If every motor unit contracts fully or not at all, how does a performer vary the force of a contraction?

Force is graded by changing the number and size of motor units recruited. Force also depends on how often the units are stimulated.

What is spatial summation?

Spatial summation is the recruitment of more motor units, and larger ones, to increase force. A stronger contraction involves more fibres working at the same time.

A twitch is a single brief contraction of a motor unit in response to one impulse. A twitch is followed by complete relaxation.

CCEA A-Level Sports Science and the Active Leisure Industry 7210

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The Muscular System

Motor units, summation and the all-or-none law

3 things to know
A2 2

Overview

Motor units and the all-or-none law, spatial and wave summation, twitches and tetanic contraction, the size principle, fine and gross control, asynchronous recruitment and the neuromuscular junction.

What you need to know

Answer each one before you reveal it.

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Key term

What is a motor unit?

Key term

What is the all-or-none law, sometimes called the all-or-nothing law?

Key term

Why does a stronger stimulus not make a single motor unit contract harder?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a motor unit?
Model answer
A motor unit is a single motor neurone and all the muscle fibres it stimulates. All the fibres in one motor unit are of the same type.
2
What is the all-or-none law, sometimes called the all-or-nothing law?
Model answer
A motor unit stimulated at or above its threshold makes all of its fibres contract fully. A stimulus below the threshold produces no contraction at all.
3
Why does a stronger stimulus not make a single motor unit contract harder?
Model answer
A stimulus at threshold already produces a full contraction under the all-or-none law. Extra stimulus strength adds nothing to the force of that unit.
4
If every motor unit contracts fully or not at all, how does a performer vary the force of a contraction?
Model answer
Force is graded by changing the number and size of motor units recruited. Force also depends on how often the units are stimulated.
5
What is spatial summation?
Model answer
Spatial summation is the recruitment of more motor units, and larger ones, to increase force. A stronger contraction involves more fibres working at the same time.
6
What is a twitch?
Model answer
A twitch is a single brief contraction of a motor unit in response to one impulse. A twitch is followed by complete relaxation.

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AThe target heart rate is about 168 bpm.

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Motor units, summation and the all-or-none law

Overview

What you need to know

What is a motor unit?

What is the all-or-none law, sometimes called the all-or-nothing law?

Why does a stronger stimulus not make a single motor unit contract harder?

Questions from this subtopic

What is a motor unit?

What is the all-or-none law, sometimes called the all-or-nothing law?

Why does a stronger stimulus not make a single motor unit contract harder?

If every motor unit contracts fully or not at all, how does a performer vary the force of a contraction?

What is spatial summation?

What is a twitch?

Other ways to work this

What is inside Levo

See a huge jump in your memorisation

Cram a whole paper in one sitting

Blank Blitz consolidates the facts

Build It trains answer structure

Test It catches misconceptions

Brain Breaks between sessions

Every question in The Muscular System, on a schedule.