cost. Air resistance becomes a factor at the higher
speeds involved in cycling, and reducing the
energy needed to overcome air resistance is a
crucial factor in improving performance. In
swimming, more so than in the other types of
activity, technique is important in determining
the energy cost of covering a fixed distance or
moving at a fixed speed. In most sporting situa-
tions, as in most daily activities, the exercise
intensity is not constant, but consists of intermit-
tent activity of varying intensity and duration.
Elite marathon runners can sustain speeds that
result in rates of heat production in the order of
1200 W for a little over 2 h, which is the time it
takes for the top performers to complete a
marathon race (Maughan 1994a). In spite of this,
however, the rise in body temperature that is
observed seldom exceeds 2–3°C, indicating that
the rate of heat loss from the body has been
increased to match the increased rate of heat pro-
duction. In general, the rise in body temperature
during exercise is proportional to the exercise
intensity, whether this is expressed in absolute
terms as a power output or in relative terms as a
proportion of each individual’s aerobic capacity.
This observation indicates that the balance
between heat production and heat loss is not
perfect, but the relationship is none the less
rather precise.
Heat exchange between the body surface and
the environment occurs by conduction, convec-
tion and radiation (Fig. 15.1), and each of these
physical processes can result in either heat gain
or heat loss: in addition, evaporation can cause
heat to be lost from the body (Leithead & Lind
1964). Air has a low thermal conductivity, but the
thermal conductivity of water is high, which is
why an air temperature of, say, 28°C feels warm
but water at the same temperature feels cool or
even cold. The pool temperature is therefore of
critical importance for swimmers. Convection
and radiation are effective methods of heat loss
when the temperature gradient between the skin
and the environment is large and positive, i.e.
when the skin temperature is much higher than
the ambient temperature. Under such condi-
tions, these two processes will account for a
204 nutrition and exercise
major part of the heat loss even during intense
exercise. As ambient temperature rises, however,
the gradient from skin to environment falls, and
above about 35°C, the temperature gradient from
skin to environment is reversed so that heat is
gained by the body. In these conditions, evapora-
tion is therefore the only means of heat loss.
The heat balance equations are described by
Kenney (1998) and are usually described by the
following equation:
S=M±R±K±C–E±Wk
This indicates that the rate of body heat storage
(S) is equal to the metabolic heat production (M)
corrected for the net heat exchange by radiation
(R), conduction (K), convection (C) and evapora-
tion (E). A further correction must be applied to
allow for work (Wk) done: this may be negative
in the case of external work done, or positive
when eccentric exercise is performed.
A high rate of evaporative heat loss is clearly
essential when the rate of metabolic heat produc-
tion is high and when physical transfer is limited
or actually results in a net heat gain by the body.
Evaporation of water from the skin surface will
result in the loss from the body of about 2.6 MJ
(620 kcal) of heat energy for each litre of water
evaporated. If we again use our marathon runner
as an example, and again assume a rate of heat
production of 1200 W, the effectiveness of evapo-
ration is readily apparent. Assuming no other
mechanisms of heat exchange, body temperature
would rise rapidly and would reach an intoler-
able level within only about 20 min of exercise.
Evaporation of sweat at a rate of 1 l · h–1would
result in heat loss by evaporation occurring at a
rate of 2.6 MJ · h–1(620 kcal · h–1), which is equiva-
lent to 722 J · s–1(172 cal · s–1), or 722 W. The entire
metabolic heat load would therefore be balanced
by the evaporation of about 1.7 l sweat · h–1, and
this is well within the range of sweat rates nor-
mally observed in various sports during exercise
(Rehrer & Burke 1997).
Although the potential for heat loss by evapo-
ration of water from the skin is high, this will
only be the case if the skin surface is kept wet by
constant replacement of the sweat that evapo-