<prompt> Starlings law of the heartDoes not operate in the failing heart.Does not operate during exercise.Explains the increase in heart rate produced by exercise.Explains the increase in cardiac output that occurs when venous return is increased.  <response>: Starling pointed this out when he stated that the energy of contraction is proportional to the initial length of the cardiac muscle fiber Starlings law of the heart or the FrankStarling law .For the heart, the length of the muscle fibers ie, the extent of the preload is proportional to the end-diastolic volume.The relation between ventricular stroke volume and end diastolic volume is called the FrankStarling curve. 