Unlike other muscles that get tired and sore after working out for sometime, how come the muscles of the heart never get tired even after beating continuously?

Even though it seems as if the heart is beating without a break, the heart does rest for a very short time in between two beats. The human heart has four chambers — the left and right atria on top, which drain, into two corresponding ventricles below, through valves. The atria receive blood from the entire body through veins. This blood flows through the connecting valves into the ventricles, and the ventricles contract and push blood out into arteries, which branch and supply to the whole body. 

The heart beats in 'cycles' and each cycle consist of a systole and a diastole. Taking the average heart rate to be 75 beats per minute each cycle lasts just 0.8 seconds. In the systole, which lasts about 0.27 seconds, the ventricles contract pumping blood out into the arteries and in the diastole, which lasts around 0.53 seconds, the ventricles receive blood from the atria. It is during diastole that the heart muscle rests. Also, if we were to plot the contraction of a single heart muscle fibre on a graph it can be seen that there is a refractory period for the fibre during which it will not contract on applying a second stimulus. This prevents the heart muscles from developing tetanus.

Source: thehindu.com