Some parts of your body are more and other less efficient to bring your temperature down. But which body spots are best suitable to apply cooling?
Lets start looking at the neck/head area first:
- Despite the fact that the head and the neck comprise only 10% of the total body surface area, head and neck cooling was found to be as effective as cooling large other BSA surfaces in alleviating heat strain and thereby enhanced exercise performance in an efficient way. (*19)
- In a recent meta-analysis, hand cooling with cooling of both the head and the neck by using small cooling packs in the context of exercising in the heat were compared. They found that head and neck cooling significantly and positively impacted human performance (6.8%, EFS = 0.36), whereas hand cooling produced a negligible effect (−5%, EFS = 0.13). (*24)
- Cooling the face resulted in a two-to five-fold more powerful suppression of sweating and thermal discomfort than did the cooling of a surface area of an equivalent size. (*25)
In the above studies head/neck cooling are considered together as being 1 area. But what is more effective; cooling the neck or the head?
When looking into cooling of the head in more detail, there are scientific studies and reviews that provide insight into the effects of localized cooling on the forehead and temples and the impact on core temperature reduction and heat perception.
- The effectiveness of cooling the forehead and neck during exercise in a hot environment was studied in 2 different studies. They found that cooling the forehead improved feelings of comfort and thermal well-being, but had only a modest effect on actual core temperature. The neck was found to be more effective in regulating thermal comfort through blood flow in the region. (*39), (*40)
- Cooling the temples and face (where larger blood vessels are present) provided better subjective cooling than the forehead alone. Blood vessels at the temples provided more efficient heat removal. (*41)
- The skin around the temples is more sensitive to temperature changes, which explains why cooling here often affects heat perception more than other areas such as the forehead. (*42)
- Arm cooling is significantly more effective in lowering core temperature than face or head cooling. The face helped with the feeling of comfort but contributed less to core temperature reduction. (*43)
- Studies showed that although face cooling improves the feeling of cooling, the influence on core temperature is minimal without additional, large-scale cooling methods such as immersion of extremities. (*44)
The above leads to the conclusion that cooling of the temples due to their proximity to large blood vessels may be slightly more effective in improving the sensation of cooling than the forehead. Cooling of the face and head mainly improves perception of thermal comfort, but has limited impact on actual core temperature.
So, for significant core temperature reduction, broader cooling techniques are more effective than cooling specific parts of the head.
This finding was also confirmed during a recent study of the OMIUS headband. This headband was tested during a 5-km running time trial preceded by 70 min of moderate-intensity running. It was found that in hot/humid conditions the OMIUS headband:
- Does not improve heart rate
- Does not improve forehead and rectal temperatures
- Does not improve perceived exertion
Surprisingly, the OMIUS headband did also not increase whole-body and forehead thermal comfort and performance. So the OMIUS did not even contribute to perception of thermal comfort. (*45)
Earlier it was concluded though that neck/head cooling was effective in bringing core temperature down. So, when the head is not effective in cooling the core temperature, the neck is. Unfortunately neck-cooling is not very practical, especially DURING exercise. It was found though (by using a cooling water (8.3 °C) system), that cooling the chest is as efficient as cooling the neck. (*23) This leads to the conclusion that chest cooling should be effective in bringing core temperature down.
So, lets have a closer look at the chest area.
Tests demonstrated that a cooling vest (torso cooling) with a cooling area of only 3.3% of the BSA (Body Surface Area) was effective in alleviating heat strain. (3,3% of 1,7M2 BSA for an adult man, means 561cm2, which is around 20x28cm). The beneficial effects of chest cooling may be explained by the large vascularization in this area, which allows the cooling of substantial quantities of blood. (*22), (*19)
Next to the chest area, it is in different studies suggested that to effectively lower the body's core temperature, cooling large perfused surfaces, such as the forearms, usually works best.
So let's have a look at the underarms.
For many years, it was generally assumed that heat transfer is uniform across the entire body surface, however recent studies have shown this to be false. Glabrous (hairless) skin regions with specialized anatomy for heat exchange play a critical role in thermoregulation because glabrous skin cooling provides the largest benefit, reducing subjects’ core body temperatures at the fastest rate. (*15)
Now it gets a little technical;
Arterio-venous anastomoses (AVAs) are direct connections between small arteries and small veins, with no capillary section between them. Since they contain no capillary segment, they cannot transport dissolved substances to or from the tissues. The only transport function they could possibly have is the transport of heat from the body core to surface areas containing AVAs. *17
The retia venosa are specialized heat exchange structures underneath glabrous skin areas of the body, and vasomotion of the AVAs determine the volume and velocity of warm blood flowing near the skin surface, ultimately facilitating heat exchange with the environment. (*15)
Only in recent years has it been widely accepted that the AVA and retia venosa are greatly responsible for transporting large volumes of blood to the skin surface and facilitating heat transfer from the human body. (*15)
So… we need to focus on these hairless skin / AVA regions.
The arteriovenous anastomoses together with the superficial venous retes in the hand, forearm and parts of the upper arm and the similar structures in the leg may be described as an organ, the AVA-organ. As no similar and synchronous surface temperature changes, to those of the proposed organ, are found in other skin areas of the body, the AVA-organ should be regarded as the main moment to moment regulator of the physical heat exchange over the skin. (*18)
In the IR-photos (Figure 1 and 1b) it is seen that the surface skin temperatures are high over the superficial veins of the AVA-organ in the warm person, while in the cold exposed person these veins disappear into the background of the heat that is brought to the surface from the underlying structures. The function of AVAs is demonstrated in an experiment with intermittent exercise in a cool room (Figure 1c).
So, in conclusion; the underarm and parts of the upper arm as well as parts in the (inner-upper) leg are perfect spots for heat-transfer; meaning cooling the core of the body. Since the legs are difficult to cool during exercise, it is logical to focus on cooling the underarm / part of the upper arm.
Cooling larger parts of the chest/torso and the underarms seem the perfect area’s for decreasing core temperature. Does it mean we can forget about the head?
In individuals with hyperthermia (increase in one’s core temperature above 38 °C during moderate-intensity exercise), the brain temperature may become higher than the core temperature and cognitive effort cannot be sustained over time. To address this problem and protect the brain, cooling methods such as head and neck cooling may be of primary importance for sport performance and/or medical assistance. *19
Cooling interventions such as head cooling and external cooling seem to restore cognitive resources disturbed by hyperthermia, increase performance in cognitive tasks and lower the perceived load of high temperature. *19
Although cooling the head is not related to decreasing the core temperature, it plays an important role in cognitive and lowering the perceived heat load.
In conclusion:
- Cooling that covers underarms and parts of the chest (560cm2 (around 20x28cm) is sufficient according to studies) appear to be the most effective for improving performance.
- Cooling the head (temples predominantly) help to increase cognitive tasks and decrease perceived heat.
- Cooling needs to be continuous to be effective in reducing core temperature.