Autonomic nervous control of the heart rate during isometric exercise in normal man

Abstract

The relative contribution of the efferent components of the autonomic nervous system to the regulation of tachycardia induced by isometric exercise was assessed in 23 normal males. The isometric exercise (handgrip) was performed at the maximum intensity tolerated by the individual over a period of 10 s (maximal voluntary contraction-MVC) and at levels equivalent to 75, 50 and 25% of MVC for 20, 40 and 10 s, respectively. The study was performed both under control conditions and after pharmacological blockade with atropine (12 individuals) or propranolol (11 individuals). Under control conditions, the heart rate (HR) responses to isometric effort were dependent on the intensity and duration of the exercise, showing a tendency towards progressive elevation with the maintenance of muscular contraction at the levels studied. The tachycardia evoked by this effort was of considerable magnitude and of rapid onset, especially at the more intense levels of activity. Parasympathetic blockade markedly decreased tachycardia, which manifested itself during the first 10 s of exercise at all levels of intensity, whereas sympathetic blockade markedly modified the HR response after 10 s of effort at the 75 and 50% MVC levels. A slight depression of the tachycardiac response could be observed already after 10 s of maximum effort after propranolol. The present results suggest that the autonomic regulation of these responses is based on a biphasic mechanism, with the initial phase depending on the rapid withdrawal of the parasympathetic influence, followed by a marked sympathetic contribution to the induction of tachycardia after 10 s of isometric contraction or even a little before at maximum exertion.

Identification of anaerobic threshold using heart rate response during dynamic exercise

“…At medium and high powers, sympathetic stimulation of the sinus node is responsible for the appearance of a slow tachycardia from 1 to 4 min of DE. In this situation the HR response does not attain a steady-state condition (8)(9)(10)(11)). An important finding related to the autonomic control of HR during DE is the observation that the sympathetic contribution of exercise tachycardia appears above the AT (4).…”

Muscle Metaboreflex Contribution to Sinus Node Regulation During Static Exercise

“…2 This general view came from the observation that during postexercise circulatory occlusion, a maneuver that maintains muscle metaboreflex activation while removing the central command, the increases in AP, vascular resistance, and sympathetic nerve activity to resting muscles are kept elevated above resting levels, whereas HR fully recovers. [3][4][5] However, Maciel et al 6 and Martin et al 7 reported a reduced HR response to static exercise after administration of ␤-adrenergic blocking drugs, suggesting an involvement of the sympathetic nervous system in HR regulation, although the mechanism underlying the sympathetic contribution (ie, central versus reflex) has not been determined. More recently, O'Leary 8 provided evidence that sympathetic activation originating from the muscle metaboreflex contributes substantially to the HR increase during exercise in the conscious dog, inasmuch as parasympathetic blockade with atropine did not affect the increase in AP and HR that occurred during exercise but did prevent the fall in HR during postexercise circulatory occlusion.…”

Muscarinic antibodies and heart rate responses to dynamic exercise and to the Valsalva maneuver in chronic chagasic patients

“…Reactivation of parasympathetic activity, on the other hand, determines the magnitude of heart rate deceleration following termination of the maneuver and immediately after cessation of dynamic exercise. These two tests assess the responsiveness of the cardiac autonomic nervous system, particularly the negative chronotropic effects of the parasympathetic system on heart rate 5,19,26,30,35 .…”

“…The heart rate changes during the early stages 30,35 and recovery phases of dynamic exercise were assessed by performing a maximal standard stress test 5 . Heart rate prior to, during and after exercise was stored in a hard disk and played back for analysis.…”