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How muscles respond to training and physical activity are influenced by these muscle fibre distinctions. And, each fibre type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fibre types. On average, you have about 50 percent slow twitch and 50 percent fast twitch fibres in most of the muscles used for movement.

Slow Twitch (Type I)

The slow twitch muscles are more efficient at using oxygen to generate more fuel (ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibres and can last a long time before fatigue. Slow twitch fibres are beneficial in helping athletes run marathons or paddle for hours.

Fast Twitch (Type II)

Because fast twitch fibres use anaerobic metabolism to create fuel, they can generate short bursts of strength or speed much better than slow muscles. However, they fatigue more quickly. Although fast twitch fibres generally produce the same amount of force per contraction as slow muscles, they earn their name by firing more rapidly. Given this, more fast twitch fibres can be an asset to sprinters who need to quickly generate a lot of force.