Transcript Surface EMG for Characterization of Muscle

Differential Amplifier
Surface Electromyography for
Noninvasive Characterization
of Muscle
R Merletti, A Rainodi and D Farina
Features of Myoelectric Signal
Typical amplitude 10500 μV
Frequency 10-400 Hz
Mean Frequency 70130 HZ
Median Frequency 50110 Hz
MonoPolar
Needle
Geometrical and Anatomical Factors
Electrode size, shape, and interelectrode distance
Electrode location with respect to the innveration
zone (IZ) and the muscle tendon junction
Thickness of the skin and subcutaneous layers
Misalignment between the muscle fibers and
electrodes
Physiological Factors
Muscle fiber conduction velocity (CV) as a global
average
Statistical distribution of muscle fiber CV (dispersion
or scatter of CV around the average
Number of Motor Units (Mu’s), territory, number of
fibers, fiber size, and histological type of each MU
Blood flow and temperature
Rate of metabolite production, intramuscular pH, ion
concentration, and shifts across the muscle cell
membrane
Type and level of contraction (iso, con, ecc, stim)
Mean and SD of interpulse intervals of the Mu’s
Degree of MU synchronization
Effects of Geometry and Physiology
The EMG signal contains information about many physical
and physiological factors or variables whose contributions
to the signal are not easy to separate:
Ex. 1: A change in signal amplitude from one test to
another may be due to a change of electrode position,
MU’s activated, thickness of subcutaneous tissue,
conduction velocity, electrode alignment with the direction
of muscle fibers, or level of muscle activation.
Ex. 2: The decrement in MNF or MDF during a sustained
contraction may be due to a decrement of mean CV, an
increase in CV dispersion, the dropout of superficial MU’s
(or those with higher CV) or the recruitment of deeper ones
(or those with lower CV), or the widening of the
depolarization zones, a different degree of MU
synchronization.
A Linear Electrode Array
When a single EMG channel is used it is not possible
to separate the contributions of physiological and
geometrical factors
A linear array of equally spaced electrodes makes
available many EMG channels and provides
additional information.
A linear array allows the identification of single MU
action potentials (MUAPs), the location of innervation
and tendon zones, and the estimation of CV of the
individual MUAPs and of their firing patterns
A linear array is also required to learn where a single
electrode pair should be located on the muscle and to
determine the effects of placing it in different
positions
Effects of Electrode Placement on EMG
Consider the two monopolar potentials
propagating in opposite directions, if a
differential detection by placing one
electrode on each side of the IZ the
resulting voltage will be:
Small with respect to the monopolar
Sensitive to differences in shape
between the two waves, which depend
on the distribution of the NMJs within the
IZ
Sensitive to electrode location and to the
spread of CV values among MU’s
Sensitive to relative movements
between the electrodes and the muscle
fibers, which will alter the symmetry of
the detectors relative to the IZ
Effects of Electrode
Placement on EMG
Consider a differential detection by placing
both electrodes on one side of the IZ the
resulting voltage is the difference between to
very similar monopolar time-shifted voltages
and will show:
Smaller or comparable amplitude with respect
to the monopolar potentials, depending on
the interelectrode distance
Small sensitivity to the spatial distribution of
the NMJ within the IZ because the two
monopolar potentials will be equally affected
by it and the effect will cancel the difference
between the signals
Small sensitivity to electrode location and
electrode-muscle relative movement as long
as the electrodes are sufficiently distant from
the IZ and termination zone
High sensitivity to CV and to CV distribution,
which will affect the time duration and the
time delay of the differential signals
Figure 2 Surface EMG from a biceps
brachii muscle detected using a linear array
of equally spaced electrodes made of silver
bars 1 mm diameter, 5 mm long, and 10 mm
apart. (a) Fifteen single differential channels
are detected between adjacent electrodes.
The firings of different Mus and the
propagation and extinction of their action
potentials (MUAPs) at the tendons are
evident. (b) When only two electrodes are
used, most information is lost, and the
features of the signal are strongly affected
by the electrode location with respect to the
IZ. Note the small amplitude and random
components of the signals detected with
electrodes placed symmetrically with
respect to the IZ and the larger signal
detected with both electrodes on one side of
such a zone. The letters labeling the traces
in b refer to the electrode pairs indicated
next to the array.
Fig 2a: a very small signal is
detected by the seventh pair
which is over the IZ
Each firing generates a welldefined propagating signal
that begins at the IZ and
terminates at the muscletendon junction
Fig 2b: Shows how signals
detected with a single pair of
electrodes can change
depending on electrode
location and distance
Pairs placed symmetrically
over the IZ (B, F, G) give
small noisy signals
Pairs that are on one side of
the IZ give larger amplitude
signals
Fig 3 shows changes in the EMG
amplitude detected along an
electrode array
Large variations are observed
near the IZ, indicating that reliable
estimate may be obtained only
with electrodes placed between
the innervation and tendon zones
Similar observations apply to
spectral variables and CV
Because the IZ is not known a
priori and it differs across
individuals, the zone should be
identified in each muscle and each
individual using
 A linear electrode array
 Two adjacent pairs of
electrodes and verifying that
the two signals are sufficiently
similar to justify the
assumption of monodirectional
propagation (r > .7)
Cross Correlation Technique
The Cross Correlation
algorithm can be used to
estimate muscle fiber
conduction velocity.
The Cross Correlation
technique finds the optimal
time lag (+ or -) based upon
the highest correlation
between the two signals.
Electrode Placement
It is evident that improper electrode placement may
lead to total inconsistencies in the detection of signal
amplitude and spectral features
Improper electrode repositioning after training or
treatment could lead to large variations of amplitude
and/or spectral variables that could be attributed to
the effect of training or treatment
Effects of Misalignment
Fig 4 shows a single
MUAP detected with a
linear array and
simulated with three
angles of alignment
between the fiber and
electrode (0, 5, 10 deg)
Depending upon the
location of the
electrodes along the
muscle, amplitude,
MNF, and MDF may be
overestimated or
underestimated.
Proper alignment is
characterized by a
symmetric waveform
pattern of the action
potential propagating in
the two directions
Myoelectric Manifestations of Fatigue
Muscle fatigue could be considered as associated to:
 Change in fiber excitability and MUAP propagation
 Alteration in metabolic conditions
 Failure of E-C coupling
Well known EMG-Fatigue relations:
 Decrease in CV
 Decrease MDF and MNF
 Increase (followed by a decrease) in Amplitude
Direct Myoelectric Fatigue Parameters
Fig 5 shows a system
that estimates the
amplitude and spectral
features from a single
differential signal and
CV from two double
differential signals (DD1
and DD2)
Relationship Between Fiber Type and EMG-Fatigue for
Median Frequency
Fig 6 shows a
strong
relationship
between MDF
and percent of
type I and type II
fibers in the
muscle
Relationship Between Fiber Type and Muscle Fiber
Conduction Velocity
Strong
relationship
between fiber
type and muscle
fiber conduction
velocity
Estimation of Conduction Velocity
Fig 8 depicts a set of
differential signals detected
with an array electrode
The signal shows three
MU’s with two IZ’s, one
between pair 8 and 9 (MU’s
1 & 2) and one under pair 4
(MU 3)
It is clear that under certain
electrode pairs (4, 5, 6, & 7)
signals travel sometimes in
one direction (when Mus 1
and 2 fire) and sometimes
in the opposite direction
(when MU 3 fires)
Note that a global
estimation of CV based on
only two electrode pairs
would be completely
incorrect if propagation is
not always in the same
direction under the
electrode pairs