The motor unit
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Module 1: Basic muscle physiology

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Physical Education - The motor unit

The motor unit

Motor units (right click for closer view)

Each fibre of a muscle can contribute to force production only if it is
recruited by the brain. One motor nerve can branch into tens, hundreds or
even a thousand branches, each one terminating on a different muscle fibre.
A single muscle can consist of hundreds of motor units. For example, your
rectus femoris (one of the 4 quadriceps muscles) might contain 1 million
muscle fibres controlled by 1,000 motor nerves. So, on average, each motor
unit contains 1,000 fibres. The fibre type composition of a single motor
unit will always be all one type of fibre. So a single motor unit will
consist entirely of either slow twitch or fast twitch. The composition of
the entire muscle will contain some combination of slow and fast motor
units.

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