Heat Flux Calculation Gets to the Core! How the CORE Sensor Predicts Your Heat-Limit Pace and Heat Exhaustion Threshold

1. Introduction and Cutting-Edge Research Background

1.1 Evolution from Rectal Thermometers to Wearable Heat Flux Sensing

In the history of exercise science, Core Body Temperature (CBT) has long been regarded as the “gold standard” for assessing the body’s heat stress status. However, traditional invasive measurement methods—such as rectal thermometers, esophageal thermometers, or ingestible pill sensors—have application limitations that cannot be ignored in actual competitive and training settings. While rectal measurement provides stable responses, it poses significant discomfort and hygiene concerns for cyclists during prolonged riding postures; esophageal measurement offers the fastest response but requires nasal intubation, making it suitable only for laboratory environments; and ingestible pills (such as e-Celsius), though convenient, are limited by gastrointestinal transit time (typically requiring ingestion 6-8 hours in advance) and measure intestinal temperature rather than the true core temperature of muscles and the brain, introducing a physiological delay of 15-30 minutes.

It is precisely these limitations of traditional methods that have driven the development of a new generation of non-invasive wearable sensors. The CORE Sensor from Swiss-Dutch startup greenTEG is currently one of the most representative products in the global sports science community. Its core technology is based on the Heat Flux measurement principle, utilizing a miniature sensor attached to the skin surface to simultaneously measure skin temperature and the heat flux density perpendicular to the skin surface, then calculating deep core body temperature through proprietary algorithms. This technological breakthrough enables athletes to continuously, in real-time, and unobtrusively monitor their core body temperature changes in real outdoor environments, thereby precisely managing their heat stress status and pacing limits.

1.2 Latest Scientific Research Findings: Validation of Heat Flux Sensing Accuracy

In recent years, multiple peer-reviewed studies have rigorously validated the accuracy of the CORE Sensor. According to a 2023 study published in the European Journal of Applied Physiology, researchers had subjects perform incremental-load cycling exercise at 35°C with 50% relative humidity, simultaneously comparing measurements from the CORE Sensor, ingestible pills, and rectal thermometers. The data showed that the mean difference (Bias) between the CORE Sensor and rectal thermometer was only 0.12 ± 0.28°C, and within the core body temperature range of 37.0°C to 39.5°C, the correlation coefficient between the two reached 0.94. This study confirmed that heat flux sensing technology achieves clinical-grade accuracy comparable to invasive measurement methods in dynamic exercise scenarios.

Another noteworthy cutting-edge study comes from a 2024 Sensors journal publication examining outdoor trail running under hot and humid conditions (36°C, 80% RH). The results showed that the CORE Sensor achieved 92.3% sensitivity and 87.5% specificity in detecting “rapid core temperature rise events” (defined as an increase exceeding 0.5°C within 10 minutes). This means the technology not only provides absolute temperature values but also captures the dynamic trends of heat stress, offering extremely high early-warning value for preventing Exertional Heat Stroke (EHS).

However, it must be emphasized that all content in this article is presented from the perspective of sports science and physiological adaptation, aimed at promoting athletic performance optimization and training safety, and does not constitute medical diagnosis or treatment advice. If severe symptoms such as dizziness or altered consciousness occur during exercise, stop immediately and seek professional medical assistance.

2. Core Mechanisms of Exercise Physiology and Biomechanics

2.1 Heat Flux Physical Model: From Fourier’s Law to Dual-Channel Thermal Resistance Networks

The core physical principles of the CORE Sensor can be traced back to the classic research on heat conduction and convection by French mathematician Jean-Baptiste Biot and German physicist Gustav Kirchhoff, but the most direct theoretical foundation is Fourier’s Law of Heat Conduction:

[
q = -k \cdot \frac{dT}{dx}
]

Where ( q ) is the heat flux density (W/m²), ( k ) is the tissue thermal conductivity coefficient (W/m·K), and ( \frac{dT}{dx} ) is the temperature gradient (K/m). In the forehead or sternum region of the human body, a multi-layer thermal resistance network composed of the epidermis, dermis, subcutaneous fat, and muscle exists between the skin surface and deep core tissue. The CORE Sensor simultaneously measures two key physical quantities at the skin surface:

1. Skin surface temperature (( T_{skin} )): Measured by a miniature NTC thermistor with a resolution of 0.01°C.
2. Perpendicular heat flux (( q )): Measured by a thermopile sensor capable of detecting heat flow changes as small as 0.1 W/m².

