peratures during activities such as walking and
running (MacDougall et al. 1983). In an attempt
to control for differences in stature between
adults and children, sweating rate is normalized
to body surface area, but, even after this adjust-
ment, children demonstrate a lower sweating
rate than adults (Bar-Or 1980; Falk et al. 1992).
This decrease exists in spite of the fact that chil-
dren have a greater number of heat-activated
sweat glands per unit skin area (Falk et al. 1992).
The sweating threshold is considerably higher in
children than in adults (Araki et al. 1979). Meyer
et al. (1992) demonstrated that adults have el-
evated sodium (Na) and chloride (Cl) concentra-
tions in sweat. It has also been shown that body
core temperature increases at a higher rate for
any given level of hypohydration in children
than in adults (Bar-Or et al. 1980). Despite the
multitude of differences in the physiological
responses of the child, the critical question is
whether these characteristics will limit perfor-
mance in children. There is no definite answer,
but it is clear that in a hot environment, children
are at a disadvantage compared with adults. In
adult studies, it has been found that there is a
clear effect of temperature on exercise capacity
which appears to follow an inverted-U relation-
ship. Galloway and Maughan (1997) found
under their study conditions that exercise dura-
tion was longest at 11°C: below this temperature
(at 4°C) and above this temperature (at 21°
and 31°C), a reduction in exercise capacity was
observed.
Bar-Or et al. (1992) identified that children, like
adults, do not drink enough when offered fluids
ad libitum during exercise in the heat, a condition
known as voluntary dehydration. The phy-
siological consequences for the child athlete are
serious; at any given level of hypohydration,
children’s core temperature rises faster than that
of adults, and it is therefore critically important
to reduce voluntary dehydration (Bar-Or et al.
1992). The general guidelines that should be
issued to children exercising in the heat are to
drink until the child does not feel thirsty, and
then to drink an additional half a glass (100–125
ml); for adolescents, a full glass extra is recom-
436 special considerations
mended. However, in order to implement these
guidelines for sporting competitions under cli-
matic heat stress conditions, competition regula-
tions need to be altered for the child athlete.
Suggestions include allowing the child to leave
the field of play periodically, or, as in the 1994
soccer World Cup, the positioning of drinking
bottles on the perimeter of the field to allow for
fluid intake during natural stoppages in play.
In order to encourage the child to take on
board sufficient fluid to offset voluntary dehy-
dration, the fluid of choice has to be palatable
and should stimulate further thirst. Thirst per-
ception is influenced by drink flavour and drink
composition. Meyer et al. (1994) demonstrated
in prepubertal children, at rest after a maximal
aerobic test and for rehydration purposes after
prolonged exercise in the heat, that grape
flavouring was preferred to apple, orange
and unflavoured water. Wilk and Bar-Or (1996)
attempted to determine which of the two factors
played the more important role. Trials were
undertaken using flavoured water and an identi-
cally flavoured carbohydrate (6%)–electrolyte
(NaCl) drink. It was shown that the flavoured
water (grape) maintained euhydration over a
90-min exercise period under heat stress condi-
tions. The carbohydrate–electrolyte drink pro-
duced a slight overhydration over the same time
period. These studies suggest that voluntary
dehydration could be reduced by drinking
flavoured water and prevented by drinking a
carbohydrate–electrolyte drink (Fig. 32.4).
The concentration of sodium ions in the
extracellular fluid is critical to the rate of
replenishment of body fluids. Nose et al. (1988)
demonstrated that the ingestion of 0.45 g NaCl
in capsule form per 100 ml of water enhanced
volume restoration after dehydration relative to
water alone. Wilk and Bar-Or (1996) demon-
strated that there was a 45% increase in drinking
volume in favour of the grape flavoured water,
and an additional 47% increase in voluntary
drinking on the addition of carbohydrate and
NaCl (Fig. 32.5). Their study design did not allow
the partitioning out of the carbohydrate and
NaCl effects, but previous studies (Nose et al.