Advanced Biomechanics of Physical Activity (KIN 831)

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Advanced Biomechanics of Physical Activity (KIN
831)

• Electromyography (EMG)
• Material included in this presentation is derived
  primarily from two sources
• http//www.delsys.com/library/tutorials.ht
  m
• Nigg, B. M. Herzog, W. (1994).
  Biomechanics of the musculo-skeletal system. New
  York Wiley Sons
• Winter, D.A. (1990). Biomechanical and
  motor control of human movement. (2nd ed.). New
  York Wiley
• Sons

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Electromyography (EMG)

• Electro electrical
• Myo muscle
• Graphy record
• --------------------------------------------------
  ------
• Electromyography involves recording the
  electrical activity of muscle
• Electromyogram electrical signal associated
  with the contraction of a muscle

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Selected Historical Events Related to EMG

• Andreas Vesalius, father of modern anatomy,
  appearance and geography of dead muscle, 1555

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Selected Historical Events Related to EMG

• William Croone, in De Ratione Motus Musculorum
  concluded from nerve section experiments that the
  brain must send a signal to the muscles to cause
  contraction, 1664
• Physiologists became excited over the phenomena
  produced by electrical stimulation of muscles,
  ?1740

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Selected Historical Events Related to EMG

• Albrecht von Haller (1708-1777) summarized many
  of the earlier studies in his treatise on
  muscular irritability. 
• Robert Whyatt (1714-1766) reported clinical
  observations on a patient treated by
  electrotherapy.
• Animal electricity was proposed as a substitute
  for the animal spirits which earlier
  experiments believed to be the activating force
  in muscular movement.

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Selected Historical Events Related to EMG

• Luigi Galvani (1737-1798) studied the effects of
  atmospheric electricity upon dissected frog
  muscles. He concluded that the movement of the
  muscle was the result of its exterior negative
  charge uniting with the positive electricity
  which proceeded along the nerve (1786).
  Galvanis Commentary on the Effects of
  Electricity on Muscular Motion (1791 or 1792) is
  probably the earliest statement of the presence
  of electrical potentials in nerve and muscle. He
  showed that electrical stimulation of muscular
  tissues produced contraction and force. He is
  considered the father of experimental neurology.

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Galvanis demonstrations of the effects of
electricity on muscles of frogs and sheep (De
viribus electricitatis in motu musculari
commentarius, 1792)
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Selected Historical Events Related to EMG

• animal electricity became the absorbing
  interest of the physiological world. The
  greatest name among the early students of the
  subject was Emil DuBois-Reymond (1818-1896). He
  laid the foundation of modern electrophysiology.
  He was probably the first to discover and
  describe that contraction and force production of
  skeletal muscle were associated with electrical
  signals originating from the muscles (1849).

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Selected Historical Events Related to EMG

• Guillaume Benjamin Amand Duchenne (1806-1875) set
  out to classify the functions of individual
  muscles through electrical stimulation. He
  recognized the problem of attempting to isolate
  muscle contractions.

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• Duchennes book, Physiology des Movements (1865),
  has been acclaimed one of the greatest books of
  all time. He was probably the first to perform
  systematic investigations of muscular function
  using an electrical stimulation approach.

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Guillaume Benjamin Amand Duchenne de Boulogne
investigating the effect of electrical
stimulation of the left frontalis muscle on one
of his cooperative (prisoner) subjects
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Selected Historical Events Related to EMG

• Wedinski (1880) demonstrated the existence of
  action currents in human muscle. Practical use
  had to await the invention of a sensitive string
  galvanometer (W. Einthoven - 1906).

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Selected Historical Events Related to EMG

• The physiological aspects of EMG were first
  discussed (1910-1912) by H. Piper of Germany
• E.D. Adrian, in an article in Lancet (1925, vol.
  2, pp. 1229-1233) entitled Interpretation of the
  Electromyogram demonstrated for the first time
  that it was possible to determine the amount of
  activity in a human muscle at any stage of
  movement.

