Effects Of Aerobic Exercise Training On Hemodynamic Responses

Effects of Aerobic Exercise Training on Hemodynamic Responses During Psychosocial Stress in Normotensive and Borderline Hypertensive Type A Men: A Preliminary Report ANDREW SHERWOOD, PHD, KATHLEEN C. LIGHT, PHD, AND JAMES A. BLUMENTHAL, PHD This study assessed the effects of aerobic exercise training on cardiovascular responses to a 5min reaction time competition task. Twenty-seven Type A men (aged 30-56) participated in this randomized study in which 14 underwent supervised aerobic training and 13 strength training, with sessions scheduled three times per week for 12 consecutive weeks. Aerobic exercise training was associated with a 13.6% increase in VO2max compared to 2.9% for the strength group. The effects of aerobic exercise training were most evident in subjects whose initial casual blood pressure readings fell in the borderline hypertensive range (N = 5). Those individuals exhibited a general reduction in diastolic blood pressure (i.e., during rest, competition, and recovery) which was associated with a fall in both heart rate and total peripheral vascular resistance. Furthermore, diastolic pressure reactivity to the competition task was attenuated in borderline hypertensive subjects who underwent aerobic conditioning. These data are interpreted as preliminary findings suggesting that borderline hypertensives may be particularly responsive to the cardiovascular benefits of aerobic conditioning. For patients who have progressed to this stage of hypertensive disease, aerobic exercise may be of ameliorative value.

INTRODUCTION

In a recent epidemiological study of a large cohort of Harvard alumni, habitual physical activity was shown to be a lifestyle characteristic associated with increased longevity, due primarily to a decrease in mortality from cardiovascular and respiratory diseases (1). Aerobic exercise training may be especially beneficial to cardiovascular health because of the resulting physiological adaptations, with both an improved potential for en-

From the Department of Psychiatry, University of North Carolina, Chapel Hill, North Carolina (A.S., K.C.L.) and Duke University Medical Center, Durham, North Carolina (J.A.B.). Address reprint requests to Andrew Sherwood, Ph.D., CB #7175, Medical Research Building A, University of North Carolina, Chapel Hill NC 27599. Received June 28,1988; revision received November 9, 1988.

Psychosomatic Medicine 51:123-136 (1989) 0033-3174/89/5102-0123$O2.OO/O Copyright© 1989 by the American Psychosomatic Society

ergy delivery by the cardiovascular system and energy utilization by the skeletal muscles leading to increased physical work capacity (2). Of the numerous mechanisms that underlie these adaptations, modifications in autonomic nervous system control of the cardiovascular system are of particular interest in the context of the present study. Resting and exercise bradycardia is characteristic of the exercise-trained individual and is considered to be at least partially attributable to a decreased level of sympathetic and increased level of parasympathetic influences on the heart (3). The converse state of autonomic balance, with heightened sympathetic and reduced parasympathetic control of the heart, associated with abnormally high cardiac output (CO), is responsible for elevated blood pressure (BP) in some cases of borderline hypertension, a condition that frequently progresses to established hypertension (4). 123

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In contrast to the beneficial aspects of aerobic exercise, psychological stress is generally considered to be deleterious to cardiovascular health. Several models have been proposed that implicate the sympathetic nervous system in the pathological process by which behavioral influences may contribute to both coronary heart disease (CHD) (5, 6) and hypertension (7). According to these models, individuals who repeatedly exhibit pronounced sympathetically mediated arousal of the cardiovascular system, in association with their responses during psychological stress, may be at particular risk for the development of cardiovascular disease (8). It seems plausible that aerobic exercise training may ameliorate the risk of cardiovascular disease associated with behavioral influences, both by attentuation of sympathetic responses and/or through modification of personality traits and behavior patterns. Relatively few studies have been conducted to examine directly the effects of aerobic exercise on physiological reactivity during psychological stress. Positive findings from psychophysiological studies include reports that heart rate (HR) responses during psychological stress are attenuated in physically fit individuals (9-11). Other studies have found negative results regarding differences in HR reactivity but have found that HR recovery from stress tends to occur more rapidly in the aerobically fit (12-15). Myocardial contractility and systolic blood pressure (SBP) responses during stress, in addition to HR, were found in a recent study to be less pronounced in college students categorized as high in aerobic fitness, relative to low fit students (16). Diastolic blood pressure (DBP) increases during stress have also been found to be attenuated in association with physical fitness, in a 124

