Physiology in sport: keeping your hands cool can help your performance

Why cooling your hands provides performance benefits in hot conditions
This article:
Explains why cooling the hands can provide performance benefits in hot conditions;
Shows how athletes can implement hand-cooling techniques in a range of sporting situations.
Our hands are a biological marvel and have probably played more of a part in our evolutionary development than the human brain!  But as Alan Ruddock explains, keeping your hands cool can also help your sport performance
The development of our human hand can be traced back millions of years and attributed to improvements in how primates use tools. The most successful primates developed a ‘precision grip’ for throwing and a ‘power grip’ for clubbing. These new techniques were utilised for fighting and hunting and through ‘survival of the fittest’ our ancestors bred an anatomical advantage in our species over millions of years.

The human hand is now significantly different to our primate cousins’; our thumbs and first two fingers are stronger and more flexible and agile, and we also have highly sensitive nerve endings and precise neuromotor control of our fingers, both required for throwing accuracy and grip strength. But our hands are also a key heat exchange centre and play an important role in maintaining our core body temperature.

Hands and temperature regulation
The hand is an ideal structure for high rates of heat exchange (gain and loss) due to its large surface to mass ratio, low metabolic heat production, large blood supply and rapid cooling rates. Specialised blood vessels within the hand control the rate of blood flow and are very sensitive to changes in the environment and core temperature. For example, on a cold day the blood vessels near your skin constrict and divert the warm blood to your central organs. However, when you are getting hotter, blood will flow to your skin and coupled with sweat responses will help you to cool you down.

Hand cooling
The idea of hand cooling to aid thermoregulation was introduced by the Royal Navy in the late 1990s. The first study conducted by Navy researchers aimed to quickly reduce heat strain by immersing hands in cold water following exercise in 40°C heat whilst wearing full fire fighting clothing(1). After exercise the participants rested with their hands in water maintained at 10°C, 20°C, 30°C or were rested without hand immersion.
The results showed that the colder the water, the quicker the rate of cooling. After 20 minutes of recovery, core temperature had decreased from 38.5°C to 36.9°C in the 10°C water and to 37.3°C and 37.8°C in the 20°C and 30°C water respectively. The authors concluded that this simple technique could be applied to many industrial and military tasks where personnel alternate between work and rest for extended periods of time.

Alternating between work and rest is a feature of many sports. Building upon previous military research, researchers investigated the effects of hand cooling by immersing hands in 10°C cooled water during a 10-min recovery period from a 60-min exercise bout on subsequent high-intensity time trials at 30°C and 60% humidity(2). The participants in this study were able-bodied athletes whose time trial distance was 3km and wheelchair athletes whose time trial distance was 1km. The results showed that hand cooling improved both the time trial over 3km by 14 seconds and 1km by over 20 seconds. (NB: the greater improvement in the 1km time trial was attributed to the mix of elite and non-elite wheelchair participants used and the mechanics of wheelchairing).

But what about athletes who don’t have rest periods or those who are required to wear protective clothing which might impair theromoregulation? The problem with hand immersion is that it restricts the mobility and use of the hands during work. British researchers sought a solution to these impracticalities by designing water-perfused cuffs around the forearm, which circulated water at 12°C(3).

The participants in this study were asked to exercise (box stepping) in 40°C heat until their core temperature had reached 38.5°C. The mean work time until a core temperature of 38.5°C was reached was 34 minutes with no cooling and 47-minutes wearing the cuffs.

Afterwards participants rested in the same environment for 40 minutes while investigators measured their core temperature responses to different stimuli. During the 40-minute rest some participants continued to wear cuffs while one group immersed their hands in 15°C water. In agreement with the study above(1), the researchers observed the greatest core temperature reduction with hand immersion.

The average reduction in core temperature after 10 minutes was 0.8°C compared to 0.2°C with the water-perfused cuff and just 0.1°C with no treatment. These trends continued throughout the recovery period; after 30 minutes, core temperature had returned to resting levels (between 37 and 37.5°C) but after 40 minutes of data collection, both the cuff and control conditions had failed to drop core temperature below 38°C.