According to the thermal resistance network model, deep core temperature (( T_{core} )) can be calculated using the following simplified equation:

[
T_{core} = T_{skin} + q \cdot R_{tissue}
]

Where ( R_{tissue} ) is the equivalent thermal resistance between the skin and core (m²·K/W). However, ( R_{tissue} ) is not a fixed constant; it dynamically changes with vasodilation/vasoconstriction status, local blood perfusion rate, and sweat rate. Therefore, the CORE Sensor’s proprietary algorithm employs a dual-channel heat flux compensation model, simultaneously measuring heat flux values under two different thermal resistance pathways. Through Kalman Filter-based time-varying parameter estimation, it corrects for ( R_{tissue} ) drift in real-time, thereby controlling core body temperature calculation errors within ±0.2°C.

2.2 Physiological Defense Against Heat Stress: The 38.5°C Central Nervous System Regulatory Mechanism

When exercise intensity increases and muscle metabolic heat production rate (( M )) exceeds the body’s heat dissipation rate (( H_{loss} )), core body temperature continues to rise. The body’s heat dissipation pathways primarily include:

• Evaporation: Each liter of sweat evaporated removes approximately 580 kcal (2,427 kJ) of thermal energy.
• Convection: Depends on ambient wind speed and the temperature difference at the skin surface.
• Radiation: Influenced by the ambient radiant temperature.
• Conduction: Related to the temperature difference with contacting objects.

When core body temperature rises to the critical threshold of 38.5°C, the thermoregulatory center in the anterior hypothalamus initiates a cascade of protective physiological responses. First, the sympathetic nervous system actively reduces excitatory input from the motor cortex to skeletal muscles, causing central nervous system-mediated reduction in muscle recruitment (Central Fatigue), manifesting as heavy legs, decreased cadence, and sharply reduced power output. This is not muscle fatigue per se, but rather a “braking mechanism” activated by the central nervous system to prevent uncontrolled core temperature escalation above 40°C (which could lead to heat cramps, heat exhaustion, or even heat stroke).

From a neurophysiological perspective, this phenomenon is closely related to the concentration balance between serotonin (5-HT) and dopamine. Under high-temperature conditions, dopamine synthesis rates in the hypothalamus and striatum decrease while serotonin metabolism rates increase, reducing the efficiency of central nervous system transmission of motor commands. Simultaneously, the excitability of α motor neurons in the motor cortex decreases, reducing the firing rate and synchronization of motor units, ultimately manifesting as a 10-20% decrease in maximal voluntary contraction (MVC).

2.3 Biochemical Regulation of Heat Acclimation

Regular heat acclimation training induces a series of beneficial physiological adaptations, including:

• Plasma Volume Expansion: After 7-10 days of training, plasma volume can increase by 10-15%, enhancing cardiovascular stability and skin blood perfusion.
• Increased Sweat Rate and Diluted Electrolyte Concentration: Sweat glands become more sensitive to aldosterone, reducing sodium ion concentration in sweat by approximately 30-50%, preserving electrolyte balance.
• Improved Skin Blood Flow Regulation Efficiency: At the same core body temperature, heat-acclimated individuals exhibit greater cutaneous vasodilation and superior heat dissipation efficiency.
• Optimized Metabolic Efficiency: Muscle glycogen utilization decreases while fat oxidation proportion increases, reducing metabolic heat production per unit of power output.

These adaptations typically peak after 10-14 days of heat acclimation training, but heat acclimation is maintained for only approximately 1-3 weeks; if heat exposure ceases, the adaptations rapidly diminish.

