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Lecture 12
Electromyography
EXS 587
Dr. Moran
Outline
• Finish Lecture 11 (Muscle Moment – Moment Arm)
• Review of Muscle Contraction Physiology
• Physiological Basis and Concepts of EMG (Alwin Luttmann)
• Methods of EMG Collection
• Electromyograhy in Ergonomics (Shrawan Kumar)
• Limitations & Uses
• Journal of Electromyography and Kinesiology (full-text in ScienceDirect)
Physiological Basis
• Muscle contraction due to a change in the
relative sliding of thread-like molecules or
filaments
• Actin and Myosin
• Filament sliding triggered by electrical
phenomenon (ACTION POTENTIAL, AP)
• The recording of muscle APs is called
electromyography
• The record is known as an electromyogram
What can be learned from an EMG?
• Time course of muscle contraction
• Contraction force
• Coordination of several muscles in a movement
sequence
• These parameters are DERIVED from the amplitude,
frequency, and change of these over time of the EMG signal
• Field of Ergonomics: from the EMG conclusions
about muscle strain and the occurrence of
muscular fatigue can be derived as well
Excitable Membranes
• Cell membrane separates intracellular from
extracellular space
• Diffusion barrier which restricts ION flow
• Cell Membrane Structure
– Double layer of phospholipids (both surfaces covered in proteins)
•
•
Hydrophyllic Head
Hydrophobic Tail
– Role of Proteins
• Transport
– “carrier molecules”
• Receptor
– Transfer information
http://www.longevity.ca/images/cell_membrane3.gif
Fluid Distribution
• Concentration of ions different inside vs.
outside of cell membrane
• This results in an electrical potential difference
known as a MEMBRANE POTENTIAL
• Typical magnitude of membrane potential is -60 and 90 mV (interior of cell is negatively charged)
• This potential can change within fractions of seconds
to +20 to +50 mV
• This rapid change is called an ACTION POTENTIAL
Ion Concentration
• Intracellular Fluid
• High concentration of Potassium cations (K+) and
Protein anions (A-)
• Extracellular Space
• High concentrations of Sodium cations (Na+) and
chloride anions (Cl-)
Ions
[Intracellular]
mmol/l
[Extracellular]
mmol/l
Ratio:
Inside/outside
Na+
12
145
1:12
K+
155
4
40:1
Cl-
4
120
1:30
A-
155
---
---
Uneven distribution the work of active transport that pushes Na+ from inside to outside and K+ from outside to
inside (ION PUMP, requires ATP)
-
+
FCON
FEL
INSIDE
High [K+]
OUTSIDE
CELL
MEMBRANE
(permeable to K+)
Low [K+]
Nernst Equation
• Used to determine resting membrane
potential
RT
» Vm = z F
ln (ci/co)
• Nernst Extension (Goldman 1943):
considered the effect of only K+, Na+, and
Cl• Based on their permeabilities and [ ] values in
preceding slide the resting membrane potential is
-75 mV
Action Potential
• Active response of excitable membranes in
nerve and muscle fibers produced by sodium
and potassium channels opening in response to
a stimulus
• AP abide by the all-or-none principle
• If MP reaches threshold voltage then Na+ channels open at
first (Which direction will Na+ flow?)
