Getting hot, red and sweaty is something everyone experiences during exercise and competition – a phenomenon that might not be welcome to everyone and every time. As clothing also influences the athlete’s temperature it is important for STEAKS® to understand thermoregulation and the factors influencing it. So we asked a fellow roller derby skater, who is also an engineer working in prosthetic/orthotic research to help us with this research. This post is the first of two and puts together some background about the underlying mechanisms on what happens when we heat up and explains the factors that influence our ability to regulate heat and gives some insights on what athletes might be able to do.
Thermoregulation – what is it and how does it work?
Humans, like most mammals, are warm-blooded, which means we keep a core temperature within a constant range at which our organism functions optimally. In humans the normal core temperature range at rest is typically between 36.5 °C – 37.5 °C (97.7 °F to 99.5 °F) depending on age, sex, health state, emotional state, state of consciousness (waking, sleeping or sedated), body location at which the measurement is taken1,2.
Thermoregulation is what our body does to keep the temperature in this range despite variations caused by environmental conditions or physical work3. Core temperature is regulated by the hypothalamus as part of our brain. It acts as the body’s thermostat and responds to various temperature receptors located throughout the body. It triggers thermoregulatory mechanisms that help to release heat back to the environment if needed. These mechanisms are a dilation of skin vessels and an increase in cardiac output which leads to an increased blood circulation at the skin. This makes a rapid distribution of heat to the body surface possible.
The skin then has the fundamental role of regulating heat exchange trough:
- Radiation e.g. like the sun, or sometimes you can feel heat radiation from a person if you are standing close
- Conduction eg. transferring heat with direct contact, like to the surface you are sitting on
- Convection e.g. headwinds, heat is taken by airstream moving over your skin
- Evaporation e.g. sweat converts from fluid to gas: therefore energy, in the form of heat is needed to break hydrogen bonds in molecules. This causes the cooling effect. Alcohol, for example, needs a lower temperature to turn into gas – this is how rubbing alcohol cools as well (it does not mean that you can cool better by using alcohol internally 😜)
What happens during physical exercise?
With physical exercise, heat production increases as a result of energy turnover in the activated muscles. Blood flow in muscular areas like a skater’s legs increases and takes up the heat while the blood volume in internal and peripheral parts of the body decreases. The hypothalamus is sensing these changes and to compensate for an increasing core-temperature regulates that the warmer blood is transferred to the body surface and makes us sweat.
Sweat production and evaporation is the most effective way to lose heat in exercise 4,5. Under ambient temperatures around 18 °C radiation, convection and conduction together contribute about ¼ to the total heat loss. Sweat dripping from the body and not evaporating directly on the skin does not contribute to heat dissipation.
Beyond 30 °C (86 °F) outside temperature those three mechanisms aren’t effective anymore. In such conditions, the athlete depends exclusively on heat dissipation over sweating and evaporation. This is also limited because with a more humid climate the evaporation rate decreases.
The Heat Is On
Factors influencing our temperature
Skin temperature is lower than core temperature and can differ regarding different body parts. During semi-nude exercise (shorts, socks, shoes) between 10 °C – 30 °C (50 °F – 86 °F) local skin temperature is more dependent on ambient air temperature than metabolic rate, while core temperature is essentially dependent of metabolism6. Whether we can keep up with heat dissipation depends on the amount of heat generated through muscle work and the ambient conditions such as temperature, humidity and airflow. If we can’t balance these factors via thermoregulation anymore it will lead to non-compensatory heat stress with a continuous increase in body core temperature. This promotes fatigue, a lack of coordination and concentration and will, therefore, influence performance and health7. However, a controlled increase in body core-temperature during sports is normal. In physical activities like football or running we can heat up to core temperatures up to 39.7 °C (103.5°F). The actual temperature sensation differs from the measured ambient temperature. It depends on humidity, exposure to solar radiation, airflow and individual factors. Besides ambient factors, there are some more individual aspects influencing our ability to regulate heat1,2,8:
Stay hydrated – always! Replace lost electrolytes. This is what you need to sweat. Furthermore, thermoregulation is supported by a sufficient amount of blood plasma volume, which also needs to be fluid
A good training condition will reduce the increase in heart rate, you are getting better in sweating and less sensitive to heat.
Sport under heat conditions
Our organism has the ability to adapt. If we are used to hot conditions, we sweat more, our sweat contains less sodium and we start sweating at a lower body core temperature and in case of thermos regulation, these are good things.
Extensive lack of sleep can influence the ability of thermoregulation.
Skin lotions before exercise
Sweating after using lotion does not feel good. And it is sitting in your pores, which is hindering the evaporative mechanisms.
Our capability to regulate heat can be negatively influenced when alcohol is consumed.
