Associations of physical activity and fitness with stress reactivity and indices of inhibitory control under stress

Background: Acute psychosocial stress impairs top-down cognitive processes, including aspects of executive functioning. Particularly negative effects on inhibitory control have been highlighted in the literature. During adolescence, the ability to maintain high levels of executive functioning under acute stress is extremely important, because performance in major school exams and finals determines future career opportunities. Moreover, in this age group, brain areas associated with executive functions, such as the dorsolateral prefrontal cortex (DLPFC) are still developing and can be more vulnerable to negative effects of stress. Consequently, research on variables with the potential to mitigate negative effects of acute stress on executive functions is required. Physical activity, fitness, and acute exercise are promising candidates, as research so far suggests that they can reduce stress reactivity and improve executive functioning, including inhibitory control. However, so far these effects have only been investigated separately, and no information is available on associations with executive functioning under stress.
Aims: The overall aims of this dissertation were to investigate whether in male adolescents, physical activity, fitness and acute exercise have health-beneficial effects on stress reactivity, and if these factors improve behavioral and neurocognitive inhibitory control under psychosocial stress.
Methods: One systematic review and two studies were conducted within this research project. The systematic review focused on studies investigating effects of physical activity and fitness on stress reactivity as measured with the Trier Social Stress Test (TSST). Associations of regular exercise and fitness (Study 1, N=42) and acute exercise (Study 2, N=60) with stress reactivity and behavioral as well as neurophysiological inhibitory control under stress were investigated. In both studies, healthy male, right-handed adolescents aged 16-20 years were recruited from local academic high schools. In Study 1, two appointments were scheduled one week apart, with control variables, aerobic fitness and inhibitory control (low stress) being assessed at the first, and stress reactivity and inhibitory control (high stress) at the second appointment. A modified TSST served as the stressor, and endocrine (salivary cortisol), autonomic (salivary alpha-amylase, heart rate) and psychological stress reactivity (state-anxiety) were measured. Inhibitory control was assessed with a computerized Stroop task. The simultaneous measurement of functional near-infrared spectroscopy and electroencephalo-graphy allowed for an analysis of DLPFC oxygenation and the N450 component of event-related potentials, respectively. In Study 2, after assessment of control variables and a Stroop task (low stress), participants were randomly assigned to a moderate exercise (30 min on a bicycle ergometer at 70% of maximum heart rate) or a control group (30 min reading). Subsequently, a modified TSST, which included a Stroop task (high stress), was conducted. Stress reactivity and DLPFC oxygenation were measured as in Study 1. In both studies, anthropometric, sociodemographic and psychological control variables were assessed.
Results: Higher aerobic fitness was associated with lower alpha-amylase reactivity, but not with changes in cortisol or psychological stress reactivity. After an acute bout of exercise, compared to a control condition, alpha-amylase and psychological stress reactivity were reduced. Better inhibitory performance at baseline (low stress) was associated with greater N450 negativity and more left-lateralized DLPFC activation. Furthermore, higher aerobic fitness was associated with better inhibitory control at baseline (low stress), which was mediated by N450 negativity, but not by DLPFC lateralization. When comparing high- and low-stress situations, we observed differences in DLPFC oxygenation during tasks demanding inhibitory control. However, inhibitory performance remained unchanged between low and high stress conditions. Acute and chronic exercise had no significant influence on inhibitory control and corresponding DLPFC activity under stress.
Conclusions: We found potentially health-beneficial associations of aerobic fitness and acute exercise with stress reactivity. Our results suggest that exercise might be recommendable to reduce psychological and ANS reactions to acute stress in adolescents, and to improve inhibitory control in low-stress situations. Better conflict monitoring, as indicated by N450 negativity, is suggested as a mechanism underlying the beneficial effects of fitness on inhibitory control. Finally, acute stress had no negative effect on behavioral inhibitory control in our sample of male adolescents, and our data do not support the implementation of acute and chronic exercise to improve inhibitory control under stress.