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Selected Historical Events Related to EMG

• Toward the end of WWII, with marked improvement
  of electronic apparatus anatomists,
  kinesiologists, and orthopedic surgeons began to
  make increasing use of EMG. The first study that
  gained wide acceptance was that of Inman,
  Saunders, and Abbott who reported their work on
  the movements of the shoulder region in
  Observation on the Function of the Shoulder
  Joint in the Journal of Bone and Joint Surgery
  (1944, vol. 26, pp. 1-30).

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We have come a long way!!!
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Selected Historical Events Related to EMG

• During the 1950s and beyond , EMG for
  kinesiological studies became widespread.

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EMG of normal gait??? Note the use of event
markers in the foot.
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Selected Historical Events Related to EMG

• John Basmajian (1921- ) wrote the bible of
  electromyography entitled Muscles Alive. He and
  Carlo De Luca summarized the existing knowledge
  and research on muscle function as revealed by
  EMG studies.

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Copper screen cage to inhibit noise in the EMG
signal
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• Typical multifactorial gait-recording showing
• Angular accelerometer on the left leg
• Vertical accelerometer
• Horizontal accelerometer
• Strain gauge tensiometer on left gastrocnemius
• EMG of left gastrocnemius

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Electromyography is a seductive muse because it
provides an easy access to physiological
processes that cause the muscle to generate
force, produce movement and accomplish the
countless functions which allow us to interact
with the world around us. The current state of
Surface Electromyography is enigmatic. It
provides many important and useful applications,
but it has many limitations which must be
understood, considered and eventually removed so
that the discipline is more scientifically based
and less reliant on the art of use. To its
detriment, electromyography is too easy to use
and consequently too easy to abuse.C. J. De
Luca, 1993
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Schematic Representation of a Recording an EMG
Signal from a Single Muscle Fiber

• Measure of changes in electrical potential across
  the muscle fiber
• At rest, potential -90mv
• With sufficient stimulation potential inside cell
  rises to 30-40mv
• Change in potential (fiber action potential) can
  be recorded
• Action potentials from multiple fibers in a motor
  unit are simultaneously recorded
• Signal from depolarization of a motor unit is
  called motor unit action potential

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Electrophysiology of Muscle Contraction

1. Motor unit action potential (muap) change in
   electrical potential across the muscle fiber
   membranes when a motor unit is stimulated beyond
   a critical threshold
2. Electrodes placed inside (indwelling) or on the
   surface of a muscle record the algebraic sum of
   all muaps transmitted along muscle fibers that
   reach the electrodes
3. Motor units far away from the electrode have
   their muap attenuated (i.e., are smaller)
4. Motor units of a muscle are controlled by motor
   neurons activating them at their motor end plates

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Electrophysiology of Muscle Contraction

1. End plate potential (EPP) depolarization of
   post synaptic membrane
2. EPP that reach a threshold initiate action
   potential in muscle fiber membrane
3. Depolarization of the transverse tubular system
   and sarcoplasmic reticulum results in a
   depolarization wavealong the direction of the
   muscle fibers
4. EMG records the depolarization and subsequent
   repolarization

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Two Categories of Electrodes

• 1. By placement of electrode
• Surface
• Indwelling (needle)

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Delsys Surface Electrodes
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Delsys Surface Electrodes
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Comparison between Recording Areas of Two Types
of Surface Electrodes
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• Indwelling (needle)

Steps in making a bipolar fine-wire electrode
(Basmajian and Stecko, 1962) ?
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Surface vs. Indwelling Electrodes

• Surface
• Non-invasive
• Detect average activity of superficial muscles
  and give more reproducible results
• Metal (silver/silver chloride) disk or bar
• May be subject to cross-talk (EMG signals from
  motor units of other muscles near by

• Indwelling
• Invasive
• Used to detect EMG signal from small muscles and
  deep muscles
• Fine hypodermic needle with insulated wire
  conductors
• May be subject to cross-talk