It was suggested by the authors that cooling the forearm represented an effective method of limiting heat strain during exercise but hand immersion should be the preferred method of cooling during rest.

Cold-induced vasodilation
Have you ever noticed your hands heating up in the cold? On a chilly winter’s morning you set off walking to work and experience the normal constrictive response in your hands to the cold. But a few minutes after setting off you notice that your hands heat up – quite the opposite of what you might think should be occurring. This natural phenomenon is called ‘cold-induced vasodilation’ (CIVD) or the ‘hunting response’ and occurs to prevent your hands from cold-injury (damage to finger cells). It is this exact response that coaches and athletes can exploit and use to their advantage to provide a powerful cooling effect to the rest of the body.

Undoubtedly the cooling mechanisms developed by our body have been designed to prevent heat stroke. Our nomadic hunter-gatherer ancestors would have had to endure marathon treks in scorching heat whilst searching for food. Consequently, they adapted the main thermoregulatory mechanisms that we utilise during heat stress – diversion of blood from the core to periphery to aid radiation of heat and sweat responses to aid convective and evaporative heat loss.

Vacuum cooling
It seems that rate of cooling is related to peripheral circulation and that the amount of blood flow to the extremities may determine the rate of heat extraction. With this in mind, researchers at  Stanford University in the US sought to improve blood flow to the hand by using a low pressure vacuum to draw blood to the area(4). The system they developed also cooled the hand by gripping a metal surface that was maintained at 18-22°C.
Participants were asked to walk uphill on a treadmill at 5.63km/h in 40°C heat until they reached 90% of their maximum heart rate (MHR). Three conditions were used:
No cooling;
Hand cooling only;
Hand cooling with additional vacuum.
The researchers found that condition #2 had little impact on exercise time to 90% MHR (an increase from 34 to 38 minutes). However, the third condition substantially increased time to 90%MHR, which was found to be on average 57 minutes. The theory is that the vacuum condition may assist the blood vessels to stay open for longer and more consistently, resulting in a greater rate of heat extraction than the other methods. Another study using the same device also showed improved endurance performance in the heat(5). Male triathletes were able to complete a 30km time trial in 32°C heat 6% faster with the cooling device than with no cooling at all.

In practice
Major championships often occur in the summer, when high environmental temperatures increase the potential for heat stress. Limiting increases in core temperature is imperative for performance and the health of athletes. Here are some tips for hand cooling, which may help to reduce the risk of heat stroke and improve performance.

Cooling application – Although seemingly very effective, the vacuum hand cooling device mentioned here is expensive (￡1,400) and water-perfused forearm cuffs are probably not a practically viable option in sport. By contrast, hand immersion in cooled water requires minimal equipment so could easily be set up in almost any environment.

Water temperature – The research above shows that the cooler the water, the faster the heat extraction. Water temperatures of around 10°C have proved to be the most effective in drawing heat from the body and lowering core temperature.

Duration of cold application – Studies have shown that core temperature can be reduced by around 1°C after 10 minutes of hand immersion in 15°C water. A lower water temperature could potentially increase the rate of heat extraction.

Intermittent sports (eg football/rugby/hockey) – The optimal time for hand cooling would be at half-time, when players have the potential for five minutes of hand cooling. A water temperature of 15°C during this time has the potential to lower core temperature by 0.5°C, which would significantly lower the risk of heat illness and potentially benefit performance.

Endurance sports (eg running/cycling) – Pre-cooling prior to an event with hand cooling would provide the body with the potential to store more heat before reaching critical core temperature levels. This may assist performance by enabling athletes to work harder before the level of heat storage becomes incapacitating.

Summary
We use our hands for more than just manipulating tools. Thanks to their large surface area, our hands are key thermal exchange centres and when exploited can assist in the process of rapid whole-body cooling.

The available research suggests that immersion of the hands for 10 minutes in cold water at a temperature of 15°C can lower core temperature and aid recovery/performance. Although greater cooling benefits may be possible when additional vacuum cooling is employed, the equipment required means that for most athletes for most of the time, simple immersion will be the most easily utilised hand cooling method.

Alan Ruddock MSc, CSCS, YCS is a researcher in exercise physiology at Sheffield Hallam University, UK