study which otherwise reported negative findings (17). Methodological differences between studies which may partly account for the inconsistency of findings to date include cross-sectional versus longitudinal designs, differences in the laboratory stressors employed, and differences in the definition and measurement of physical fitness. Also, benefits may differ depending on whether subjects under study have borderline or sustained hypertension versus normal resting BPs. Persons with borderline hypertension are a population of special interest: two recent five-year prospective follow-up studies have shown that a high DBP response during and after stress is a powerful predictor of the development of sustained hypertension in such individuals (18, 19). Overall, further research seems justified to help clarify if, how, and in which individuals exercise might modify stress reactivity. In a previous study of Type A men (20) we found that a 12-week program of aerobic exercise was associated with significant attenuation of HR and BP responses to mental arithmetic. In the present study, on a new cohort of Type A men, some of whom were borderline hypertensive, we provided a more comprehensive assessment of cardiovascular function, using impedance cardiography to assess the hemodynamic basis of BP responses. Cardiovascular responses to a reaction time task, in which pairs of subjects were in direct competition for a monetary bonus, were monitored both before and after a 12-week supervised program of either aerobic or strength training. A number of studies have shown that resting blood pressure is reduced in hypertensive and borderline hypertensive individuals following aerobic exercise training (21, 22). Since greater reactivity during mental Psychosomatic Medicine 51:123-136 (1989)

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stress has been reported to occur in borderline hypertensives (23-25), we were particularly interested in assessing whether subjects in this category would exhibit heightened reactivity to the competition task; and, if so, whether attenuation of cardiovascular reactivity following aerobic training might be particularly evident in borderline hypertensive subjects. METHODS Subjects Data based upon 27 men, aged between 33 and 56 years (mean = 41.4 years) and who were initially rated as Type A by the Structured Interview (26), are described in the present study. (Note: thirty-four men comprised the initial sample, but three failed to complete the exercise training program, one failed to show for follow-up testing, and physiological measurement problems led to the loss of data on another three.) All subjects were employed and had at least a high school education Clinical manifestations of CHD were absent in all subjects as determined by medical history, physical examination, and exercise treadmill testing. A number of subjects participating in this study could be considered borderline hypertensive according to casual (ausculatatory) BP readings taken during the initial screening physical examination. Some subjects also showed borderline hypertensive pressures during the first three seated readings taken

after instrumentation, just before the initial rest period of the pre-training laboratory competition task test session. Subjects were categorized as borderline hypertensive if resting SBP exceeded 140 mm Hg and/or DPB exceeded 90 mm Hg during the latter period while all other subjects were considered normotensive. According to these criteria, in the Strength training group, six subjects were categorized as borderline hypertensive and seven as normotensive, while in the Aerobic group there were five subjects classified as borderline hypertensive and nine normotensive. Group mean casual blood pressures taken at the screening physical examination and immediately after instrumentation, before the competition task protocol, are summarized in Table 1.

Exercise Intervention Program Subjects were randomly assigned to either an Aerobic exercise training group (N=14) or a Strength and Flexibility training group (N=13). Subjects in the Aerobic group attended three supervised exercise training sessions per week for 12 consecutive weeks. Aerobic exercise sessions began with a 15min stretching period followed by 35 min of continuous aerobic exercise which included walking, jogging, and stair-climbing at an intensity of at least 70% of subjects' initial maximal aerobic power (V02max). Subjects were instructed to monitor their HRs by taking their radial pulses. A HR monitor (Exersentry 3) was also worn twice a week during the first two weeks of the training program, and once a week thereafter, to ensure that subjects' HRs were kept within the prescribed training ranges. Subjects in the Strength group participated in 20 min of flexibility exercises, followed by 30 min of

TABLE 1. Means and SEMs for SBP and Casual DBP in Borderline Hypertensive and Normotensive Subject Groups Aerobic Training Group

Strength Training Croup

Borderline Borderline .. , Normotensives, Normotensives hypertensives . hypertensives (N = 9) (N=7) (N = 6) Casual auscultatory BPa

Systolic (mm Hg) Diastolic (mm Hg) BP readings taken immediately Systolic (mm Hg) 6 after instrumentation Diastolic (mm Hg)

