Heat, Cold and Altitude

01What the measurements show

The Numbers Behind Heat, Cold and Altitude

Eight findings that describe what an environment does to an athlete and how fast the athlete answers.

02One brain, three climates

Heat, Cold and Altitude Are Answered by One Regulating Brain

Heat, cold and altitude are three different loads on one control system. Body temperature regulation is one of the most critical functions of the nervous system Tan 2018. Molecules and cells measure temperature in the periphery. Neural pathways carry that reading to the brain, and central circuits coordinate what the body does about it.

The hypothalamus is where those lines converge. It reads the temperature of the blood passing through it and the reports coming up from skin thermoreceptors, then sets what the body does about the gap. Heat leaves the athlete by radiation, conduction, convection, evaporation and respiration. Cold is met by closing the skin down and by burning fuel to make heat.

One of those exits carries the day in sport. As air temperature climbs toward skin temperature, the gradient that drives radiation and convection collapses, and evaporation of sweat is what is left. Humidity is the limit on that last defense. When the vapor pressure of the air approaches the vapor pressure of the skin, sweat stops evaporating and rolls off, taking its cooling with it Periard 2021.

This is why a humid day is more dangerous than a merely hot one. The athlete is producing sweat at full cost and getting almost none of the cooling return. What that sweat costs in fluid and sodium, and where the replacement thresholds sit, belongs to Hydration and Electrolytes.

The defense competes with the performance

Cooling is not free, and the bill is cardiovascular. Humans stand upright and carry a large skin surface, which makes blood pressure regulation a problem during exercise in the heat Periard 2016. One cardiac output has to supply working muscle and supply the skin for heat dissipation and sweat secretion. Skin blood flow and muscle blood flow are drawing on one account.

That competition is the mechanism behind the drift every coach recognizes. Heart rate rises through a hot session at a work rate that felt easy in cool air, and pace falls at the same effort. The athlete is not weaker. A share of the circulation has been reassigned to temperature.

The two thresholds worth memorizing

Two core temperature numbers mark where regulation has been overrun. In heat stroke, marked hyperthermia, usually above 40.5 degrees Celsius, occurs together with encephalopathy Epstein 2011. It is the most lethal failure of thermoregulation in sport.

Hypothermia is defined at the other end, by convention, at a core body temperature below 35 degrees Celsius Sorodoc 2002. Between those two numbers sits every environment an athlete will ever compete in.

A threshold only means something if it is measured at the core. External thermometry is unreliable under heat stress, so oral, ear, temporal and axillary readings are not adequate for the diagnosis. Direct internal measurement by rectal or esophageal probe is what the number refers to Pryor 2015.

The early signs are neurological, because the brain registers the damage first. Clumsiness, stumbling, headache, confusion and apathy arrive before an athlete looks unwell. Recognition on the field and the cooling protocol that follows belong to Emergency and Field Procedures, which carries the full sequence. This page stays with the physiology that decides whether an athlete ever reaches those numbers.

03The heat acclimation clock

Near Complete Heat Acclimation Takes 7 to 10 Days

Heat acclimation runs on a known clock. Nearly complete exercise-heat acclimation appears after 7 to 10 days of exposure Pandolf 1998. Two-thirds to 75 percent of the physiological adjustment, and of the improvement in performance, is already present at day 4 to 6. The first week carries most of the change, which is why a preseason built around it looks so different from one that is not.

The pooled literature agrees on the window. A meta-analysis of 96 studies found 7 to 14 days the most common duration and the controlled work-rate approach the most common method Tyler 2016. Heat adaptation had a moderately beneficial effect on exercise capacity and performance. An international consensus panel put it more directly still. Of everything an athlete can do before competing in the heat, repeated exercise-heat exposure over one to two weeks is the single most effective Racinais 2015.

