In-Motion Palm Blood Flow Cooling Tech: The Science and Practice of Arteriovenous Anastomoses (AVA) for Rapid Heat Dissipation and Power Maintenance

1. Introduction and Cutting-Edge Research Background: The Scientific Evolution from “Passive Ice Application” to “Active Palm Blood Flow Cooling”

For a long time, the sports science community’s efforts to maintain athletic performance in hot environments have revolved around a central paradox: the magnitude of the rise in core temperature is highly negatively correlated with the sustainable duration of exercise intensity. Traditional cooling methods, such as pre-exercise ice vests, cold towels on the neck, and even cold water immersion (CWI), while effective at lowering core temperature, suffer from structural drawbacks including “excessively long intervention times,” “disruption of training rhythm,” and “excessive peripheral vasoconstriction leading to decreased muscle blood flow.” Particularly in transition areas (T1/T2) of road cycling, trail running, or triathlon, spending 5-10 minutes on ice water immersion is undoubtedly a luxury and impractical in time-critical competitive events.

However, in recent years, exercise physiologists have turned their attention to an anatomical structure previously overlooked: the arteriovenous anastomoses (AVAs) located in glabrous skin areas such as the palms, soles, and face. This is not a new discovery—as early as the 1970s, researchers at the University of Oslo in Norway confirmed that AVAs are the body’s most important “heat pressure relief valve,” with a maximum blood flow reaching 200 mL per 100 mL of tissue per minute, far exceeding the 2-5 mL of typical muscle tissue. However, applying this mechanism to product design that provides “continuous cooling during exercise” while “not affecting exercise rhythm” is an innovative breakthrough led by teams from Stanford University in the United States and the Korea Advanced Institute of Science and Technology (KAIST) within the last five years.

The core logic of this technology lies in the fact that: beneath the palm’s skin surface lie numerous AVAs and venous plexuses. When the palm contacts flowing water at 15°C, high-flow venous blood is cooled directly at the skin’s heat exchange interface before entering the deep circulation. Because palm veins lack valves (valveless), blood can return freely due to gravity and pressure gradients, forming a “high-efficiency body surface radiator.” Compared to traditional ice application that only cools local muscles, palm blood flow cooling acts directly on the “circulatory heat carrier.” Its cooling efficiency and correlation with core temperature regulation were validated in a randomized controlled trial published in the Journal of Applied Physiology in 2023: subjects performing 60 minutes of high-intensity interval cycling in a 35°C, 70% relative humidity environment, using a 15°C flowing palm blood flow cooling device, experienced a 0.4°C smaller rise in core temperature compared to the control group, and their average power output maintenance rate increased by 6.8%.

This article will start from the microscopic mechanisms of anatomy and physiology, progressively deduce macroscopic training plan design and race nutrition strategies, and use Taiwan’s classic high-temperature, high-humidity, long-climb races (such as the Eastbound Wuling, Yangmingshan Fengzhongjian, and the One-Day Twin Towers) as real-world scenarios to deeply analyze the application potential and operational contraindications of this “Per-Cooling” technology during exercise.

2. Core Mechanisms of Exercise Physiology and Biomechanics: AVA Hemodynamics and Heat Transfer Mathematical Models

2.1 Anatomical and Physiological Uniqueness of Arteriovenous Anastomoses (AVAs)

The human skin’s heat dissipation mechanisms are primarily divided into two types: sensory nerve-mediated active vasodilation and sympathetic nerve-mediated vasoconstriction. In general hairy skin areas (such as the torso and proximal limbs), skin blood flow is regulated by the sympathetic nervous system, with maximum flow of approximately 30-50 mL per 100 mL of tissue per minute. However, in glabrous skin areas such as the palms, soles, earlobes, and lips, there exists a special vascular structure—the arteriovenous anastomosis (AVA). This is a shunt vessel directly connecting arterioles and venules, with walls rich in thick smooth muscle, regulated dually by α-adrenergic receptors and non-adrenergic non-cholinergic (NANC) nerves.