140 98 140 96

±7.6 ± 2.7 ±4.0 ± 3.0

122 79 122 82

± 2.2 ± 2.8 ±2.3 ±1.2

146 93 138 90

+ 6.4 + 3.3 ±6.0 ± 3.7

132 86 122 80

± 2.0 ±1.8 ±2.5 + 2.2

Baseline BPs, reported under Results, are lower since they were recorded following 15 min of quiet rest. Taken during screening physical examination. Prior to onset of competition task protocol.

a b

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A. SHERWOOD et al. Nautilus circuit training. Subjects participated in the supervised group sessions two to three times per week for 12 consecutive weeks. The Strength training group was employed to serve as a control for the effects of social stimulation and attention, since both groups were supervised. Cardiovascular adaptations characteristic of aerobic conditioning are not produced by Nautilus circuit training (27). All training sessions were concluded with a 10-min "cool-down" period. Subjects in the Strength group were requested not to engage in any aerobic exercise during their participation in this study. Subjects in both groups were instructed to maintain their dietary habits until the completion of the study, and no suggestions for dietary modification were offered. All participants underwent comprehensive physiological evaluations as well as behavioral and psychophysiological assessments. Evaluations were conducted prior to the beginning of the exercise training program and after three months of exercise conditioning. The focus of the present report is on the hemodynamic adjustments to the psychosocial challenge presented by performance on a reaction time task in which subjects were placed in direct competition with each other.

Treadmill Exercise Test During a one-week evaluation period, pre- and post-exercise training, subjects' V02max was measured during a symptom-limited maximum treadmill (Pacesetter R-9) exercise test following the Duke protocol of 1-Met/mi n increments (28). Oxygen consumption (VO2) was measured using a Beckman Metabolic Cart (Sensormedics), and HR was measured from the continuous electrocardiogram.

Psychosocial Stress Test Protocol Subjects participating in this study were tested in pairs and were exposed to essentially identical experimental protocols on each of two sessions. The same two subjects were never paired together on both the pre- and post-exercise training sessions. After signing an informed consent form and following instrumentation for physiological recording, subjects were seated 6 feet apart, facing a projector screen, in an electrically shielded, sound-attenuated experimental chamber. At this time three BP readings were taken in the presence of a technician, using our automated auscultatory system. The experimental session was initiated by a 15-min rest

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period, during which subjects were asked to relax and remain quiet. At the end of this period a technician entered the room and instructed subjects on the nature of'the competition task. The competition task was 5 min long and required subjects to depress a button, as quickly as possible, upon identifying that a target letter was present in a live-letter word, projected onto the screen in front of them. A total of 17 words were presented at varying intervals, with a mean intertrial interval of 18 sec and a range of 8-38 sec. Each subject was designated a target letter during the pre-task instruction period. However, it was explained at this time that, in order to make the task more difficult, after every few trials (two to five trials) a technician, who sat between the subjects throughout the task, would say "Switch," at which point each subject should respond only to the target letter to which his opponent had just previously been responding. Subjects' reaction times for each trial were displayed on digital stopclocks (Lafayette model 54519-A). In order to monitor their performance in an overt manner, the technician recorded each subject's reaction times on a clipboard scoresheet. It was explained that, in this "winner-take-all" competition, a $20 bonus would be awarded to the subject with the lower total reaction time score over the entire task. Since, it was explained, any incorrect response would lead to a 2-sec penalty for that trial, it was emphasized that subjects should not give up, even if they felt they were behind, as a single mistake by their opponent could profoundly affect the competition outcome. The task was designed so that subjects could be required to both respond, either respond or neither respond on a given trial, but both subjects would need to respond to 9 of the 17 trials if all responses were to be correct Following completion of the competition task, subjects were told that they would be informed of the outcome of the contest at the end of the experiment and were asked again to relax quietly for a second 15-min rest (recovery) period. At the end of this period subjects were asked to complete a brief questionnaire, asking them to indicate on a sevenpoint scale how hard they tried (one=not at all; 4=moderately; 7=very, very much) during the competition.

