Final answer:
The extra heart sound present when forceful atrial contractions move blood into a stiff ventricle is the fourth heart sound, S4, which indicates left ventricular dysfunction.
Step-by-step explanation:
When forceful atrial contractions are required to move blood into a stiff ventricle, this indicates the presence of a fourth heart sound, or S4. The occurrence of the S4 heart sound is associated with the atria pushing blood into a ventricle that has become rigid or hypertrophic, which is indicative of a failure in the ventricular function, particularly on the left side of the heart. Unlike the regular 'lub-dub' sounds (S1 and S2) heard in a healthy heart during the cardiac cycle, S4 precedes the S1 sound and may be part of a gallop rhythm known as the 'Tennessee gallop,' as it adds an additional sound component to the heartbeat and can be an important diagnostic clue of pathological changes within the heart's structure or function.