Muscle Fiber Types

Skeletal muscle fibers have metabolic and mechanical differences that allow them to be classified into different types. A motor unit, which consists of a somatic efferent neuron and all the muscle fibers it innervates, is comprised of muscle fibers that are all of the same type. Thus the classification for muscle fibers also applies for motor units.

Some of the different characteristics of muscle fiber (and motor unit) types are listed in the table. These are grouped in colors according to four basic features about muscle fibers.

	Slow-oxidative	Fast-oxidative	Fast-glycolytic
mitochondria	many	many	few
capillaries	many	many	few
myoglobin content	high	high	low
myosin ATPase activity	low	high	high
contraction velocity	slow	fast	fast
rate of fatigue	slow	intermediate	fast
muscle fiber diameter	small	intermediate	large
innervating neuron size	small	intermediate	large
motor unit size	small	intermediate	large

The principal mode of ATP synthesis

Oxidative fibers primarily use oxidative phosphorylation to generate ATP. As a consequence, they are richly endowed with mitochondria, and surrounded by capillaries that deliver oxygen. Furthermore, oxidative fibers contain a lot of myoglobin, an oxygen-binding protein that can store oxygen and speed its delivery to mitochondria within the muscle cell. Myoglobin imparts a red color to the cell, hence the terms “red muscle” for oxidative fibers and “white muscle” for glycolytic fibers that have low levels of myoglobin.

The type of myosin expressed

The different muscle fibers express different myosins with different levels of myosin ATPase activity. The rate at which myosin can hydrolyze ATP will determine how rapidly cross-bridge cycling can occur, and thus determines the contraction velocity of the fiber.

The rate of fatigue

Fast glycolytic fibers fatigue rapidly, while slow oxidative fibers are highly resistant to fatigue. Muscles that need to be active continuously, such as weight-supporting postural muscles, contain a higher percentage of fatigue-resistant slow oxidative muscle fibers.

Size

Slow oxidative muscle fibers have the smallest diameter, fast oxidative fibers are intermediate in size, and fast glycolytic fibers are the largest. Consequently, the fast glycolytic fibers produce the most force, since they contain the most myofibrils. Fast glycolytic motor units also produce more force because they tend to have more muscle fibers in each motor unit.

Another effect of size relates to recruitment. Recruitment refers to the process of increasing activation of motor units to increase the force produced by a whole muscle. The slow oxidative motor units are innervated by somatic efferent neurons with the smallest cell bodies. It turns out that the smallest neurons are the easiest to excite. So slow oxidative motor units are recruited first, for low-intensity activity such as standing. As activity intensifies and excitatory drive increases, larger and larger somatic efferent neurons will be excited and so fast oxidative, and finally fast glycolytic motor units will be recruited. This orderly recruitment of motor units according to size of somatic efferent neurons is known as the size principle.

Quick Quiz

	Fill in Answer	Correct	False	Correct Answer
1. Name the oxygen binding protein found in high quantities in oxidative muscle fibers.
2. Fast glycolytic fibers have a fast contraction velocity due to high activity of _________.
3. Fast glycolytic fibers produce more tension in a twitch because they have more _________.
4. What type of muscle fiber is innervated by the smallest somatic efferent neurons?
5. Slow oxidative fibers are resistant to _______.
6. Through recruitment, an increase in muscle force is achieved by increased activation of _________.
	(Spelling must be correct)