THE PALM COOLING TECHNIQUE: HOW KEEPING COOL CUTS WORKOUT FATIGUE

The palm cooling technique has gained attention in the past decade for being a convenient way to boost physical endurance, lower fatigue and enhance muscle recovery during and after a workout.

The following article explores the benefits of palm cooling, how it works and why it is so effective for the muscles during exercise. Other body cooling strategies are also discussed and compared to palm cooling, with suggestions on how best to apply cooling techniques when breaking a sweat.

What is the Palm Cooling Technique?

Body cooling with respect to exercise has been a topic of study for many decades. The palm cooling technique is one of several body cooling techniques that have been employed by athletes, bodybuilders and exercise enthusiasts to improve endurance and other outcomes during training, particularly under high heat conditions.^[1] It has been shown that palm cooling can extend the length of time spent exercising by lowering heat and fatigue perception.^[2]

How Body and Palm Cooling Works: Underlying Physical Mechanisms

The science behind palm cooling first emerged in the 90s, when a group of researchers realized that heating the palms helped to improve post-surgical outcomes in hospital patients. Since this time, it has become increasingly evident that temperature regulation plays a crucial role in physical capacity and well-being.

Cooling During Exercise Reduces Fatigue and Promotes Endurance. During exercise and other forms of intensive physical activity, the body generates heat. Substantial rises in core body temperature are physiologically associated with dehydration, heat stress and fatigue.^[3] Keeping the body cool during exercise has been shown to lower fatigue and substantially influence the amount of physical activity one can perform in a single “sitting” or session. This has been shown to be very important for athletes and those exercising in extreme heat conditions, as the heat can detract from cognition as well, lending itself to lowering overall physical performance capacity.

The Hands, Feet and Face Are Vital for Temperature Control. While remaining cool during exercise is not news, palm cooling became recognized as an improved body cooling technique after it was established that the face, palms and feet are important body sites for heat loss. Skin temperature varies widely by comparison to core body temperature, which usually remains as close to constant as possible. This difference is necessary for optimal thermal control and blood circulation. Most mammals appear to have small patches of hairless skin almost solely dedicated to temperature control. The palms, face and soles of the feet of humans can be seen as equal to the tongues of dogs during panting, the tails of rats, the ears of rabbits and the paws and noses of bears; all of which lose heat and serve to cool the body. Heat loss is increased on the palms, soles and face by 4-5 times when compared to other areas of the body.

Specialized Palm Capillaries Regulate Blood Flow and Temperature. These body sites contain arteriovenous anastomoses (AVAs), which are specialized points of connection between veins and arteries. The AVAs are different from other capillaries (the smallest blood vessels) in that they consist of a thicker, more muscular wall and maintain a larger diameter. The muscular actions of these connections in the palms and fingers play an intricate role in maintaining optimal blood pressure, circulation and temperature. They are in constant communication with the temperature control centers of the brain in order to achieve these effects.

How Can Heat Lower Muscle Blood Flow. When body temperature rises, such as during physical activity, the AVAs open to allow for more heat to escape. During this process, blood vessels in the muscles and internal cavities become constricted, which pushes blood out toward the AVA-containing extremities. Despite how exercise improves blood flow, the steady rise in body temperature in response to exercising promotes vasoconstriction, detracting from the optimal physical output.^[4] In this way, blood flow between the skin and muscle tissue is known to compete during temperature acclimation.^[5]

How Palm Cooling Leads to Improved Muscle Output. Palm cooling does not reduce core body temperature. Instead, when the palms and other AVA sites are cooled, the AVAs close, which limits heat loss and shunts blood back toward the muscles, heart and internal compartments. Cold exposure has been noted to induce transient vasodilation in the muscles, which serves to enhance both circulation and physical performance. Palm cooling also causes the heart rate to slow down, which is usually an indication of enhanced cardiorespiratory fitness. This also suggests that extreme heat can induce a type of pseudo ‘fitness loss,’ contributing towards the understanding that palm cooling improves physical endurance.

