Hana Hicks’ MSc Thesis Defence

Title: Effects of Exercise Intensities on Cortisol Release Following the Induction of a Psychological Stressor

Thesis Supervisor: Dr. Eli Puterman
Committee members: Dr. Frances Chen, Dr. Mark Beauchamp
Defence Chair: Dr. Desmond McEwan

Abstract: Long-term physical activity and exercise have been shown to mitigate neurobiological responses to acute psychosocial stressors. There is also evidence that engaging in a single bout of exercise has immediate effects on reducing neurobiological reactivity to acute psychosocial stressors. However, much of this previous work applied definitions of intensity zones on American College of Sports Medicine’s global prescription recommendations, not based on individualized, physiologically meaningful cut-off points for demarcating the intensities at which participants exercised.

This project expands on prior evidence by investigating the extent to which a single bout of exercise, performed at light, moderate, or vigorous intensities – established with meaningful indicators of changes in metabolic needs – alters the neurobiological response to a subsequently induced psychosocial stressor. It is hypothesized that vigorous intensity, defined as the midway point between the first and second ventilatory thresholds, will dampen cortisol output following a subsequently induced psychological stressor, compared to moderate (defined as being 10% below the VT1) and light (set at 1.7km/hour) intensities. Moderate intensity is expected to dampen the cortisol trajectory as compared to the light intensity, but to a lesser extent than vigorous.

To achieve this, participants complete a modified maximal cardiopulmonary exercise test (CPET) to determine their individual ventilatory thresholds. Participants then return to the lab a second time to complete the experiment, whereby they are randomized to complete a 30-minute exercise at their randomized intensity zone, followed by the Trier Social Stress Test 45 minutes later.

Multilevel Growth Curve Analysis determined that participants in the vigorous group had significantly slower rates of increased cortisol compared to both the light and moderate groups. Results also indicated no significant differences in cortisol trajectories between the light and moderate groups, apart from the fact that those in the moderate group had lower estimated starting points prior to starting the TSST. Results suggest that the current findings may be a result of the negative feedback loop, if they hold when the complete sample is analyzed. Examination of other psychological and physiological responses to psychosocial stressors will help elucidate the extent to which exercise mitigates stress reactivity in its more comprehensive conceptualization.