Definition: Our bodies are made up of different types of muscle fibers, all of which rely on different energy systems and support the body in different ways. Fast twitch fibers, as the name suggests, contract more more quickly than slow twitch muscle fibers and, as a result, they produce more power and strength. Fast twitch muscle fibers are activated during short bursts of speed and strength such as in sprinting or bodybuilding. These muscle fibers are also designed to grow larger in response to training unlike slow twitch muscle fibers.
Strength training with heavy weights for fewer reps will target these fibers as will explosive training. But, heavy strength training won't necessarily make you bulk up unless you eat more calories than you burn.
In general, many of us will have about 50% slow twitch fibers and 50% fast twitch fibers, although it will differ between individuals. Successful long-distance runners will often have more slow twitch fibers while sprinters or bodybuilders will often have more fast twitch fibers. Strength training in different rep ranges (from 1-16) will allow you to work different muscle fibers and energy systems in your body.
