The present study examined whether being physically active moderates the effects of rumination on the trajectory of cortisol responses to acute stress. We hypothesized that one pathway through which physical activity could lower the stress response is by mitigating the effects of stress-related cognitions on the body’s stress arousal systems. Our findings support this notion by demonstrating that the cortisol trajectory was a function of stress-induced rumination only in those who were sedentary, and unrelated to rumination in those who were active. Specifically, sedentary participants who had higher self-reported levels of rumination in response to the stressor had faster and prolonged reactivity and delayed recovery to stress, evidenced by a more rapid initial increase, a later peak, and a delayed return to baseline cortisol compared to the sedentary lower ruminating participants. For active participants, cortisol trajectory was not a function of rumination, regardless of level of stressor-induced rumination.
It is interesting to note that we may be detecting a trait-like effect rather than just the immediate effect of state rumination on cortisol. Rumination was determined after the acute stressor had terminated. Sedentary ruminators were not just high in cortisol during the recovery phase when we measured their levels of rumination, but also during the initial onset of the stressor. Therefore, our measure of state rumination may be differentiating people who are different in their initial acute stress reactivity, possibly due to differential appraisal of the stressor. Furthermore, activity alone may not shape cortisol responses. Rather, activity may be especially beneficial to those responding to stress with rumination, given the findings that activity moderates the relationship between rumination and cortisol reactivity and recovery, but does not directly predict cortisol reactivity to and recovery from stress.
We have recently demonstrated that being physically active, as defined here, moderates the relationship between chronic stress and short leukocyte telomere length, a marker increasingly understood to capture the accumulated burden of genetics, life stressors, and health behaviors (22
). Understanding the psychological and physiological mechanisms through which physical activity confers such effects to those most vulnerable to stress is critical for the development of interventions to enhance health, and for identifying those individuals for whom new interventions would be beneficial. The present findings address these issues and suggest that one pathway that may explain the physiological benefits of physical activity is reduced activation and enhanced recovery of stress arousal systems tied to psychological responses to stress. Overall, these findings lend support to the idea that being physically active may confer stress resistance, at least in post-menopausal women. Importantly, these findings appear to extend to elderly women.
Chronic stress is documented to produce different profiles of HPA axis dysregulation, including either hyperactive or blunted cortisol responses to stressors (29
). In this study, we examined HPA axis reactions to acute stress in the context of rumination, a potential psychological mechanism of chronic stress. A recent meta-analysis by Denson and colleagues (36
) provides evidence that rumination may induce increased cortisol levels in order to maintain vigilance toward unresolved stressors. Repeatedly ruminating in response to stress in the real world may repeatedly stimulate the HPA axis resulting in chronically elevated levels of cortisol. Chronic exposure to elevated cortisol is in turn linked with insulin resistance, accumulation of abdominal fat, and increased risk for cardiovascular disease (29
). Healthy levels of physical activity may differentially affect high ruminating individuals by creating a physiological eustress state (76
), and attenuating biological stress responses including cortisol responses (50
). A caveat of our study is that the heightened cortisol reactivity and delayed recovery observed in high ruminators was defined as such based on comparisons with low ruminators. At present, to our knowledge, there are no standard trajectories that are understood to represent excessively high cortisol reactivity or delayed cortisol recovery.
Our study adds to the current literature on the benefits of physical activity in that it tests if being physically active moderates the physiological effects of commonly experienced stress-related cognitions. There are a number of neurobiological and physiological mechanisms by which exercise may confer benefits, including increases in the expression of genes that encode brain-derived neurotrophic factor (BDNF) leading to increased cognitive functioning and neural plasticity (77
), a heightened anti-inflammatory environment in the body (80
), enhanced insulin sensitivity (83
), and enhanced oxidative buffering capacity (76
). Physical activity reduces depressive symptoms among those genetically predisposed to lower BDNF levels (20
), and reduces symptom reporting and doctor visits in those with increased numbers of stressful life events (19
). Here, we demonstrate that physical activity may attenuate the acute physiological response to stress, as indexed by HPA axis reactivity, in those particularly vulnerable to stress – ruminators.
