Hello everyone. My name is Julien Guéritée. I got my Ph.D. a
couple of years ago from the University of Portsmouth (UK). The aim of my work
was to understand some of the physiological mechanisms driving thermal comfort
(and the loss of it!) around water sports. After some time as a post-doc
researcher, and, later as a R&D engineer, I founded a consulting firm where
I share what I have learned with sports clothing manufacturers.
Davide gave me the opportunity to write a few words about
what I have discovered when I was at university. I would like to emphasize that
nothing would have been possible without great support from the technical team
and the experts in human and applied physiology who work there.
In water, before we started our studies at Portsmouth,
research had focused on the safety aspects of cold water (below 15 °C) immersions
[1]. Consequently, little was known about thermal comfort in cool water where
water sports are undertaken and where maintaining thermal comfort becomes more critical
as it affects both the behavioural and pleasure responses [2]. After some
extensive literature review, we hypothesized that the hands and feet would be
responsible for the loss of overall thermal comfort.
We thus decided to test this hypothesis. However, we could
not immerse participants in cold water and expect them to give relevant,
accurate and informative answers regarding their thermal comfort state. We all
know it, when you jump in cold water, you just feel cold, uncomfortable, and
miserable. At that stage, you don’t really care whether your hands or your
abdomen is responsible. To avoid this, and get the most of our data, we
immersed our participants in a comfortable water temperature of 34.5 °C. After
a few minutes, the temperature was decreased to 19.5 °C over 20 min.
During this cooling phase, our resting or exercising volunteers
(depending on the day) reported when they no longer felt comfortable and which
region was responsible. In practice, they looked at the scale in front of them
(going like this: very uncomfortable > uncomfortable > just uncomfortable
> just comfortable > comfortable > very comfortable) and they just
told me “Julien, I am now ‘just uncomfortable’, and this is because of my arms
[they were allowed to give any body region]”.
To be able to explain this subjective event (the loss of
overall thermal comfort) with physiological mechanisms, we had to record more “objective”
data whilst water (and skin) temperatures were cooling. We know that thermal
comfort is equally driven by core and skin temperature [3]. This is why we had
decided to continuously collect core and local skin temperatures. By doing so,
when participants lost their overall thermal comfort, we had a fairly accurate
idea of their thermophysiological status.
First of all, when overall thermal comfort was lost, and in
contrast with skin temperatures, core temperature had not changed. Although
this was not surprising, it was good to verify it. Secondly, in most cases,
when volunteers in swimming briefs became uncomfortable, water (and skin!)
temperature was around 30°C.
Surprisingly, the hands and feet were not responsible for the
loss of overall thermal comfort. Instead, the chest and the lower back were
reported to cause this event. Now this was intriguing and we needed to
understand why the extremities were not involved.
You may already know it: our body is not really good at
sensing temperature. It is much more sensitive to temperature changes. This is
due to the way our thermoreceptors work [see 4 for in-depth description]. When
a dynamic thermal stimulus is applied to the skin, the frequency of discharge
of the thermoreceptors (the signal our brain eventually receives) is increased
and can reach maximum levels depending on the adapting temperature, which can be
defined as the steady state discharge frequency observed at constant
temperatures (what happens now, as you are reading, if you are thermally
comfortable). The faster the rate of change of skin temperature for a given
adapting temperature, the greater the dynamic response to cooling up to maximum
levels.
The idea was thus to try to explain our findings in the
light of our adapting temperatures, the thermal profile of our skin when
nothing particular happens (when we are comfortable). We believed that the environmental
conditions of a working office on a normal day are those under which many modern
humans spend most of their time. The skin temperature distribution across the
body in such conditions would therefore be the one the most frequently experienced.
In addition, we knew that humans evolved in, and seek, “comfortable”
thermoneutral air or microclimate temperatures of 26–28 °C [5].
Therefore, the skin temperature distribution of a resting
human, in a thermoneutral environment (a mix of 26-28 °C in minimum clothing
and 21 °C with office clothes on) could be the reference upon which subjective
thermal responses are based. We thus expected that the influence of each body
region on overall thermal comfort would be driven by local adapting
temperatures in such environments.
An assessment of the “reference” skin temperature distribution
in thermoneutral air indicated that the extremities (hands and feet) were
warmer when the loss of overall thermal comfort was reported during immersion
than when volunteers were in thermoneutral and comfortable air. We therefore
suggested that the regions where temperature remained above the “reference”
thermoneutral temperature in air would not determine the onset of overall
thermal discomfort, mainly because the stimulation of these regions would not
cause a sufficient increase in the frequency of discharge of the cold cutaneous
thermoreceptors.
We concluded that in cooling water, or when the skin is more
uniform in temperature and cools slowly from a warm stating point, the chest
and the lower back rather than the extremities are responsible for the loss of overall
thermal comfort. In these situations, hands and feet are already adapted to
colder air temperatures while the chest and lower back cool by more than
normal.
As I mentioned earlier, this occurred fairly early in the
cooling phase of the water. However, we had decided to keep it cooling to be
able to observe other responses. Once water temperature had reached 20 °C, it
was maintained at that level until the end of the experiments. Only then the
influence of the extremities on overall thermal comfort became important. At
that point, local skin temperatures of around 21 °C on these regions may have
constituted a more “specific” stimulus than that in the warmer temperature of the
cooling phase. Hands and feet were colder than what they are “naturally” in
air, and sent neurophysiological signals interpreted as very uncomfortable.
This work should have an impact on future research, as it
may help understand variations in thermal comfort responses to stimuli across
the body. What I report here is only a fraction of what has been investigated. If
you want to read more about it, check out the original article “The
determinants of thermal comfort in cool water” published in the Scandinavian
journal of medicine and science in sports. If you have any questions, please
get in touch!
The next topic should deal with the effect of swimming on
thermal comfort, and the impact of evaporative cooling on thermal regulation,
perception and comfort in air.
References:
[1]: Golden F, Tipton M. Essentials of sea survival.
Champaign, IL, USA: Human Kinetics, 2002: 120–139.
[2]: Chatonnet J, Cabanac M. The perception of thermal
comfort. Int J Biometeorol 1965: 9: 183–193.
[3]: Frank MS, Raja SN, Bulcao CF, Goldstein DS. Relative
contribution of core and cutaneous temperatures to thermal comfort and
autonomic responses in humans. J Appl Physiol 1999: 86: 1588–1593.
[4]: Hensel H. Thermoreception and temperature regulation.
London: Academic Press, 1981: 33–49. Monographs of the physiological society;
nr. 38.
[5]: Lahr MM, Foley R. Multiple dispersals and modern human
origins. Evol Anthropol 1994: 3: 48–60.
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