GLOSSARY

Type I (slow-twitch) muscle fibers are skeletal muscle fibers that contract slowly and produce relatively little force but are highly resistant to fatigue.
They are also called slow oxidative fibers because they obtain their energy mainly from aerobic metabolism.

Characteristics

These fibers have abundant mitochondria, a dense capillary network, and large amounts of myoglobin, which stores oxygen in the muscle and gives the fibers their red color.
Their diameter is usually smaller, they are supplied by small motor units, and according to the size principle they are the first fibers recruited in any activity.
Their myosin type, myosin heavy chain type I, uses ATP more slowly.

Relevance to Sport

Postural muscles such as the soleus, which are active almost constantly, contain a high percentage of type I fibers.
Elite endurance athletes such as marathon runners and road cyclists usually have a higher percentage of these fibers in their leg muscles; this proportion is largely genetic.
Endurance training increases the aerobic capacity of all fiber types and can shift some type IIx fibers toward more oxidative characteristics.

Key Teaching Point

Most human muscles contain a mixture of slow and fast fibers, and their proportion varies between individuals and even between different muscles of the same person; training design should therefore match the demands of the sport and the characteristics of the individual.