Muscles and Muscle Fiber Types

The limbs of the body are able to move because muscles contract and excert force on the skeleton. Muscles consist of muscle fibers and a single muscle fiber in turn is made up of smaller units called sarcomeres Muscle and muscle fiber structure

There are different types of muscle fibers, which differ in contractile properties. Muscle fibers are divided into type I (slow twitch or red muscle fibers) and type II (fast twitch or white muscle fibers). These types differ in contraction velocity (the speed with which the fiber can contract), contraction force and endurance.

Type I fibers are slow and can not deliver much force, but have greater endurance and can therefore sustain force for longer periods of time, thanks to a well developed blood supply, which also explains the name red fibers. Type II muscle fibers have a greater contraction velocity and can deliver more force, but are more quickly exhausted. Their blood supply is less well developed, hence the name white fiber.

Type II muscle fibers can be further divided into Type IIa and Type IIX. Type IIX are the larger, stronger and faster muscle fibers, with little endurance. Type II fibers are intermediates between Type I and Type IIx fibers, leaning more towards Type IIx in speed, force but having a reasonably developed endurance capacity, although still less than that of Type I fibers. The table underneath outlines the greatest differences between the different fiber types.


Property Type I Type IIa Type IIx
Motor neuron Small Large Large
Nerve conduction velocity Slow Fast Fast
Contraction velocity Slow Fast Fast
Relaxation speed Slow Fast Fast
Fatigue resistance Large Average/low Low
Strength Low Average High
Power Low Average/high High
Endurance Large Average/low Low
Amount of aerobic enzymes Large Average/low Low
Amount of anaerobic enzymes Small Large Large
Capillary density High Average Low
Amount of myoglobin Large Small Small
Mitochondrial density High Average Low
Fiber diameter Small Average Large
Color Red White/red White



-Baechle, T.R., Earle, R.W. (2008). Muscular, Neuromuscular, Cardiovascular, and respiratory systems. Essentials of Strength Training and Conditioning. Third Edition. USA Human Kintetics.