Endurance exercise training is associated with elevated basal sympathetic nerve activity in healthy older humans

Exercise and autonomic function in health and cardiovascular disease

Autonomic nervous system activity contributes to the regulation of cardiac output during rest, exercise, and cardiovascular disease. Measurement of HRV has been particularly useful in assessing parasympathetic activity, while its utility for assessing sympathetic function and overall sympathovagal balance remains controversial. Studies have revealed that parasympathetic tone dominates the resting state, while exercise is associated with prompt withdrawal of vagal tone and subsequent sympathetic activation. Conversely, recovery is characterized by parasympathetic activation followed by sympathetic withdrawal, although clarification of the normal trajectory and autonomic basis of heart rate decay following exercise is needed. Abnormalities in autonomic physiology--especially increased sympathetic activity, attenuated vagal tone, and delayed heart rate recovery--have been associated with increased mortality. Exercise training is associated with a relative enhancement of vagal tone, improved heart rate recovery after exercise, and reduced morbidity in patients with cardiovascular disease. However, whether exercise training leads to reduced mortality in this population because of its ability to specifically modulate autonomic function is unknown at the present time. Although the results of a recent randomized study in patients with CHF and a meta-analysis in the setting of a recent myocardial infarction determined that exercise training leads to improved outcomes in these populations, neither study measured autonomic function. Improved autonomic function due to exercise training is a promising rationale for explaining improvements in outcome, although more research is needed to confirm this hypothesis.

Regular endurance exercise induces expansive arterial remodelling in the trained limbs of healthy men

1. In experimental animals chronic elevations in arterial blood flow increase the lumen diameter and reduce the intima-media thickness (IMT) of the arterial segment involved. We determined whether intermittent elevations in active muscle blood flow associated with regular aerobic leg exercise induced such expansive arterial remodelling in the common femoral artery of humans. 2. In the cross-sectional study 53 sedentary (47 +/- 2 years) and 55 endurance exercise-trained (47 +/- 2 years) men were studied. Common femoral artery lumen diameter (B-mode ultrasound) was 7 % greater (9.62 +/- 0.12 vs. 9.03 +/- 0.13 mm), and femoral IMT (0.46 +/- 0.02 vs. 0.55 +/- 0.02 mm) and IMT/lumen ratio were 16-21 % smaller in the endurance-trained men (all P < 0.001). Basal femoral artery blood flow (duplex ultrasound) was not different, shear stress tended to be lower (P = 0.08), and mean femoral tangential wall stress was 30 % higher in the endurance-trained men (P < 0.001). 3. In the intervention study 22 men (51 +/- 2 years) were studied before and after 3 months of regular aerobic leg exercise (primarily walking). After training, the femoral diameter increased by 9 % (8.82 +/- 0.18 vs. 9.60 +/- 0.20 mm), and IMT (0.65 +/- 0.05 vs. 0.56 +/- 0.05 mm) and the IMT/lumen ratio were approximately 15-20 % smaller (all P < 0.001). Basal femoral blood flow and shear stress were not different after training, whereas the mean femoral tangential wall stress increased by 31 %. The changes in arterial structure were not related to changes in risk factors for atherosclerosis. 4. Our results are consistent with the concept that regular aerobic leg exercise induces expansive arterial remodelling in the femoral artery of healthy men. This adaptive process is produced by even a moderate training stimulus, is not obviously dependent on corresponding improvements in risk factors for atherosclerosis, and is robust, occurring in healthy men of different ages.

Age-related decreases in basal limb blood flow in humans: time course, determinants and habitual exercise effects

Basal whole-limb blood flow is lower in older than in young healthy sedentary men due to a lower limb vascular conductance. In Study 1, we determined whether age-associated reductions in basal whole-leg (femoral artery) blood flow and vascular conductance are modulated by habitual physical activity by studying 89 healthy men aged 20-35 or 55-75 years (26 sedentary, 31 physically active and 32 endurance exercise trained). Femoral blood flow (duplex Doppler) and vascular conductance were approximately 20-30 % lower (P < 0.01) in the older men in all three physical activity groups. In Study 2, to determine the temporal pattern and relation to local metabolism and lean tissue mass of the age-associated reductions in femoral blood flow, we studied 142 healthy men aged 18-79 years. Femoral blood flow (r = -0.40) and vascular conductance (r = -0.51) were linearly and inversely related to age (both P < 0.001). Leg fat-free mass (r = -0.48) and estimated leg oxygen consumption (r = -0.49) declined with advancing age (both P < 0.001), and were strongly and positively related (r = 0.75; P < 0.001). The age-associated decline in femoral blood flow correlated with the corresponding reductions in leg fat-free mass and estimated leg oxygen consumption (both r = 0.47; P < 0.001). We concluded that: (1) basal whole-limb blood flow and vascular conductance decrease progressively with advancing age in healthy men; (2) reductions in both limb fat-free mass and oxygen consumption are related to the decline in whole-limb blood flow with age; and (3) habitual aerobic exercise does not appear to modulate the age-related reductions in basal limb blood flow and vascular conductance.

