Chapter 9
Muscles and Muscle Tissue
303
9
Has few mitochondria, little myoglobin and few capillar-
ies (making it white), and is a much thicker cell (because it
doesn’t depend on continuous oxygen and nutrient diffusion
from the blood)
For these reasons, a fast glycolytic fiber is best suited for short-
term, rapid, intense movements (moving furniture across the
room, for example).
Resists fatigue and has high endurance (typical of fibers that
depend on aerobic metabolism)
Is thin (a large amount of cytoplasm impedes diffusion of O
2
and nutrients from the blood)
Has relatively little power (a thin cell can contain only a lim-
ited number of myofibrils)
Has many mitochondria (actual sites of oxygen use)
Has a rich capillary supply (the better to deliver bloodborne O
2
)
Is red (its color stems from an abundant supply of myoglobin,
muscle’s oxygen-binding pigment that stores O
2
reserves in
the cell and helps O
2
diffuse through the cell)
Add these features together and you have a muscle fiber best
suited to endurance-type activities.
Now think about a
fast glycolytic fiber
(Table 9.2, third col-
umn, and Figure 9.23, le± side). In contrast, it
Contracts
rapidly
due to the activity of fast myosin ATPases
Does not use oxygen
Depends on plentiful
glycogen
reserves for fuel rather than
on blood-delivered nutrients
Tires quickly because glycogen reserves are short-lived and
lactic acid accumulates quickly, making it a fatigable fiber
Has a large diameter, indicating the plentiful myofilaments
that allow it to contract powerfully before it “poops out”
Predominance
of fast glycolytic
(fatigable) fibers
Predominance
of slow oxidative
(fatigue-resistant)
fibers
Small load
Contractile
velocity
Contractile
duration
Figure 9.23
Factors influencing velocity and duration of
skeletal muscle contraction.
Table 9.2
Structural and Functional Characteristics of the Three Types of Skeletal Muscle Fibers
 
SLOW OXIDATIVE FIBERS
FAST OXIDATIVE FIBERS
FAST GLYCOLYTIC FIBERS
Metabolic Characteristics
Speed of contraction
Slow
Fast
Fast
Myosin ATPase activity
Slow
Fast
Fast
Primary pathway for ATP
synthesis
Aerobic
Aerobic (some anaerobic
glycolysis)
Anaerobic glycolysis
Myoglobin content
High
High
Low
Glycogen stores
Low
Intermediate
High
Recruitment order
First
Second
Third
Rate of fatigue
Slow (fatigue-resistant)
Intermediate (moderately
fatigue-resistant)
Fast (fatigable)
Activities Best Suited For
 
Endurance-type activities—e.g.,
running a marathon; maintaining
posture (antigravity muscles)
Sprinting, walking
Short-term intense or powerful
movements, e.g., hitting a
baseball
Structural Characteristics
Color
Red
Red to pink
White (pale)
Fiber diameter
Small
Intermediate
Large
Mitochondria
Many
Many
Few
Capillaries
Many
Many
Few
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