Prolonged stress exposure is associated with cardiovascular disease, depression, anxiety, back pain, poor immune system, obesity and metabolic syndrome. Due to the health-enhancing effects of regular exercise, it is suggested that regular exercise buffers the deleterious effects of psychological stressors by reduced physiological and emotional stress reactivity. However, previous studies only partly support the assumption that regular exercise can reduce physiological and emotional stress reactivity. Most of these previous studies used between-subject designs and artificial stressors, and did not consider the habitual physical activity level of participants. Thus, the current thesis investigated the effects of a 20-week aerobic exercise training (AET) on physiological and emotional responses to real-life stress using a randomized controlled trial and an inactive sample. To assess participants' physiological and psychological responses during every-day life, ambulatory assessment was used. The main part of the thesis consists of the following three articles, published in peer reviewed international journals:

The activity-affect association in inactive people
This publication analyzes whether the feel-better effect of unstructured physical activity observed in previous studies can be confirmed during every-day life in sedentary people.

Regular exercise and emotional stress reactivity
This publication reports on the effects of the randomized controlled trial on emotional responses to real-life stress.

Regular exercise and physiological stress reactivity
This publication reports on the effects of the randomized controlled trial on physiological stress responses during real-life stress.