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Preparation of Skin for Surface Electrodes

• Reduce electrical impedance of skin
• Shave the area
• Apply rubbing alcohol or abrasives to remove dead
  skin and oils
• Use electrode gel and pressure, adhesive tapes
  and/or elastic bands to affix electrode to skin

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Categories of Electrodes

• 2. By electrode configuration
• Monopolar records difference in voltage
  relative to ground
• Bipolar two contacts to measure electrical
  potential, each relative to a common ground, most
  common electrode type
• Multipolar

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Biphasic Signal
Signal associated with single electrode and ground
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Triphasic Signal
Signal associated with voltage difference when
two electrodes are used at one site
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Factors Affecting EMG Signal

• Propagation velocity of wave front ( 4m/s)
• Fatigue results in decreased propagation velocity
  
• Distance between electrodes
• Depth of muscle fibers being recorded
• Electrode surface area
• Larger surface area ? longer duration of muap
• ?surface electrodes record longer muap than
  indwelling electrodes ( 3-20ms)
• Size of muscle fibers being recorded
• Larger fibers have larger signals

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Preferred electrode location is between motor
point (innervation zone) and the tendonous
insertion.
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Amplitude and frequency spectrum of EMG signal
affected by electrode placement with respect
to A Myotendonous junction B, C Edge of muscle
D
B
C
Preferred location D Midline of belly between
innervation zone and myotendonous junction -
greatest amplitude detected
A
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Factors to Consider in Recording EMG Signals

• EMG signal is summation of muaps
• Goal is to have signals that are undistorted
  (linear amplification) and free of noise
  (biological ECG, other muscles man-made
  power lines, machinery) and artifacts (false
  signals from electrodes and cabling movement
  artifacts from touching electrodes or moving
  cables)
• Large signals ? 5-10 mV small signals ? 100 ?V

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Factors to Consider in Amplifying EMG Signals

• Amplifier gain ratio of output voltage to input
  voltage (gain of 1000 2 mV ? 2 V)
• Linear amplification over entire band width
• Do not overdrive the amplifier system (large
  signals clipped off)
• Full range frequency response for amplifier
  should be fast enough to handle highest EMG
  frequencies
• Amplifier input impedance resistance
• High so as not to attenuate the EMG signal
• Report magnitudes of voltage as they are sensed
  at the electrodes not amplified signal

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Factors to Consider in Amplifying EMG Signals

• Frequency response
• Amplify without attenuation all frequencies
• Frequency spectrum of EMG signals from 5 to 2000
  Hz
• Recommended range for surface electrodes 10 to
  1000 Hz
• Recommended range for indwelling electrodes 20
  to 2000 Hz
• Bandwidth of amplifier difference between upper
  and lower cutoff frequencies
• Possible filtering of signals to avoid unwanted
  noise

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Want frequencies of EMG signals to fall within
range where all frequencies are linearly
influenced by gain
2000 Hz
5 Hz
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• Power density spectrum mathematical conversion
  of EMG signals from time to frequency domain for
  analysis of the frequency content of the signal
• Higher frequency content of indwelling electrodes
  because of closer spacing of electrodes and their
  closer proximity to active muscle fibers
• Most of EMG signal concentrated in band width
  between 20 and 200 Hz
• Problem with power lines because frequency is in
  middle of band width
• Movement artifact (0-10 Hz) can be filtered
  without adversely affecting desired EMG signal

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Factors to Consider in Amplifying EMG Signals

• Common mode rejection
• Human body good conductor acts as antenna to
  electromagnetic radiation
• Want to eliminate extraneous signals
• Unwanted signals picked up simultaneously at two
  locations can be eliminated resulting in
  amplification of only difference in voltage
  associated with EMG signal
• Desired amplified signal A(Vhum emg1) -
  (Vhum emg2) Aemg1 emg2