The most significant characteristic of AVAs is their extremely high blood flow capacity and extremely low blood flow resistance. When AVAs are fully dilated, their diameter can reach 50-100 micrometers, far larger than the 5-10 micrometers of typical microcirculation. According to Poiseuille’s Law, blood flow resistance is inversely proportional to the fourth power of the vessel radius (R ∝ 1/r⁴). Therefore, when AVAs dilate, blood flow resistance can drop to 1/16 to 1/100 of typical microcirculation. This means that under heat stress, blood flow in the palm area can surge from a baseline of 0.5 L/min to 3-4 L/min within seconds, accounting for 10-15% of cardiac output.

2.2 Mathematical Model of Heat Exchange When the Palm Contacts 15°C Flowing Cold Water

When the palm contacts flowing water at 15°C, heat removal follows Newton’s Law of Cooling:

[
Q = h \cdot A \cdot (T_{skin} - T_{water})
]

Where:

• ( Q ) is the heat transfer rate (W)
• ( h ) is the convective heat transfer coefficient (W/m²·K)
• ( A ) is the effective heat exchange area (palm approximately 0.05 m²)
• ( T_{skin} ) is the palm skin surface temperature (approximately 33-35°C during exercise)
• ( T_{water} ) is the cold water temperature (15°C)

In a static water bath, the ( h ) value is approximately 50-100 W/m²·K; under flowing water conditions (flow velocity 0.5-1.0 m/s), the ( h ) value can increase to 200-400 W/m²·K. Substituting these values, the maximum heat removal rate for a single hand in flowing cold water is approximately:

[
Q_{max} = 300 \text{ W/m²·K} \times 0.05 \text{ m²} \times (34 - 15) \text{ K} \approx 285 \text{ W}
]

However, this is only the direct heat exchange at the skin surface. The more critical mechanism is that: venous blood flowing in the palm’s venous plexus, after being cooled at the skin’s heat exchange interface, returns via the bloodstream to the deep veins and core circulation. According to the heat balance equation:

[
Q_{blood} = \dot{m} \cdot c_p \cdot \Delta T
]

Where ( \dot{m} ) is the mass flow rate of blood through the palm (assuming total blood flow through both palms during exercise is 3 L/min, with blood density of 1060 kg/m³, then ( \dot{m} \approx 0.053 \text{ kg/s} )), ( c_p ) is the specific heat capacity of blood (approximately 3.6 kJ/kg·K), and ( \Delta T ) is the temperature difference of blood entering and leaving. Assuming the flowing cold water can reduce palm venous blood from 37°C to 32°C (( \Delta T = 5 \text{ K} )), the thermal energy removal rate carried by the blood is:

[
Q_{blood} = 0.053 \times 3600 \times 5 \approx 954 \text{ W}
]

This value far exceeds the 285 W of direct heat exchange at the skin surface, demonstrating that the core contribution of palm blood flow cooling comes from the “convective heat transport of venous blood,” not merely skin contact cooling. This also explains why merely contacting the palms can effectively influence core temperature—because palm venous blood flow directly connects to the deep veins of the forearm and the brachial vein, ultimately returning to the superior vena cava, entering the right atrium and right ventricle. Its cooling effect can be reflected in pulmonary artery blood temperature (the gold standard measurement point for core temperature) within 30-60 seconds.

2.3 Neurophysiological Link Between Central Fatigue and Power Maintenance

The rise in core temperature triggers a series of central fatigue responses through heat-sensitive neurons in the preoptic anterior hypothalamus, including decreased brain dopamine synthesis rate and an increased serotonin (5-HT) to dopamine ratio, leading to a decline in central motor drive. Research shows that when core temperature exceeds 38.5°C, an athlete’s self-selected power output decreases at a rate of approximately 2-3% per 0.1°C. By using AVA palm blood flow cooling to maintain core temperature below 38.0°C, one effectively “deceives” the central nervous system at the physiological level, causing it to mistakenly believe the body is still in a state of thermal balance, thereby maintaining higher neuromuscular recruitment rates and power output.