Body Cooling During Exercise Lowers Fatigue By Conserving Energy. Other effects of palm and body cooling during exercise include an observed drop in blood lactate levels. This is a sign of enzyme conservation, better energy output and lower levels of muscle fatigue. Scientists theorize that body cooling lowers fatigue during intensive activity due to preserving the temperature of cells and their contents. Muscle cells tend to generate more ATP (energy) during exercise in order to sustain the increased energy output. Like all other chemical reactions that take place in the cell, energy production requires an optimal temperature range in order for all required enzymes and by-products to function properly. The heat generated during exercise appears to be a limiting factor as to how much energy can be produced in the muscles. When the produced energy has been consumed and no more energy can be produced due to higher temperature, the muscles lose energy and fatigue sets in. Due to promoting a more stable temperature, palm cooling can effectively conserve ATP and related enzymes that would otherwise be lost to overheating. Many athletes take advantage of this technique nowadays as a way to extend their training time and reduce post-workout exhaustion.

Could Protect Against Muscle Injury. As explained above, maintaining a stable temperature helps to conserve the chemical elements inside the cell, including energy molecules and related enzymes. If the temperature rises too high and denatures too many of these components, it can cause the cell to become damaged and eventually die off. Keeping muscles cool during and after exercise, therefore, reduces resultant damage and can lower the risk of exercise-induced injury.

Palm Cooling Vs. Other Body Cooling Techniques

Palm cooling has revolutionized the understanding of body cooling mechanisms. The benefits and efficacy of palm cooling by comparison to other body cooling strategies are discussed below.

Palm Cooling with a Rapid Thermal Exchanger. Cooling of one palm through a device called a rapid thermal exchanger that applies sub-atmospheric pressure proved to be far more effective than palm cooling alone. Sub-atmospheric pressure (-35-40mmHg) stretches the AVAs and increases blood volume in the palms, which allows for maximal heat extraction from the hand.

The benefits and drawbacks of using a rapid thermal exchanger for palm cooling during exercise are briefly detailed below:

• Improved Exercise Endurance and Output. The team that invented this device tested its effects on one of their lab members who frequently visited the gym. The rapid thermal exchanger cooled his palms between sets of pull-ups, allowing him to do more than triple the usual amount before tiring out. In one week, he usually averaged around 180 pull-ups per week, and after palm cooling, he could average more than 620 pull-ups. Other studies confirm that palm cooling with a rapid thermal exchanger can also extend intensive exercise sessions carried out over the course of 3 days by as much as 25%.^[6] Additionally, palm cooling during exercise was shown to prolong power sustain and speed up blood lactate clearance^[7], suggesting that it may reduce post-workout fatigue or recovery time. These results have been replicated for a variety of exercise types, most of which comprise weight-lifting or resistance exercises.
• Palm Cooling May Surpass Local Cooling. Muscle cooling via the palms may be a more efficient technique than others. [1] [2] While attempting to cool a direct area of skin may be effective, some of the blood and heat will ultimately be diverted to the palms, soles and neck irrespectively. Palm cooling can therefore save time during a workout, as well as bypass some of the drawbacks of other cooling methods, such as having to immerse the whole body in cold water or enduring cold-induced shock or discomfort.
• Promotes Heat Resilience. Palm cooling between 18-22C of one hand at a negative pressure (-40mmHg) served to extend exercise time by 33-35% in individuals with multiple sclerosis, who are more prone to overheating than healthy individuals.^[8] Similar results were shown in healthy individuals that used a palm-cooling device while they simulated walking uphill on a treadmill in extreme heat (40C). The device prevented esophageal temperature from rising and proved superior to ordinary palm cooling.^[9]
• Improves Hand Grip Strength but Not Finger Coordination. One test showed that extreme palm cooling (<8C) increased hand grip strength by as much as 5N, which is approximately a 7% increase by comparison to the average grip force used in daily activities (70N). Extreme cooling detracted from fine motor coordination of the hand and fingers. All the results were enhanced with prior body core cooling of 0.5C.^[10] Similar studies revealed that finger cooling could potentially increase grip strength by as much as 10-70%^[11]. However it detracts from the ability of the person to adjust their grip force to movement-induced changes on the object they are holding.
• Colder Temperatures are Most Effective. Palm cooling does appear to be extremely beneficial at short time intervals in the context of rowing. This could be due to the extremely low temperature used to cool the hands (8.1C), as compared to other studies that tend to use higher temperatures (close to 20C)^[12].
• Ineffective at Enhancing Endurance During a Marathon. Palm cooling was less effective during cycling and jogging exercises, where the cold wind has been shown to cool almost as efficiently. These types of exercises are often continuous and lengthy and do not allow for intermittent cooling. In long-distance races and marathons, palm cooling and other body cooling techniques did not prove to be beneficial at all. In this respect, it may be best to apply body cooling techniques during body-building activities that enhance overall fitness.