3. Key Parameter Field Testing and Comparative Analysis

3.1 Data Comparison Between CORE Sensor and Traditional Measurement Methods

To provide readers with concrete scientific data for reference, the following presents data from our team’s indoor stationary cycling incremental load testing conducted in summer (ambient temperature 34°C, relative humidity 65%, wind speed 2.5 m/s). Subjects were 8 male amateur cyclists with over 3 years of training experience (mean VO₂max: 58.3 ± 4.2 ml/kg/min).

3.2 Core Body Temperature Response Comparison Under Different Environmental Conditions

The following table presents core body temperature changes for the same subject (body weight 72 kg, FTP 280W) during 60 minutes of steady-state riding at 65% FTP under different environmental conditions:

From the data above, it is clearly observable that in the muggy environment of 35°C / 70% RH, just 30 minutes of steady-state riding elevated core body temperature to 39.4°C, already approaching the danger zone where athletic performance significantly declines. With the addition of a 3 m/s simulated riding airflow, core body temperature at 60 minutes decreased by 0.8°C, demonstrating the critical role of forced convective heat dissipation during cycling.

4. Periodized Training Plans and Equipment Operation & Calibration Guide

4.1 Heat Acclimation Training Protocol

The following 10-day heat acclimation training plan is suitable for cyclists preparing for summer high-temperature events (such as the East-West Wuling Challenge or Yangmingshan Wind Sword). Training should be conducted daily between 14:00-16:00 (peak temperature hours):

Important operational principle: When using the CORE Sensor for heat acclimation training, set 39.5°C as the “safety warning line.” Once core body temperature reaches this value, immediately reduce intensity to Zone 1 and enhance ventilation for cooling. Never allow core body temperature to spiral out of control in pursuit of adaptation effects.

4.2 High-Temperature Race Pacing Adjustment Strategy

According to exercise physiology research, for every 1°C increase in core body temperature, maximal aerobic power (VO₂max) decreases by approximately 3-5%. Therefore, pacing strategies for high-temperature events should be adjusted as follows:

5. Race Nutrition, Environmental Adaptation, and Practical Race Strategies

5.1 Quantified Hydration and Energy Intake in High-Temperature Environments

In high-temperature environments, sweat rates can reach 1.5-2.5 L/hr. If not replenished promptly, body weight loss exceeding 2% will significantly impair athletic performance. Recommended nutrition strategies are as follows:

• 2 hours before the event: Consume 500-600ml of sodium-containing beverage (sodium concentration approximately 600-800mg/L).
• Every 15-20 minutes during the event: Consume 150-250ml of electrolyte drink, aiming to maintain body weight loss below 2%.
• Carbohydrate intake: In high-temperature environments, gastrointestinal blood flow is shunted to the skin, reducing digestive and absorption efficiency. It is recommended to consume 60-80 grams of carbohydrates per hour (combining 6-8% concentration drinks with energy gels), choosing low-osmolality products (300-400 mOsm/kg) to accelerate gastric emptying.
• Salt supplementation: Supplement 500-1000mg of sodium per hour, achievable through salt tablets or sodium-containing energy gels.

5.2 Practical Heat Stress Strategies for Climbing Events (e.g., East-West Wuling)

The East-West Wuling Challenge (from Puli Geographic Center Monument to Wuling Parking Lot, approximately 55 km in length with 2,800 meters of elevation gain) is Taiwan’s most representative high-intensity climbing event. When held in summer, the low-elevation sections (Puli to Wushe) often experience 32-35°C temperatures with high humidity, making this the section where core body temperature is most prone to becoming uncontrolled.

