Biomedical Engineering Reference
In-Depth Information
A.4
SKELETAL MUSCLE FIBER TYPES
Skeletal muscle fibers are not alike in structure or function. They vary in color
depending on their content of myoglobin, the oxygen-rich protein. Skeletal muscle
fibers that have a high myoglobin content are termed red muscle fibers. Those that
have a low content of myoglobin are called white muscle fibers. Red muscle fibers
also have more mitochondria and more blood capillaries than white muscle fibers do.
Skeletal muscle fibers contract and relax with different velocities. Whether a
fiber is slow twitch or fast twitch depends on how rapidly it splits ATP. Fast-twitch
fibers split ATP more quickly. In addition, skeletal muscle fibers vary in metabolic
reactions they use to generate ATP and in how quickly they fatigue. Based on these
structural and functional characteristics, skeletal muscle fibers are classified into
three types:
Slow oxidative fibers (type I). These fibers, also called slow-twitch or fatigue-
resistant fibers, contain large amounts of myoglobin, many mitochondria,
and many blood capillaries. They thus look red and have a high capacity
to generate ATP by the aerobic system; this is why they are called oxidative
fibers. They split ATP at a slow rate and as a result have a slow contraction
velocity. These fibers are very resistant to fatigue. Such fibers are found in
large numbers in the postural muscles of the neck.
Fast oxidative fibers (type IIA). These fibers, also called fast-twitch A or
fatigue-resistant fibers, contain large amounts of myoglobin, many mito-
chondria, and many blood capillaries. They are red and have a high capacity
for generating ATP by oxidative processes. Such fibers also split ATP at a
very rapid rate and, as a result, contraction velocity is fast. Fast oxidative
fibers are resistant to fatigue but not quite as much as slow oxidative fibers.
Sprinters tend to have a large proportion of fast oxidative fibers in their leg
muscles.
Fast glycolytic fibers (type II B). These fibers, also called fast-twitch B or
fatigable fibers, have a low myoglobin content, relatively few mitochondria,
and relatively few blood capillaries. They do, however, contain large
amounts of glycogen. Fast glycolytic fibers are white and geared to generate
ATP by anaerobic processes (glycolysis) that are not able to supply skeletal
muscle fibers continuously with sufficient ATP. Accordingly, these fibers
fatigue easily. They are the largest diameter fibers and they split ATP at a
fast rate so that contraction is strong and rapid. Muscles of the arms contain
many of these fibers.
In summary, factors influencing muscle tension in the body are the frequency
of stimulation by motor neurons (wave summation and tetanus), length of fiber just
before contraction (length-tension relation), diameter of fiber (larger diameter, fast
glycolytic fibers contract more forcefully than smaller diameter, slow oxidative and
fast oxidative fibers), and extent of fatigue (availability of nutrients and oxygen
versus buildup of lactic acid).