Physiological Measurements Arterial BP was measured noninvasively using the auscultatory method. A standard inflatable BP cuff, positioned around the subject's left arm, was rapidly inflated and then slowly deflated at a linear Psychosomatic Medicine 51:123-136 (1989)

AEROBIC TRAINING EFFECTS ON STRESS RESPONSES rate of 3 mm Hg/sec, using a custom-designed and -built automated device. Kortkoff (K) sounds were detected using a piezoelectric microphone positioned over the brachial artery and under the lower edge of the cuff. Cuff pressure and the microphone output were displayed on adjacent channels of a Beckman Dynograph chart recorder, permitting the determination of SBP, as the cuff pressure corresponding to the onset (Phase I), and DBP, as the disappearance (Phase V) of K-sounds. Mean arterial blood pressure (MAP) was derived for each set of pressure readings by computing V> pulse pressure added to DBP. Impedance cardiography was utilized to permit noninvasive monitoring of cardiac performance (29). A Minnesota Impedance Cardiograph (model 304B) was used in conjunction with a tetrapolar electrode band configuration. All four electrodes (Contact Products No. M6001) were used in conjunction with an electrode gel (Aquasonic 100) to facilitate electrical transmission. The inner two recording electrode bands were positioned around the base of the neck and around the thorax over the tip of the xiphoid process. The outer two current electrode bands were positioned to encompass the neck and thorax, at least 4 cm away from each of the recording electrodes. The EKG was recorded independently using Beckman biopotential electrodes. The basal thoracic impedance (Zo), the first derivative of the pulsatile impedance (dZ/dt) and the EKG waveforms were recorded on a FM tape recorder (TEAC model MR30). Processing of the impedance cardiogram was accomplished using a computer-based system developed in the laboratory. This system, which has previously been described in detail (30), was used to derive CO, HR, stroke volume (SV), pre-ejection period (PEP), and left ventricular ejection time (LVET). Each of these indices was generated to represent minute-mean values, based upon 30-sec continuous data samples, taken from within the 1-min periods of interest. Total peripheral resistance (TPR) of the systemic vasculature was derived on the basis of the concurrently recorded blood pressure and cardiac output measurements using the equation: TPR (dynesec cm"5) = (MAP/CO)*80.

RESULTS

Treadmill Exercise Test Training effects on V02max were evaluated using a three-way repeated measPsychosomatic Medicine 51:123-136 (1989)

ures analysis of variance (ANOVA) (BP status x Exercise Group x Session), which gave rise to a Group x Session effect (F(l,23) = 9.27, p < 0.01]. Tukey's Honestly Significant Difference (HSD] test revealed that this interaction emerged because there was a significant (+13.6%) improvement in V02max in the Aerobic group (mean pre-training = 33.76 ml/kg/ min; post-training = 38.36 ml/kg/min), while the change in the Strength group (+2.9%) was nonsignificant (mean pretraining = 34.08 ml/kg/min; post-training = 35.07 ml/kg/min). Training effects on HR responses to the treadmill test session were evaluated using a four-way repeated measures ANOVA (BP status x Exercise group x Session x Phase (Rest, Exercise at 3.5, 5, and 8 Mets)). Post-training HRs were significantly lower at rest and during exercise in the Aerobic group only (Groups X Sessions F(l,21) = 7.34, p < 0.02). Cardiovascular Responses during the Competition Task, Pre- and Posttraining Cardiovascular responses were summarized into five phases within sessions: Baseline (average of the last 5 min of the initial 15-min rest period); Comp 1 (first minute of the competition task); Comp 2 (second minute of the competition task); Recovery 1 (average of the first 5 min following competition); and Recovery 2 (average of min 11-15 following competition). The first 2 min of Competition were selected because, during this time, subjects were most uncertain about their chances of winning. Although the resulting cell sizes were small, BP status was included as grouping factor in the ANOVAs and analyses of covariance (ANCOVAs) described below, in order to provide a preliminary assessment of the possibil127

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ity that there may be differential effects of exercise training associated with preexisting BP status. Four-way repeated measures ANOVAs (BP status x Groups x Sessions x Phases) were adopted as the primary means of statistical analysis and were conducted for each cardiovascular measure. The Greenhouse-Geisser correction procedure for repeated measures was applied, and adjusted p values only are reported. Tukey's HSD tests were applied where appropriate to identify the basis of main and interactive effects. The effects of exercise training on cardiovascular reactivity (defined as change from resting Baseline to Competition) and recovery rates were assessed using ANCOVA tests with Baseline values as the covariate. BP responses. Mean SBP and DBP, preand post-training, are presented in Figure 1. For DBP there was a significant Phases effect (F(4,92) = 18.95, p < 0.01), which was found to be due to a rise in DBP during the Competition task, with levels returning to within Baseline range by the first Recovery period. DBP was significantly higher overall for borderline hypertensives (BP status F(l,23) = 5.55, p < 0.05). There was a general fall in DBP following training (Sessions F(l,23) = 4.85, p < 0.05), but this effect was primarily due to the dramatic change occurring in borderline hypertensive subjects who underwent aerobic exercise training (BP status X Exercise groups X Sessions F(l,23) = 6.28, p < 0.02), for whom posttraining DBP levels were reduced to normotensive control levels. Furthermore, a significant BP status X Exercise Groups X Sessions X Phases interaction (F(4,92) = 2.65, p < 0.05) supported the effect suggested in Figure 1, that aerobic training also attenuated the DBP response to the Competition task in borderline hyperten128