Shorter blocks are not wasted. A five day short-term protocol produces measurable adaptation in moderately and highly trained athletes, and it is cheaper and less disruptive to a taper than a two week program Garrett 2011. Inducible heat shock protein 70 rises across those five days, which is a cellular protective change and not only a cardiovascular one.

What the athlete actually gains

The adaptations are specific and they are all improvements in regulation rather than in tissue. Heat acclimation improves sweating and skin blood flow, lowers body temperatures, reduces cardiovascular strain, improves fluid balance, alters metabolism and enhances cellular protection Periard 2015.

The cardiovascular half of that list is the part an athlete feels. Total body water rises and plasma volume expands. Stroke volume is better sustained, heart rate at a given work rate falls, ventricular filling and myocardial efficiency improve, and skin blood flow and sweating responses are enhanced Periard 2016. The acclimatized athlete earns a lower sweating threshold, a larger plasma volume and a steadier system under load.

Controlled hyperthermia regimens, where a target core temperature is held rather than a fixed work rate, produce more rapid and more complete adaptation than the traditional constant work rate approach Periard 2015. The dose that matters is the thermal one, not the wattage.

The clock runs backward too

Acclimatization decays, and the rate is now measured rather than assumed. Pooling twelve studies, roughly one day of heat acclimation is lost for every two days without heat exposure Daanen 2018. Adaptation in end-exercise heart rate fell 2.3 percent for every decay day, and core temperature adaptation fell 2.6 percent.

How the block was built decides how long it lasts. Core temperature adaptations were more durable when the daily heat exposure was longer and less intense, while sweat rate adaptations held better after protracted regimens and after high intensity exposures. Two athletes can finish the same ten day program with the same numbers and lose them at different rates.

Why August is the dangerous month

The clock leaves a mark in the surveillance data. Across four school years and 100 sampled American high schools, time-loss heat illness occurred at 1.6 cases per 100,000 athlete-exposures, and 66.3 percent of those cases fell in August CDC 2010. Football carried a rate of 4.5 per 100,000, ten times the average of 0.4 across the eight other sports studied, and no deaths were reported in the sample.

August is not the hottest month everywhere it happens. August is when the unacclimatized state meets the first hard heat exposure of the year. The consensus recommendation names the same window, asking organizers to schedule with the health risk in mind during the first hot days of the year and in mass participation events Racinais 2015.

04One climate, many answers

The Same Heat Protocol Produced a Six-Fold Spread in Adaptation

Environment is the cleanest natural experiment in sport, because the input can be held constant while the athletes cannot. Seventeen men with a mean maximal oxygen uptake of 58.8 mL per kg per minute completed the same ten day heat acclimation at 40 degrees Celsius and 50 percent relative humidity Corbett 2018. The mean reduction in end-exercise rectal temperature was 0.70 degrees Celsius.

The mean is the least interesting number in that study. Individual reductions ran from 0.20 to 1.32 degrees Celsius. One protocol, one chamber, one duration, and a six-fold difference in what came out. The variance was not explained by aerobic capacity, by any history of previous heat acclimation, or by the accrued thermal dose.

The heat physiology literature reaches the same place from its own side. Exertional heat stroke is described as a function of both intrinsic and extrinsic modulators Epstein 2011. Genetics, fitness, acclimatization, illness, medications and sleep quality sit on one side. Exercise intensity, clothing, ambient temperature, humidity and solar radiation sit on the other.

That is the finding the Unified Model of Tone was built to expect. A chamber set to 40 degrees Celsius is not one input. It is seventeen different inputs, because what an exposure does is decided by the organization it lands in. Heat is a demand. What answers it is a state, and the state was different in each of those men before the door closed.

The controlled version of the same experiment

Altitude supplies the version with the state deliberately manipulated. Unacclimatized adults were taken to 4300 m by four routes. Direct ascent produced acute mountain sickness in 83 percent. Two days of staging at 3000 m before the ascent brought that to 43 percent, and staging at 3500 m to 40 percent Beidleman 2018.