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Analog to Digital Conversion and Sampling an
Analog Signal
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Analog EMG signal
Digital display of analog EMG signal sampled at 2
kHz
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Sampling a 1 V, 1 Hz sinusoid at 10 Hz
Recreating the sinusoid at 10 Hz
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Sampling a 1 V, 1 Hz sinusoid at 2 Hz
Recreating the sinusoid at 2 Hz
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Sampling a 1 V, 1 Hz sinusoid at 4/3 Hz
Recreating the sinusoid sampled at 4/3 yields a
1/3 Hz signal. The original 1 Hz signal is
undersampled.
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The Nyquist FrequencySignals should be sampled
at no less than twice the original frequency.
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• Fourier decomposition of maup
• Original signal in red
• Superimposed signal in blue is the mathematical
  summation of the 10 sinusoids above
• Exact reconstruction would require an infinite
  number of sinusoids, but 10 provides appropriate
  accuracy

time
Signal is in time domain because it expresses
voltage as a function of time.
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Signal of muap from previous slide is in the
frequency domain because it describes amplitudes
of the frequency contained in it.
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Unprocessed EMG Signals

• Useful for determining
• Onset and turn-off of muscle contraction
• Pattern of contraction of muscles
• Electromechanical delay (EMD)

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Why Process EMG Signals?

• Raw signals resemble noise (stochastic)
• Raw signals fluctuate around 0 voltage (? V over
  time ? 0) ? ? V over time for all EMG records are
  the same no differentiation
• Processed signals may be correlated to parameters
  of muscle contraction being studied (e.g., force,
  fatigue)

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Processing EMG Signals in the Time Domain

• Rectification
• Half wave eliminate negative values only
  positive signals are used
• Full wave absolute value of all signals used
• Preferred because no information is eliminated
• Often used in further processing
• Smoothing
• Filtering signal to eliminate selected
  frequencies
• Low pass filter allows low frequencies to pass
  untenanted, but removes most of the high
  frequencies
• High pass filter allows high frequencies to
  pass untenanted, but removes most of the low
  frequencies
• Window or notch filter

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Some Common EMG Processing
Absolute value of EMG signal?
Full wave rectified and low pass filter?
Area under voltage time curve?
Area under voltage time curve with time reset ?
Area under voltage time curve with time reset ?
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Examples of EMG Signal Processed in the Time
Domain
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Processing EMG Signals in the Time Domain

• Integration
• Integration measures the area under the
  volt-time curve
• IEMG
• Reset at regular intervals of time
• Reset at regular intervals of pre-established
  area (Vsec)

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Processing EMG Signals in the Time Domain

• Root Mean Square
• Frequently used in studying muscular fatigue
• Calculation
• Sum of squared raw data values of EMG signal
• Determine mean of sum
• Take square root of the mean
• RMS

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Processing EMG Signals in the Frequency Domain

• Power density spectra
• Frequency domain important because frequency
  content of EMG signal shown to be reduced with
  fatigue
• Power density spectra of EMG signal obtained
  using Fast Fourier Transformation technique
• Mean and median frequency, bandwidth, and peak
  power frequency examples of use of power density
  spectra

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Example of EMG Signal Processed in the Frequency
Domain
Frequency spectrum of EMG signal detected from
the tibialis anterior muscle during a constant
force isometric contraction at 50 voluntary
maximum.
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Power density spectrum of EMG signal obtained
from Fast Fourier Transformation (FFT)
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Mean and Median Frequencies

• Mean frequency that frequency where the product
  of the frequency value and the amplitude of the
  spectrum is equal to the average of all such
  products throughout the complete spectrum used
  mainly to monitor muscle fatigue
• Median frequency that frequency that divides
  the power density spectrum into two regions
  having the same amount of power preferred for
  detecting muscle fatigue
• Less sensitive to signal noise
• Less sensitive to aliasing
• More often more sensitive to biochemical and
  physiological factors in muscle during sustained
  contractions

