heat acclimatisation. Optimal heat acclimatisation requires a minimum daily heat exposure of about 90 minutes combined with aerobic exercise, rather than resistance training. More specifically, during the initial exercise-heat exposure, physiological strain is high, as manifested by elevated core temperature and heart rate, but the physiological strain induced by the same exercise-heat stress decreases each day of acclimatisation. That decreasing strain is the adaptation. That is what you are chasing.
Passive heating increased haemoglobin mass, total blood volume, and left-ventricular end-diastolic volume. These haematological and cardiac adaptations explained the observed improvement in maximal oxygen uptake, indicating that heat exposure can enhance aerobic performance via coordinated effects across multiple components of the oxygen transport chain. A 2025 study published in The Journal of Physiology found this was achievable even through passive methods such as hot-water immersion alongside normal training, no suffering required.
Why “pushing through” defeats the purpose
The crucial misunderstanding in those twelve summers was about intensity. Athletes should gradually increase exercise intensity and duration, or just the heat exposure duration, each day of heat acclimatisation. Arriving at a session in 32-degree heat and treating it like a temperate interval session ignores the thermal load entirely, which means the cardiovascular system is asked to manage two stresses simultaneously, recovering from neither properly.
Favourable performance and physiological benefits can be realised from short-term programmes of seven days or fewer, but greater benefits are likely from longer protocols of seven to fourteen days. The irony is that doing less, with more intention, produces more. Supervision by an exercise physiologist or coach well-versed in heat training is strongly recommended, because too often people try to do too much too quickly.
Heat acclimatisation gradually disappears if not maintained by continued repeated exercise-heat exposures, with benefits retained for roughly one week and then decaying, with about 75% lost by three weeks once heat exposure ends. This explains something else the data revealed: fitness built through chaotic, unstructured summer training was never properly consolidated. The gains dissolved each autumn before they had been properly absorbed into baseline fitness.
What structured heat training actually looks like
The shift in approach is less dramatic than it sounds. The core principle is simple: treat heat as a training variable, not an ambient nuisance. Performing ten to fourteen days of a heat acclimatisation protocol four to six weeks prior to a key race allows the plasma volume expansion and cardiovascular adaptations to consolidate before competition. Doing hard intervals in the middle of a heatwave without that framework is just creating fatigue.
On ordinary hot training days, the practical rule is straightforward. If easy pace is forcing a higher heart rate, slow down, shorten the session, or move it earlier in the day. This is not a concession to weakness; it is a recognition that heat is real training load and needs to be accounted for in the total stress budget. Monitoring trends over days rather than reacting to single-session data also matters: monitoring autonomic responses to training load is essential for optimising performance and preventing maladaptation.
One further nuance worth knowing: heat acclimation has been shown to significantly improve VO2peak in hot conditions and time trial performance in both hot and cooler conditions. The benefits are not confined to race day in a heatwave. A well-executed acclimatisation block in summer can raise the ceiling for cooler-weather performance in autumn, a transfer effect that unstructured “suffer through it” training rarely produces, because the adaptations never had a chance to fully develop.
The twelve years were not wasted. But a few of those summers, approached with a proper protocol and a willingness to run slower before running faster, would have been worth considerably more.
Sources : physoc.onlinelibrary.wiley.com | ncbi.nlm.nih.gov