Force-length relationship
Muscle strength as a function of the
initial length of the sarcomere
Muscle
muscle fibers
myofibrils
actin and myosin filaments.
Structure of actin and myosin:
The Sliding Filament Model of Muscle Contraction:
The curve represents what is called the force-length of the active muscle
Force generated based on the sarcomere
length when the muscle fiber is stimulated.
Note the presence of a maximum force
corresponding to the optimum overlap
between the contractile filaments of actin and
myosin. It was at that length that can form the
maximum bridges between actin and myosin.
Below or above, the force that the muscle
fiber can generate decreases or becomes zero
(the right side of the curve).
During the lengthening of the sarcomere the Z
lines move away from each other, the bridges
between actin and myosin are fewer in
number and the force produced for a given
length decreases gradually.
When shortening the sarcomere,
myofilaments overlap more and more, thus
limiting the possibilities of bridging and
thereby decreasing the force the muscle can
develop.
The active length-force relationship is unequivalent for the whole muscle
When the muscle is at rest (unstimulated) and
that is subjected to stretching, it generates a
resistance force or passive tension that rely on
the length of the sarcomere elongation. This
resistance force increases exponentially with
the voltage applied.
The sum of these two curves, active and
passive tension voltage, gives the total voltage
curve that can be produced in the muscle
according to the length to which it is
stimulated.
This curve is a theoretical curve. It can change
depending on the type of muscle we're
dealing with, because the amount of tissue
can vary depending on the proportions of
each type of fiber it contains (fiber type I or
type II).