What indicates that the heart is in the relaxation phase of the heartbeat?

The relaxation phase of the heartbeat is known as diastole. During diastole, the heart muscles relax, allowing the chambers of the heart to fill with blood. This phase is crucial for the effective pumping of blood, as it ensures that sufficient volume is present in the heart when it contracts again during the subsequent phase, systole.

In contrast, systole refers to the contraction phase of the heartbeat, where the heart pumps blood out to the circulation. Myocardial contraction specifically indicates the muscle activity that occurs during systole rather than relaxation. Arrhythmia denotes an abnormal heart rhythm, which can occur during either phase of the heartbeat and does not specifically relate to the relaxation or contraction state. Therefore, diastole is the correct term to describe the heart's relaxation phase.