2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
water before retiring, and to eat an evening meal and breakfast high in carbohydrate and low
in fat. Subjects also refrained from using caffeine for 2 h prior to each trial. On arrival at the
laboratory, subjects were provided with supplementary water (10 mL.kg -1 ), and within each
trial, they consumed 300 mL of water (at chamber temperature) during each rest period. Prior
to departure, fluid replacement equal to 150% of the body mass loss was provided.
Physiological measurements:
Heart rate: Steady-state data were monitored every 15 s from ventricular depolarisation
(Polar Electro Sports Tester, Finland). Oxygen uptake: Steady-state data were determined
during the last 5 min of each exercise level (2900 Sensormedics system, U.S.A.). Core
temperature: Auditory canal temperature (insulated: Edale instruments Ltd., Cambridge,
U.K.) was recorded at 15-s intervals (1206 Series Squirrel, Grant Instruments Pty Ltd.,
Cambridge, U.K.). Sweat rate: Local sweat rates (15 s) were recorded from the chest and
scapula (left side) using sweat capsules (3.16 cm 2 ) attached to each skin site (Clinical
Engineering Solutions, NSW, Australia). Sweat sodium concentration: This was determined
using flame photometery (Corning M410, Ciba Corning, U.K.). Sweat samples were collected
into chilled vials from the chest and back during the final 2 min of each steady-state exercise
period (30 min), then stored under refrigeration for subsequent analysis.
RESULTS
This experiment resulted in data being collected at five different relative work rates for each
subject, with data at the lightest work rate collected in both trials. These forcing functions
elicited steady-state data across the following physiological ranges: (a) heart rate: 119-173
beat.min -1 ; (b) oxygen uptake: 0.55-2.22 L.min -1 ; (c) core temperature: 37.4-38.7 o C; (d) sweat
rate: 0.56-2.37 mg.cm -2 .min -1 ; and (e) sweat sodium loss: 0.3-3.18 g.h -1 .
Data were plotted such that the four inter-dependent relationships could be combined into a
single quadrant diagram, with each relationship sharing a common origin, and adjacent
quadrants sharing common axes (Figure 1). Of course, this does not follow the convention of
plotting dependent variables on the ordinate, but it does allow one to move sequentially
around the diagram. Such a schema is not new. Indeed, the logic used in developing this
representation of these four relationships has previously been also applied to help describe
carbon dioxide transport from the lungs and through blood during exercise (McHardy et al.,
1967), and to model thermoeffector control and feedback within mammalian temperature
regulation systems (Werner, 1990).
If one determines the steady-state heart rate for the activity of interest, then one can enter the
upper left quadrant at position one. A vertical line allows prediction of the steady-state
oxygen uptake (position two), with a horizontal line into the upper right quadrant providing
the projected steady-state core temperature for this activity (position three). Another vertical
line, this time into the lower right quadrant, intercepts the asymptotic relation between core
temperature and sweat rate (position four). A horizontal line passes from the lower right to the
lower left quadrant, intercepting the sweat rate and sodium loss relationship, thereby enabling
one to predict both the steady-state sweat sodium loss (position five) and sweat rate (position
six). These predictions can be used to prescribe sodium and fluid replacement. Of course,
each of these four relationships can easily be modelled mathematically, with a series of
algorithms being combined to obtain the desired predictions from just a few keystrokes on a
computer.
302