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Lecture 12 Electromyography

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Title: Lecture 12 Electromyography


1
Lecture 12Electromyography
http//www.delsys.com/images/library/teresa.jpg
  • EXS 587
  • Dr. Moran

2
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)

3
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

4
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

5
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
6
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

7
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-)

Uneven distribution the work of active transport
that pushes Na from inside to outside and K
from outside to inside (ION PUMP, requires ATP)
8
-

FCON
FEL
OUTSIDE Low K
INSIDE High K
CELL MEMBRANE (permeable to K)
9
Nernst Equation
  • Used to determine resting membrane potential
  • Vm

R T
ln (ci/co)
z F
  • 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

10
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)

11
Action Potential(continued)
http//upload.wikimedia.org/wikipedia/en/thumb/7/7
8/Apshoot.jpg/300px-Apshoot.jpg
12
Release of Action Potentials
  • AP occur at muscle fibers from two processes
  • AP propagation along muscle fibers
  • 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)

13
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

14
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
15
Recording Methodology
  • Sweep of AP ? similar to a wave
  • Height of wave and the density of the wave can be
    recorded
  • Represented graphically ? electromyogram

16
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
17
Fine Wire
http//educ.ubc.ca/faculty/sanderson/EMG/Index.htm
18
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

19
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

20
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

21
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?

22
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

23
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
    positive-peak amplitude

24
Average Rectified Amplitude
  • EMG contains a varying negative, positive
    alternative current (AC) signal
  • Rectified all negative values converted to
    positive values (absolute value)

25
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)

26
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
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