O o d h e a d p u b L i s h I n g L i m I t e d



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11.2
The human system
Sport imposes some rather severe demands on clothing over and above the
basic requirements of general comfort, modesty and appearance. Sports apparel
must contend with a much wider range of heat and moisture loads than
conventional apparel and is often subject to far greater physical abuse and
© 2009 Woodhead Publishing Limited


Wool performance apparel for sport
267
put through more cleaning cycles during its life cycle. Perhaps the most
challenging requirements are the maintenance of thermal and subjective
comfort.
11.2.1 Metabolic heat loss and thermal comfort
The human body is an exothermic system, constantly generating and releasing
heat to the surroundings. In a healthy state the temperature of its internal
organs, known as the core temperature, is maintained at around 37
°C. This
varies by a degree or so depending on the time of the day, the level of
physical activity and the emotional state. Greater changes occur during illness,
in extreme environmental conditions and during vigorous activity. When
heat is generated more rapidly than it can be dissipated, the core temperature
rises. An increase of around 2
°C can be safely tolerated but further sustained
rises can lead to heat-related illness (Nielsen, 1928).
Work done to move the body around during exercise is derived from
biochemical reactions that occur within the muscles. These processes are
accompanied by the release of heat, the rate of heat production depending
largely on the level of physical activity involved. Muscles are very inefficient
at converting stored energy into physical work done and waste energy released
into the tissue as heat is typically as much as five times the actual energy
expended in moving the body around (Coyle et al., 1992; De Prampero
et al., 1971).
Highly trained athletes can maintain work rates of around 500 watts for
periods as long as 30 minutes with a total heat output of the order of 2000
watts – more than 20 times greater than the resting metabolic output (Padilla
et al., 2000). Such a wide range of heat generation requires a very sophisticated
temperature control system in order to maintain thermal or physiological
comfort.
Heat is transferred from the muscles to the skin via the cardiovascular
system. An increase in body core temperature stimulates expansion of the
blood vessels near the skin and increases the blood flow to the skin surface.
During sport, blood flow is further accelerated by an increased heart rate.
The result is elevation of the skin temperature. Heat loss from the skin to the
surroundings normally occurs by a combination of infrared radiation, air
convection and as latent heat from the evaporation of moisture. Each of
these modes relies on a difference in either temperature or moisture
concentration between the body and the environment to drive the heat transfer
process.
The skin loses moisture continually, sourced from evaporation within the
tissue and diffusing as moisture vapour through the dermal structure. During
sleep and very low activity conditions, this amounts to about 30 millilitres
per square metre per hour (Hendrie et al., 1997). When the heat lost by
© 2009 Woodhead Publishing Limited


Advances in wool technology
268
radiation and convection is not sufficient to prevent the core temperature
from rising, sweat glands within the skin are activated, releasing moisture
for evaporation directly at the skin surface. The dissipation of energy as
latent heat of vaporisation in this way significantly increases the rate of body
heat loss. In extreme conditions sweat evaporation becomes the major route
for heat loss by the body. With sweat glands fully activated, the moisture
output may be one to two litres per hour or more (Hendrie et al., 1997),
occurring both as moisture vapour and if it cannot evaporate quickly enough,
liquid sweat.
11.2.2 Skin sensory behaviour and subjective comfort
The physiology of the skin plays an important role in the subjective comfort
of clothing. Just below the skin surface are sensitive nerve endings that
respond to movement, temperature and pain. The nerve endings that detect
temperature are very much more sensitive to changes in the temperature of
the skin (or more correctly, rate of change of temperature) than they are to its
actual temperature.
The lowest rate of change of temperature that the skin is able to detect is
about one hundredth of a degree Celsius per second (Hensel, 1981). A rise in
ambient temperature of one degree Celsius over a period of an hour is unlikely
to be noticed because the rate of change is very low. The same change in just
1 second produces an instant reaction – something akin to the experience
encountered when first entering a coolroom or a sauna.
The sensations we understand as warmth or coolness to the touch are the
result of physical processes that occur at the skin and trigger responses in the
skin’s thermal sensors. When an object contacts the skin, the resulting heat
flow between the two depends on how much warmer or cooler the temperature
of the object is than the skin and how well it conducts heat. The faster the
heat flow into or out of the skin, the greater the rate of change of skin
temperature and the warmer or cooler the object feels.
A brushed knit fabric with a hairy surface feels warmer than the smooth
surface of a dense, tightly woven sateen because for the same temperature
difference, the rate of heat flow from the skin into the knit is lower and
produces a weaker thermal sensation. Smooth fabrics have more fibres in
contact with the skin than hairy fabrics, giving greater contact area and more
rapid heat flow, cooling the skin faster and producing a stronger coolness
sensation.
Human skin has no known nerve endings to detect moisture directly, and
yet discomfort associated with clamminess and stickiness is invariably attributed
to moisture. It is likely that these sensations result from indirect changes in
the skin that the brain associates with moisture although the actual mechanisms
involved are yet to be identified.
© 2009 Woodhead Publishing Limited


Wool performance apparel for sport
269
Coarse fibres tend to indent the skin rather than buckle when under external
pressure, triggering pain receptors and causing irritation (Naylor et al., 1992).
Thus parameters such as fibre diameter and fabric stiffness are important
considerations in the comfort of active sportswear. Like wool, the skin contains
proteins that make it hygroscopic so that, like wool, it absorbs and desorbs
moisture as the humidity around it changes. As its moisture content increases
it becomes softer, making it easier for textiles on the skin surface to activate
the underlying nerve receptors. As a consequence, the skin is more sensitive
during active sports or in warm climates than when dry and cold.
For wool products to be worn in skin contact during active sports the issue
of fibre diameter is quite important. Wool is commercially available in a
wide range of diameters and diameter distributions and the fine wools required
to ensure universal acceptance in active sportswear (typically 18.5
µ and
finer) come at a significant cost disadvantage. This is one of the factors that
have discouraged the use of wool fabrics in active sportswear, particularly in
the high-volume look-alike sector where base fabric cost is a critical issue.

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