302
UNIT 2
Covering, Support, and Movement of the Body
9
joints normally prevent bone movements that would stretch at-
tached muscles beyond their optimal range.
Velocity and Duration of Contraction
Muscles vary in how fast they can contract and how long they
can continue to contract before they fatigue. Tese characteris-
tics are influenced by muscle fiber type, load, and recruitment.
Muscle Fiber Type
Tere are several ways of classifying muscle fibers, but learning
about these classes will be easier if you pay attention to just two
functional characteristics:
Speed of contraction.
On the basis of speed (velocity) of
fiber shortening, there are
slow fibers
and
fast fibers
. Te
difference reflects how fast their myosin A±Pases split A±P,
and the pattern of electrical activity of their motor neurons.
Contraction duration also varies with fiber type and depends
on how quickly Ca
2
1
moves from the cytosol into the SR.
Major pathways for forming ATP.
Te cells that rely mostly
on the oxygen-using aerobic pathways for A±P generation
are
oxidative fibers
. Tose that rely more on anaerobic gly-
colysis are
glycolytic fibers
.
Using these two criteria, we can classify skeletal muscle cells as:
slow
oxidative fibers
,
fast oxidative fibers
, or
fast glycolytic fibers
.
Table 9.2
gives details about each group, but a word to the
wise: Do not approach this information by rote memorization—
you’ll just get frustrated. Instead, start with what you know for
any category and see how the characteristics listed support that.
For example, think about a
slow oxidative fiber
(±able 9.2,
first column, and
Figure 9.23
, right side). We can see that it
Contracts
slowly
because its myosin A±Pases are slow (a
criterion)
Depends on
oxygen
delivery and aerobic pathways (its major
pathways for forming A±P give it
high oxidative capacity
—a
criterion)
(muscle insertion). When the contraction ends, the noncontractile
components recoil and help return the muscle to its resting length.
±ime is required to take up slack and stretch the noncon-
tractile components, and while this is happening, the internal
tension is already declining. So, in brief twitch contractions,
the external tension is always less than the internal tension.
However, when a muscle is stimulated rapidly, contractions are
summed, becoming stronger and more vigorous and ultimately
producing tetanus (see Figure 9.15).
During tetanic contractions more time is available to stretch
the noncontractile components, and external tension ap-
proaches the internal tension. So, the more rapidly a muscle is
stimulated, the greater the force it exerts.
Degree of Muscle Stretch
Te optimal operating length for a muscle fiber is the length
at which it can generate maximum force (Figure 9.21 and
Figure 9.22
). Within a sarcomere, the ideal
length-tension rela-
tionship
occurs when a muscle is slightly stretched and the thin
and thick filaments overlap optimally, because this relationship
permits sliding along nearly the entire length of the thin filaments.
If a muscle fiber stretches so much that the filaments do
not overlap, the myosin heads have nothing to attach to and
cannot generate tension. Alternatively, if the sarcomeres are so
compressed and cramped that the Z discs abut the thick myo-
filaments, and the thin filaments touch and interfere with one
another, little or no further shortening can occur.
If you stretch a muscle to various extents and stimulate it te-
tanically, the active tension the muscle can generate varies with
length (Figure 9.22). A severely stretched muscle (say one over
180% of its optimal length) cannot develop tension. Likewise,
at 75% of a muscle’s resting length, its ability to generate force
(or shorten) is limited because the actin myofilaments in its sar-
comeres overlap and the thick filaments run into the Z discs,
restricting further shortening.
In the body, skeletal muscles are maintained near their opti-
mal operating length by the way they are attached to bones. Te
Sarcomeres
greatly
shortened
Sarcomeres at
resting length
Sarcomeres excessively
stretched
170%
Optimal sarcomere
operating length
(80%–120% of
resting length)
0
60
80
100
120
140
160
180
50
100
Tension (percent of maximum)
Percent of resting sarcomere length
100%
75%
Figure 9.22
Length-tension relationships
of sarcomeres in skeletal muscles.
A muscle
generates maximum force when it is between
80 and 120% of its optimal resting length.
Increases and decreases beyond this optimal
range reduce its force and ability to generate
tension.
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