<prompt> What is the result of increasing preload on cardiac muscle?Shoening of muscle fibreLengthening of muscle fibreNo effectVariable effect  <response>: Preload is the degree to which the myocardium is stretched before it contracts. Increasing the preload therefore increases the length of muscle fiber. The force of contraction of cardiac muscle depends on its preload and afterload. According to Frank Starlings law: Energy of contraction is propoional to the initial length of cardiac muscle fiber. When cardiac output is regulated by changes in cardiac muscle fiber length, this is referred to as heterometric regulation. Regulation of cardiac output due to changes in contractility independent of the length is sometimes called homometric regulation. Ref: Ganongs Review of Medical Physiology, 24e, chapter 30 