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Motor Unit

A motor neuron and all the skeletal muscle fibers it innervates (stimulates).

Muscle Tone

A small number of motor units are involuntarily activated to produce a sustained contraction of their muscle fibers.

Fused (complete) tetany

A sustained contraction with no relaxation observed between twitches. Individual twitches can not be observed.

Unfused (incomplete) tetany

When a skeletal muscle fiber is stimulated at 20-30/second, it can only partially relax between stimuli, resulting in a sustained, but wavering contraction.

Muscle twitch

A brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron.

Motor unit summation

Contraction strength increases by increasing the number of motor units stimulated.

Motor unit recruitment

Increasing the number of motor units contracting at the same time; also increases the amount of force generated.

Wave summation

Increased muscle force due to an increase in the frequency of nerve impulses. (stimuli arriving, one after the other causes larger contractions)

Latent period

Lasts about 2 milliseconds (msec) and is the time between stimulation of muscle cells and force generation.

Contraction period

Lasts about 10 to 100 milliseconds (msec) and is the period during which force is increasing.

Relaxation period

This period, which lasts 10 to 100 milliseconds, (msec) is the period when muscle force is decreasing. Acetylcholine is rapidly broken down by the enzyme acetylcholinesterase. This ends the generation of muscle action potentials and the calcium channels in the sarcoplasmic reticulum membrane close.

Threshold stimulus

The voltage at which the first perceptible contractile response is obtained.


The inability of a muscle to contract forcefully after prolonged activity. Due to depletion of creatine-phosphate, glycogen and ATP; buildup of lactic acid and ADP; failure of nerve implulses to release enough acetylcholine.


Recording of a muscle contraction.


Electromyography (EMG) is a test that checks the health of the muscles and the nerves that control the muscles


An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction.

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