Vagal and sympathetic contributions to the heart rate at rest and during isometric and dynamic exercise in young healthy men.

Six young healthy males received on four different occasions 0.4 mg/kg i.v. of propranolol. 0.04 mg/kg i.v. of atropine, atropine following propranolol or saline. heart rate and blood pressure were recorded at rest (supine, sitting, and standing) and during isometric and dynamic exercise. On the basis of heart rate changes under the influence of drugs, the sympathetic contribution to heart rate was 25% at supine rest, 33% in the standing position, 45% during handgrip, and 74% during bicycle exercise that raised heart rate to 153/min. These results suggest that with increasing muscular activity the contribution of parasympathetic tone to heart rate decreases from about 75% at supine rest to about 25% at submaximal exercise. Extrapolation of the heart rate plots indicate that parasympathetic tone is virtually abolished at exercise loads resulting in a heart rate of about 200 beats/min.