sive men. This reduction in DBP reactivity during stress was confirmed by an ANCOVA adjusting for Baseline levels, which gave rise to a significant BP status x Exercise Groups x Sessions effect (F(l,22=6.23, p < 0.05) showing that DBP responses were reduced both during (Comp min 1 and 2) and following (Recovery 1 and 2) the Competition task. SBP showed a significant Phases effect (F(4,92) = 60.17, p < 0.001), accounted for by a rise in SBP during Competition which remained elevated during Recovery 1, but returned to Baseline by Recovery 2. A significant main effect of BP status (/(1,23) = 11.91, p < 0.002) indicated that SBP was higher in the borderline hypertensive subjects across all phases of the stress test protocol, both before and after training. There were no other significant main or interactive effects for SBP. However, it is of note, as shown in Figure 1, that though nonsignificant, mean SBP in the borderline hypertensives who underwent Aerobic training showed a tendency to fall post-training. The ANCOVA test indicated that neither training program significantly affected SBP reactivity to the Competition task or rate of recovery of SBP to Baseline levels. Myocardiai responses. HR means are presented in Table 2. The Competition task gave rise to reliable increases in HR as indicated by a Phases effect (F(4,92) = 36.66, p < 0.001), with HR remaining elevated above Baseline during the first Recovery period, returning to Baseline by Recovery 2. Normotensive subjects showed greater HR reactivity to the Competition task than the borderline hypertensives (ANOVA BP status X Phases F(4,92) = 5.37, p < 0.001; ANCOVA BP status main effect F(l,22) = 5.66, p < 0.05). Post-training HRs were generally lower in both exercise groups (Sessions F(l,23) = Psychosomatic Medicine 51:123-136 (1989)

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STRENGTH GROUP 155

AEROBIC GROUP

150 145 140 X 135 E E. o. 130 m CO

125 120 115 110 105 100 95 90

Bose 10-15

Compete 1 2

Recov Recov 1-5 10-15

Base 10-15

Compete I 2

Recov 1-5

Recov 10-15

Fig. 1. SBP and DBP means for borderline hypertensive and normotensive men during resting baseline (Base), competition task (Compete), and post-task recovery (Recov), before and after 12 weeks of aerobic or strength training.

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TABLE 2. Mean HR, CO, SV, PEP, and TPRS and Total Responses during the Stress Test Protocol, Before and After Exercise Training Strength Training Croup

Aerobic Training Croup Borderline hypertensives (n = 5)

HR(bpm) Baseline Comp 1 Comp 2 Recov 1 Recov 2 CO (liters/mm) Baseline Comp 1 Comp 2 Recov 1 Recov 2

Normotensives (n = 9)

Borderline hypertensives (n = 6)

Normotensives (n = 7)