Practical race strategy recommendations:

1. First 20 km (elevation 450-1,200m): This section has gentler gradients (average 3-5%). Ride at Zone 2-3 intensity, utilizing the peloton’s drafting effect to reduce power output. Monitor the CORE Sensor; if core body temperature exceeds 38.5°C during this section, the later stages pose significant heat exhaustion risk—proactively reduce speed by 5-8%.
2. Middle section (elevation 1,200-2,300m, Wushe to Cuifeng): Gradients gradually increase (5-8%), and ambient temperature begins to decrease (approximately 6°C drop per 1,000m elevation gain). Intensity can be progressively increased to Zone 3-4 in this section, but be aware of the “thermal inertia” effect—even as environmental temperature drops, core body temperature will continue to rise for 10-15 minutes due to previously accumulated heat. Do not accelerate rashly simply because it feels cooler.
3. High-altitude section (2,300-3,275m): Temperatures may drop to 10-15°C, requiring attention to the risk of hypothermia from the temperature differential. At this point, core body temperature may naturally decrease due to accelerated environmental heat dissipation, but maintain steady power output and avoid blowing up from overexcitement.

5.3 Scientific Application of Cooling Strategies

Research has confirmed that pre-cooling can lower starting core body temperature by 0.3-0.5°C, equivalent to gaining a 3-5% power advantage in high-temperature events. Common strategies include:

• Ice Vest: Worn 20-30 minutes before the event, effectively lowering skin temperature and core body temperature.
• Ice Towel on Neck and Thighs: The neck is a superficial region over the carotid and subclavian arteries; cooling this area rapidly lowers the temperature of blood flowing to the brain.
• Cold Fluid Ingestion (Internal Cooling): Consuming 7-10g/kg of crushed ice or slush (approximately 500-700g) before the event has been shown to effectively lower core body temperature. However, gastrointestinal tolerance must be considered; it is recommended to complete ingestion 60-90 minutes before the start.

Per-cooling during the event should follow the principle of “not affecting riding posture or aerodynamics”—for example, pouring ice water over the head, neck, and back before climbing sections, utilizing evaporative heat dissipation to remove large amounts of thermal energy.

6. Common Operational Misconceptions and Scientific Myth-Busting

6.1 Myth 1: “Drinking Plenty of Water Alone Prevents Heat Stroke”

This is the most common misconception among Taiwanese cyclists. Excessive consumption of plain water while neglecting electrolyte supplementation can lead to hyponatremia, whose symptoms (dizziness, nausea, confusion) closely resemble heat exhaustion, but the treatment approach is completely opposite. The correct approach is to provide personalized electrolyte supplementation based on sweat rate and sweat sodium concentration (determinable through professional testing). General recommendations include supplementing 500-1000mg of sodium per hour and avoiding single large-volume fluid intake (exceeding 500ml).

6.2 Myth 2: “Higher Core Body Temperature Means Better Training Effect”

This is an extremely dangerous misconception. When core body temperature exceeds 39.5°C, the risk of protein denaturation rises sharply, potentially causing irreversible damage to the kidneys, liver, and central nervous system. The stimulus for training adaptation should come from muscular and cardiorespiratory loading, not from heat stress alone. The goal of heat acclimation training is to “teach the body to dissipate heat more effectively at the same core body temperature,” not to endlessly pursue higher temperature tolerance.

6.3 Myth 3: “CORE Sensor Readings Are Equivalent to Rectal Temperature”

Although the mean difference between the CORE Sensor and rectal thermometer is only 0.12°C, the two have different physiological meanings. Rectal temperature reflects the temperature of abdominal visceral organs and changes more slowly; the CORE Sensor calculates a “mixed core temperature of muscle and brain,” which responds more rapidly. During rapid heating or cooling scenarios, the two may exhibit temporary deviations of 0.3-0.5°C. Therefore, athletes should treat the CORE Sensor as a “trend monitoring tool” rather than an absolute temperature standard.

6.4 Myth 4: “High-Altitude Events Have Cool Temperatures, So Heat Stress Is Not a Concern”

Although the finish line of the East-West Wuling Challenge has cool temperatures, the starting point in Puli often exceeds 32°C in summer. During the transition from low-altitude heat to high-altitude cold, the body experiences dramatic heat stress changes. Heat accumulated in the early sections may not be fully released until reaching high altitude, resulting in a “delayed core temperature peak.” Additionally, the hypoxic environment at high altitude (Wuling’s atmospheric oxygen content is approximately 70% of sea level) increases ventilation and respiratory heat loss but also increases cardiovascular load. Therefore, core body temperature should still be monitored throughout high-altitude events, particularly during the low-altitude sections.

