But they work. If you keep up a regimen of two sessions a week of HIIT, your muscles will gradually improve their ability to produce high, rapid forces in part by augmenting how many fibers contract simultaneously when stimulated by nerves. In addition, your muscles will change composition. Although HIIT cannot stimulate your body to produce more fast-twitch muscle fibers, the ones you have will thicken, making you stronger and hence faster.