Pretraining

Posttraining

Pretraining

Posttraining

Pretraining

Posttraining

Pretraining

Posttraining

67 77 79 75 72

61 75 72 68 66

68 86 85 73 71

59 79 75 66 63

69 75 76 72 70

62 73 72 67 65

71 94 92 78 75

70 93 89 75 73

3.57 5.09 5.35 4.15 3.59

3.95 4.63 4.88 4.42 3.94

5.00 6.59 6.75 5.53 4.86

4.97 6.54 6.49 5.57 4.85

4.35 4.83 5.14 4.76 4.58

4.78 5.86 6.05 5.20 4.60

4.39 7.34 7.54 4.95 4.54

4.38 7.64 7.57 4.72 4.28

SV (ml) Baseline Comp 1 Comp 2 Recov 1 Recov 2

57.0 68.9 71.4 58.7 53.5

64.8 62.2 68.7 65.3 60.1

74.2 76.0 79.3 76.3 69.6

82.9 81.5 88.1 82.2 76.3

61.1 61.7 64.8 64.7 63.5

78 9 80.6 82.6 78.7 71.4

62.5 76.6 79.5 63.4 61.2

PEP (msec) Baseline Comp 1 Comp 2 Recov 1 Recov 2

98 88 84 95 98

95 81 78 89 93

87 64 67 81 87

89 80 74 84 88

92 88 86 89 91

81 78 78 82 83

94 69 67 89 92

106 71 71 99 106

2640 2421 2315 2669 2882

2065 1961 1811 1884 2103

1516 1304 1299 1421 1585

1541 1459 1343 1466 1609

2199 2433 2218 2133 2170

1824 1728 1811 1812 1997

1803 1338 1386 1857 1820

1685 1231 1211 1708 1803

TPR (dyne-sec-cm"5) Baseline Comp 1 Comp 2 Recov 1 Recov 2

9.64, p < 0.005). Although the latter effect appeared to be more pronounced in the Aerobic group (see Table 2), there was no significant interaction to indicate that it was reliably more so than in the Strength group. HR reactivity to the Competition task and subsequent rate of HR recovery 130

63.4 79.2 84.7 63.1 58.5

to Baseline were not affected by either exercise training program. CO (see Table 2) was also significantly elevated during the Competition task (Phases F(4,92) = 23.22, p < 0.001) but returned to baseline range by Recovery 1. Normotensive subjects showed greater Psychosomatic Medicine 51:123-136 (1989)

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CO reactivity to the Competition task than the borderline hypertensives (ANOVA BP status X Phases F(4,92) = 4.08, p < 0.05; ANCOVA BP status X Phases F(3,69) = 4.36, p < 0.05). Although exercise training led to lower overall HRs, there was no significant change in CO post-training in either group. SV also showed a significant Phases effect Cf(4,92) = 11.29, p < 0.001), which was due to SV generally increasing during the Competition task but returning to baseline by the first recovery period (see Table 2). As would be expected from the posttraining reduction in HR, which was not associated with an overall change in CO, there was a significant overall increase in SV post-training {Sessions F(l,23) = 4.66, p < 0.05). There was also a significant Exercise Groups x Sessions x Phases interaction for SV (F(4,92) = 3.82, p < 0.05), which multiple comparisons (Tukey's HSD) revealed to be predominantly due to the pre-training increase in SV in response to the Competition task persisting following exercise training in the Strength group, but showing a post-training diminution in the Aerobic group. Myocardial PEP showed a significant decrease in response to the Competition task (Phases F(4,92) = 25.92, p < 0.001), indicating a significant increase in contractility during Competition, but returned to baseline levels by the first Recovery period (see Table 2). Exercise training did not affect resting Baseline or Recovery PEP values, but during the first minute of competition PEP tended to show a lesser decrease post-training in the Aerobic group, in contrast to a greater decrease following Strength training, giving rise to an Exercise Groups X Sessions X Phases effect (F(4,92) = 3.37, p < 0.005). There was also a significant BP status X Exercise Groups x Sessions X Phases inPsychosomatic Medicine 51:123-136 (1989)

teraction (F(4,92) = 5.30, p < 0.01) which post-hoc analyses showed to be primarily due to a reduction in PEP response to Competition in the normotensive men following Aerobic exercise training. This attenuation of PEP reactivity in normotensives who underwent Aerobic training, but not in those who underwent Strength training, was further supported by an ANCOVA BP status X Exercise Groups x Sessions interaction (F(l,22) = 4.68, p < 0.05). Vascular responses. TPR values, presented in Table 2, decreased significantly during the Competition task (Phases F(4,92) = 9.63, p < 0.001); this decrease in response to the task showed a nonsignificant trend to be more pronounced in normotensive than in hypertensive subjects (BP status X Phases F(4,92) = 2.21, p = 0.074). There was an overall decrease in TPR following Exercise training in both groups (Sessions F(l,23) = 4.44 p < 0.05). There was also a significant Groups X Sessions X Phases interaction (F(4,92) = 3.58, p < 0.05) which was associated with the absence of a significant fall in TPR during the first minute of Competition on the post-training session in the Aerobic group (while the decrease in TPR was greater during the same period following Strength training). TPR was generally higher in borderline hypertensive than in normotensive subjects (BP status F(l,23) = 6.50, p < 0.02), and there was a nonsignificant trend toward lower overall TPR levels in hypertensives following exercise training (BP status X Sessions F(l,23) = 3.58, p = 0.07).