Effect of muscle mass and intensity of isometric contraction on heart rate

The purpose of this study was to determine the effect of muscle mass and the level of force on the contraction-induced rise in heart rate. We conducted an experimental study in a sample of 28 healthy men between 20 and 30 yr of age (power: 95%, alpha: 5%). Smokers, obese subjects, and those who performed regular physical activity over a certain amount of energetic expenditure were excluded from the study. The participants exerted two types of isometric contractions: handgrip and turning a 40-cm-diameter wheel. Both were sustained to exhaustion at 20 and 50% of maximal force. Twenty-five subjects finished the experiment. Heart rate increased a mean of 15.1 beats/min [95% confidence interval (CI): 5.5-24.6] from 20 to 50% handgrip contractions, and 20.7 beats/min (95% CI: 11.9-29.5) from 20 to 50% wheel-turn contractions. Heart rate also increased a mean of 13.3 beats/min (95% CI: 10.4-16.1) from handgrip to wheel-turn contractions at 20% maximal force, and 18.9 beats/min (95% CI: 9. 8-28.0) from handgrip to wheel-turn contractions at 50% maximal force. We conclude that the magnitude of the heart rate increase during isometric exercise is related to the intensity of the contraction and the mass of the contracted muscle.

Sympathetic adaptations to one-legged training

The purpose of the present study was to determine the effect of leg exercise training on sympathetic nerve responses at rest and during dynamic exercise. Six men were trained by using high-intensity interval and prolonged continuous one-legged cycling 4 day/wk, 40 min/day, for 6 wk. Heart rate, mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA; peroneal nerve) were measured during 3 min of upright dynamic one-legged knee extensions at 40 W before and after training. After training, peak oxygen uptake in the trained leg increased 19 +/- 2% (P < 0.01). At rest, heart rate decreased from 77 +/- 3 to 71 +/- 6 beats/min (P < 0.01) with no significant changes in MAP (91 +/- 7 to 91 +/- 11 mmHg) and MSNA (29 +/- 3 to 28 +/- 1 bursts/min). During exercise, both heart rate and MAP were lower after training (108 +/- 5 to 96 +/- 5 beats/min and 132 +/- 8 to 119 +/- 4 mmHg, respectively, during the third minute of exercise; P < 0.01). MSNA decreased similarly from rest during the first 2 min of exercise both before and after training. However, MSNA was significantly less during the third minute of exercise after training (32 +/- 2 to 22 +/- 3 bursts/min; P < 0.01). This training effect on MSNA remained when MSNA was expressed as bursts per 100 heartbeats. Responses to exercise in five untrained control subjects were not different at 0 and 6 wk. These results demonstrate that exercise training prolongs the decrease in MSNA during upright leg exercise and indicates that attenuation of MSNA to exercise reported with forearm training also occurs with leg training.

Influence of age on arterial baroreflex inhibition of sympathetic nerve activity in healthy adult humans

Resting levels of muscle sympathetic nerve activity (MSNA) increase markedly with age in healthy adult humans. An age-related reduction in arterial baroreflex inhibition of MSNA could contribute to these elevations. To test this hypothesis, we measured MSNA using peroneal microneurography in young (age, 25 +/- 1 yr; n = 8) and older (69 +/- 1 yr; n = 7) healthy normotensive men before (baseline control) and during graded constant infusion of phenylephrine hydrochloride (0.5-2.0 microgram . kg-1. min-1) that produced a sustained approximately 10-mmHg increase in arterial blood pressure. Central venous pressure was controlled at baseline levels (+/-1 mmHg) using lower body negative pressure. Resting MSNA was approximately 95% higher in the older compared with the young subjects (43 +/- 5 vs. 22 +/- 3 bursts/min; P < 0.05). However, arterial baroreflex MSNA inhibitory responsiveness was similar in the older compared with the young subjects (254 +/- 112 vs. 259 +/- 40 arbitrary integration units/mmHg, respectively), although the percent reduction in MSNA was smaller in the older men (8.9 +/- 0.7 vs. 5.2 +/- 1.1%/mmHg), due to their elevated baseline levels. The reflex increase in the R-R interval was not different in the two groups (13 +/- 10 vs. 16 +/- 7 ms/mmHg). In summary, our findings suggest that arterial baroreflex inhibition of MSNA is preserved with age in healthy normotensive adult humans. As such, this mechanism does not appear to contribute to the age-related rise in tonic MSNA.

