Central Fatigue Under Heat Stress: Neuroscience Research on How Elevated Brain Temperature Influences Exercise Termination

This article is based on international peer-reviewed journals such as the Journal of Applied Physiology, Medicine & Science in Sports & Exercise, and Sports Medicine. It provides an in-depth analysis of the environmental physiology mechanisms behind “heat-induced central fatigue,” combining Taiwan’s subtropical climate, mountainous terrain, and local racing contexts to offer evidence-based training and preparation strategies.

Within the field of environmental physiology, “heat-induced central fatigue” is one of the key variables determining whether an endurance athlete can perform at their best on a real race course. Many amateur and elite athletes focus their training on power, pace, and equipment, underestimating the massive impact of the environment—heat, cold, altitude, humidity, and air quality—on human physiology. In fact, when two athletes have similar fitness and equipment, the one who better understands how to adjust strategies to the environment is often the key to maintaining pace in the latter stages of a race and avoiding collapse or mishaps. Taiwan’s geographical environment is particularly unique: featuring hot and humid summers, cold and damp winters, extreme altitude variations from sea level straight up to 3,275 meters at Mount Wuling, plus urban air pollution and typhoon seasons, making the practical value of environmental physiology especially prominent in the local sports community. This article will guide you from cellular and systemic physiological mechanisms, through empirical research from top international journals, quantitative relationships between dose and effect, differences in responses among different populations, to directly actionable training applications and Taiwan’s local context, finally debunking long-standing myths to ensure your understanding of “heat-induced central fatigue” is truly grounded in science rather than hearsay.

Review of Academic Research

Regarding the scientific exploration of “heat-induced central fatigue,” the field of environmental physiology has accumulated rigorous and rich evidence. The following selection of representative studies, each valuable in methodological design, subject populations, and conclusion strength, collectively builds our current understanding:

1. Nybo & Nielsen (2001). In the Journal of Applied Physiology, central motor drive was confirmed to decrease when core body temperature reached 40°C, verified via EEG and electromyography.

2. Nielsen et al. (2001). In the Journal of Physiology, exhaustion was found to be highly correlated with an absolute core body temperature threshold, supporting the “critical temperature” hypothesis.

3. Rasmussen et al. (2004). In the Journal of Physiology, the impact of changes in cerebral blood flow and metabolism on fatigue perception under high heat was quantified.

4. Roelands & Meeusen (2010). In Sports Medicine, a review was conducted on the roles of dopamine and serotonin in the brain regarding heat-induced central fatigue.

Overall, these studies show that the scientific picture of “heat-induced central fatigue” has deepened continuously with advancements in measurement technology. Early research often manipulated temperature, humidity, or oxygen partial pressure in environmental chambers to observe changes in VO2max, time to exhaustion, or time-trial performance. Subsequent research introduced ingestible core temperature capsules, near-infrared spectroscopy, stable isotope tracing, muscle biopsies, and molecular markers, allowing us to move from ‘phenomenological observation’ to ‘mechanistic explanation.’ Notably, most high-quality studies employed randomized crossover designs, where each subject served as both experimental and control condition, significantly reducing noise from individual differences. However, extrapolating research conclusions must be done cautiously: single environmental variables controlled in the lab do not necessarily equate to the complex scenarios on a real race course involving heat, humidity, wind, radiation, and fatigue; responses from trained subjects may not apply to general amateurs. When interpreting ‘statistical significance’ and ‘effect size,’ it is crucial to distinguish between average trends in the lab and actual significance on an individual’s race course—a 3% performance difference may decide rankings in elite competition but holds relatively limited significance for recreational riders. It is this caution regarding evidence hierarchy and applicability that forms the foundation of scientific training.

Core Mechanisms

Fatigue under heat is not solely from muscles but originates from the central nervous system. When core body temperature approaches approximately 40°C and brain temperature rises, the brain’s voluntary motor drive decreases. Even if the muscles themselves still have capacity, the central nervous system actively reduces motor unit recruitment to protect the body from overheating; this is the “critical temperature” hypothesis. At the level of neurotransmitters, heat alters the balance of dopamine and serotonin in the brain: increased serotonin is associated with fatigue and decreased motivation, while maintaining dopamine activity helps delay central fatigue. Additionally, heat causes reduced cerebral blood flow and decreased cerebral oxygenation, further weakening central function. This explains why feeling “very tired” often precedes true metabolic exhaustion in hot environments.

