By imposing such breaks, we shall aim at increasing the stroke volume and the cardiac output, as well as the number of functional pulmonary alveoli. Fast-twitch fibres will thus have time to replace the muscle glycogen supply, since slow-twitch fibres do most of the work. At the same time, the increase of hemoglobin levels is also important, although apparently a person can gain this effect only by swimming at high altitudes (Wilmore, & Costill, 1999).