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The force a muscle generates is dependent on the length of the muscle and the shortening velocity of the muscle.
Differentiate between force-length and force-velocity of muscle contraction
The force-length relationship in muscle indicates that muscles generate the greatest force when at their resting, or ideal length, and the least amount of force when shortened or stretched relative to the resting length.
The force-velocity relationship in muscle demonstrates that power produced is controlled by the velocity and force of muscle contraction, with an optimum power output at one third of maximum velocity.
The force a muscle generates is dependent on the length of the muscle and its shortening velocity. These two fundamental properties of muscles limit many key biomechanical properties including running speed, strength, and jumping distance.
Due to the presence of titin muscles are innately elastic,
skeletal muscles are attached to bones via tendons which maintains the muscle under
a constant level of stretch; termed the resting length. If this attachment was
removed, for example if the biceps was detached from the scapula or radius it
would shorten in length.
Muscles exist in this state to optimize the force produced
during contraction, which is modulated by the interlaced myofilaments of the sarcomere.
When a sarcomere contracts, myosin heads attach to actin to form cross-bridges
followed by the thin filaments sliding over the thick filaments as the heads
pull the actin, and this results in sarcomere shortening, creating the tension
of the muscle contraction. If a sarcomere is stretched too far then there will
be insufficient overlap of the myofilaments and so the force produced will be
reduced. Alternatively, if the muscle is over contracted then the potential for
further contraction is reduced which will similarly reduce the amount of force
the tension generated in skeletal muscle is a function of the magnitude of overlap between actin and myosin myofilaments.
In mammals there is a strong overlap between the optimum and actual resting length of sarcomeres.
The force-velocity relationship in muscle relates the speed
at which a muscle changes length to the force of this contraction and the
resultant power output (force x velocity = power). The force generated by a muscle
depends on the number of actin and myosin cross-bridges formed, a larger number
of cross-bridges results in a larger amount of force. However, cross-bridge
formation is not immediate and so if myofilaments slide over each other at a
faster rate their ability to form cross bridges is, and so force is reduced.
At a maximum velocity no cross-bridges can form, so no force
is generated resulting in the production of zero power (right edge of graph).
The reverse is true for stretching of muscle, although the force of the muscle
is increased there is no velocity of contraction and so zero power is generated
(left edge of graph). Maximum power is in fact generated at approximately
one-third of maximum shortening velocity.