The Muscular System 39repeated hundreds of times in a second to produce
the shortening of the sarcomere associated with
muscle contraction. And the amount of tension
generated by a muscle depends on the average
number of links between actin and myosin at a given
moment.
As soon as activation of the muscle fiber ends,
calcium is rapidly pumped from the vicinity of the
myofilaments. This drop in calcium concentration
restores the blocking of the sites on actin, returning
the filaments to their relaxed, “resting” state.
Muscle Fiber Type
Although all muscle fibers are capable of contract-
ing as just described, there are some important dif-
ferences in their contractile properties and energy
use that are reflected by their classification into two
basic types—slow-twitch (Type I) and fast-twitch
(Type II) (American College of Sports Medicine
[ACSM], 2001). Type II fibers can be further divided
into two major subtypes: Type IIa and Type IIb.
A third subtype has been identified but appears to
occur infrequently, and its characteristics are still
under investigation (Wilmore and Costill, 2004).
Hence, it will not be further addressed in this text.
Type I fibers emphasize energy systems that utilize
oxygen (aerobic metabolism), allowing them to
remain active for prolonged periods. Type I fibers
have a slower contraction time and smaller cross-
sectional area, and produce less tension, but have
a higher resistance to fatigue than Type II fibers.
These fibers are particularly important for carrying
out sustained contractions or repetitive low-intensity
muscle contractions such as those used with pos-
ture, walking, or endurance events (e.g., running
a marathon).
In contrast, Type IIb fibers favor energy systems
that do not depend on oxygen (anaerobic pathways)
and are characterized by having the fastest contrac-
tion time, the largest diameter, and the greatest force
production, but the greatest fatigability of all fiber
types. These fibers are particularly important for
carrying out short-duration, high-intensity muscle
contractions such as used with weight training or
with sprint or power events such as the 100 m dash.
Lastly, Type IIa fiber properties lie between those of
Type I and Type IIb fibers, with generally a slightly
slower contraction time, smaller diameter, and less
force production, but a higher resistance to fatigue
than Type IIb fibers. Type IIb fibers become key when
a limited endurance element is added to activities,
such as with running a mile. Because the character-
istics of Type IIa fibers are in between those of Type
I and Type IIb fibers, some texts list their charac-
teristics as intermediate. However, because many of
these characteristics are still closer to those of Type
IIb than Type I fibers, other texts list many of their
characteristics as the same as those of Type II. This
text combines these approaches to reflect character-
istics more closely aligned to Type IIb (hence their
name) but different in the direction of Type I fibers
as seen in table 2.2.
Although most human muscles contain both
slow-twitch and fast-twitch fibers, different propor-
tions exist in line with functional demands. For
example, the gastrocnemius is used predominantly
for powerful activity such as jumping and has about
50% fast-twitch and 50% slow-twitch fibers in the
average individual. In contrast, in the soleus muscle,
which is used in more sustained activity and serves
key postural functions, the proportion of slow-twitch
fibers may be as great as 85% (Smith, Weiss, and
Lehmkuhl, 1996). The difference in the role ofTABLE 2.2 Skeletal Muscle Fiber Characteristics
Muscle fiber type
Characteristics Type I Type IIa Type IIb
Muscle fiber diameter Small Intermediate/large Large
Color Red (dark) Red White (pale)
Contractile speed Slow Fast Fast
Force production Low Intermediate/high High
Fatigue resistance High Moderate Low
Energy efficiency High Intermediate/low Low
Aerobic capacity High Moderate Low
Anaerobic capacity Low High Highest