Limitations of the study included our short-term and self-reported measure of physical activity. Physical activity was reported on a daily basis across three days, and participants reported the number of minutes they engaged in activities in which their heart rate was increased and/or they perspired. We assume that these three days of reporting represent typical exercise behaviors, and thus, a long-term lifestyle factor. However, we do not know if the short-term activity specifically occurring in the days before the TSST was important in the results here, or whether this was a proxy measure for general fitness. Although our measure represents an important advance over the usual retrospective report of physical activity, future studies should measure participants’ baseline fitness levels and objectively capture daily physical activity with the use of accelerometers or assessments of current physical fitness. Our measurement across three days may be an over or under representation of actual engagement in physical activity per week, and thus extending measurement to longer than 3 days to a week or two would benefit future studies that aim to use daily measurement of vigorous activity as a measure of high activity level. These limitations of our measure of physical activity may explain why we did not detect a main effect of physical activity on cortisol trajectory, in contrast with other studies on fitness and cortisol output (52
). Self-report of physical activity is not as strong a predictor of health as measurements of actual fitness levels (82
). Finally, only a randomized controlled aerobic exercise intervention study can truly test the buffering effects of physical activity on HPA axis activation as a result of stress-induction. Our results are thus limited to between-group differences that can also be a function of personality and other between group differences.
Findings of the present study are also limited to post-menopausal women, and should be replicated in larger samples, including men as well as individuals with diverse ages and ethnic backgrounds. Of particular importance, our sample size is limited to 18 participants in the active group, and 28 in the sedentary one. While mixed models with REML estimation are considered unbiased and robust, it may be possible that our findings are limited to our specific sample. We thus consider these findings preliminary, and suggest that follow up studies with larger sample sizes and more diverse groups and ages may be worthwhile. Given the sample size, we were unable to stratify our results by caregiver group, age and BMI. Exposure to ongoing chronic stress may change psychological responses including ruminative responses to acute stressors. However, in our sample, caregivers and controls showed similar levels of rumination in response to the acute laboratory stress, and our findings were significant over and above the effects of caregiver status. Exposure to chronic stress may also influence physical activity levels. Although not statistically significant, the caregivers in our study were disproportionately sedentary compared with controls. While significant differences between caregivers and controls in our study were not apparent, possibly due to sample size limitations, the current findings may be particularly relevant to caregivers, given their lower rates of physical activity. Caregivers who ruminate about everyday stressors may benefit from physical exercise interventions.
Further, there may be sex differences, which we could not test in this all-female sample. It may be that rumination is not only more common in women but has different physiological effects in women. Thus including men in future studies is important to further our understanding of the moderating potential of being active on physiological consequences of rumination. Comparing these trajectories to younger, fit and unfit women who are high and low on rumination will also deepen our understanding of the interplay of age with psychological and physiological response to stress.
In summary, the findings reported in the present study are the first, to our knowledge, that demonstrate the moderating effects of being physically active on the physiological responses associated with rumination following acute laboratory stressors. We applied a statistical model, namely growth curve modeling, which better captures the nature of physiological responses by modeling initial increases, minutes to peak responses, and speed of recovery. The effects found in the present study were apparent, even after covarying age, BMI, caregiving group, and depressive symptomatology. Heightened increases in cortisol responses and delayed recoveries to repeated stressors across the day would possibly lead to sustained exposure to elevated cortisol levels, ultimately affecting physical health outcomes(60
). Our study demonstrates the potential for physical activity to allow rapid recovery of the HPA axis after the induction of stress, especially in those who are ruminating, thus reducing the heightened cortisol response earlier and potentially protecting individuals from continued, prolonged exposure. It has been increasingly clear that it is important to understand factors that enhance physiological recovery from stressors in the service of promoting health. Exercise appears to be one promising way to promote physiological stress resistance, particularly in stressed ruminators who are prone to affective and physiological disorders.