EMG
The Muscle Physiology of
Electromyography
Jessica Zarndt
Department of Kinesiology
UNLV
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
EMG and Muscle Physiology
I.
Muscle Contraction
•
•
II.
Brief Anatomical Review
Emphasis on the electrical potential
Physiological Explanation of an EMG signal
•
III.
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
Skeletal Muscle Organization
• Series Elastic
Components
– Tendons & Bones
– Fascia, Endomysium,
Perimysium and the
Epimysium
• Excitable Vs NonExcitable
– Muscle tissue IS
– Connective is NOT
Skeletal Muscle Organization
• The Muscle Fiber
(Cell) is excitable
• The Muscle Fiber is
what Contracts
Skeletal Muscle Organization The
Muscle Fiber
The Muscle Fiber at the
electrophysiological level
•
•
•
Resting Potential – the voltage across an
unstimulated cell
Muscle Cell = -90mV
Established by
1. Active Transport of Ions
– The Na+/K+ pump
– 3Na+ out / 2K+ in
2. Potassium Diffusion Potential
– K+ diffuses in; sarcollemma is 100 x more
permeable to K+ than Na+
The Muscle Fiber at the
electrophysiological level
• What does this mean?
– High [Na+] ~140mEq/L
outside the c
EMG and Muscle Physiology
•
How does the muscle
fiber become excited &
contract?
1. Neuro-Stimulation
2. Electrochemical
changes in the muscle
3. Proteins of the muscle
move-the muscle moves
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
Action Potential
of 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
Action Potential
of a Neuron
Action Potential
of a Neuron
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
The Neuromuscular Junction
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
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
Electrochemical Changes in the
Muscle
5) End Plate Potential
-Bidirectional
-Local
-Leads to AP if
large enough
-Usually 50-70mV
Electrochemical Changes in the
Muscle
6) When threshold is met in the End Plate,
an Action Potential will initiate
- Threshold = -55mV
7) Action Potential
EMG’s
Action potential from one skeletal muscle cell
An EMG
• EMG’s allow recording of the action potentials
from an entire muscle (or at least significant
portion of one)
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
EMG’S
2
1
At Rest
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
EMG’S
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
EMG’S
• 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
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
EMG’S
• 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 AP’s do not have to
be in phase
– Some may fire multiple
times…others only once
– The amplitudes of the
AP’s can be different, too
– The position of the
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
The result is a signal that looks a lot like
noise….but is it?
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
Magnitude (dB)
“frequency” domain,
we can identify the
individual waves that
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf
1
2
4
Frequency (Hz)
•
FAST FOURIER
TRANSFORMS
Computational
means of
decomposing nonperiodic signals into
individual
components
• Fortunately, a lot
programs are
This FFT of an EMG shows definite peaks
atavailable
33, 45, 55, 65, 75,
and 94 Hz.
to90,
perform
FFT’s (Matlab, for
This implies that there are large numbers
ofinstance…also
motor units firing at these the
frequencies!
Biopac software!)
http://www.holycross.edu/departments/biology/kprestwi/phys'02/labs/emg_lab/Phys'02_L1_Intro_E-myo&FFT.pdf