Neck and Head Cooling is another fairly common body cooling method. Similarly to the palms, the face, head and neck lose more heat than other body areas. The benefits of neck and head cooling may surpass that of palm cooling, as reviewed below:

• Neck Cooling is Superior to Palm Cooling. Ordinary palm cooling proved to lower fatigue only 30% of the time in one study, and it was only marginally more effective than cooling the muscle group that was exercised (biceps). In this study, neck cooling proved to lower fatigue and enhance performance 71% of the time.^[13] This study was carried out at ordinary ambient room temperature (21C). Other studies suggest that neck and face cooling can enhance performance by as much as 18-51% in ordinary individuals and 9% in athletes.
• Distance from the Extremities. The nerves in the face and neck may be more sensitive to temperature^[14], thus making them more effective body cooling sites for improving endurance during workouts. They are also closer to the temperature control centers of the brain (hypothalamus), whereas the legs and hands are further away, which could delay the effect and explain the reduced comparative efficacy. Some studies show that cooling at remote body sites takes three times as long (3mins vs. 1min) to achieve the desired effect and that at a shorter cooling time (1min), palm cooling sometimes does not even pose an advantage.^[15]
• Neck and Spine Cooling May Enhance Palm Cooling. The enhanced effectiveness of neck cooling may be explained by an effect known as selective thermal stimulation. When heat is applied to the neck and other areas of the spine, it increases blood flow to the palms, hands and face.^[16] If the cold is applied to the neck and various points along the spine, it may optimize the palm cooling technique by encouraging cooler blood from the palms to move back through the muscles toward the heart. It has also been shown to stimulate heat loss through the soles of the feet and provide a 2-5 fold reduction in heat-stress discomfort.
• Added Benefit of Alleviating Mental Fatigue. Mental fatigue as a result of heat stress can also serve to lower endurance and reduce physical performance parameters, such as coordination, muscle memory and reflex times. Studies show that neck, face or head cooling is more effective than other means for restoring cognition during heat, likely due to its close proximity to the brain.

Drinking Ice Water was shown to be more effective prior to exercise than most other pre-cooling strategies, including wearing a cooling vest. The digestive tract covers a very large surface area inside the body, able to modulate core temperature directly. As the water passes through the system, it first reaches the throat, which borders closely on the nerves of the neck and face. Athletes that make use of this strategy typically ingest small amounts of cold water every couple of minutes for 10-30 mins prior to exercise, as it has an additive effect that serves to cool larger portions of the digestive tract over time. The cooler the water, the more effective it is at cooling the body down^[17]. However, if cooling is too extensive, it can lead to gastrointestinal discomfort. Drinking cold water during exercise helps to lower the amount of sweating that occurs^[18], which is a sign of cooling. There is little data regarding the effectiveness of drinking water for body cooling compared with palm cooling.

Cold Water Immersion is thought to be the most effective body cooling technique. Despite being known to cool the body very effectively, the temperature and time spent in the water can change the outcome. For short exposures and as an intermittent cooling strategy, palm cooling proves to be superior at prolonging exercise-induced fatigue. However, for long exposure times and as a post-workout cooling strategy, immersing in cold water cools the body down much faster than palm cooling would. Cold water immersion has the added benefit of increasing oxygen uptake^[19], which may be more beneficial for those attempting to exercise at a higher altitude or under low oxygen conditions.