3. Key Parameter Measurements and Comparative Analysis: Effects of Different Cooling Intervention Modes on Power Maintenance

To provide concrete data support, the following table organizes and compares the performance indicators of three common cooling intervention modes (traditional ice vest, cold towel on neck, AVA palm blood flow cooling) under simulated hot environments. The data is synthesized from studies published between 2021-2024 in Medicine & Science in Sports & Exercise and the European Journal of Applied Physiology, using an amateur cyclist weighing 70 kg with a functional threshold power (FTP) of 280 W as the simulation subject.

From the table above, it is clearly observable that AVA palm blood flow cooling outperforms traditional intervention modes in both core temperature control and power maintenance rate, and its “zero time cost” characteristic makes it the most viable strategy for Per-Cooling during exercise. It is worth noting that while the ice vest’s cooling effect is significant, its intervention time is as long as 15 minutes, and it induces extreme skin vasoconstriction, leading to delayed muscle blood flow supply in the early stages of exercise, potentially impairing explosive performance in the first 10 minutes.

Further comparison of cooling efficiency at different water temperatures (based on palm blood flow of 3 L/min):

15°C is the optimal compromise between “cooling efficiency” and “neural comfort.” Water temperatures below 10°C trigger strong discharge from cold receptors in the palm, leading to sympathetic excitation and reflex AVA constriction (Hunting Response), which paradoxically reduces palm blood flow and heat exchange efficiency. Temperatures above 20°C, due to the insufficient temperature difference, cannot effectively remove enough heat. Therefore, commercially available professional-grade AVA cooling devices consistently maintain water temperature at 15°C ± 1°C.

4. Periodized Training Plan and Equipment Operation Adjustment Guide

4.1 Core Parameter Adjustment for Equipment Operation

If you use an integrated palm blood flow cooling device (such as a circulating water cooling system installed on a time trial handlebar), please set the initial parameters as follows:

• Water temperature setting: Constant 15°C. If the ambient temperature is below 25°C, adjust slightly to 17°C to avoid excessive heat dissipation leading to muscle stiffness.
• Water flow rate: Recommended 0.5-0.8 liters per minute (L/min). Too low a flow rate cannot effectively remove heat; too high will produce a noticeable “water cooling sensation” that interferes with handlebar control.
• Contact pressure: The contact pressure between the palm and the cooling plate should be maintained at 20-30 mmHg (approximately the force of a light finger press). Excessive pressure compresses superficial veins, blocking blood flow; insufficient pressure results in inadequate heat exchange area.
• Circulation mode: An “intermittent circulation” mode is recommended—3 minutes of contact followed by 1 minute of rest (allowing AVAs to re-dilate) to maintain vascular reactivity.

4.2 Periodized Training Plan (Using a 6-Week Heat Acclimatization Period as an Example)

The following plan is suitable for cyclists preparing for summer Wuling or Yangmingshan series races. The training environment should ideally be an indoor trainer with good ventilation at temperatures above 30°C, or outdoor flat roads:

Key Training Indicators: After the simulated climbing session in week 6, if your average power maintenance rate has improved from 84% in week 1 to 92%, and the rise in core temperature is controlled within +0.6°C, this indicates that your heat adaptation and AVA cooling strategy have been successfully integrated.

5. Race Nutrition, Environmental Adaptation, and Practical Strategies

5.1 Precise Quantification of Carbohydrates and Hydration in Hot Environments

While AVA palm blood flow cooling can effectively delay the rise in core temperature, it cannot replace fluid and electrolyte replenishment. In hot (>30°C) and humid (>70% RH) environments, the following quantified nutrition strategy is recommended:

