The cardiac cycle time during normal heartbeat is 0.8 second, which includes both systole and diastole. In the event of fast running, the heart starts beating faster leading to decrease in cardiac cycle timings. This decrease is due to:

The cardiac cycle is the event during, which one heartbeat or one cycle of contraction(systole) and relaxation (diastole) of cardiac muscle occurs. A cardiac cycle consists of systole and diastole of atria and systole and diastole of ventricles. The time of cardiac cycle is in reverse ratio of the rate of heartbeat. In man, the heart rate is about 72 times / min, therefore the time of a cardiac cycle is 60/72=0.8 sec approx. The total time of the cardiac cycle is divided into atrial systole 0.1 sec, atrial diastole 0.7 sec, ventricular systole 0.3 seconds and ventricular diastole 0.5 seconds.