Type I muscle fibreSlow-twitch fibre · Type I muscle fiber

Type I fibres contract relatively slowly, produce less peak force, and resist fatigue for a long time. They are dense in mitochondria and capillaries and rich in myoglobin, which gives them their red colour and their reliance on aerobic metabolism.

It matters because fibre-type distribution differs between people and between muscles, and it partly explains why some people take naturally to endurance work and others to sprinting. Postural muscles such as the soleus are heavily type I in almost everyone.

In practice, training changes fibre properties considerably but wholesale conversion between the major types is limited — most of the shifting happens within the type II subtypes. The practical lesson is to train the quality you want rather than to hope for a conversion.