emg lecture

8/10/2019 emg lecture

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7th LectureDimitar Stefanov


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RecappingThree types electrodes are used for sensing of EMG signals:

1. indwelling (intramuscular) electrodes (single fiber electrodes, monopolar

electrodes, concentric electrodes)

2. Wire electrodes

3. surface electrodesnon-invasive recordings

Potential of surface electrode (V)

Differential voltage waveform


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Velocity of propagation of the m.a.p.4 m/s

There is a delaybetween the EMG and muscle contraction (30-80 milliseconds).

In case of isometric muscle tension, a linear dependencybetween the muscle tensionand the rectified EMG output is observed.

Important parameters of the EMG amplifiers:1. Gain and dynamic range

2. Input impedance

3. Frequency response4. Common mode rejection.

EMG signal:

contains certain level of noises

has specific spectral densityfunction.

Fatigue

(1) If we assume that the EMG is stimulation rate remains constant then the muscle

tension deceases in case of fatigue.

(2) The shape of the m.a.p. is altered in case of fatigue.(3) tremor occurs.


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Problem with the electrodes: polarization

The electric conductivity of the body involves ions as charge carrier.

Electrodes can be considered as electrical conductors in contact with the aqueous

ionic solutions of the body.The interaction between electrons in the electrodes and ions in the body can affect

the EMG signal

Half -cell potential(HCP) is called thepotential differencebetween the metal of

the electrode and the bulk of the electrolyte.

HCP depends on the ionic concentration

HCP can be measured when no electric current flows between an

electrode and the electrolyte


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Problem with the electrodes: polarization

Polarizationarises in case when current flows between the electrodeand the solution.

Perfectly polarizable electrodesno actual current crosses the

electrode- electrolyte interface

Nonpolarized electrodesallow the current to pass freely in

electrode-electrolyte interface.

Silversilver chloride electrode (Ag/AgCl)it possesses characteristics which

are similar to a perfect nonpolarizable electrode.


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AgCl film

insulated lead wire

Ag metal

Ag lead wiresintered Ag and AgCl

(Ag and AgCl powder mechanically pressed)

greater mechanical stability

Silversilver chloride electrodes

Low noise electrodes


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Equivalent circuit of a biopotential electrode

Ehchalf-cell potential

Rdand Cdrepresent the impedance associated with the electrode-electrolyte

interfaceRsseries resistance.

Biopotential electrode impedance

as a function of frequency


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EMG amplifiers

Amplifier gainthe ratio of the output voltage to the input voltage

Surface EMG electrodes- maximum amplitude of 5 mV peak-to-peak

Indwelling electrodesamplitude of up to 10 mV

Single m.a.p. electrodesamplitude of 100 mV

Amplitudes of the EMG signal :

Noise level of the amplifieris the amplitude of the higher frequency random

signal on the output of the amplifier when the electrodes are shorten together.

Noise level of the amplifier should not exceed 50 mV,

(preferably 20mV).


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Input impedance of an amplifier of biosignals

The resistance of the electrode-skin interface depends on:

thickness of the skin layer,

the cleaning of the skin prior to the attachment of the electrodes,the area of the electrode surface,

temperature.

Electrode pastedecreases the resistance

between the electrode

and the skin.


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Input impedance of an amplifier of biosignals

The capacitancebetween the electrode and the skin causes

frequency distortions.

EMG amplifiers should possess high input resistance


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Frequency response of the EMG amplifier

Frequency bandwidth

All frequencies presentin the EMG should be

amplified at one and the

same level.

Bandwidththe difference between upper cutoff frequencyf2and the lower cutoff

frequencyf1.The gain of the amplifier atf1andf2is 0.707 from the gain of the gain in the mid-

frequency region (half-power).

Amplifier gain:

Example: linear gain 1000, or 60 dB; gain at the cutoff frequencies

57 dB (3dB less than that atthe mid-frequencies).


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The EMG amplifier should amplify equally all EMG frequency components.

Most of the EMG signalsare concentrated in the band

between 20 and 200 Hz.


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Recommended range of the EMG amplifiers:

from 10 Hz to 1000 Hzwhen the signal is collected with surface

electrodes;

from 20 Hz to 2000 Hzwhen the signal is collected withindwelling electrodes.

Interferences:

Hum from power line (60 Hz in the USA and 50 Hz in Europe)-in the middle of the EMG spectrum

Movement artifactstheir frequency lies in the 0 to 10 Hz range

dont cause big problems

Noise from low quality cabling systemsinterfere with thebaseline of the EMG signal; can be eliminated by good low

frequency filtering (by setting of f1 to about 20 Hz).


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Single-ended amplifier

Differential amplifier

A perfect subtraction

never occurs.


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Common mode rejection ratio (CMRR)

CMRR is measured in dB.

In good quality EMG amplifiers CMRR should be 10,000 (80 dB) or higher.


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Processing of EMG

Example:

1. Half of full-wave rectification (absolute value)

2. Linear envelope (low-pass filtering of the rectified signal)

main decision here is the choice of the low pass filter!

3. Integration of the signal from (2)over the period of the musclecontractionarea under the curve

4. Integration of the signal from (2) for a fixed time, reset to zero,

and repeating the integration cyclesuch scheme represents the

trend of the EMG amplitude with time

5. Integration of the signal from (2) to a present level, reset to zero,

and repeating the integration cyclerepresents the level of the

muscle activity (high or low muscle activity).


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Diagram of several common EMG processing systems and the processing results


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Biopotential amplifiers

Basic amplifier requirements:1. The physiological process to be monitored should not be influenced in any way

by the amplifier

2. The measured signal should be not distorted

3. The amplifier should provide the best possible separation of signal and

interferences

4. The amplifier should offer protection of the patient from any hazard and electric

shock

5. The amplifier should be protected against damages due to high input voltages.

The input signal to the amplifier consists of 5 components:

1. Desired biopotential

2. Undesired biopotentials

3. A power line interference signal and its harmonics

4. Interference signals generated by the tissue-electrode interface

5. Noise.


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Block diagram of a biopotential amplifier

FET transistorsGalvanic

decoupling of

the patient

Motion artifactsthe contact between the electrode and the tissue changes

during the relative motions between the electrodes and the tissue.

Measures for decreasing the motion artifacts:

High input resistance of the amplifier

Usage of non-polarized electrodes (Ag/AgCl)

Reduction of the source impedance by usage of electrode gel.

Artifactsdue to electric and magnetic fieldsExample.


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Amplitude/frequency characteristics of the

bioamplifiers used in different applications

S i l i i hi h b il h bi i l lifi


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Special circuits which built the biopotential amplifier

Instrumentation amplifiers

DCinstrumentation amplifiers


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AC instrumentation amplifiers

AC amplifiers eliminate the electrode offset potential, permit high gain and

permits higher CMRR.

The capacitors between the electrodes and the input stage of the amplifier causecharging effects from the input bias current.


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Isolation amplifier

Isolation is realized in the following technologies:Transformer isolation

Opto-isolation.

Isolation provides a complete galvanic separation between the input stage

(patient) and the other part of the measure equipment.


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Surge protection of the bioamplifiers

Protection of the amplifier from damage due to surge input potentials.

Diodes

Zener diodes

Gas-discharge tubes


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Input guarding

Instrumentation

amplifier providing

input guarding

Technique for increase both the input impedance of the amplifier of biopotentials and

the CMRR

Dr iven-r ight-leg circui t

reducing common-mode

interference

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