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Every single motor nerve entering a muscle
can stimulate anywhere from one to several thousand muscle fibers. All
of the muscle fibers activated by an individual motor nerve react
together to its impulses, contracting and relaxing in unison. A single
motor nerve together with the muscle fibers it activates creates a motor
unit.
When a motor nerve is
stimulated, the impulse sent to the muscle fibers within the motor unit
either spreads completely or does not spread at all. This is the
all-or-none law, which means that a weak impulse creates the same
tension within the motor unit as a strong impulse.
This all-or-none law does not apply to
the muscle as a whole. While all muscle fibers within a single motor
unit respond to the stimulation of the motor nerve, not all motor units
are activated during a muscular contraction. The number of motor units
involved in a contraction depends on the load imposed upon the muscle,
and this has a direct correlation to the force produced. For example, if
the load is light, only a small number of motor units will be recruited
and the strength of contraction will be low. On the other hand, if the
load imposed upon the muscle is extremely heavy, then all or almost all
of the motor units will be recruited resulting in a maximal force
output. Since a muscle's motor units are recruited in sequential order,
the only way to train the entire muscle is to expose it to maximum
loads, so every motor unit is used for muscle contraction.
While the force exerted by a muscle
depends upon the number of motor units recruited during a contraction,
it also depends upon the number of muscle fibers within a motor unit.
The number of fibers can vary between 20-500, with the average number
being around 200. The more fibers in a motor unit, the higher the force
output. The genetic factor that determines the number of fibers helps to
explain why some people can increase muscle size and strength quite
easily, while others have to fight for every small gain.
When a nerve impulse stimulates a motor
unit, it responds by giving a twitch or a very quick contraction,
followed by relaxation. If another impulses reaches the motor unit
before it has time to relax, the two twitches summate (join forces) and
produce greater tension than that produced by a single twitch.
The summation of motor units depends
upon the load imposed on the muscle. During maximum loads, all of the
muscle's fibers summate in synchronization leading to maximum force
output; during medium loads, some motor units are twitching while others
are relaxing, leading to medium force output. This is one of the main
reasons why heavy loads lead to higher gains in maximum strength.
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