Summer changes everything. Rising temperature and humidity dramatically affect our physiology during exercise, especially in endurance sports. To keep training safely and effectively, we need to understand the basic principles of thermoregulation and adapt our strategies accordingly.
Goals of this article:
The human body has an extremely effective thermoregulatory mechanism whose goal is to keep core temperature within a narrow range of 36.5–37.5°C, regardless of external conditions.
This homeostasis is achieved through the combined action of three levels of control:
Neurophysiological (via the hypothalamus and the autonomic nervous system). It receives signals from skin thermoreceptors (which detect ambient temperature) as well as from central receptors (in the spinal cord, brain ventricles, etc.) that monitor internal body temperature. It detects deviations of less than 0.5°C and triggers autonomic responses such as sweating, shivering, changes in breathing rate, and vasomotor responses. The hypothalamus compares actual temperature with the target temperature and issues corrective commands through the autonomic nervous system (mainly the sympathetic branch).
Circulatory (by regulating blood flow to the skin). It adjusts blood flow to the skin so that heat is transferred from the core to the surface, or retained internally. During overheating, the sympathetic nervous system causes vasodilation of skin vessels, increasing blood flow and moving heat outward. During cold exposure, vasoconstriction occurs, reducing peripheral blood flow and heat loss.
Behavioral (through changes in clothing, environment, or activity).
The body's ability to respond precisely to thermal stimuli is critical for protecting cellular function, maintaining the stability of enzymatic reactions, and preserving performance during exercise.
Which of these mechanisms dominates depends on the relative intensity of exercise and the ambient temperature.
Our muscles produce heat both during exercise — due to the energy released as a result of muscular contraction — and during rest and recovery, as a result of ongoing metabolic processes.
Of the energy we obtain from the food we consume (chemical energy), about 55% is released as heat at rest, while the remaining 45% is stored as ATP (adenosine triphosphate) and phosphocreatine.
At rest, part of that stored 45% is used to perform useful biological work. During muscular effort, however, about half of it — roughly 25% — is used to perform external mechanical work, while the rest is again released as heat. This means that during exercise, approximately 75% of the energy produced is lost as heat.
Body temperature (36–37°C) rises by about 1 degree when muscular effort corresponds to 50% of VO2max sustained for 20–30 minutes, and by 2 degrees when effort corresponds to 100% of VO2max sustained for 10–15 minutes. Temperature in the working muscles themselves also runs about 0.7 degrees higher than in the rest of the body.
Heat production during muscular effort is always greater than heat loss, which is why body temperature rises during exercise. This rise is proportional to the relative intensity of the muscular effort — not its absolute intensity.
The highest core temperature ever recorded in an athlete is 41.1°C, considered the upper limit of heat tolerance. This was measured in trained athletes under race conditions. Although regarded as the outer physiological limit, exceeding it dramatically increases the risk of heat stroke.
1. Ambient temperature
Significantly affects the body's ability to dissipate heat through sweating, increasing heart rate and perceived exertion.
2. Humidity
When humidity is high, sweat does not evaporate easily, reducing the effectiveness of thermoregulation and increasing the risk of overheating and heat exhaustion.
3. Solar radiation
Direct sun exposure adds to the body's heat load and increases the risk of sunburn and heat stroke.
4. Air pollutants and ozone (mainly in cities)
Pollution increases with heat and can make breathing more difficult, reducing VO2max, especially in sensitive individuals (e.g. those with asthma).
5. Wind
Cool wind helps carry heat away from the body, while hot wind adds to the thermal load.
6. Ground surface and reflected heat
Asphalt or unshaded surfaces absorb and radiate heat. Temperature near the ground can be 5–10°C higher than what is recorded at a weather station.
Dehydration
The rate of dehydration depends on heat production and ambient temperature. In a marathon, average water loss is about 3.7% of body weight, ranging from 1.7% to 6.2%. Dehydration reaching 5% of body weight reduces the capacity for intense, sustained muscular effort by 20–30%. This is why fluids lost during exercise need to be replaced. Sweat also carries away electrolytes, which likewise need to be replenished. Heavy sweating combined with inadequate hydration can lead to cramps, heat exhaustion, and heat stroke.
How much fluid to drink during exercise — especially in hot conditions — is not the same for everyone. It depends on body weight, sweat rate, the duration and intensity of exercise, and ambient temperature and humidity.
Hyponatremia
Excessive sodium loss through sweat, or drinking plain water without electrolytes. Symptoms: dizziness, cramps, nausea, confusion, seizures. Can become life-threatening in severe cases.
Heat exhaustion
The body cannot dissipate heat fast enough. Core temperature: 38.5–40°C. Symptoms: intense fatigue, tachycardia, headache, faintness, pallor, muscle weakness. Requires immediate cessation of exercise and cooling.
Heat stroke — medical emergency
Core temperature above 40.5°C. Collapse of the thermoregulatory mechanism. Symptoms: confusion, tachycardia, seizures, loss of consciousness, potentially death. Requires immediate cooling of the body and emergency medical care.
The body's tolerance to heat can improve significantly when training in warm environments. This heat acclimation is largely the result of increased sweat production and cardiovascular adaptations.
Acclimation is accelerated by exercising in a warm environment, but exercise alone — without heat exposure — does not trigger these thermal adaptations.
What heat acclimation actually means:
Why this matters: the body dissipates more heat through evaporation and keeps core temperature lower, delaying premature fatigue; the risk of hyponatremia and cramping is reduced and electrolyte balance is better maintained, supporting muscular and neural function; and there is less cardiovascular strain at a given intensity, delaying fatigue and improving heat tolerance overall.
Classic (active) heat acclimation
Training in a warm environment (≥30°C) with humidity above 40%, for 60–90 minutes per day over 7–14 consecutive days, with duration and intensity progressively increasing (50–70% VO2max).
Post-exercise hot water immersion (HWI)
Immersion in hot water (~40°C) for 30–40 minutes, starting about 10 minutes after training, with water at chest height, repeated over 6–10 consecutive sessions.
Sauna after running
5–7 consecutive sessions of 20–30 minutes in a sauna at roughly 80°C, immediately after running.
Practical tips for safety and performance
Understanding your physiology is the key to safe and effective training through the summer.
Constantinos Pallikaris
Contact: info@matrixendurance.com
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