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Mechanical
properties. Length-tension relationship
The
force developed by a muscle, whether it is contracting or
resting, is strongly dependent on the length of the muscle.
Resting skeletal muscle does not exert any force at lengths
less thanthe normal length of the resting muscle in the
body (l o ). When resting skeletal muscle is extended somewhat
beyond l o , however, a passive force begins to assert itself.
The exact length at which this passive force occurs depends
on the particular muscle. This force is characterized as
passive because it is developed in noncontracting or inactive
muscles by the elastic elements of the muscle.
Skeletal
muscles do not develop any force when they are stimulated
at lengths of less than about 50 percent l o . The amount
of force developed, however, increases during isometric
contractions at lengths from 50 to 100 percent of l o .
Once beyond l o , the passive force becomes a factor: the
total force has two components, a passive one, resulting
from elasticity, and an active one, resulting from contraction.
For isometric contractions the active force developed during
contraction decreases beyond l o ; at about 150 percent
l o , the muscle fails to develop any active force.
The
structural basis for the dependence of the magnitude of
active force on the length of skeletal muscle has been established
in experiments with single fibres rather than whole muscles.
The active force has been correlated with the relative position
of thick and thin filaments during contraction at each sarcomere
length. The tension increases as the sarcomere length shortens
from 3.65 to 2.25 ? because more sites for actin-myosin
cross-bridge interactionbecome available as the overlap
of the thick and thin filaments increases. The force is
constant from sarcomere length 2.25 to 2.00 ? as the thin
filaments move over the bare region devoid of cross bridges
in the centre of the thick filaments. As the sarcomeres
shorten further, from 2.00 to 1.67 ?, the thin filaments
overlap each other, preventing effective interaction with
myosin in the thick filaments and consequently diminishing
the force. Finally, from sarcomere length 1.67 to 1.27 ?
the thick filaments run up against the Z lines, and internal
resistance causes an even greater diminution of the force.
Load–velocity
relation
When a muscle is to lift a constant load (isotonic conditions)
after stimulation starts, the force increases, just as in
an isometric contraction, and, when the force is equal to
the load, the muscle begins to shorten and lifts the load.
When the activity of both the muscle and the force in it
begin to decline, the load stretches the muscle back to
its initial length. The tension in the muscle is equal to
the load during the shortening and the lengthening of the
muscle, except during brief periods of acceleration as the
muscle begins to move. Only after the muscle has returned
to its initial length does the tension begin to diminish.
The size of the load also determines the velocity of shortening,
and this relationship between load and velocity also applies
to cardiac and smooth musclein which V is the velocity of
shortening;
P o is the maximum force developed under isometric conditions;
P is the force developed at a particular muscle length;
and b and a are physical constants. A graphic interpretation
of these values results in a rectilinear hyperbola called
the force–velocity curve (Figure 8). From this curve can
be determined both the velocity of shortening when the load
is known and the force developed to overcome the load when
the velocity is known.
