2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
298
ABSENCE OF A GENDER AFFECT ON THE FLOW-DEPENDENT
NATURE OF SWEAT SODIUM LOSS.
Anne M.J. van den Heuvel, Lara S. van den Wijngaart, Nigel A.S. Taylor
Human Performance Laboratories, University of Wollongong, Wollongong, Australia
Contact person: nigel_taylor@uow.edu.au
INTRODUCTION
In other communications at this conference, we have described regional differences in human
eccrine sweat secretion rates at rest and during exercise. Such sweat is isotonic, with its
principal solutes being sodium, chloride, lactate and potassium (Robinson and Robinson,
1954; Patterson et al., 2000), and it is well established that its electrolyte concentration is a
function of sweat secretion rate, both before (Cage and Dobson, 1965) and after heat
adaptation (Allan and Wilson, 1971). Indeed, this is a flow-dependent, linear relationship.
Herein we focus upon gender differences in the sweat rate of the torso, and how these rates
affect the concentration of sodium within that sweat.
METHODS
Twelve fully-hydrated and unacclimatised subjects (6 males and 6 females) participated in
two cycling trials, each conducted at the same time of day. Trials took place in a heated
climate chamber (36 o C, 50% relative humidity), and subjects were exposed to a series of three
step increases in work rate, designed to elicit steady-state heart rates between 120-170
beats.min -1 . The first work rate was set to obtain a heart rate of 120 beats.min -1 (30 min),
while the order of next two work rates was randomised within each trial (25 min): trial 1:
120, 140, 170 beats.min -1 ; trial 2: 120, 150, 160 beats.min -1 . Subjects rested for 5 min
between successive steps.
Subjects were asked to refrain from strenuous exercise and the consumption of alcohol and
tobacco during the 12 h before to each trial. Prior to testing, subjects were instructed to drink
15 mL.kg -1 of additional 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: Data were monitored every 15 s from ventricular depolarisation (Polar Electro
Sports Tester, Finland). 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, then stored under refrigeration for subsequent
analysis.