Stress-Buffering Effects of Physical Activity and Cardiorespiratory Fitness in Police Officers - A Real-Life Study

Background: In modern society, psychosocial stress is a major health threat, related to cardiovascular disease and impaired mental well-being. The workplace constitutes a main source of stress in western countries. The related physiological and mental health impairments are manifold and welldocumented, and the associated costs for individuals and society are considerable. Promising evidence suggests physical activity (PA) and cardiorespiratory fitness (CRF) may be public health resources that buffer the negative effects of stress on health. However, evidence from laboratory studies is conflicting to some extent, and research to elaborate previous findings in externally valid conditions is required. To date, research has primarly been based on retrospective self-reports, and studies in realistic circumstances are scarce. This project was designed to examine the stress-buffering effects of PA and CRF on physiological and psychological health in realistic circumstances with a comprehensive methodology. A central goal of the current study was to investigate underlying mechanisms in line with the cross-stressor adaptation hypothesis (CSA) on psychophysiological stress responses. Dynamic psychophysiological processes were captured using newest methodological developments of Ecological Momentary Assessment (EMA) and Ambulatory Assessment (AA), as well as sophisticated statistical models. Research objectives: In a one-year prospective study with a sample of 201 police officers, exposure to chronic and acute work stress was examined, along with cardiometabolic and mental health parameters. PA was assessed over seven consecutive days via accelerometry, substantiated by a recently developed and validated questionnaire (Simple Physical Activity Questionnaire; SIMPAQ). CRF was objectively measured with the submaximal Åstrand cycling test. Chronic work stress was observed based on the job-demand and control (JDC) model, as well as the effort-reward imbalance (ERI) model. Acute work stress was measured using smartphone-based EMA of current stress experiences, whereas physiological responses were captured using AA of heart rate variability (HRV). Cardiovascular risk factors (waist circumference, blood pressure, blood sugar, blood lipids) were assessed in a laboratory setting and psychological health outcomes (Burnout, sleep health, psychological distress) via online-questionnaire. The first hypothesis posited moderating effects of PA and CRF on the association between chronic work stress, cardiometabolic health, and mental health. Higher levels of PA and CRF were suggested to be related to improved cardiometabolic and mental health, with stronger effects when levels of chronic work stress were higher. Hypothesis testing was performed using cross-sectional and prospective regression analyses. The second research question focussed on the CSA hypothesis. Lower physiological stress reactivity and increased physiological stress recovery were expected for individuals with higher levels of PA (results not VIII shown), as well as CRF. Physiological stress reactivity was defined as changes in HRV which were matched to the individuals’ specific stress experiences. Physiological recovery was investigated by measuring HRV during the following night. Hypothesis testing was performed using multilevel models and regression analysis. Results: The validation studies supported the suitability of the applied questionnaires for the assessment of PA and burnout. The main finding was that no stress-buffering effects of PA appeared in the data. Furthermore, CRF partially occurred as a stress-buffer on the cross-sectional association between ERI scores and cardiovascular risk factors, whereas no stress-buffering effects occurred on metabolic syndrome and mental health. The second major finding was that PA did not appear to influence physiological stress reactivity or recovery. However, CRF partially buffered physiological stress reactivity, whereas no effect occurred on physiological stress recovery. CRF was related to a more favorable stress reactivity regarding feelings of stress, whereas no effects occurred for feelings of anger, positive or negative affect. Aside from these main findings, PA was significantly and negatively correlated with metabolic syndrome scores after one year. CRF was a reliable significant predictor for lowered cardiovascular risk factors, including metabolic syndrome after one year. Additionally, CRF showed significant associations with more favorable day and night HRV levels. The present findings must be cautiously interpreted because of non-probability sampling. Furthermore, an inconsistent relationship between work stress and physiological outcomes might have lowered the detectable effects. This could be due to the high activity and fitness levels in the present sample, which may have entailed a ceiling effect. Conclusion: This is one of the first real-life studies examining the stress-buffering effects of PA and CRF in realistic circumstances and with such comprehensive methodology. In summary, CRF appeared as an important health resource with the potential to buffer some of the associations between stress and health risk factors. Therefore, the consistent and routine measurement of CRF should be addressed in occupational health programs for police officers, and constant efforts to encourage individuals to improve and preserve CRF levels should be made. PA has to be measured more precisely to elaborate acute and regular differences and interactions with fitness, and differences in leisure time and work-related PA, especially in a physically demanding work environment. Research with longer follow-up periods including continuous/multiple measurements is warranted to elaborate important health related effects and mechanisms more precisely.