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Meaning of EMG Signals

• Logical to assume that EMG signals relate to
  biomechanical variables (e.g., muscle contraction
  force, muscle fatigue)
• Quandary EMG signal is the result of many
  physiological, anatomical, and technical factors

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Meaning of EMG Signals

• 5 cardinal questions
• Is the signal detected and recorded with maximum
  fidelity?
• How should signal be analyzed?
• Where does the detected signal originate? (cross
  talk, electrode placement on muscle)
• Is signal stationary?
• Where does the measured force originate?
  (influence of synergists and antagonists)

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Relationships between EMG Signals and
Biomechanical Variables - Force

• Qualitative relationship not questioned in
  scientific literature quantitative nature hotly
  debated
• Quantitative relationship difficult to show
• Difficulties measuring EMG and force of muscle
  contraction
• Problem with temporal disassociation of muscular
  contraction and EMG signal (EMD)

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Relationships between EMG Signals and
Biomechanical Variables Force

• Isometric contraction
• Can eliminate problems with problems with
  measurement of force of contraction and EMG
• Can eliminate temporal dissociation by sampling
  in middle of steady state contraction
• Despite ability to eliminate or reduce problems
• Different relations between force and EMG seen
• Muscle specific relationships with EMG?
• Force measured indirectly?
• Activity of antagonists or synergists?
• Signal processed differently in each study
• Linear and non-linear relationships found

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Electromechanical Delay (EMD)
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Rat Muscle
Soleus slow twitch, high aerobic, slow
fatiguing Extensor digitorum longus fast
twitch, high glycolytic, fast fatiguing Note
dramatic delay of force time rise under same
stimulation conditions
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Relationships between EMG Signals and
Biomechanical Variables Force

• Dynamic contractions (concentric, eccentric,
  isokinetic)
• Few studies with unrestrained movement
• Because of problems, most studies of isokinetic
  contraction
• Constant angular velocity ? constant velocity of
  muscle shortening
• Constant angular velocity ? constant velocity of
  contractile element shortening
• EMG amplitude associated with negative work
  considerably less than positive work

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Relationships between EMG Signals and
Biomechanical Variables Fatigue

• Fatigue point at which force of contraction
  can not be maintained
• Problems in measuring fatigue
• Which muscle is fatigued?
• Variable recruitment and utilization of motor
  units
• Fatigue both psychological and physiological
  phenomena

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Relationships between EMG Signals and
Biomechanical Variables Fatigue

• Fatigue is associated with a shift in the
  frequency spectrum of the EMG signals to lower
  frequencies
• Lower conduction velocities of some or all action
  potentials
• Slower motor units remain active while faster
  motor units drop out
• Motor units tend to fire more synchronously

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• Diagrammatic explanation of spectral modification
  which occurs in EMG signal during sustained
  contractions
• Muscle fatigue index is represented by the median
  frequency of the spectrum

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Factors
EMG Signal
Inter-pretation
Causative
Intermediate
Deterministic

• Extrinsic
• Electrode
• Configuration
• Motor point
• Muscle edge
• Fiber orientation
• Tendon
• Intrinsic
• Number of active motor units
• Motor unit firing rate (synchronization)
• Fiber type Lactic acid (pH)
• Blood flow
• Fiber diameter
• Electrode Fiber location
• Subcutaneous tissue
• Other factors

• Differential electrode filter
• Detection volume
• Superposition
• Signal crosstalk
• Conduction velocity
• Spatial filtering

• Number of active motor units
• Motor unit twitch force
• Muscle fiber interactions
• Motor unit firing rate
• Number of motor units detected
• MUAP amplitude
• MUAP duration
• MUAP shape
• Recruitment stability

• Amplitude (RMS/ARV)
• Spectral variables (median/mean frequency)

• Muscle fiber (net force/torque)
• Muscle activation (on/off)
• Muscle fatigue
• Muscle biochemistry

Schematic of factors affecting EMG signal
influences and interactions, C.J. De Luca, 1993