Norepinephrine kinetics in older women: relationship to physical activity and blood pressure

The sympathetic nervous system participates in the regulation of carbohydrate, lipid, and energy metabolism, and has been implicated in the pathogenesis of hypertension and obesity. Increased sympathetic nervous system activity with age may alter disease risk and contribute to the development of certain chronic diseases. Thus, we examined possible determinants of sympathetic nervous system activity in older normotensive women from infusions of tritiated norepinephrine (NE) to estimate rates of norepinephrine appearance and clearance. A secondary aim was to examine the association between norepinephrine kinetics and mean supine arterial blood pressure. Twenty-two older women (65.7 +/- 5.7 years) were characterized for resting NE kinetics, body composition, body fat distribution, peak aerobic capacity, leisure time physical activity energy expenditure (LTA), dietary carbohydrates, and daily energy intake. Analysis of univariate correlations revealed that only the LTA was significantly correlated with plasma NE appearance (r = 0.54, p < 0.01). Stepwise regression analysis identified LTA as the only significant predictor of plasma NE appearance rate with a total R2 = 0.29. The waist-to-hip ratio was selected as the only significant predictor of mean arterial blood pressure with an R2 = 0.30. When forced into the model, plasma NE appearance explained only 1% of the unique variance in mean arterial blood pressure. In summary, we found that: (1) higher levels of physical activity are related to higher plasma NE appearance in older women; (2) greater central body fatness is an independent predictor of mean arterial blood pressure; and (3) plasma NE appearance rate is a minor contributor to variation in mean arterial blood pressure in older, normotensive women.

Sympathetic neural adaptations to exercise training in humans: insights from microneurography

Sympathetic nerve activity has long been regarded as an important regulator of blood flow and blood pressure. Its importance has been especially recognized during exercise. The present review examines sympathetic neural adaptations to exercise training in humans obtained by sympathetic nerve recordings to nonactive skeletal muscle. Little evidence exists from both cross-sectional and longitudinal studies indicating that training alters resting muscle sympathetic nerve activity (MSNA). However, MSNA responses during exercise appear to be attenuated after training. This attenuation of MSNA seems to be specific to the trained muscle and not generalizable to other muscle groups. The mechanisms for the decrease in exercise-induced MSNA have been attributed to changes in both the muscle metaboreflex and muscle mechanoreflex. In addition to exercise, training has generally not altered MSNA responses to other stressors such as cold pressor test, lower body negative pressure, and upright tilting. However, the effect of training on baroreflex control of MSNA is equivocal. These conclusions are based on few studies. More comprehensive training studies are needed to better understand the role of training on sympathetic neural outflow.

Randomized trial of a hospital-based exercise training program after acute myocardial infarction: cardiac autonomic effects

OBJECTIVES

This study sought to determine whether a moderate intensity supervised exercise training program, performed immediately after an uncomplicated acute myocardial infarction, improves recovery in cardiac autonomic function compared with standard advice about activity at home.

BACKGROUND

Exercise training has beneficial effects on cardiac autonomic function and may improve prognosis after acute myocardial infarction.

METHODS

Thirty-nine male and 10 female patients, mean (+/-SE) age 57 +/- 1 years, with an uncomplicated acute myocardial infarction were randomized to either a 6-week moderate intensity supervised hospital-based exercise training program (exercise group) or to an unsupervised low intensity home walking program (control group). Outcome measures included changes in baroreflex sensitivity (phenylephrine bolus method) and heart rate variability (24-h Holter monitoring) and the endurance time at 85% of peak oxygen consumption.

