Definition: Our bodies are made up of different types of muscle fibers, all of which rely on different energy systems. Slow twitch fibers, as the name suggests, contract more slowly than fast twitch muscle fibers and they can contract for a longer period of time. We often call these 'endurance' muscles because they're great for aerobic activities that require endurance like running, cycling and swimming.
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 4-16) will allow you to work different muscle fibers and energy systems in your body.