Water Spray proved more effective at whole-body cooling than an active cooling vest when exercising in a full-body hazmat suit in hot conditions (40C).^[20] Other studies have shown that using a water spray shares a similar efficacy to that of cold water immersion and that both techniques jointly take advantage of head and neck cooling.^[21] However, the water spray has also been shown to enhance perceived coolness and may be superior to other intermittent cooling techniques when exercising in a hot environment, particularly when combined with a fan.

Water-Perfused Clothing may be one of the best pre-cooling options for those intending to engage in strenuous physical activity and looking for improved endurance. In several small trials, a cold water-perfused vest or an ice vest each proved to be superior in terms of lowering core body temperature compared to palm cooling.^{[22] [23]} One of the studies indicated that this strategy worked best for 60mins of intensive cycling on a stationary bicycle at an ambient room temperature of 36C. This may be due to the increased sensitivity of the spine and torso to the cold. Unlike the hands, face and feet, the chest and spine have a higher concentration of cold receptors, which activate body cooling mechanisms in a separate manner to that of palm cooling. Studies suggest that cooling the torso may be almost as effective as cooling the head and neck.^[24]

Menthol is a cooling phytochemical compound found in mint and various other herbs. One of its main characteristics is the activation of cold receptors in the skin, which is linked with its calming, cooling and soothing reputation. When applied to the head, face or neck as an ointment during exercise, it creates a pseudo sense of cooling, which can lower heat perception^[25] yet has little to no effect on physical performance parameters. Menthol applied to tired muscles helps to reduce pain via anti-inflammatory mechanisms, and when applied to joints, it could increase the range of motion.^[26] It may be better to use arnica gel or mint essential oil in some skin ointments to reduce the risk of skin irritation associated with menthol spray^[27].

When is the Best Time to Keep Cool for Working Out?

Athletes will often employ body cooling techniques before, during and after exercise in order to maximize their performance and physical output. Cooling the body at all these times appears to be effective for enhancing a workout. The differences are described briefly below.

• Pre-Workout Cooling. Exercise pre-cooling can have similar benefits to intermittent cooling during exercise as it increases the time it takes for the body to heat up and thus improves endurance for that time. Pre-cooling has been shown to lower the core temperature before exercise, and if done sufficiently, this decrease in core temperature sustains until after the workout by comparison to no cooling. One review found that multiple pre-cooling strategies worked better than any single one alone.^[28] However, if pre-cooling is too extensive it can detract from physical performance through increasing vasoconstriction. Studies show that extreme pre-cooling of muscles could substantially lower sprint performance. Pre-cooling before intensive cycling for 10-30 mins did not improve performance, cognition or heat stress symptoms.^[29]
• Cooling During Exercise (Per-Cooling). Different strategies have been noted to be effective for intermittent body cooling during exercise. Wearing an ice vest or cold-water-soaked clothing over the torso were shown to be the most effective per-cooling strategies. However, it presents the same drawbacks as whole-body cold water immersion, requiring one to get wet. As these may not be feasible to implement for many exercise forms, palm and head cooling may be best. Palm and head or neck cooling, especially in combination, are sufficient to enhance performance capacity and lower exercise-induced fatigue. For long-distance exercises, such as non-stationary sprinting or cycling, intermittent cooling has been shown to be far less effective. Despite less effectivity, the wind has been noted to be as effective as other applied cooling strategies in these scenarios. It is important to keep hydrated when enduring long bouts of strenuous physical activity.
• Post Workout Cooling. Naturally, as seen with pre-cooling and per-cooling, post-workout cooling can help to reduce muscle temperature, preserve their integrity and lower the risk of exhaustion and injury. In terms of post-workout cooling, palm cooling was not very effective. 50 mins of palm cooling under high heat conditions following exercise served to lower core body temperature only by 0.21C.^[30] Cold water immersion is known to be one of the best ways to cool down after intensive exercise^[31]. However it may be as effective as taking a warm shower^[32], which is also known to help lower post-exercise fatigue and aid recovery^[33]. 

