SPALDING, T. W., L. S. JEFFERS, S. W. PORGES, and B. D. HATFIELD. Vagal and cardiac reactivity to psychological stressors in trained and untrained men. Med. Sci. Sports Exerc., Vol. 32, No. 3, pp. 581–591, 2000.
Purpose: The aim of this study was to determine whether higher aerobic fitness is associated with enhanced vagal influences on the myocardium, resulting in moderation of chronotropic cardiac activity during psychological stress and recovery.
Method: Heart period (HP) and respiratory sinus arrhythmia (RSA) were obtained from 10 aerobically trained (AT) and 10 untrained (UT) college-aged men at rest and during three contiguous psychological challenges and 3 min of recovery. Ratings of perceived stress were obtained at the end of the rest period, at the midpoint of each stressor, and at 30 s into recovery. Time series methods were used to quantify RSA from the beat-to-beat HP series. Responsivity was assessed both in terms of absolute levels of activity and phasic changes in activity (task or recovery minus baseline).
Results: Both groups reported similar levels of subjective stress throughout the experiment. The AT exhibited longer HP at rest and during psychological stress and recovery than did the UT. However, the groups did not differ on RSA at rest or during psychological stress and recovery, nor did they differ on phasic changes in RSA or HP during stress or recovery. Additionally, aerobic capacity was not correlated with absolute levels or phasic changes in RSA during psychological challenge for either group and, except in Min 2 for the UT, similar results were obtained for recovery.
Conclusions: The results supported the hypothesis that, among young men, higher aerobic fitness is associated with longer HP at rest and during psychological stress and recovery. However, the lower cardiac chronotropic activation observed among the AT relative to the UT was not paralleled by a group difference in the amplitude of RSA. These results suggest that the group difference in HP was not mediated directly by the vagal mechanisms manifested in the amplitude of RSA.
Epidemiological evidence indicates that physical activity is associated with a reduced risk of coronary heart disease (7,63,80). A number of mechanisms may be responsible for this effect of physical activity. One such mechanism may involve cardiac vagal tone. Links between aerobic exercise, cardiac vagal tone, and cardiovascular disease are suggested by findings from two lines of research. First, evidence from several studies suggests that reduced cardiac vagal tone and/or impaired vagal function is a risk factor for hypertension (20,35,36,40,98), coronary artery disease (1,45,46), episodes of myocardial ischemia (94), and sudden cardiac death (47). Second, the findings of a number of studies have provided support for the association of higher aerobic fitness and enhanced resting cardiac vagal tone relative to lower levels of fitness (5,15,32,33,59,69,74,82,86,90), although not all of the studies are in agreement (19,37,44,58,68,81). If enhanced cardiac vagal tone is, in fact, an adaptation of chronic aerobic exercise involvement, then it may partially account for the exercise-related reductions in the risk of heart disease.
One specific way that exercise-related increases in cardiac vagal tone may reduce the risk of heart disease is by attenuating cardiac reactivity to psychological stressors, as it is hypothesized that cardiovascular hyperreactivity to real-life stressors contributes to the development of cardiovascular disease (60,70). It is well documented that heart rate and blood pressure increase during stressful psychological challenges in both the laboratory (3,66) and real-life situations (67,99). Such changes in heart rate are typically attributed to increases in sympathetic influence on the heart (34,75); however, a withdrawal of cardiac vagal tone may also contribute to the increased cardiac activation (3,42,89). In light of the fact that vagal influences on the heart can offset those of the sympathetic nervous system, the maintenance of higher cardiac vagal tone may attenuate the increases in heart rate that occur during stress (40) and thereby attenuate the associated increases in cardiac output and blood pressure (56,84).
In light of the evidence that aerobically fit individuals exhibit greater cardiac vagal tone at rest than those who are less fit, it is possible that aerobically fit individuals also tend to exhibit greater vagal tone during psychological stress and recovery. Although Hatfield et al. (44) did not find differences between trained and untrained men on respiratory sinus arrhythmia (RSA), an index of cardiac vagal tone, during ergogenic stress, it is possible that group differences in the parasympathetic response to psychogenic stressors may emerge in light of the possibility of specific cardiovascular response patterns to ergogenic versus psychogenic challenges (24,57). Beyond the possibility of higher vagal tone levels during psychological stress, aerobically fit individuals may also exhibit a smaller phasic vagal withdrawal than less fit individuals, and they may restore vagal tone more rapidly during recovery. Concomitantly, we would expect aerobically fit individuals to exhibit longer heart periods (HP) and smaller phasic changes in HP during stress and recovery than less fit individuals.