To truly understand how “heat-induced central fatigue” affects athletic performance, one must return to the systemic integration of thermoregulation, cardiovascular, respiratory, metabolic, and central nervous systems. Environmental factors can be translated into measurable performance differences precisely because they act on one or more critical links in this physiological chain—either limiting the delivery of oxygen and fuel, interfering with heat dissipation and fluid balance, or altering central fatigue perception and motor drive. The table below summarizes key points of action at different physiological levels for this topic, helping you build a complete mechanistic picture:

Physiological Level	Key Mechanism	Significance for Athletic Performance
Cardiovascular System	Affects blood flow distribution, stroke volume, and circulating volume	Determines oxygen delivery and stability for prolonged exercise
Temperature/Fluid Regulation	Regulates heat dissipation pathways, sweat rate, and electrolyte balance	Affects the rate of core temperature rise and dehydration risk
Muscles and Metabolism	Alters substrate utilization, enzyme activity, and contraction function	Determines sustainable power and timing of fatigue onset
Central Nervous System	Regulates fatigue perception, motor drive, and cognitive decision-making	Affects ‘how tired one feels’ and ability to persist and make safe judgments

Special emphasis must be placed on the dimensions of “dose-response” and “time dynamics.” The same environmental exposure can produce vastly different, even opposite effects under different intensities, durations, and adaptation states—this is precisely why many popular suggestions are one-sided. Limiting factors for athletic performance also shift dynamically with context: short-duration high-intensity efforts are limited mostly by anaerobic energy supply and local metabolism, while multi-hour long-distance endurance events shift towards the combined effects of rising body temperature, fluid imbalance, glycogen depletion, and central fatigue. “Heat-induced central fatigue” is worth in-depth exploration precisely because it can target certain of these limiting factors. Understanding the mechanisms allows us to judge ‘what environment requires adjustment, how much to adjust, and when to adjust,’ rather than being led by the environment on the race course. The more thoroughly one understands the mechanisms, the more flexibly one can adapt between different contexts such as heat, cold, altitude, or pollution. This ability to adjust according to context is the dividing line between those who understand environmental physiology and those who train blindly.

Dose-Response Relationship

In environmental physiology, the principle that “dose determines effect” is central. Environmental stimuli that are too low fail to reach the physiological threshold, producing no adaptation or effect; exposure that is too high may exceed the body’s compensatory capacity, triggering risks such as heat injury, hypothermia, and altitude sickness. The table below summarizes the dose-response relationship for “heat-induced central fatigue,” serving as the most critical quantitative reference when formulating training and competition plans:

| Dose / Condition | Physiological State | Effect & Key Points |

|—|—|—|

| Core Body Temperature 38.5°C | Increased Perceived Load | Slight pace adjustment maintains performance |

| 39.0–39.5°C | Central Drive Begins to Decline | Perceived Exertion Increases Significantly |

| ~40°C | Critical Body Temperature | Motor Drive Strongly Suppressed, Near Exhaustion |

| >40°C | Danger Zone | Risk of Heat Injury, Must Stop Immediately |

From the table above, it can be seen that the effects of environmental exposure often follow a threshold-type or inverted U-shaped curve: before reaching an effective dose, adaptation or effect increases with dose; but beyond a certain critical point, there are no additional benefits, and risks and costs rise sharply. This means that “finding your optimal exposure” is far more important than “pursuing extremes at all costs.” Whether it is the daily heat dose for heat acclimatization, the altitude and duration for high-altitude training, or the exposure time in cold environments, there exists a sweet spot that balances efficacy and safety. It is recommended to gradually test responses under different environmental conditions during training (rather than on competition days), recording core body temperature or heart rate, perceived exertion, power data, and recovery status to build a personal environmental response profile. Remember: the average from the laboratory is a starting point, not an endpoint; individual factors such as body type, sweat rate, adaptation level, and genetic background will cause individualized shifts in the optimal dose. Only by calibrating with your own data can population science be transformed into a personalized prescription.