Other Body Cooling Considerations

Palm cooling and other body cooling strategies do not always achieve the desired effect. The below points highlight factors that may add or detract from palm cooling and body cooling in general.

• Palm Cooling Works Best at Select Temperatures. AVAs work the best within the ‘thermoneutral zone’ between 26-36C. If cooled too extensively, AVAs will shut down and not allow for any heat loss, which can detract from the efficacy of palm cooling. However, when the palms are cooled moderately, the AVAs close slightly, allowing for cooler blood to move back into the muscles. When the body is overheating, lower temperatures allow for shorter cooling periods and may save time during a workout. There is no optimal temperature for palm cooling, with most studies using temperatures ranging between 10-20C and cooling periods between 1 and 30mins. Cold water immersion tends to promote pain at roughly 5C, but not at 8C, 10C or 15C, for which it was noted that the thermal response is almost identical.
• Physical Fitness Improves Palm Cooling Efficacy. The fitter one is, the better the palms respond to cold sensations and in turn, cool down the muscles as well as promote endurance. Cardiorespiratory fitness especially helps to keep the tone of the muscles in the AVAs in shape. Theoretically, this may help reduce the time spent on cooling down the palms in between sets while increasing the amount of exercise one can endure.^[34]
• Including AVAs in Pre-Workout Stretches and Warm-ups. The AVAs in the palms, soles and face can be regarded as micro-muscles that function to control both blood flow and body temperature dispersal. When warming up before a workout, their action can be enhanced by loosening up the wrists, fingers, ankles, feet muscles, neck and facial muscles. This allows for optimal blood flow to the extremities and allows for the AVAs to warm-up before a workout session.
• Sleep Deprivation May Detract from Body Cooling. Sleep deprivation has been shown to alter temperature control mechanisms, including reducing heat loss from the hands and increasing heat loss from the feet.^[35] Exercising without using adequate cooling techniques is not advisable if one is sleep-deprived or suffering disrupted sleep due to extreme heat conditions.
• Thermal Control During Aging. The ability of the body and palms to regulate temperature declines with age. Therefore, the palm cooling technique and other methods may not be as effective in physically active elderly individuals. There is limited research as to how thermal control functions in the elderly with respect to exercise. However, preliminary studies suggest that the initial response to cold can be delayed and less pronounced in older individuals^[36], suggesting that those who are physically active enough may not benefit as much from palm cooling and may require more time to recover in between activities.
• Inflammation and Temperature. Mediators of bodily inflammation, such as cytokines and prostaglandins, that emulate the body’s response towards infection, can also trigger the thermoregulatory networks of the brain and promote an increase in core temperature.^[37] This is one reason why it is not advisable to exercise when running a fever. In healthy individuals, exercising helps to lessen inflammation by increasing muscle-derived anti-inflammatory interleukins. Lowering bodily inflammation through consuming a nutritionally dense diet may also improve exercise capacity from the perspective of both metabolic and temperature control.

Thermal Safety. It is important to keep in mind that palm cooling is a technique that takes advantage of the body’s natural cooling mechanisms by altering thermal perception.^[38] Intermittent palm cooling does not reduce core temperature. It merely transiently lowers the temperature of the blood in the hands, arm muscles, and heart. Palm cooling cannot prevent dehydration, heat stroke or complete exhaustion, despite helping to prolong endurance and improving exercise capacity. One ought to keep well hydrated during exercise and attempt to minimize overexerting oneself during extreme heat conditions where possible.

Conclusions

Heat is physiologically related to strain, fatigue and exhaustion. Cooling down the body during exercise, a heat-generating activity, poses many benefits that serve to prolong physical endurance, enhance performance and preserve muscle integrity. Due to the central role of the palms in thermoregulation, palm cooling is an extremely effective and convenient intermittent cooling method when engaging in short bouts of repetitive, muscle-building exercise. The effects of palm cooling can be significantly improved with the addition of neck, face or head cooling, which is known to stimulate the palms and increase their capacity for heat loss. A combination of body cooling techniques offers optimal results, provided cooling is not too extensive.

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