Infections like a cold, hidden inflammation, thyroid dysfunctions (over or under) can increase heat sensitivity. Certain medications like antihistamines, prescribed for allergic reactions can reduce the amount of sweat.
The female cycle
During the second half of the cycle, the body core temperature at rest is about 0.4 °C higher. This goes with a higher threshold for the onset of thermoregulatory mechanisms, such as sweating or increasing blood circulation to the skin.
How clothing influences our temperature
Besides the above-mentioned factors, another major influence on our thermoregulation mechanism comes from clothing. Besides in semi-nude studies, we are usually wearing some kind of clothing in most of our physical exercises. Generally, this is a good thing since clothes protect us from environmental influences like:
- physical influences e.g. scratches, UV-rays
- biological influences e.g. virus, bacteria
- chemical influences e.g. acids
On the downside, as an extra layer covering our skin, heat loss is limited because of the thermal insulation and a mechanical barrier5,9.
When talking about garment, another aspect is how it feels to the user. The difference between measured and perceived temperature can be high. When sweat evaporation on the skin surface is limited, but sweat production is ongoing, the skin becomes saturated with sweat and the garment will also get wet. Discomfort due to stickiness coming from wet sweaty clothing is reported to influence wellbeing and also the commitment to performance10. After physical exercise, skin and clothing-wetness cause a fast decrease in body temperature which can lead to thermal discomfort and cold sensations10. To address this discomfort, sweat absorbing material is advertised a lot, aiming for dry sensation. BUT the cooling efficiency for the body of such absorbed sweat will be much less than the sweat evaporated from the skin. Therefore sports garments should ideally:
- allow air flow
- not insulate
- allow vapour permeability
- allow moisture absorption
A study comparing semi-nude, cotton and synthetic garment promoting sweat evaporation showed no statistically significant differences between the conditions regarding sweating sensation or skin and body temperature.
An improved sweating efficiency and reduced sweat absorption were found for synthetics11. Another 20 year old study showed a higher mean body temperature in polyester than in a cotton and cotton/wool blend12. A more recent study compared shirts made from 100% cotton, a 50/50% cotton/soybean blend and 100% polyester constructed with mesh loops. Results showed no effect on thermoregulation or clothing comfort with the three different materials. But the thermal sensation was rated more positive with the synthetic fibre and microclimate temperature at the chest was a little lower wearing this13.
The ambient temperature in these studies is mostly around 30 °C (86 °F) and several other factors like wind speed, humidity, training intensity have to be controlled while subjects run on a treadmill. The studied groups are typically quite small (< 10 persons). Laboratory tests in materials and subjects are hard to support by real-life data from humans during exercise10 and it is unknown how draping, air gap and contact-area between the garment and the body impact sweat absorption. The chest, neck and the back are areas where we sweat a lot during exercise. This can be more easily defined for tight-fitting clothes where the sweat transfer between skin and garment would be expected to be similar to the body sweat pattern10,14.
So clothes should either have a tight fit made of breathable material to allow sweat evaporation happening close to the skin or a loose fit, to give space for air flow and to allow direct evaporation on the skin.
Research is clear that the greater amount of clothing worn, the lower the evaporative sweat rate is, so additional clothing increases thermal stress15. When thinking about more gear demanding sports additional layers pile up quickly. In contact sport, we easily count up to 3 layers adding underwear, protectors and team jerseys. This is where we can take action and limit our layers to reduce thermal stress. Built-in bras, as well as direct prints on contact wear and protectors instead of team jerseys, could save a layer in favour of thermoregulation.
Thus saying, please don’t give up on necessary protective gear in favour of your thermoregulation! Protection should be used where it is needed but should leave areas which are not prone to impact. So gear should be selected specifically to the requirements of a certain sport and maybe even the position played. Sometimes even rule changes can help reduce layers. So instead of using spine protectors in a full-contact sport where engaging in the back is not allowed, penalizing hits to the back stricter is beneficial in several ways. Additionally, when training outdoors, light-coloured clothes can reduce heat radiating from the sun.
Check out part II of this STEAKS Story
In the second part of this blog, we share some skin temperature observations we collected from various skaters with and without STEAKS® Contact Wear. Get answers to the question: Does STEAKS® Contact Wear heat up your body?
About the author:
Block Sabbath is an engineer working in prosthetic/orthotic research using movement analysis and a skater with the Rhein-Neckar Delta Quads, Mannheim (Germany).
She enjoys transferring the biomechanics and material properties from work to roller derby gear and exercise. Some say that makes her a nerd. In her first year of roller derby/doing sport it was a relief to learn that getting hot and sweaty in situations where that didn’t occur before (like rushing to the train) was a sign of effective training.
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- Podcast: More or less: Behind the Stats “The Mathematics of Fever”, Tim Harford BBCRadio 4 12.01.2019