Secondly, the hearts of the clenbuterol-taking, trained rats increased dramatically
in size compared to the hearts of sedentary
rats, but the heart expansion was probably due to the infiltration of collagen fibres into the heart walls, not an increase in heart-muscle cells. Collagen is a tough connective tissue which doesn't augment heart-muscle power but in fact stiffens the heart, potentially leading to a decrease in cardiac output.