The destination was identical in every arm. Neither activity level during staging nor sex changed the outcome. What changed was the state of the athlete arriving, and the incidence of illness moved by more than half. Acclimatization is the state changing to meet the input, and the staged groups had simply started that change earlier.

The starting state can be read before the dose is given

The heat data carry the same lesson in a form a practitioner can use. In the acclimation study, responses to the very first heat stress test were related to how much adaptation followed. The change in mean skin temperature during that first test correlated with the later reduction in the rise in rectal temperature at a coefficient of minus 0.676 Corbett 2018.

The authors are careful that some of those initial responses were themselves shaped by how the test was designed, and that metabolic heat production correlated with both. The signal is real and it is not clean. Read that way, a first exposure is not only a dose. It is a measurement of the system about to be dosed.

One result in that paper points the other way and belongs on the page in full. The adaptation indices were mainly independent of one another across individuals, so a high responder on the thermal index was not reliably a high responder on the sudomotor, cardiovascular or hematological ones. Those are four effectors adapting on four schedules, which is a different question from whether the regulator behind them is one thing.

05The cold answer

Cold Is Met by Vasoconstriction First and Shivering Second

Cold exposure produces two acute responses, and they arrive in order. Cutaneous vasoconstriction reduces heat loss, and shivering thermogenesis raises heat production Castellani 2016. Vasoconstriction is elicited both by reflex and by local cooling of the tissue itself, and together the two responses hold thermal balance while the body is losing heat.

The first response is the body pulling blood inward to make skin, fat and muscle into an insulating shell that protects the core at the expense of the limbs. The second is fuel being burned for warmth in skeletal muscle, involuntarily as shivering and voluntarily as movement. Hypothermia is defined at a core temperature below 35 degrees Celsius, and severity climbs as shivering fails and coordination and judgment go with it.

Who holds core temperature is not decided by fitness alone. Anthropometry, sex, race, fitness and thermoregulatory fatigue all shape the acute response to cold Castellani 2016. Body build sits in that list alongside conditioning rather than beneath it, which is why a lean, extremely fit athlete can be the first one shivering.

Cold acclimatization is habituation, not toughness

Repeated cold exposure produces three distinct patterns, and only one of them is what an athlete usually means by getting tougher. Habituation attenuates the response compared with the unacclimatized state. Metabolic acclimatization raises thermogenesis. Insulative acclimatization strengthens the mechanisms that conserve heat Castellani 2016.

Which pattern appears depends on how skin and core temperature actually changed and for how long. That is the input law again in a second environment. The same winter produces a habituated athlete, a thermogenic one, or a better insulated one, according to what the exposure did to the state that met it.

The governing position stand draws the same conclusion and turns it into policy. Cold-weather clothing should be chosen for each individual, and standardized clothing ensembles should not be mandated for a whole group Castellani 2006.

That is an unusual sentence to find in a position stand, and it is the input law written as a rule. The same panel sets a hard environmental number: heightened surveillance of exercisers is recommended at wind-chill temperatures below minus 27 degrees Celsius, where relative frostbite risk climbs.

Water is the variable that changes everything

Water changes the arithmetic of cold. Immersion is a precursor to drowning, cardiac arrest and hypothermia, and the same immersion is used deliberately to extend exercise time in the heat and to cool hyperthermic casualties Tipton 2017. One intervention, opposite consequences, decided entirely by the state of the person entering the water.

That review is candid about how uneven the evidence is across the claims made for cold water immersion, with some supported and some resting on anecdote. For an athlete the practical reading is narrow and firm. Cold water is a powerful input in both directions, and which direction it takes is a property of the athlete, not of the water.

06Thin air

Altitude Illness Begins Far Below Mountaineering Altitude

Altitude is an oxygen problem before it is a mountain problem. Among 454 people attending week-long programs at Rocky Mountain resorts with base elevations of about 2000 m, 25 percent reported acute mountain sickness by symptom criteria Montgomery 1989. In a sea level comparison group the figure was 5 percent. Half of those with symptoms took medication for them.

