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Muscular contraction involves the two
contractile proteins, actin and myosin, in a mechanical series of events
called the sliding filament theory of contraction. Each myosin filament
is surrounded by six actin filaments. The myosin filaments contain
cross-bridges, which are tiny extensions that reach toward the actin
filaments.
When an impulse from the motor nerve
reaches the muscle cell, it stimulates the entire fiber, creating
chemical changes that allow the actin filaments to join with myosin
cross-bridges. The binding of myosin to actin via cross-bridges results
in a release of energy that causes the cross-bridges to swivel, pulling
or sliding the myosin filament over the actin filament. This sliding
motion causes the muscle to shorten (contract), producing force. Once
the stimulation ceases, the actin and myosin filaments separate,
lengthening the muscle to its resting length and the contraction ceases.
This cross-bridge activity explains why the muscular force generated
depends upon the initial length of the muscle prior to contraction. The
optimal length for muscular contraction is resting length (or slightly
greater), because all of the cross-bridges can connect with the actin
filaments, slowing maximal tension development.
The contractile force decreases when the
length of the muscle prior to contraction is significantly shorter than
the resting length (i.e. already partially contracted). This is due to
the fact that in an already shortened muscle the actin and myosin
filaments overlap to a great degree, which leaves fewer cross-bridges
open to "pull" on the actin filaments. With fewer available
cross-bridges less tension and force can be produced.
When the muscle lengthens too far beyond
resting length the force potential will also be small, because the actin
filaments are too far away from the cross-bridges to the able to join
and shorten the muscle.
Contractile force diminishes as the muscle
length becomes either shorter or longer than resting length. The highest
force output occurs when the contraction begins at a joint angle of
approximately 110-120 degrees (resting length).
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