Question

Explain the Length-tension property of a muscle contraction.

Answer 1

Tension or force developed in the muscle during resting
condition and during contraction varies with the length
of the muscle.
Tension developed in the muscle during resting
condition is known as passive tension. Tension
developed in the muscle during isometric contraction is
called total tension.

Active Tension -
Difference between the passive tension and total
tension at a particular length of the muscle is called
active tension. Active tension is considered as the real
tension that is generated in the muscle during contractile
process. It can be determined by the length-tension
curve.

Length-Tension Curve -
Length-tension curve is the curve that determines
the relationship between length of muscle fibers and
the tension developed by the muscle. It is also called
length-force curve. The curve is obtained by using frog
gastrocnemius-sciatic preparation. Muscle is attached
to micrometer on one end and to a force transducer on
other end. Muscle is not allowed to shorten because of
its attachment on both the ends .
A micrometer is used to set length of the muscle
fibers. Force transducer is connected to a polygraph.
Polygraph is used to measure the tension developed by
the muscle during isometric contraction.

To begin with, the minimum length of the muscle
is set by using the micrometer. The passive tension is
determined by using force transducer. Then the muscle
is stimulated and total tension is determined. From these
two values the active tension is calculated. Then the
length of muscle is increased gradually. At every length,
both passive tension and total tension are determined
followed by calculation of active tension. All the values
of active tension at different lengths are plotted to obtain
the length-tension curve . From the curve the
resting length is determined.

Resting Length-
Resting length is the length of the muscle at which the
active tension is maximum. Active tension is proportional
to the length of the muscle up to resting length. Beyond
resting length, the active tension decreases.

Tension Vs Overlap of Myofilaments -
Length-tension relationship is explained on the basis of
sliding of actin filaments over the myosin filaments during
muscular contraction. The active tension is proportional
to overlap between actin and myosin filaments in the
sarcomere and the number of cross bridges formed
between actin and myosin filaments. When the length
of the muscle is less than the resting length, there is
increase in the overlap between the actin and myosin
filaments and the number of cross bridges. The active
tension gradually increases up to the resting length.
During stretching of the muscle beyond resting length,
there is reduction in the overlap between the actin and
myosin filaments and the number of cross bridges.
And the active tension starts declining beyond resting
length.