268
Chapter 9
bound cross-bridges do exert a force on the thin fi laments,
but they are unable to move it. Rather than the fi laments slid-
ing, the rotation during the power stroke is absorbed within
the structure of the cross-bridge in this circumstance. If iso-
metric contraction is prolonged, cycling cross-bridges repeat-
edly re-bind to the same actin molecule. During a lengthening
contraction, the load pulls the cross-bridges in step 2 back-
ward toward the Z lines while they are still bound to actin and
exerting force. The events of steps 1, 3, and 4 are the same in
all three types of contractions. Thus, the chemical changes in
the contractile proteins during each type of contraction are
the same. The end result (shortening, no length change, or
lengthening) is determined by the magnitude of the load on
the muscle.
Contraction terminology applies to both single fi bers
and whole muscles. In this section, we describe the mechanics
of single-fi ber contractions. Later we will discuss the factors
controlling the mechanics of whole-muscle contraction.
Twitch Contractions
The mechanical response of a muscle fi ber to a single action
potential is known as a
twitch.
Figure 9–16a
shows the main
features of an isometric twitch. Following the action poten-
tial, there is an interval of a few milliseconds, known as the
0
20
40
60
80
100
120
140
Time (ms)
Single action
potential
Tension (mg)
30
20
10
Distance shortened (mm)
3
2
1
0
20
40
60
80
100
120
140
Time (ms)
Single action
potential
(b) Isotonic contraction
Latent period
Latent period
Stimulator
Contraction time
(a)
Isometric contraction
Motor neuron
4
3
2
1
0
mm
4
3
2
1
0
mm
Muscle fiber
Force transducer
Load
Shortening
distance
Figure 9–16
(a) Measurement of tension during a single isometric twitch of a skeletal muscle fi ber. (b) Measurement of shortening during a single isotonic
twitch of a skeletal muscle fi ber.
Figure 9–16
physiological
inquiry
Assuming that the same muscle fi ber is used in these two experiments, estimate the magnitude of the load (in mg) being lifted in the
isotonic experiment.
Answer can be found at end of chapter.
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