Lecture 12 Electromyography

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Lecture 12Electromyography
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• EXS 587
• Dr. Moran

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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)

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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

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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

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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

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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

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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)
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-

FCON
FEL
OUTSIDE Low K
INSIDE High K
CELL MEMBRANE (permeable to K)
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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

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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)

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Action Potential(continued)
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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)

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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

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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
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Recording Methodology

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

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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

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

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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

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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

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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?

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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

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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

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Average Rectified Amplitude

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

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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)

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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