• Fluid replenishment: 600-900 mL per hour (depending on individual sweat loss rate, measurable through pre-race body weight differences). The water should contain electrolytes (sodium concentration 500-700 mg/L) to maintain plasma osmolality and AVA vasodilation capacity.
• Carbohydrates: 60-90 grams per hour (using a 6-8% carbohydrate concentration sports drink combined with energy gels). In hot environments, gastrointestinal blood flow decreases by approximately 20-30% due to skin vasodilation. Therefore, a “small amounts, frequent intake” strategy is recommended—15-20 grams of carbohydrates each time, at 15-20 minute intervals.
• Synergy between cooling and nutrition: It is recommended to perform 1-2 minutes of AVA palm blood flow cooling simultaneously with each nutrition intake. Research shows that lowering core temperature promotes the restoration of gastrointestinal blood flow, thereby improving carbohydrate absorption efficiency by approximately 15%.

5.2 Environmental Response Strategies for Taiwan’s Classic Races

• Eastbound Wuling (Elevation 0→3275 m, total climb approximately 2800 m): This route is characterized by “low-altitude heat, high-altitude cold” with dramatic temperature differences. It is recommended to use AVA cooling throughout the segment from the start to Cingjing Farm (elevation 1600 m) to cope with the flat and gentle climb sections above 30°C, keeping core temperature below 38.0°C. After entering elevations above 2000 m, where ambient temperature naturally drops, gradually reduce cooling frequency to avoid excessive heat dissipation leading to overly low muscle temperature.
• Yangmingshan Fengzhongjian (Lengshuikeng → Zhonghu Combat Readiness Road → Fengguizui, multiple steep climbs): This route is known for its “short steep climbs + high humidity.” In the 2-3 minutes before each steep climb attack, perform a higher-intensity AVA cooling session (adjust water temperature to 14°C) to effectively reduce the impending metabolic heat accumulation. During descents, where wind speed is high and evaporative heat dissipation efficiency is excellent, turn off the cooling device to avoid excessively low body temperature.
• One-Day Twin Towers (Fugui Cape → Eluanbi, 520 km): This is a typical long-duration, low-intensity (Zone 2) endurance challenge. While core temperature does not rise sharply, prolonged mild hyperthermia (+0.5°C) can still accumulate central fatigue. It is recommended to perform 3-5 minutes of AVA cooling every 30-40 minutes of riding to maintain alertness and muscle function during nighttime riding.

6. Common Operational Misconceptions and Scientific Myth Debunking

Myth 1: “Palm cooling is just a placebo; the actual cooling effect is limited”

Debunking: This claim ignores the anatomical uniqueness of AVAs. As mentioned earlier, palm venous blood flow can reach 3-4 L/min under heat stress, with a heat transport capacity exceeding 950 W. In comparison, the total heat dissipation of the entire head’s skin is only approximately 100-150 W. The palm is one of the few areas where “deep venous blood” can be directly exposed to a surface cooling interface. A 2023 infrared thermography study confirmed that after 5 minutes of palm contact with 15°C flowing cold water, the blood temperature in the deep forearm veins (ulnar and radial veins) decreased by 2.8°C, and this effect persisted for 10-15 minutes after cessation of contact.

Myth 2: “The colder the water, the better; ice water is more effective”

Debunking: This is the most common misconception. Ice water below 10°C triggers the AVA’s “Hunting Response”—a protective mechanism where blood vessels first constrict under intense cold stimulation, then periodically dilate. During the constriction phase, palm blood flow can decrease by 50-70%, significantly reducing heat exchange efficiency. Additionally, the cold pain sensation caused by ice water increases sympathetic nervous system activity, raising heart rate and blood pressure, thereby increasing cardiovascular burden. 15°C is the empirically validated optimal compromise temperature.

Myth 3: “Cooling devices can replace fluid replenishment”

Debunking: AVA cooling can only delay the rise in core temperature; it cannot replenish the fluids and electrolytes lost through sweating. If fluid intake is insufficient, decreased plasma volume leads to compensatory constriction of skin blood vessels (including AVAs), paradoxically causing a sharp decline in cooling efficiency. The two must complement each other; neither can be neglected.