Competition Task Performance Approximately half the subjects in each Exercise training group won the Compe131

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which appears to be characteristic of tasks that encourage effortful active coping (32), has previously been demonstrated to be mediated via the sympathetic nervous system and, more specifically, through stimulation of the /3-adrenergic receptors (30, 33). Systolic BP, HR, and CO reactivity during the competition task remained essentially unchanged following aerobic training. However, there was some evidence that /3-adrenergic responses during the Competition task were attenuated following aerobic training, particularly at the onset of the task, where augmented myocardial contracatility, as indexed by PEP and SV changes, was initially less pronounced and the vascular vasodilatation, DISCUSSION indexed by TPR decrease, was also less In this study there was a significant marked. In contrast, following training in improvement in aerobic fitness in sub- the Strength group, myocardial and vasjects assigned to the Aerobic training con- cular responses to the Competition task dition, as verified by the 13.6% average tended to remain unaltered or were acincrease in directly measured VO2niax. In tually augmented. contrast, subjects who underwent 3 Regarding the mechanisms by which months of strength training showed an aerobic exercise training may potentially improvement in V02max of only 2.9%, alter cardiovascular responses during which was not statistically significant. As mental stress, the modification of central expected, subjects in the Aerobic exercise autonomic balance toward reduced symcondition exhibited lower HRs at rest and pathetic and increased parasympathetic during treadmill exercise at submaximal influences has been of particular interest. workloads following training, while no One assumption has been that mental such effects were evident in the Strength stressors that evoke cardiovascular retraining group. Thus, our experimental sponses mediated via the sympathetic manipulation was successful in inducing nervous system may be attenuated if symdifferential changes in levels of aerobic pathetic activity is generally suppressed. fitness. However, this proposition may be too simplistic, since it is generally accepted that Consistent with previous findings, the there is a peripheral systemic contributypical rise in BP during the stressful tion to training-induced adaptations in Competition task was characterized by an cardiovascular function. A clear illustraincrease in CO, due primarily to HR ele- tion of this is provided by evidence that vation with some accompanying augmen- improved maximal aerobic capacity, as tation of SV, while there was a decrease well as altered cardiovascular adjustin TPR ( 30, 31). This response pattern, ments to exercise, are specific to the type tition task on each of the two sessions. In both the Aerobic and Strength groups average reaction times were similar on both sessions (overall mean reaction time = 330 msec). Subjects' ratings of how hard they tried during the Competition task (scale of 1 to 7) were similar in both Exercise groups and similar pre- and posttraining (Strength pre-training = 6.4, posttraining = 6.2; Aerobics pre-training = 6.4, post-training = 6.4). These results indicate that task performance was similar in each of the two groups at both pre- and posttraining test sessions.

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of exercise engaged in during the training program. For example, attenuated HR responses to submaximal bicycle exercise are evident during bicycle exercise testing but are not pronounced during testing of responses to exercise in untrained arm muscles (34). This phenomenon is thought to be a reflection of modification in both vascular and enzymatic processes related to aerobic metabolism in the trained skeletal muscles (34). On the basis of this evidence, it is interesting to speculate that altered reactivity during mental stress might therefore be expected to occur only if the hemodynamic response pattern to the mental stressor includes peripheral responses that invoke the trained muscle vascular beds. Studies incorporating forearm blood flow measurement have shown that reaction time tasks and mental arithmetic both appear to be associated with increased blood flow to the skeletal muscles of the forearm (31, 35, 36). However, in one study where calf blood flow was measured in addition to the more standard forearm measurement, it was found that augmented blood flow was present in the forearm muscle and absent in the calf (37). If the relationship of exercise training to performance or exercise testing generalizes to mental stress testing, then it is possible that the running exercise used for aerobic conditioning in the present study may lead to less conspicuous effects on cardiovascular reactivity than might occur following aerobic conditioning resulting from upper body exercise training. Sinyor et al. (15) have made the point that the nature of the mentally stressful task may be a relevant consideration in understanding the relation of aerobic fitness to stress reactivity. Extending this point, for longitudinal studies, changes in stress reactivity associated with changes in aerobic fitness may depend on both the nature Psychosomatic Medicine 51:123-136 (1989)

