EMG

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EMG
The Muscle Physiology of Electromyography
Jessica Zarndt Department of Kinesiology UNLV
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EMG
Electromyography (EMG) the measurement of
electrical activity that brings about muscle
contractions
5. Plowman SA, Smith DL. Exercise Physiology for
Health Fitness and Performance. Benjamin
Cummings, 2003
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EMG and Muscle Physiology

• Muscle Contraction
• Brief Anatomical Review
• Emphasis on the electrical potential
• Physiological Explanation of an EMG signal
• What corresponds to what we see on a signal
• Physiological Factors that can Influence an EMG
  Signal
• How do things like fiber type, size and disease
  affect the EMG

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Skeletal Muscle Organization

• Series Elastic Components
• Tendons Bones
• Fascia, Endomysium, Perimysium and the Epimysium
• Excitable Vs Non-Excitable
• Muscle tissue IS
• Connective is NOT

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Skeletal Muscle Organization

• The Muscle Fiber (Cell) is excitable
• The Muscle Fiber is what Contracts

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Skeletal Muscle Organization The Muscle Fiber
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The Muscle Fiber at the electrophysiological level

• Resting Potential the voltage across an
  unstimulated cell
• Muscle Cell -90mV
• Established by
• Active Transport of Ions
• The Na/K pump
• 3Na out / 2K in
• Potassium Diffusion Potential
• K diffuses in sarcollemma is 100 x more
  permeable to K than Na

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The Muscle Fiber at the electrophysiological level

• What does this mean?
• High Na 140mEq/L outside the c

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EMG and Muscle Physiology

• How does the muscle fiber become excited
  contract?
• Neuro-Stimulation
• Electrochemical changes in the muscle
• Proteins of the muscle move-the muscle moves

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1. Nervous System Signal

• Originates in a Motor Neuron
• Activated by conscious thought or afferent input
  (i.e. reflex)
• Travels through the nervous system to the target
  muscle(s) via, depolarization (action potential)
  and neurotransmitters
• Action Potential - a reversal in relative
  polarity or change in electrical potential of a
  cell
• Neurotransmitters- chemical messengers

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Action Potentialof a Neuron

• Resting Potential -70mv
• Excited to 35mv
• The change in polarity travels down a neuron to
  the next
• Neurotransmitter is released from terminal end

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Action Potentialof a Neuron
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Action Potentialof a Neuron
Pre Synaptic Stimulation
Post Synaptic Stimulation
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The Neuromuscular Junction

• A specific synapse
• Synapse the junction at the terminal end of a
  neuron and another cell
• The Neuromuscular Synapse
• Motor Neuron and Muscle Cell

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The Neuromuscular Junction
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2. Electrochemical Changes in the Muscle

• 1) Ca are released in the terminal end of
  Neuron
• 2) Neurotransmitter is released Acetylcholine
  (Ach)
• 3) Ach travels to receptors on muscle end plate
  (50million per fiber)
• Muscle End Plate area of muscle cell innervated
  by neuron

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Electrochemical Changes in the Muscle

• 4) Na channels open in the muscle cell
• -Na flows into the cell
• -Voltage begins to raise from -90mv
• 5) End Plate Potential- local positive potential
  inside a muscle fiber

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Electrochemical Changes in the Muscle

• 5) End Plate Potential
• -Bidirectional
• -Local
• -Leads to AP if large enough
• -Usually 50-70mV

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Electrochemical Changes in the Muscle

• 6) When threshold is met in the End Plate, an
  Action Potential will initiate
• - Threshold -55mV
• 7) Action Potential

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EMGs
Action potential from one skeletal muscle cell
An EMG

• EMGs allow recording of the action potentials
  from an entire muscle (or at least significant
  portion of one)
• The signal is a compound action potential

http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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EMGS
2
1
At Rest
http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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EMGS
http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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EMGS

• Evoked field potential from a single motor unit
  is actually (usually) triphasic
• Duration is between 3 and 15 msec
• Magnitude is between 20-2000 microvolts,
  depending on the size of the motor unit
• Frequency of discharge varies from 6 30 per
  second

http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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EMGS

• Of course, when we measure an EMG, we are not
  recording the AP from a single motor unit, but
  rather we are recording from multiple
  cells/fibrils that
• Each generate an AP
• The APs do not have to be in phase
• Some may fire multiple timesothers only once
• The amplitudes of the APs can be different, too
• The position of the electrode relative to other
  muscles may also cause interference

The result is a signal that looks a lot like
noise.but is it?
http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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FOURIER ANALYSIS

• We can think of an EMG as the result of the
  superposition of many, many waves that may or may
  not be in phase
• By changing from the time domain to the
  frequency domain, we can identify the
  individual waves that comprise the final signal

Magnitude (dB)
1
2
4
Frequency (Hz)
http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf
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FAST FOURIER TRANSFORMS

• Computational means of decomposing non-periodic
  signals into individual components
• Fortunately, a lot programs are available to
  perform FFTs (Matlab, for instancealso the
  Biopac software!)

This FFT of an EMG shows definite peaks at 33,
45, 55, 65, 75, 90, and 94 Hz. This implies that
there are large numbers of motor units firing at
these frequencies!
http//www.holycross.edu/departments/biology/kpres
twi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myoFF
T.pdf