Differences Among Populations

The impact of “heat-induced central fatigue” is not equal for everyone. Age, gender, training level, body type, adaptation status, and genetic background all significantly modulate an individual’s response magnitude to the environment. Applying a single recommendation while ignoring these differences is one of the most common errors in environmental physiology applications.

| Population Aspect | Response Characteristics | Practical Recommendation |

|—|—|—|

| Beginner vs. Advanced | Advanced athletes have more mature environmental adaptation and better tolerance, but less room for marginal adaptation | Beginners should adopt a conservative, progressive approach, first establishing basic tolerance before increasing load |

| Male vs. Female | Differences in body surface area to weight ratio, hormonal cycles, and sweat composition | Women should undergo individualized assessment of heat dissipation and hydration, paying attention to energy availability |

| Young vs. Older | Older adults have reduced thermoregulation, sweat gland function, and compensatory capacity | Older adults are more sensitive to extreme environments and require greater caution and longer adaptation periods |

| Body Type Differences | Body surface area, body fat, and muscle mass affect heat dissipation and heat production | Larger body types dissipate heat more slowly, while smaller body types lose heat faster, each carrying its own risks |

When interpreting individual differences, one must also be wary of a statistical trap: research reports mostly reflect “average group responses,” but beneath the average often lies huge individual variability. Within the same environmental intervention, some individuals may be strong responders, some may show almost no response, and some may even respond in the opposite direction. This is why, even if a study shows “average effectiveness,” you still need to confirm through your own trials which category you belong to. It is recommended to conduct personalized control tests: in two training sessions under similar conditions, compare performance using and not using a certain environmental strategy (such as pre-cooling or heat acclimatization), comparing power, core body temperature, heart rate, and subjective feelings, repeating several times before drawing conclusions.

Taking the common amateur endurance population in Taiwan as an example, many are middle-aged riders and runners over 35 years old who train after work. This group’s thermoregulation and recovery capabilities are slightly inferior to young elites. When facing humid and hot summers and high-altitude challenges, correct environmental strategies (heat acclimatization, individualized hydration, progressive ascent) can bring relatively greater safety and performance benefits. Female athletes need to pay attention to the impact of the menstrual cycle on core body temperature baseline and fluid regulation, as well as whether energy availability is sufficient. Understanding population differences leads to the realization that truly professional environmental physiology advice is always an “individualized” prescription tailored to the person, not a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately be implemented into training schedules and race courses. The following provides a practical framework for translating “heat-induced central fatigue” into specific training and competition operations:

• Pre-Race Environmental Drills: All environmental strategies must first be practiced in training; “never try new methods on race day” is a golden rule. Whether for heat acclimatization, pre-cooling, or hydration pacing, the body needs time to build tolerance and proficiency.

• Periodized Thinking: Align environmental adaptation with training cycles—build tolerance during the base phase and conduct specialized environmental adaptation for target race conditions (temperature, humidity, or altitude) during the pre-competition phase.

• Progressive Exposure: Start with mild environmental stimuli and gradually increase the dose based on the body’s response, building a personalized exposure dose and timing profile to avoid injury from a single excessive exposure.

• Objective Data Monitoring: Combine core body temperature (or heart rate drift), power, perceived fatigue (RPE), and urine/body weight changes to objectively evaluate whether environmental strategies are truly effective.

• Overall Context: Environmental adaptation is part of training, sleep, recovery, and nutrition; a single strategy cannot compensate for lack of sleep, dehydration, or flaws in training design.

Taking a weekly training plan as an example, different environmental scenarios can be practiced in key weekday workouts and weekend long rides: high-intensity days focus on maintaining power quality under target environmental conditions and testing heat dissipation or insulation gear; long-distance days focus on hydration and sodium pacing, thermoregulation, and prolonged gear tolerance. Through repeated drills, the body can respond to environmental conditions in a nearly automated manner on race day, leaving mental resources for pacing and tactical judgment. One of the most common mistakes is “seriously facing the environment only on race day, while casually handling it during daily training”—this is precisely putting the cart before the horse. Daily training is the best laboratory for building heat tolerance, testing hydration doses, familiarizing with equipment, and cultivating environmental response rhythms.

