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Electromyography:

Physiology

D. Gordon E. Robertson, Ph.D.

Biomechanics Laboratory,School of Human Kinetics,

University of Ottawa, Ottawa, CANADA

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Nervous System

Central Nervous System (cerebellum, cerebrum, brainstem, spinal cord)conscious control, motor programs

Peripheral Nervous System (afferent and efferent motor

nerves, various sensory organs)reflex control, sensoryfeedback

Somatic nervous system skeletal muscles

Muscles can be excited (contracted) by either system

E.g., messages can travel from motor cortex directly toa-motoneurons via pyramidal nerve cells in spine, Or a stretched muscle (tendon tap) can send a message

via Ia-afferent nerves attached to muscle spindlesdirectly to a-motoneurons to cause a reflex contraction

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Reflexes (Examples)

Stretch reflexes(AKA monosynaptic,myotatic) are fastest

skeletal reflex sincethere are nointerneurons. Theycause a stretched

muscle to contract.

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Reflexes (Examples)

Flexion reflexes have an interneuron and act tocause flexor muscles to contract after a painfulstimulus

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Reflexes (Examples)

Crossed-extensor reflexes occur to antagonists causingrelaxation when a flexor is contracting.

Tonic neck reflexes. Flexion of neck facilitates flexor

muscles of extremities; neck extension does vice versa.

Long-loop reflex (AKA functional stretch reflex,transcortical reflex). Acts like the stretch reflex buttakes longer and is trainable. Is part of the reason that

pre-stretching a muscle (via a counter-movement)creates a stronger contraction.

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Muscle Spindles vs.

Golgi Tendon Organs Muscle spindles (Ia-afferent & g-efferent nerves)

sense muscle length and velocity changes

have excitable muscle fibers called intrafusal fibers that allow

spindle excitability to be modulated

in parallel with muscle fibers

act primarily to excite muscles to contract

Golgi tendon organs (Ib-afferent nerves)

located in tendons and thus are able to sense force changes

in series with muscle fibers

act primarily to prevent tearing of muscle by inhibitingcontractions

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Motor Unit

One a-motoneuron plus all the muscle fibers it enervates Innervation ratio varies with number of fibers per

motor unit (large leg muscles have many fibers per

motoneuron for stronger responses, facial and eyemuscles have few fibers and therefore permit finermovements but weaker contractions)

All-or-none ruleonce a motoneuron fires all its musclefibers must fire

Graded muscle responses occur because oforderlyrecruitment of motor units, i.e., lowest threshold motorunits fire first followed by next lowest threshold. Highestthreshold and strongest motor units fire last.

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Motor Unit Action Potential

When an action potential reaches the muscle atlocalized motor points (AKA innervation points)sarcoplasmic reticulum and t-tubule system carries the

message to all parts of the muscle fiber A rapid electrochemical wave ofdepolarization travels

from the motor point causing the muscle to contract

Followed by a slower wave of repolarization and a brief

refractory period when it cannot contract The wave of depolarization can be sensed by an

electrode and is called the electromyogram (EMG). Therepolarization wave is too small to detect.

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Electrodes

An electrode detects the wave of depolarization as it passesbelow.

As the wave approaches, the voltage increases; as it passes

underneath the voltage goes to zero; finally as it departsthe voltage reverses polarity and gradually declines.

This yields a biphasic signal.

The biphasic signals are so small that other electrical

signals from the environment (called interference) maskthem.

Solution is to use a differential amplifier.

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Differential Amplifier

Subtracts one signal from another.

By placing two electrodes in series over the muscle,the wave of depolarization passes under each

electrode one after the other (i.e., with a slight delay).

Subtracting the two electrode signals from each othermakes any common signal disappear and identicalbiphasic signals that arrive at different times become

a triphasic signal. This is what we call the electromyogram (EMG).

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Electromyogram

Each motor units wave of depolarization may bedetected by the electrode pair and will haveapproximately the same shape if the electrode stays at

the same place with respect to the muscle. Thus, it ispossible to detect the recruitment ofsingle motor units.

In most contractions, however, there are many motorunits some large, some small, some close and some farfrom the site so it is usual impossible to tell how many

are firing and which fibers are firing (large vs. small).

But, EMGs may be used to roughly estimate the level ofrecruitment and the timing of muscle contractions.

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Electromyogram contd

Except in very special situations it is NOT possible to useEMGs to estimate the level of FORCE in a muscle.

It is also NOT suitable to compare the magnitude of one

muscles EMG compared to a different muscle even inthe same person.

The magnitude of EMG depends of many factorsunrelated to the force and therefore is a relative measure

for each muscle. Thus, EMGs are often normalized to specific values such

as the muscles maximal voluntary contraction(MVC) orto some standard load.