Endurance exercise : effects of high ambient temperature and central acting agents
The main intent of the work described in this thesis was to gain further insights into factors that affect
performance during endurance exercise with particular emphasis on exercise in the heat. Five
different studies are reported. The two first studies (Chapters 1 and 2) assessed the role of skin
temperature modulating the negative impact of high ambient temperature. In Study Ia cap was
developed that cooled the top of the head. The results showed, however, that cooling the top of the
head had no effect on performance during a simulated cycling time trial conducted in the heat (35°C),
despite the fact that the cold stimulation was effective in blunting prolactin secretion during exercise.
In Study 2 the effect of body heating on the ratings of perceived exertion (RPE) was assessed during
a short sub maximal exercise test in the heat. Body heating significantly increased RPEs from the
start of exercise whereas it had no effect on thermal comfort. Cold stimulation of the face
counteracted the effect of body heating on RPEs.
Studies 3 and 4 (Chapters 4 and 5) explored the effect of high ambient temperature on the capacity to
perform during subsequent exercise. A repeated exercise paradigm was used in Study 3. The results
showed that, after a 2-h recovery, fatigue occurred earlier during a second bout of exercise in the heat
(35°C) occurred earlier than in the first bout of exercise performed at the same ambient temperature.
No changes in thermoregulation, plasma glucose, lactate, IL-6, prolactin or cortisol were seen that
could explain the earlier fatigue onset. It was thought that exercise in the heat might affect
hypothalamic neuroendocrine function and this was tested in Study 4 by conducting an insulin
challenge 2h after either rest or strenuous exercise in the heat. However, no differences were seen in
the magnitude of plasma prolactin and cortisol responses to acute hypoglycaemia. However, previous
exercise in the heat increased reactivity of the autonomic innervation of the heart during the insulin
test, as assessed by shifts in the power spectra of the R-R interval.
The final study (Chapter 6) examined a possible mechanism for the ergogenic action of caffeine
during prolonged exercise. Baclofen, a -aminobutyric acid B (GABAB) receptor agonist, was coingested
with caffeine as a means of preventing the presumptive release of brain dopamine. The
results, however, failed to demonstrate any effect of baclofen preventing the ergogenic effect of