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

Type I fibres are the slow twitch half of your muscle: fibres that contract more slowly, produce less peak force, and keep going for a very long time. They are dense with mitochondria and capillaries, rich in the enzymes of aerobic metabolism, and coloured red by the myoglobin that ferries oxygen inside them.

They matter because they do most of the work in most of your life. Standing, walking, holding posture and any sustained aerobic effort run mainly on these fibres, and they are recruited first in almost every movement because the nervous system calls on small motor units before large ones.

Their advantage is fatigue resistance rather than force. They regenerate ATP aerobically, which is slower but effectively unlimited while fuel and oxygen last, so a muscle rich in them can hold a submaximal effort for hours where a fast twitch dominant one would fade in minutes.

Endurance athletes tend to have a higher proportion of them in the muscles they use, and that proportion is substantially inherited. Training changes the properties within a fibre type far more than it converts one type into another, so the honest framing is that you train what you have rather than rebuilding it.

They grow too, which surprises people. Hypertrophy is often described as a fast twitch phenomenon, and the fast fibres do have the greater capacity, but slow twitch fibres enlarge in response to training as well, particularly with higher rep work taken close to failure.

They also age better than their fast twitch neighbours. Sarcopenia preferentially strips fast twitch area, which is why power fades faster than endurance in later life and why deliberately training for power becomes more important, not less, as the decades pass.

In practice: most of your aerobic training uses these fibres, and hard sets of any rep range end up recruiting everything you have. Our heart rate zone calculator sets the intensities that lean on them.