RESULTS

At baseline, there were no significant differences in left ventricular ejection fraction (57 +/- 2% vs. 53 +/- 2%), frequency of anterior infarction (27% vs. 18%) and peak creatine kinase (1,256 +/- 170 vs. 2,599 +/- 295 IU) between the exercise and control groups. Baroreflex sensitivity (10.5 +/- 1.0 vs. 8.4 +/- 1.2 ms/mm Hg) and time domain measures of heart rate variability were also similar. After completion of the program, the exercise group exercised for a median of 15 min (interquartile range 12 to 25) at a workload of 104 +/- 7 W compared with 7 min (interquartile range 3.5 to 12) at a workload of 89 +/- 8 W in the control group (p < 0.01). There were significant (p < 0.001) improvements in baroreflex sensitivity and heart rate variability for the 49 patients combined but no differences between the exercise and control groups. Baroreflex sensitivity improved by 3.4 +/- 1.0 and 1.7 +/- 1.0 ms/mm Hg and the standard deviation of 24-h RR intervals by 36 +/- 6 and 40 +/- 10 ms, respectively (p > 0.1).

CONCLUSIONS

A hospital-based exercise training program increased endurance capacity but did not improve recovery of cardiovascular antonomic function after uncomplicated acute myocardial infarction.

Age, fitness, and regional blood flow during exercise in the heat

During dynamic exercise in warm environments, the requisite increase in skin blood flow (SkBF) is supported by an increase in cardiac output (Qc) and decreases in splanchnic (SBF) and renal blood flows (RBF). To examine interactions between age and fitness in determining this integrated response, 24 men, i.e., 6 younger fit (YF), 6 younger sedentary (YS), 6 older fit (OF), and 6 older sedentary (OS) rested for 50 min, then exercised at 35 and 60% maximal O2 consumption (VO2max) at 36 degrees C ambient temperature. YF had a significantly higher Qc and SkBF than any other group during exercise, but fitness level had no significant effect on any measured variable in the older men. At 60% VO2max, younger subjects had significantly greater decreases in SBF and RBF than the older men, regardless of fitness level. Total flow redirected from these two vascular beds (deltaSBF + deltaRBF) followed YF >> YS > OF > OS. A rigorous 4-wk endurance training program increased exercise SkBF in OS, but deltaSBF and deltaRBF were unchanged. Under these conditions, older men distribute Qc differently to regional circulations, i.e., smaller increases in SkBF and smaller decreases in SBF and RBF. In younger subjects, the higher SkBF associated with a higher fitness level is a function of both a higher Qc and a greater redistribution of flow from splanchnic and renal circulations, but the attenuated splanchnic and renal vasoconstriction in older men does not appear to change with enhanced aerobic fitness.

Quantitation of lower physical activity in persons with multiple sclerosis

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that often affects the motor system. We tested the hypothesis that physical activity was lower in a group of 17 MS patients (mean +/- SD; age = 46 +/- 6 yr, 11 females, 6 males) compared with 15 healthy sedentary control subjects (age = 44 +/- 7 yr, 9 females, 6 males). Physical activity was measured with a three-dimensional accelerometer and with an activity questionnaire for 7 d. Vector magnitude values from the accelerometer for the MS and sedentary control subjects were 121,027 +/- 59,336 and 185,892 +/- 60,566 arbitrary units/day, respectively (P = 0.01). Estimated energy expenditure values derived from the questionnaire were 35.9 +/- 3.0 and 36.2 +/- 4.1 Kcal.kg-1.d-1 (NS), respectively. Thus, when measured directly with an accelerometer, activity was lower in MS compared with sedentary control subjects. The data also suggest that the accelerometer was more sensitive than the questionnaire for detecting differences in activity between two relatively sedentary populations, including one with neurologic disease.

Bradycardia during baroreflex stimulation and active or passive stressor tasks: cardiorespiratory fitness and hostility

We examined whether the resting bradycardia associated with cardiorespiratory fitness extends to lowered heart rate (HR) and mean arterial pressure during novel passive or active laboratory stressors and to a longer heart period during stimulation of the carotid-cardiac vagal baroreflex, independently of the Type A behavior pattern (TABP) and hostility. Forty-four normotensive Caucasian men (18-35 years of age) completed the Structured Interview for TABP, the Cook-Medley Hostility Scale, and a peak oxygen uptake (Vo2peak) test. Vo2peak was inversely related to HR prior to, during, and after each stressor and was positively related to heart period during baroreflex testing. Hostility was inversely related to heart period during baroreflex testing. TABP had no effects. Our findings indicate that cardiorespiratory fitness and low hostility are independently associated with a bradycardia during stimulation of the carotid-cardiac baroreflex, consistent with a possible enhancement of cardiac vagal tone.