Slowing of the heart at the beginning of exercise

¹ Applied Physiology Branch, U.S. Army Chemical Research and Development Laboratories, Army Chemical Center, Maryland

At 18 C the heart began to beat faster at the beginning of treadmill exercise. In the first 10 beats, the average increase above the previous standing rate amounted to 12 beats/min in walking and 28 beats/min in running. At 38 C or when atropine was injected, the standing rate before exercise was rapid; in the immediate response to exercise, the rate increased less than at 18 C without atropine, remained the same as in standing, or decreased temporarily below the standing rate by as much as 40 beats/min. These early responses were attributed to changes in vagal inhibition. After the start of the run there was a secondary increase between the 25th and 60th sec that averaged 22 beats/min at the peak of the response to atropine. This secondary increase was attributed to accelerator activity.