Cardiac acceleration in man elicited by a muscle-heart reflex.

The shortening of the R-R interval in response to voluntary and electrically induced isometric muscle contractions of short duration was investigated in 15 volunteers. In some of those experiments the effect of vagal blockade was also studied. The results show: 1) a lag time between the start of the contraction and the following decrease in R-R interval duration of 550 milliseconds; 2) a similar R-R interval response due to voluntary and electrically induced contractions of the same force; 3) no shortening of the R-R interval when the skin is stimulated without ensuing muscular contraction; 4) a complete disappearance of the response to isometric contractions during vagal blockade. A difference in lag time between the onset of arm contraction and cardiac acceleration could not be demonstrated. Most of the results give strong evidence to the existence of a muscle-heart reflex in man, involved in the instantaneous cardiac acceleration at the onset of exercise, that has its origin in the muscles and the vagal nerves as its efferent pathway.