Muscle
271
stretched rubber band. By a different mechanism, the amount
of
active
tension a muscle fi ber develops during contraction
can also be altered by changing the length of the fi ber. If you
stretch a muscle fi ber to various lengths and tetanically stim-
ulate it at each length, the magnitude of the active tension
will vary with length, as
Figure 9–21
shows. The length at
which the fi ber develops the greatest isometric active tension
is termed the
optimal length, L
0
.
When a muscle fi ber length is 60 percent of L
0
, the fi ber
develops no tension when stimulated. As the length increases
from this point, the isometric tension at each length is
increased up to a maximum at L
0
. Further lengthening leads
to a
drop
in tension. At lengths of 175 percent of L
0
or beyond,
the fi ber develops no tension when stimulated.
When skeletal muscles are relaxed, the lengths of most
bers are near L
0
and thus near the optimal lengths for force
generation. The length of a relaxed fi ber can be altered by the
load on the muscle or the contraction of other muscles that
stretch the relaxed fi bers, but the extent to which the relaxed
length will change is limited by the muscle’s attachments to
bones. It rarely exceeds a 30 percent change from L
0
and is
often much less. Over this range of lengths, the ability to
develop tension never falls below about half of the tension that
can be developed at L
0
(see Figure 9–21).
We can partially explain the relationship between fi ber
length and the fi ber’s capacity to develop active tension dur-
ing contraction in terms of the sliding-fi lament mechanism.
Stretching a relaxed muscle fi ber pulls the thin fi laments past
the thick fi laments, changing the amount of overlap between
them. Stretching a fi ber to 175 percent of L
0
pulls the fi laments
apart to the point where there is no overlap. At this point,
there can be no cross-bridge binding to actin and no devel-
opment of tension. As the fi ber shortens toward L
0
, more and
more fi lament overlap occurs, and the tension developed upon
stimulation increases in proportion to the increased number of
cross-bridges in the overlap region. Filament overlap is greatest
at L
0
, allowing the maximal number of cross-bridges to bind
to the thin fi laments, thereby producing maximal tension.
The tension decline at lengths less than L
0
is the result
of several factors. For example, (1) the overlapping sets of thin
fi laments from opposite ends of the sarcomere may interfere
with the cross-bridges’ ability to bind and exert force, and (2)
at very short lengths, the Z lines collide with the ends of the
relatively rigid thick fi laments, creating an internal resistance
to sarcomere shortening.
0
1
2
3
Relative tension
S
SSSSS
100
200
300
400
500
600
SSSSSSSSSSSSSSSS
700
800
900
1000
SS
Time (ms)
Twitch
Fused tetanus
Unfused tetanus
Figure 9–20
Isometric contractions produced by multiple stimuli (S) at 10 stimuli per second (unfused tetanus) and 100 stimuli per second (fused tetanus),
as compared with a single twitch.
Figure 9–20
physiological
inquiry
If the twitch contraction time is 35 ms and twitch duration is 150 ms, estimate the range of stimulation frequencies (stimuli per second)
over which unfused tetanic contractions will occur.
Answer can be found at end of chapter.
40
60
80
100
120
140
160
Percent of muscle length
100
80
60
40
20
0
Percent of maximum isometric tetanic tension
L
0
Figure 9–21
Variation in active isometric tetanic tension with muscle fi ber length.
Each point is the peak tension measured when the fi ber was held at
the indicated length and a fused, tetanic stimulus was applied. The
blue band represents the range of length changes that can normally
occur in the body.
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