The physiology of self-paced exercise: pharmacological intervention and cell stress response in the heat

There is an abundance of literature regarding the human response to constant-load exercise, however there is much less clarity surrounding self-paced exercise and performance. The purpose of this thesis was to examine the physiological, neuromuscular and stress responses to self-paced exercise in the heat, in healthy men and women. The first study investigated the performance and thermoregulatory response of males to a dopamine/norepinephrine (DA/NE) reuptake inhibitor during self-paced exercise in a warm environment. The second study focussed on healthy oral contraceptive (OC) users to investigate performance, thermoregulation and neuromuscular function during self-paced exercise in a warm environment. The final study investigated the cellular stress and thermoregulatory responses of healthy men and women to a self-paced cycling protocol. All studies were conducted in a cool and a warm ambient temperature and consisted of a standardised protocol: the first 30 min was fixed-intensity cycling at 50% Wmax followed by a 30 min self-paced time trial. Each 30 min period was interspersed with a maximal 30 sec sprint at 9, 19 and 29 min. Thermoregulatory, perceptual and heart rate responses were all recorded at regular intervals throughout cycling trials. Neuromuscular measures were taken pre- and post-cycling exercise to examine fatigue and attempt to identify the origin of fatigue during self-paced exercise.
The main findings of each study were 1) that a DA/NE reuptake inhibitor affected thermoregulation (38.6°C in BUP32 v 37.7°C in PLA32) but did not affect self-paced cycling exercise in males (distance covered was 12.8 km v 13.0 km for BUP20 and PLA20, respectively and 12.7 v 13.1 km for BUP32 and PLA32, respectively), in warm and moderate conditions; 2) that OC use did not affect self-paced cycling exercise, nor neurotransmission but did affect thermoregulation in warm conditions, with females reaching core temperatures of 38.5°C and 38.4°C for WARM and COOL, compared with 38.2°C for CON and 3) that women exhibited higher levels of cellular stress (HSP 246 pg.l-1 compared with 120 pg.l-1), higher core temperature (38.5°C v 37.9°C)and heart rate (177 bpm v 175 bpm), along with perceived exertion (8.1±1.5 v 7.8±0.5) than men, despite men exercising at a higher intensity than women in all conditions.
The conclusions for these studies are that a DA/NE re-uptake inhibitor influenced thermoregulation but not exercise performance in either warm or moderate conditions. We also show that the drug could act peripherally to reduce twitch characteristics of muscle. For women taking an OC pill formulation, irrespective of environmental condition, self-paced cycling performance was not different, nor was neuromuscular performance influenced across the OC cycle. Finally, in the third study, females had a two-fold higher HSP72 concentration than males, regardless of environmental condition. The mechanisms for this remain unclear due to the fact that women attained higher Tc, HR and perceived exertion, however men exercised at higher intensities in all conditions.