The Role of Fast & Slow Twitch Muscle Fibers

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. Because of the higher energy demands of slow-twitch fibers (due to their more frequent and prolonged usage), they have a higher density of mitochondria for energy and myoglobin to provide oxygen.

Meanwhile, fast-twitch fibers tend to have larger stores of glycogen so that vast quantities of energy can be provided on short notice. Fast-twitch fibers can deliver a quick burst of power. 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 of the slower twitch fibers while sprinters or bodybuilders will often have more fast twitch fibers.

Strength training with different repetition ranges along with speed tempo variations will allow you to work the different muscle fibers and challenge the different energy systems in your body.

Susan Arruda