Final answer:
An extrasystole can be induced during the ventricular diastole portion of the cardiac cycle when the heart muscle is relaxed. Inducing an extrasystole is not normal and reflects disturbances in the heart's rhythm.
Step-by-step explanation:
The portion of the cardiac muscle contraction during which it is possible to induce an extrasystole is ventricular diastole. During ventricular diastole, the heart muscle is relaxed and the ventricles are filled with blood. This relaxation period allows the cardiac muscle to respond to an additional stimulus, potentially causing an extrasystole, which is an extra heartbeat outside the regular rhythm. It is important to note that inducing an extrasystole is abnormal and typically indicates a disruption in cardiac rhythm.
The first heart sound represents the transition from atrial diastole to ventricular systole, where the ventricles contract to push blood out of the heart. The atrial diastole refers to the period when the atria are relaxed, and ventricular systole is when the ventricles are contracting forcefully. An extrasystole cannot occur during systole because the cardiac muscle cells are in a refractory state, unable to respond to further stimulation until the completion of the contraction cycle.