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The force a muscle generates is dependent on the length of the muscle and its shortening velocity. These two fundamental properties 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 that maintain the muscle under
a constant level of stretch called the resting length. If this attachment was
removed, for example if the bicep was detached from the scapula or radius, the muscle 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. Then, the thin filaments slide over the thick filaments as the heads
pull the actin. This results in sarcomere shortening, creating the tension
of the muscle contraction. If a sarcomere is stretched too far, there will
be insufficient overlap of the myofilaments and the less force will be produced. If the muscle is over-contracted, the potential for
further contraction is reduced, which in turn reduces the amount of force
Simply put, 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 with 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, so if myofilaments slide over each other at a
faster rate the ability to form cross bridges and resultant force are both reduced.
At 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 zero power is generated
(left edge of graph). Maximum power is generated at approximately
one-third of maximum shortening velocity.