of the stressful task employed and the nature of the aerobic training program. The most dramatic effects of exercise training in the present study were found to occur in subjects who were classified as borderline hypertensive. However, since analyses using BP status were based on small numbers of subjects per cell, the reported effects should be viewed as preliminary findings. Borderline hypertensive subjects in the aerobic exercise group exhibited a substantial reduction in DBP following the 3-month training program. Since CO was unaltered post-training, both the reduction in TPR and the bradycardia resulting from aerobic training are interpreted as together forming the underlying hemodynamic basis for this DBP reduction. Furthermore, DBP increase during and following the competition task were significantly reduced in the borderline hypertensives who underwent aerobic exercise training, but not in those who underwent strength training. Two studies have demonstrated a predictive relationship between high DBP responses during and following mental stress and the development of sustained hypertension over the next five years (18, 19). The substantial reduction in resting DBP together with an attenuation of DBP reactivity during psychological stress suggests that aerobic exercise training may be of ameliorative benefit to borderline hypertensive patients who might otherwise be at high risk for the development of sustained hypertension. Research supporting this possibility is provided by an animal study that employed a rat model of borderline hypertension and found that daily exercise conveyed some resistance to the development of hypertension associated with exposure to chronic stress (38). The lowered DBP levels and attenuated DBP reactivity during stress following 133

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aerobic conditioning in the borderline hypertensive subjects are suggestive of an alteration in vascular function. In a recent study it was shown that the fall in DBP associated with epinephrine infusion was greater in aerobically fit individuals, as well as being enhanced in association with improved aerobic fitness in subjects who underwent a 4-month exercise training program (39). Since no alterations in myocardial responses to epinephrine were observed, these results were interpreted as reflecting an increased sensitivity of vascular /3-adrenergic receptors and/or a proliferation of skeletal muscle precapillary blood vessels associated with aerobic fitness. Since hypertensive disease is associated with a down-regulation of/3-receptor sensitivity (40), it is possible that hypertensive individuals may be especially susceptible to a reversal of this trend brought about by aerobic conditioning. Such alterations in vascular /3-receptors provide a plausible explanation for both the initially high DBP reactivity during the competition stress seen in our borderline hypertensive subjects and the subsequent alteration of this response only in subjects who underwent aerobic exercise training. Further research investigating the effects of exercise on cardiovascular responses during psychosocial stress are clearly needed in order to fully understand the mechanisms by which aerobic exercise may reduce the stress response. Our preliminary findings indicate that aerobic training may be beneficial in attenuating pressor responses during stress, especially in borderline hypertensives. These data suggest the need for more long-term prospective studies to assess the ameliorative benefits of increased fitness, especially in those hypertensives who are hyperreactive during stress. Future stud134

ies should also assess the possible benefits of exercise in other specific populations, including blacks, adolescents, and the elderly. There is also a need to clarify which forms of exercise training are most effective in modifying cardiovascular responses during psychological stress. Possible mechanisms of reactivity attenuation, such as reduced catecholamine responses and/or altered adrenergic receptor sensitivities, need to be evaluated. Ambulatory blood pressure monitoring studies would also provide a means of assessing whether the overall attenuation of blood pressure levels, at rest as well as during physical and psychological stress, observed in the present study, generalize to a similar reduction in blood pressure responses during everyday activities. SUMMARY The effects of a 12-week aerobic exercise training program on responses to a psychologically challenging competition task were evaluated in a group of Type A men. A second matched group of men who underwent strength training served as a control group. Improved aerobic fitness was generally associated with lowered heart rates and blood pressure at rest, during exercise, and during the psychologically challenging competition task. For subjects who pre-training casual blood pressures fell in the borderline hypertensive range, aerobic conditioning led to both a substantial post-training reduction in resting blood pressure and a significant attenuation of initially high diastolic pressure reactivity to the competition task. While an overall reduction in blood pressure levels resulting from aerobic conditioning is likely to benefit most Psychosomatic Medicine 51:123-136 (1989)

AEROBIC TRAINING EFFECTS O N STRESS RESPONSES individuals, the present findings are tentatively interpreted as suggesting that borderline hypertensives are one subgroup for whom the benefits may be of immediate clinical value by abating the progress of hypertensive disease.

We thank Doris MurrelJ, Sally Schnitz, and Robin Pomeroy for assistance in data coJIection and analysis, Margaret WaJsh and Gwendy Wasser for supervision of

training procedures, and Dorothy Faulkner for manuscript preparation. We also thank Dr. Mats Frederickson for his helpful comments on an earlier draft of the manuscript. This study was supported by National Institutes of HeaJth grants HL-30675, HL-19876, HL-01096, and AGO4238, and by a grant from the John D. and Catherine T. MacArthur Foundation Research Network on the Determinants and Consequences of Health-Promoting and Health-Damaging Behavior.

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