Muscle
269
Light load
Slope = shortening velocity
Intermediate load
Heavy load
Time (ms)
Single action
potential
Distance shortened (mm)
4
3
2
1
0
20
40
60
80
100
120
140
Figure 9–17
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.
latent period,
before the tension in the muscle fi ber begins
to increase. During this latent period, the processes associ-
ated with excitation-contraction coupling are occurring. The
time interval from the beginning of tension development at
the end of the latent period to the peak tension is the
contrac-
tion time.
Not all skeletal muscle fi bers have the same twitch
contraction time. Some fast fi bers have contraction times as
short as 10 ms, whereas slower fi bers may take 100 ms or lon-
ger. The total duration of a contraction depends in part on
the time that cytosolic calcium remains elevated so that cross-
bridges can continue to cycle. This is closely related to the
Ca
2+
-ATPase activity in the sarcoplasmic reticulum; activity
is greater in fast-twitch fi bers and less in slow-twitch fi bers.
Twitch duration also depends on how long it takes for cross-
bridges to complete their cycle and detach after the removal of
calcium from the cytosol.
Comparing isotonic and isometric twitches in the
same muscle fi ber, you can see from
Figure 9–16b
that the
latent period in an isotonic twitch is longer than that in an
isometric contraction. However, the duration of the mechani-
cal event—shortening—is briefer in an isotonic twitch than
the duration of force generation in an isometric twitch.
The
reason for these differences is most easily explained by refer-
ring to the measuring devices shown in Figure 9–16. In the
isometric experiment, twitch tension begins to rise as soon as
the fi rst cross-bridge attaches, so the latent period is due only
to the excitation-contraction coupling delay. By contrast, in the
isotonic twitch experiment, the latent period includes both the
time for excitation-contraction coupling and the extra time it
takes to accumulate enough attached cross-bridges to lift the
load off of the platform. Similarly, at the end of the twitch, the
isotonic load comes back to rest on the platform well before all
of the cross-bridges have detached in the isometric experiment.
Moreover, the characteristics of an isotonic twitch depend
upon the magnitude of the load being lifted (
Figure 9–17
).
At heavier loads: (1) the latent period is longer, (2) the velocity
of shortening (distance shortened per unit of time) is slower,
(3) the duration of the twitch is shorter, and (4) the distance
shortened is less.
A closer look at the sequence of events in an isotonic
twitch explains this load-dependent behavior. As just explained,
shortening does not begin until enough cross-bridges have
attached and the muscle tension just exceeds the load on the
fi ber. 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
fi ber is increased, eventually a load is reached that the fi ber is
unable to lift, the velocity and distance of shortening decrease
to zero, and the contraction will become completely isometric.
Load-Velocity Relation
It is a common experience that light objects can be moved
faster than heavy objects. The isotonic twitch experiments
illustrated in Figure 9–17 demonstrate that this phenomenon
arises in part at the level of individual muscle fi bers. When the
initial shortening velocity (slope) of a series of isotonic twitches
is plotted as a function of the load on a single fi ber, the result
is a hyperbolic curve (
Figure 9–18
). The shortening velocity
is maximal when there is no load and is zero when the load
is equal to the maximal isometric tension. At loads greater
than the maximal isometric tension, the fi
ber will
lengthen
at
a velocity that increases with load.
The shortening velocity is determined by the rate at which
individual cross-bridges undergo their cyclical activity. Because
Maximum shortening velocity
(zero load)
Maximum isometric tension
(zero velocity)
Load
Isotonic
shortening
Lengthening
contraction
Shortening
velocity
Lengthening
velocity
0
Figure 9–18
Velocity of skeletal muscle fi ber shortening and lengthening as a
function of load. Note that the force on the cross-bridges during
a lengthening contraction is greater than the maximum isometric
tension. The center three points correspond to the rate of shortening
(slope) of the curves in Figure 9–17.
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