• Na+ channels only open for 1 ms, this causes repolarization
(K+ channels also open during this time to speed up return of
resting membrane potential)
Action Potential
(continued)
http://upload.wikimedia.org/wikipedia/en/thumb/7/78/Apshoot.jpg/300px-Apshoot.jpg
Release of Action Potentials
•
AP occur at muscle fibers from two
processes:
(1) AP propagation along muscle fibers
(2) Neuromuscular transmission of excitation at
motor end-plates
•
AP propagation velocity dependent upon:
(1) diameter of fibers (faster for thick – fast twitch)
(2) [K+] in extracellular fluid (KÖssler et lal., 1990)
Motor Unit Action Potential
• Typically, each motorneuron innervates
several hundred muscle fibers (innervation ratio)
• Motor Unit Action Potential (MUAP) =
summed electrical activity of all muscle
fibers activated within the motor unit
• Muscle force increased through higher
recruitment and increased rate coding
Physiological Basis of EMG
“The technique of electromyography is based on the
phenomenon of electromechanical coupling in muscle”
Shrawan Kumar
1.) Train of AP sweep into muscle membrane (sarcolemma)
2.) Travel INTO muscle cells through invaginations (T-tubules)
3.) AP trigger release of Ca2+ ions from sarcoplasmic reticulum into muscle cytoplasm
4.) Ca ions start the cascade of filament sliding
*this is a EXTREMELY brief synopsis of the excitation-contraction coupling (ECC)
Movie on Muscle AP Propagation
Recording Methodology
• Sweep of AP  similar to a wave
• Height of wave and the density of the wave
can be recorded
• Represented graphically  electromyogram
Recording Methodology
(continued)
• Electrical potential difference
measured between two points 
bipolar electrode configuration used
• Bipolar Electrode Types
• Fine Wire
• Needle
• Surface
– Most common, less invasive
– Silver-silver chloride electrodes
• Electrode Placement
• Overlying the muscle of interest in the
direction of predominant fiber direction
• Subject is GROUNDED by placing an
electrode in an inactive region of body
http://www.hhdev.psu.edu/atlab/EMG.jpg
Fine Wire
http://educ.ubc.ca/faculty/sanderson/EMG/Index.htm
Factors Influencing Signal Measured
• Merletti et al. (2001)
• Geometrical & Anatomical Factors
–
–
–
–
–
Electrode size
Electrode shape
Electrode separation distance with respect to muscle tendon junctions
Thickness of skin and subcutaneous fat
Misalignment between electrodes and fiber alignment
• Physiological Factors
–
–
–
–
–
Blood flow and temperature
Type and level of contraction
Muscle fiber conduction velocity
Number of motor units (MU)
Degree of MU synchronization
Factors Influencing Signal Measured
(continued)
• Merletti et al. (2001)
• Conclusions:
» Surface EMG for superficial muscles ONLY
» Muscles with parallel fiber type
» Electrode arrays should be used to determine the
most appropriate single-pair placement
» Need for methodological standardization
EMG Amplitude vs Muscle Contraction Intensity
• Amplitude increases with increased
contraction intensity
• BUT it is not a linear relationship
• Non-linear relationship between EMG
amplitude and contraction intensity
EMG Uses
• Types of questions EMG can answer
(maybe)
:
1.) Whether a muscle is active or not during a movement activity
2.) When the muscle turns ON/OFF during a movement activity
*sometimes categories of activity are used to classify EMG signal, such as: none, slight, less than
slight, more than slight, strong
3.) Phasic relationship between muscles during a movement activity
4.) Does the activation pattern indicate skill aquistion
5.) Does an increased EMG magnitude imply a higher muscular stress?
6.) Is the muscle fatigued?
Analyzing the EMG Signal
• Amplitude & Frequency
• More MU  more spikes and turns in signal
• Change in firing rate  change in frequency
• Major variables:
»
»
»
»
»
peak-to-peak amplitude (p-p)
average rectified amplitude
root-mean-square (RMS) amplitude
linear envelope
integrated EMG
Peak-to-Peak Amplitude
• One of simplest ways to describe EMG
magnitude
• M-wave = synchronous electrical activity
of all muscle fibers following an electrical
stimulus
– Calculated from the negative-peak to positivepeak amplitude
Average Rectified Amplitude
• EMG contains a varying negative, positive
alternative current (AC) signal
• Rectified = all negative values converted to
positive values (absolute value)
Other Variables
• RMS: Does not require rectification
• Linear Envelope: computed by passing a
low-pass filter (3-50 Hz) through the full
wave rectified signal
• Integrated EMG: sums the total activity
over a period of time (area under the
curve)
Normalization
• Def: calibration against a known reference
• This allows researchers ability to compare different
activities for the same muscle, different muscles,
activities on different days, different subjects for same or
different tasks, etc.
• Choices of normalization
– Maximum voluntary contraction (MVC)
» Functional activty
» Isometric activty
– Unresisted normal activity
– Submaximum contraction
• Limitations
– Variability of force generation due to motivation/physiological reasons