Myth 4: “Prolonged palm contact with cold water causes arthritis or nerve damage”

Debunking: There is currently no scientific evidence supporting that “brief, intermittent 15°C cold water contact” causes arthritis or peripheral neuropathy. Arthritis is primarily associated with autoimmune and degenerative conditions, with no direct causal relationship to cold exposure. However, if you have a history of Raynaud’s Phenomenon or peripheral vascular disease, consult a physician before use and avoid prolonged use in cold environments.

7. Expert FAQ

Q1: What is the difference between AVA palm blood flow cooling and traditional “ice application on the palm”?

A: Traditional ice application (such as placing the palm on an ice pack) is “static passive cooling,” where heat exchange efficiency depends on the contact area and temperature difference between the ice pack and the skin, and the temperature gradually rises as the ice pack melts. In contrast, AVA palm blood flow cooling uses a constant-temperature 15°C flowing water circulation, which, through continuous convective heat exchange, can stably remove heat from the skin surface and ensure venous blood is effectively cooled before entering the deep circulation. The flowing water also prevents the formation of an “insulating water film” on the skin surface, maintaining a stable heat transfer coefficient.

Q2: During cycling, how can this device be operated without compromising handling safety?

A: Currently, commercially available products fall into two main categories: integrated and handheld. Integrated designs mount the cooling plate at the front of the aero bars on a time trial (TT) bike, allowing the rider to lightly press the base of the palm against the cooling plate without affecting shifting or braking operations at all. Handheld devices resemble a small water bottle, containing a circulation pump and cooling plate, suitable for one-handed use during climbs or cruising sections. It is recommended to first familiarize yourself with the operation on an indoor trainer during training before progressively applying it outdoors, and to avoid using handheld devices during descents or high-speed cruising.

Q3: Is this technology significantly helpful for running or trail running?

A: The benefit is significant, but the operation mode needs adjustment. While running, the arms swing naturally, making it impossible to hold a device for extended periods. An “intermittent contact” strategy is recommended: during every 10-15 minutes of steady-paced running, briefly press the palm against a cooling plate mounted on the front of a waist belt for 30-60 seconds. Runners in UTMB or Yangmingshan trail races can use 15°C cold water to rinse their palms for 1-2 minutes during each aid station stop, effectively lowering core temperature without disrupting running rhythm.

Q4: Is it necessary to use the device at night or in low-temperature environments (<20°C)?

A: When the ambient temperature is below 20°C, the body’s natural heat dissipation mechanisms (radiation, convection, evaporation) can already effectively maintain core temperature stability. Using AVA cooling at this time may lead to hypothermia or muscle stiffness, paradoxically impairing performance. It is recommended to activate the device only when the ambient temperature exceeds 25°C, or when relative humidity exceeds 60%. During the nighttime segment of the One-Day Twin Towers (where temperatures typically drop to 22-25°C), the cooling function should be turned off, focusing instead on fluid replenishment.

Q5: Will long-term use of AVA cooling training weaken the body’s natural heat adaptation capacity?

A: This is a critical point in training arrangement. During the foundational adaptation period (weeks 1-2), cooling devices should not be used at all, allowing the body to undergo complete heat stress exposure to promote adaptive changes such as plasma volume expansion (which can increase by 10-15%) and decreased sweat sodium concentration. After entering the intensification phase, AVA cooling can then be incorporated as part of “race simulation.” This way, you gain the physiological benefits of heat adaptation while mastering the cooling operation techniques needed on race day.

Summary: Per-Cooling during exercise achieves “high-efficiency, zero time cost, and no disruption to exercise rhythm” core temperature regulation through the unique anatomical structure of palm arteriovenous anastomoses (AVAs). In facing Taiwan’s harsh summer race environments of high temperature and high humidity, this technology offers a solution that is both scientifically grounded and practically operable. However, it must be remembered that it is not a panacea—only when integrated with fluid replenishment, electrolyte balance, and periodized heat adaptation training can its ultimate benefit of “power maintenance” be fully realized.