Although the relationship between aerobic fitness and heart rate reactivity has been examined extensively (e.g., see 27), relatively little attention has been devoted to the specific issue of whether parasympathetic regulation of the heart during psychological stress and recovery differs between individuals who vary on aerobic fitness levels. To date, only three studies have appeared in the literature on this issue, and the results are mixed. In the first such study de Geus et al. (29) compared young men who were randomly assigned to either a brief aerobic training program or a wait-listed control group, on phasic RSA responses to two reaction time tasks and recovery. Cardiac vagal reactivity was assessed in both groups before and after the training program. When the data were analyzed with analysis of variance (ANOVA) procedures, it was found that the trained and control groups did not differ at posttest on phasic RSA withdrawal during the challenges or on the reengagement of RSA during recovery. However, when the data were subjected to correlational analysis, the investigators found, consistent with their expectations, that 1) higher aerobic fitness levels were associated with smaller phasic RSA responses (i.e., smaller withdrawal of RSA) to the tasks at pretraining and that 2) among the trained subjects, greater improvements in aerobic fitness were associated with less vagal withdrawal at the posttraining assessment. The contradictory findings yielded by the ANOVA and correlational analyses may be explained by the fact that the different statistical procedures were sensitive to different aspects of the data structure. In a preliminary report of another investigation, Nurhayati and Boutcher (74) presented findings that trained postmenopausal women maintained longer HP and higher RSA than their sedentary counterparts before, during, and after a Stroop test. However, when phasic changes in RSA were examined, they found no group difference during or after the challenge. Lastly, Boutcher and colleagues (17) examined trained young men and two groups of their untrained counterparts, one group with low resting heart rates (i.e., <62 beats·min−1) and another with typical resting heart rates (i.e., >67 beats·min−1), for differences in RSA levels as well as phasic changes in RSA during mental arithmetic and a Stroop test. There were no group differences on RSA levels during either task, nor were there any differences between the groups on phasic RSA responses to mental arithmetic. However, the trained and untrained men with low resting heart rates did exhibit a greater phasic decrease in RSA during the first half of the Stroop test than did the untrained controls with normal resting heart rates. As the trained and untrained men with low resting heart rates had higher RSA at baseline than the controls, it is possible that the group differences in phasic responses to the Stroop were simply due to the effects of the Law of Initial Values (LIV;101,102). In accordance with the LIV, individuals with higher baseline RSA would be expected to exhibit a larger decrease in RSA during stress than individuals with a lower baseline RSA. In light of the absence of appropriate tests and adjustments for the effects of the LIV, the results obtained by Boutcher et al. (17) remain equivocal.
Thus, the evidence regarding the relationship between aerobic fitness and cardiac vagal reactivity and recovery is mixed. The mixed findings may be partially explained by the different methods used to quantify RSA. de Geus et al. (29) used the peak-to-trough method (43) to quantify RSA, whereas Boutcher and colleagues (17,74) used Porges’ (77) method. Estimates of vagal tone derived with the peak-to-trough method may be influenced by changes in ventilatory activity and trends in heart rate (i.e., slow aperiodic and periodic shifts in heart rate) (23,41), which was not controlled in the study by de Geus et al. (29). In contrast, Porges’ method is robust to the influence of changes in ventilatory activity and the HP slope and is, therefore, appropriate for assessing RSA across conditions in which ventilatory activity may vary substantially. In light of the potential importance of cardiac vagal tone in moderating the effects of reactivity on the risk of cardiovascular disease, and the equivocal findings, there is a need to reexamine the association between aerobic fitness and cardiac vagal reactivity using methods which are appropriate for assessing RSA under conditions characterized by variable ventilatory activity and which account for the effects of the LIV, if present. In the present study we compared endurance-trained and untrained young men on RSA and HP during rest, three contiguous psychological challenges, and subsequent recovery. Porges’ (77) method was used to estimate RSA and tests for the effects of the LIV were made when appropriate. We predicted that trained men, as compared with untrained men, would 1) have longer HP and higher RSA levels at rest and during psychological stress and recovery, 2) have smaller phasic reductions in HP and RSA during psychological stress, and 3) show a more rapid return of HP and RSA to resting levels during recovery.