Figure 1120

(a) An isometric twitch of a skeletal-muscle fiber following a single action potential. (b) An isotonic twitch of a skeletal-muscle fiber following a single action potential. %

contraction time depends on the time that cytosolic calcium remains elevated so that cross bridges can continue to cycle. It is most closely related to the Ca-ATPase activity in the sarcoplasmic reticulum; activity is greater in fast-twitch fibers and less in slow-twitch fibers.

Comparing isotonic and isometric twitches in the same muscle fiber, one can see from Figure 11-20b that the latent period in an isotonic twitch is longer than that in an isometric contraction, while the duration of the mechanical event—shortening—is briefer in an isotonic twitch than the duration of force generation in an isometric twitch.

Moreover, the characteristics of an isotonic twitch depend upon the magnitude of the load being lifted (Figure 11-21): (1) at heavier loads, the latent period is longer, and (2) the velocity of shortening (distance shortened per unit of time), the duration of the twitch, and the distance shortened are all slower or shorter.

Light load Intermediate load Heavy load

100 120

Single action 1 potential

FIGURE 11-21

Isotonic twitches with different loads. The distance shortened, velocity of shortening, and duration of shortening all decrease with increased load, whereas the time from stimulation to the beginning of shortening increases with increasing load.

Light load Intermediate load Heavy load

100 120

Single action 1 potential

FIGURE 11-21

Isotonic twitches with different loads. The distance shortened, velocity of shortening, and duration of shortening all decrease with increased load, whereas the time from stimulation to the beginning of shortening increases with increasing load.

Let us look more closely at the sequence of events in an isotonic twitch. Following excitation, the cross bridges begin to develop force, but shortening does not begin until the muscle tension just exceeds the load on the fiber. Thus, before shortening, there is a period of isometric contraction during which the tension increases. The heavier the load, the longer it takes for the tension to increase to the value of the load, when shortening will begin. If the load on a fiber is increased, eventually a load is reached that the muscle is unable to lift, the velocity and distance of shortening will be zero, and the contraction will become completely isometric.

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Responses

  • Brigitte
    Why does shortening velocity decrease with heavy load?
    8 years ago
  • Anne
    Why does the shortening distance decrease as the load increases?
    8 years ago
  • Tesfalem
    Why does the latent period become longer as a load gets heavier?
    7 years ago

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