The cardiac cycle involves all the events that occur during heartbeat.It involves systole and diastole. The duration of each cardiac cycle is about 0.8 sec. The average adult person has 65 to 75 heart beats/minute.

Thus 65 to 75 beats $=\mathrm{ }60\mathrm{ }\mathrm{s}$

So 1 beat $\mathrm{ }=\mathrm{ }60/75=\mathrm{ }0.8\mathrm{ }\mathrm{s}\mathrm{e}\mathrm{c}$

Thus it is seen that heart beat is inversely proportional to the duration of cardiac cycle.

During exercise, heart beat increases and thus duration decreases i.e. less than 0.8 sec.