What term describes the degree to which cardiac muscle cells are stretched just before systole?

The term that describes the degree to which cardiac muscle cells are stretched just before systole is preload. Preload specifically refers to the amount of blood that fills the ventricles during diastole, which directly influences the stretching of the myocardial fibers. According to the Frank-Starling law of the heart, an increase in preload leads to an increased force of contraction during systole. This is because the greater stretch of the heart muscle enhances the overlap of actin and myosin filaments, resulting in a more powerful contraction.

In contrast, afterload refers to the pressure against which the heart must work to eject blood during systole, reflecting the resistance the heart faces rather than the degree of stretch in muscle cells. Cardiac output is the volume of blood the heart pumps per minute, which is a product of heart rate and stroke volume but does not specifically describe the stretching of muscle fibers. Stroke volume is the amount of blood ejected from the ventricle with each heartbeat, also not directly related to the extent of stretching of cardiac muscle cells prior to contraction.