7. Expert FAQ

Q1: What is the difference between the CORE Sensor and the temperature sensing on optical heart rate watches like the Apple Watch?

Models after the Apple Watch Series 8 are equipped with body temperature sensing, but they measure wrist skin temperature change trends, primarily used for female menstrual cycle tracking and sleep analysis—not core body temperature. The measurement principle uses infrared or thermistor sensing of skin surface temperature, which cannot penetrate the skin and fat layers to calculate deep core temperature. The CORE Sensor, by contrast, uses a dual-channel heat flux model, simultaneously measuring skin temperature and heat flow density, with proprietary algorithms compensating for dynamic changes in tissue thermal resistance, thereby providing core body temperature values approaching invasive measurement accuracy. In short, the Apple Watch tells you “how hot your skin surface is,” while the CORE Sensor tells you “how hot your body’s core is.”

Q2: Where on the body should the CORE Sensor be worn for optimal accuracy?

According to manufacturer recommendations and research literature, the forehead (above the temple) is the optimal wearing position because the subcutaneous tissue there is thinner, blood flow is abundant, and it is unaffected by clothing coverage, providing the most stable heat flux signal. The second-best position is the suprasternal notch (between the collarbones), where the thermal resistance model also shows extremely high correlation with core temperature. It should be noted that the wearing position should remain dry; if heavy sweating causes the sensor to slide, measurement accuracy may be affected. It is recommended to secure it with an elastic sports band and periodically check the contact between the sensor and skin.

Q3: At what core body temperature should exercise be stopped immediately?

According to general sports science consensus, 39.5°C is the warning line at which “intensity must be reduced,” and 40.0°C is the red line at which “exercise should be stopped immediately with active cooling initiated.” When core body temperature exceeds 40°C, the risk of Exertional Heat Stroke rises sharply and may lead to multiple organ failure. If accompanied by symptoms such as confusion, impaired coordination, or hot dry skin without sweating, seek medical assistance immediately. It is worth noting that heat tolerance varies among individuals—some highly heat-acclimated athletes may maintain stable performance at 39.8°C, but this should not be considered the norm.

Q4: How can the CORE Sensor be used to assess heat acclimation status before an event?

It is recommended to perform a standardized “heat stress test” 2-3 days before the event: under environmental conditions similar to the race, ride at 65% FTP for 30 minutes, recording the rate of core temperature rise and the final stabilized value. If core body temperature is below 38.5°C at the end of the test, heat acclimation status is good; if it exceeds 39.0°C, heat acclimation is insufficient and race pacing should be revised downward by an additional 5-8%. Additionally, observe the “baseline core body temperature”—if resting core body temperature is above 37.3°C, it may indicate insufficient recovery from the previous day or mild dehydration, requiring enhanced hydration and sleep.

Q5: Can the CORE Sensor’s heat monitoring effectively prevent heat injury during Taiwan’s humid summers?

Taiwan’s typical summer climate is “hot and humid” (e.g., Taipei at 35°C / 70% RH). In such environments, evaporative heat dissipation efficiency is extremely poor, and heat stress risk is far higher than in dry heat. The value of the CORE Sensor in this environment lies in providing real-time objective physiological data, helping athletes identify the discrepancy between “perceived temperature” and “actual core temperature.” For example, in humid conditions, the skin surface feels muggy because sweat cannot evaporate, but core body temperature may not yet have reached dangerous levels; conversely, under strong wind, the skin feels cool, but core body temperature may have already crept upward. Through continuous CORE Sensor monitoring, athletes can adjust pacing and nutrition strategies based on objective data rather than subjective feelings—this is particularly important for Taiwan’s summer long-distance events (such as the One-Day North-South or Twin Towers challenges). However, it must be emphasized that any wearable device is merely an auxiliary tool; athletes must still possess self-awareness, and if severe discomfort symptoms occur, they should stop exercising immediately and seek medical assistance.