It is recommended to integrate environmental logs with training logs, recording daily temperature, humidity, altitude, wind conditions, environmental strategies employed, body responses, and performance data for each key workout. After accumulation over several weeks to months, the value of this personalized database will far exceed any generic guide. Additionally, do not overlook “recovery in the environment,” an often underestimated aspect—high heat or high altitude delays recovery and exacerbates fatigue accumulation. The quality of recovery between consecutive training days often determines whether you can steadily accumulate training volume without injury, and training volume is the fundamental engine for long-term progress. Treating environmental response as a serious part of training rather than a last-minute remedy before a race will make a significant difference in your progress and safety.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and racing culture add distinct local color to the application of “heat-induced central fatigue.” Summer temperatures are high and humid, with heat index often exceeding 35°C and relative humidity frequently over 80%, resulting in low evaporative cooling efficiency. Sweat and electrolyte loss are far greater than in temperate country studies, meaning recommendations from foreign literature often need upward adjustment. The winter Northeast Monsoon brings damp cold and strong wind chill, with mountainous areas posing risks of low temperatures and hypothermia. The extreme altitude difference from sea level to Wuling Peak at 3,275 meters makes high-altitude physiological challenges immediately accessible.

Taking examples such as riding to Wuling Peak, the Hualien Island Loop, the Sun Moon Lake Lake Loop, the Taroko Marathon, various triathlons, and island-wide challenges, athletes should incorporate local and seasonal environmental factors into their planning: for summer races, start early to avoid the high WBGT of the afternoon, and take advantage of the dense convenience store network along the route to enhance water and sodium replenishment; for high-altitude sections, allow for progressive adaptation and pay attention to low-temperature insulation and signs of altitude sickness; for winter damp and cold races, enhance wind protection and insulation and hypothermia prevention. Utilizing the Central Weather Bureau’s forecasts for heat index, humidity, wind speed, and air quality to convert them into daily environmental risks and strategies will allow athletes to balance safety and optimal performance in Taiwan’s variable and harsh environment.

Myth Busting

Myth: “Hitting the wall is entirely due to depleted muscle glycogen.” Under high temperatures, central fatigue often occurs before glycogen depletion, as the brain applies the brakes first to protect body temperature. Therefore, cooling strategies (pre-cooling, water pouring) can sometimes delay exhaustion more effectively than carbohydrate replenishment.

This myth persists so widely because it sounds plausible, is easily spread orally, or is amplified by marketing and anecdotal evidence. However, the value of science lies in rigorously testing intuition with evidence: many seemingly obvious environmental concepts do not hold up under controlled environmental chamber experiments and epidemiological surveys. The field of environmental physiology is particularly rife with oversimplified statements that compress complex variables such as dosage, timing, individual differences, and risks into a single slogan. Next time you hear a definitive environmental recommendation, ask yourself: “What is the evidence level for this claim? Who is it applicable to? Are the dosage, timing, and safety margins clearly defined?” Cultivating this evidence-based critical thinking is more valuable than memorizing any single conclusion and is a crucial step for athletes to move towards scientific training and avoid environmental harm.

Conclusion

“High-temperature-induced central fatigue” is a topic in environmental physiology that combines theoretical depth with practical value. The international journal evidence reviewed in this article shows that the impact of the environment on athletic performance is real and profound, but it is certainly not uncontrollable—the key lies in understanding the mechanisms, mastering dosage, individualized adjustments, and synergy with overall training, recovery, and nutrition. For endurance sports enthusiasts in Taiwan, while mastering scientific principles, it is essential to combine them with the local humid and hot climate, extreme altitudes, and variable weather conditions, transforming general rules into personalized prescriptions suitable for oneself. May every rider and runner sweating on Wuling, in the valley, or on the环岛 road be able to protect themselves and break through limits through the wisdom of environmental physiology, enjoying the purest joy of sports amidst high temperatures, cold, high altitudes, and various challenges. Before stepping onto the track next time, remember—the true opponent, besides the stopwatch, is the entire environment beneath your feet and surrounding you.

Related Reading

• Cognitive Function Impairment in High-Temperature Environments: Impact of Heat Stress on Decision-Making Abilities
• Intermittent Training Design in Hot Environments: Body Temperature Regulation Strategies for Recovery Intervals
• Cerebral Hemodynamics in Hot Environments: Impact of High Temperatures on Brain Oxygenation
• Physiological Mechanisms of Heat Acclimatization Training: Longitudinal Study on Plasma Volume Expansion and Sweat Rate Increase