The time course in that study is the useful part for an athlete traveling to compete. Ninety percent of all reported symptoms occurred in the first 72 hours, and incidence was highest among people who had come from lower altitudes. Arriving from sea level three days before an event places the competition inside the worst window rather than after it.

More than 80 million people live permanently above 2500 m, and acute exposure can trigger acute mountain sickness, high altitude cerebral edema and high altitude pulmonary edema Gatterer 2024. All three are consequences of altitude hypoxia. Prevalence tracks acclimatization status, rate of ascent and individual susceptibility, which is the same three-part answer heat gave.

The two forms that end the trip

Acute mountain sickness is headache, nausea, dizziness and fatigue, and it is usually benign and self limiting. The two dangerous forms are not. High altitude cerebral edema follows disruption of the blood brain barrier and announces itself through the nervous system, with altered mental status and ataxia. High altitude pulmonary edema follows raised pulmonary capillary pressure and stress failure, and shows as breathlessness, cough and exercise intolerance Gatterer 2024.

Both are progressive and life threatening, and both require immediate medical intervention. Treatment is supplemental oxygen and descent, with drug therapy as appropriate. Prevention is slow ascent and pre-acclimatization. An ataxic athlete at altitude is a neurological emergency and is treated as one.

What thin air does to the nervous system

The performance question is what happens well short of illness, and the finding is narrower than the folklore. Ten healthy adults spent 30 minutes in isocapnic hypoxia at two severities. Reaction time was impaired by 4.6 percent in severe hypoxia at an end-tidal oxygen pressure of 45 mmHg, and was not impaired at 55 mmHg Caldwell 2018.

Complex cognitive performance, including psychomotor speed, cognitive flexibility, processing speed and executive function, was unaffected at either severity, and neurovascular coupling did not change. Speed of response went before thinking did. Reaction time is the readout that moves first, and Reaction Time and Motor Control carries how it is measured and why its variability matters more than its mean.

Sleep is the other casualty of altitude, and it is a large one for a traveling team. That literature, including what disrupted sleep does to next day performance, belongs to Sleep and the Athlete.

07Live high, train low

Living High and Training Low Improved 5,000 m Time by 13.4 Seconds

Altitude acclimatization can be separated from altitude training, and separating them is the whole idea. Thirty-nine competitive runners, 27 men and 12 women, completed a supervised sea level block and were then randomized into three four week camps Levine 1997. One group lived at 2500 m and trained at 1250 m. One lived and trained at 2500 m. One lived and trained at 150 m.

Both altitude groups raised maximal oxygen uptake by 5 percent, in direct proportion to a 9 percent increase in red cell mass volume, and the sea level control raised neither. Only the group that lived high and trained low improved its 5,000 m time, by 13.4 seconds. Velocity at maximal oxygen uptake and maximal steady state also improved only in that group.

The mechanism is legible in the design. Living high bought the oxygen carrying capacity. Training low preserved the running velocities that convert capacity into a race. Living high and training high bought the blood and lost the speed, which is why the older altitude camp underperformed the version that split the two.

The pooled picture, including where it is unclear

The meta-analytic view is more mixed than the single trial, and the mixture is worth stating plainly. Across 51 studies, enhancement of endurance power output in subelite athletes was very likely with artificial brief intermittent live high train low, at 2.6 percent Bonetti 2009. It was likely with natural live high train low, at 4.2 percent.

The same analysis found the effect unclear for live high train high at 0.9 percent, for artificial brief continuous exposure at 0.7 percent, and for live low train high at 0.9 percent. In elite athletes only natural live high train low reached possible enhancement at 4.0 percent, and every other protocol was unclear. The authors also report evidence that some of these effects were mediated at least partly by placebo, nocebo and training-camp effects.

