Capacity to perform prolonged exercise is reduced in high ambient temperatures, but
this premature fatigue is not adequately explained by peripheral mechanisms. The aim
of this thesis was to examine some possible underlying mechanisms of central fatigue
operating during prolonged exercise in a warm environment.
The first series of experiments investigated the effect of nutritional manipulation of
central serotonergic activity through alterations to the plasma concentration ratio of
free-tryptophan to branched-chain amino acids (f-TRP:BCAA). In contrast to
previous reports, acute BCAA supplementation failed to alter perceived exertion and
delay the onset of fatigue (Chapter 3). This response was similar when exercise was
preceded by an exercise and diet regimen designed to reduce glycogen availability
(Chapter 4). The ingestion of meals containing added carbohydrate and fat did not
alter f-TRP:BCAA at rest (Chapter 5). Acute dopaminergic / noradrenergic reuptake inhibition with bupropion increased
exercise perfonnance by 9 % in warm conditions (30C), but this effect was not
apparent at 18C (Chapter 6). This response was accompanied by attainment of a
higher core temperature and heart rate towards the end of the bupropion trial in the
heat despite no detectable difference in perceived exertion and thermal stress. These
data suggested that maintenance of catecholaminergic activity may dampen inhibitory
signals from the CNS due to the attainment of a high core temperature, allowing
power output to be maintained.
The blood-brain barrier (BBB) regulates the exchange of substances between the
cerebral interstitial fluid and the blood to maintain a stable environment for the CNS.
If the BBB is compromised this may adversely influence nonnal brain function.
Serum S1OOb, a proposed peripheral marker of BBB penneability, was increased
following exercise in a warm environment (Chapter 7). These data indicate that
exposure to combined exercise and heat stress may result in a loss of BBB integrity.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.