2007, Piran, Slovenia

Non-thermal factors
DOES A FATIGUE-INDUCED INCREASE IN RELATIVE WORK RATE
AFFECT EXERCISE THERMOREGULATION?
Alan Kacin 3,2,1 , Petra Golja 4 , Michael J. Tipton 2 , Ola Eiken 5 and Igor B. Mekjavic 1
1 Department of Automation, Biocybernetics and Robotics, Institute Jozef Stefan, Slovenia;
2 Department of Sport and Exercise Science & Institute of Biomedical and Biomolecular
Sciences, University of Portsmouth, United Kingdom; 3 Department of Physiotherapy, College
of Health Studies, University of Ljubljana, Slovenia; 4 University of Nova Gorica, Slovenia;
5 Swedish Defence Research Agency, Karolinska Institutet, Sweden
Contact person: alan.kacin@vsz.uni-lj.si
INTRODUCTION
It is now well established that non-thermal factors may interfere with thermoregulatory
mechanisms both at rest and during exercise (Mekjavic & Eiken, 2006). Findings from our
previous studies on graded ischaemia in the working muscles (Eiken & Mekjavic, 2004;
Kacin et al., 2005) and on exposure to hypoxia (Kacin et al., 2007) suggest that an acute
increase in relative work rate, indicated by increase in relative oxygen uptake (% V & O2peak) and
perception of whole-body exertion, is associated with an augmented sweating response and
attenuated skin blood flow during dynamic exercise. Both muscle ischaemia and normobaric
hypoxia increase relative work rate during constant-load exercise by limiting oxygen delivery
to the working muscles. It is thus likely that thermoregulatory responses are affected via
pathways controlling oxygen content and turnover in the body. The exact mechanisms of
these alterations in thermoregulatory responses however remaines unknown, as other nonthermal
factors associated with increased relative work rate may also play a role. In the
present study we thus investigated whether the increase in sweat secretion and decrease in
skin blood flow occurs also during steady-load exercise, if no restriction of oxygen delivery to
the active muscles is performed. The increase in %V & O2peak and perception of whole-body
exertion was induced predominantly by muscle fatigue, which progressively occurs during
prolonged steady-load exercise.
METHODS
Ten healthy males, occasionally involved in sport activities, participated in the study. Their
mean (range) age was 22 years (18-25 years), body weight 77.4 kg (61.3-92.2 kg), height 182
cm (174-193 cm) and body mass index 23.2 (21.1-25.3). In order to evaluate their initial peak
oxygen uptake (Normal V & O2peak), the first ramp-test to exhaustion was performed on cycle
ergometer with subject being well rested. On a separate day, a 120-minute cycling exercise
was performed at constant work rate corresponding to 60% of Normal V & O2peak. This was
immediately followed by the second maximal performance test (Fatigue V & O2peak). To allow
optimal regulation of body temperature during exercise, all experiments were performed in a
thermoneutral environment (Ta = 23°C, RH = 50%, wind speed = 5 m•s -1 ). To prevent
dehydration during prolonged cycling, subjects drank 0.5 L of water 1.5 hour before the
experiment and were encourage drinking water at regular intervals during exercise. Core
temperature was measured with a rectal probe, whereas mean skin temperature (T sk) and
mean heat flux (HFmean) were calculated from the values measured at 9 sites (finger, forearm,
arm, head, chest, back, abdomen, thigh and calf). Mean body temperature ( T b) was calculated
as: T b = 0.8 Tre + 0.2 T sk. Minute mass flow of secreted sweat was measured with a
ventilated capsule placed on the forehead (m& swf) and mid-back (m& swb). Skin blood flow
(BFsk) was measured with a single fibre laser-Doppler probe positioned approx. 3 cm caudal
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