A page that skipped that paragraph would be selling something. This literature holds a strong signal for one protocol in one population and a weak signal for the rest. It also holds a live question about how much of a camp effect is the mountain. Maximal oxygen uptake and the energy systems that altitude is trying to move belong to Energy Systems and Exercise Physiology.

The dose is hours per day, not only meters

Altitude has a prescription and it is measured in exposure. The optimal living altitude has been placed at 2200 to 2500 m for the erythropoietic effect and up to 3100 m for non-hematological adaptations Millet 2010. Living at 2500 m for 20 to 22 hours a day is enough to raise erythropoiesis and improve sea level performance, and the minimum daily dose for stimulating erythropoiesis appears to be 12 hours.

The two adaptation tracks run on different clocks. Roughly four weeks is required for accelerated erythropoiesis, while under three weeks is long enough for gains in economy, muscle buffering capacity, the hypoxic ventilatory response and sodium potassium ATPase activity. A three week camp and a five week camp are not the same intervention, and they are not aimed at the same adaptation.

Ventilation is the adaptation an athlete notices least and uses most. Red cell production, total blood volume and pulmonary ventilation all rise to carry and deliver more oxygen. The ventilatory change arrives well before the hematological one, since the hypoxic ventilatory response shifts inside three weeks and accelerated erythropoiesis takes about four Millet 2010.

Endurance programs use live high, train low to bank those oxygen carrying gains while keeping training velocities intact. That is the practical shape of the whole altitude argument: acclimatize where the air is thin, and do the hard work where the legs still move fast.

08Adaptation you can plan

Acclimatization Is a Trainable Adaptation, Not a Fixed Trait

Acclimatization is a trainable neural, hormonal and cardiovascular adaptation, not a fixed trait, and that is the central message for any athlete heading into a hostile climate. Heat acclimation works largely independently of general conditioning, since aerobic fitness leaves an athlete partially but not fully acclimated Pandolf 1998. Fitness is not a substitute for exposure.

The autonomic nervous system sits underneath all of it, tuning vasoconstriction, vasodilation, sweat rate and shivering. Those four effectors do not each hold a private opinion about the weather. They are driven from one central integrative state, which is why acclimatization shows up simultaneously in heart rate, plasma volume, skin blood flow and sweating Periard 2016.

What the athlete controls, and what reads it

Hydration, fueling, layered clothing, staged arrival and graded exposure are the inputs. The nervous system is the system that turns those inputs into a body that performs at the edge of the climate. Every recommendation in the heat consensus is an attempt to change either the input or the state that meets it Racinais 2015.

That is where a practice built around neurological measurement stands beside the rest of the performance staff rather than behind it. Clean sensory input, balanced autonomic tone and the regulation of core temperature and oxygen delivery are not three subjects. They are one state read through three instruments, and a clinician who records that state before the block has a baseline to read the athlete's response against.

09Figures removed

Nine Figures Removed From This Page and Three Corrected

This page previously carried numbers that could not be traced to a primary source. They are gone, and what replaced each one is named here. Radiation shedding roughly half of body heat at rest, and evaporation accounting for about thirty percent, are both removed. The mechanism they were used to teach survives without them: as air temperature rises toward skin temperature, evaporation is the avenue that remains.

Three altitude figures are removed because none could be sourced. The first two: hypoxia effects beginning at 1615 m, and acute mountain sickness appearing 6 to 96 hours after exposure. The third is a short term adaptation of two weeks at 2300 m, with one added week for each further 610 m. The measured 25 percent incidence at about 2000 m stands in their place, along with the finding that 90 percent of symptoms occur inside 72 hours Montgomery 1989.

The claim that the acclimatized state lasts only two to three weeks back at sea level is removed. The measured altitude dosing figures replace it Millet 2010. Three cold figures are removed as well: water carrying heat about seventy times faster than air, wet clothing at 5 degrees Celsius doubling heat loss, and hypoglycemia blunting shivering directly through the central nervous system.

