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.
|myosin ATPase activity||low||high||high|
|rate of fatigue||slow||intermediate||fast|
|muscle fiber diameter||small||intermediate||large|
|innervating neuron size||small||intermediate||large|
|motor unit size||small||intermediate||large|
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 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.
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.
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