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Type IIa Fibres

A sub-category of fast twitch muscles are Type IIa fibres, also known as intermediate fast-twitch fibres. These can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibres.

Type IIb Fibres

The second sub-category of fast twitch fibres are Type IIb fibres that use anaerobic metabolism to create energy. These are the “classic” fast twitch muscle fibres that produce quick, powerful bursts of speed. Of all the muscle fibre types, this one has the highest rate of contraction, or rapid firing. At the same time however, it has a much faster rate of fatigue and cannot last as long before rest is needed.

Fibre Type and Performance

Your muscle fibre type can play a part in determining which distances you are naturally good at or if you are fast or strong. Genetic makeup can determine the sport that is pursued by elite athletes. For example, Olympic sprinters have been shown to possess about 80 percent fast twitch fibres. Athletes who excel in marathons tend to have 80 percent slow twitch fibres.