Three claims are corrected rather than deleted. Heat acclimatization was given as ten to fourteen days, and near complete acclimation is reached at 7 to 10 days Pandolf 1998. The statement that body composition rather than fitness predicts who holds core temperature is narrowed: anthropometry, sex, race, fitness and thermoregulatory fatigue all shape the cold response Castellani 2016.

The heat stroke threshold is raised from 40 degrees Celsius to the figure the physiology literature actually reports. Marked hyperthermia in heat stroke is usually above 40.5 degrees Celsius, and it occurs together with encephalopathy Epstein 2011.

One more removal is not a figure. A gold pull-quote on this page was attributed to Dr. Jason Dulberg and he never said or wrote it, so it is gone. Everything left standing is either sourced to a named investigator or labeled as the model's own reading.

10The model's claim

Acclimatization Is One State Changing, Read Through Several Instruments

Two layers run through this page. The measurements belong to the investigators who made them. They include the 7 to 10 day acclimation window, the 2.3 percent per day decay in heart rate adaptation, and the 0.20 to 1.32 degree spread across seventeen men. The 9 percent rise in red cell mass belongs to them, and so does the drop in altitude sickness from 83 to 40 percent after two days of staging.

What the Unified Model of Tone supplies is an account of why an environment behaves like a dose. Heat, cold and thin air are inputs. What meets them is a nervous system already organized a particular way, and the result is a property of the meeting rather than of the weather. That is why the same chamber produced a six-fold spread, and why staging changed a mountain that never moved.

The prediction this page makes

The model treats sweating, skin blood flow, heart rate, core temperature and autonomic recovery during acclimatization as readings of one regulatory state rather than as separate systems each learning separately. That is a claim about how adaptation is organized rather than a claim about what treatment does, and the instruments to test it already sit in every heat tolerance test.

The design is a standard heat tolerance test repeated across an acclimatization block, with four things recorded in the same athletes on each occasion. Core temperature at a fixed work rate. Heart rate at that work rate. RMSSD after the test. Time for heart rate to return to baseline once work stops.

Two of the four have already been recorded together. Across a structured natural acclimatization, core temperature, heart rate variability and circulating nephrines all separated short term from long term acclimatization status in the same participants Stacey 2018. Post-test logRMSSD differed by 9.9 percent and the logLF to HF ratio by 18.6 percent between day 6 and day 23.

One published result cuts the other way and sharpens the test. Across seventeen men, the magnitudes of thermal, sudomotor, cardiovascular and hematological adaptation were largely independent of each other Corbett 2018. Those are four effectors adapting at four rates, measured once before and once after. The model's claim is about four readouts tracked repeatedly through the block, which is a different measurement and has not been made.

If core temperature at a fixed work rate, heart rate at that work rate, post-test RMSSD and time to return to baseline are shown to move together within athlete, the unification claim is confirmed.

11The tone reading

Heat, Cold and Altitude as Inputs Meeting a State

Three signatures of tone show up in this page, each in a measurement a program already takes.

The rest of the library carries the same logic through its other foundations. Gain is how hard the sweating and vasoconstrictor responses answer a given thermal error, and heat acclimation raises it while cold habituation lowers it. Coupling is why skin blood flow and muscle blood flow cannot be set independently, since one cardiac output serves both. Prediction is what pre-acclimatization exploits, because two days of staging changed a 4300 m ascent the body had not yet met. Load is the thermal or hypoxic demand itself, and constraint is what happens when a system defends temperature so hard that little capacity is left for the race. Oscillation is the beat-to-beat rhythm whose structure heart rate variability reads during acclimatization. The argument that these readings are one variable rather than several is set out in the Unified Model of Tone.

12Where this sits

How This Page Relates to the Rest of the Library

Seven places this argument continues, each with the claim that earns the link.

13Questions athletes ask

Questions Athletes Ask

14The sources

References

25 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.