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In The Name of Allah The Most Beneficent The Most Merciful

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ECE 4550:Biomedical Instrumentation

Lecture:Electro Myo Gram (EMG)

Engr. Ijlal HaiderUniversity of Lahore,

Lahore

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Electrical Activities of Muscles Similar to that of nerve fibers Except that magnitude of potentials and

time duration are different Conduction velocities are less (muscle

fibers are smaller in length, so not a big issue)

Nerve fibers opens in muscles fibers through a junction

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Muscle Potential is generated in almost the same way as a Nerve Potential is generated (l.e. due to change in ionic concentrations)

Visit following link to know more about generation of muscle potential

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

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A wave of excitation along a muscle fiber initiated at the neuromuscular endplate; accompanied by chemical and electrical changes at the surface of the muscle fiber and by activation of the contractile elements of the muscle fiber; detectable electronically (electromyographically); and followed by a transient refractory period.

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Sensory Nerves Motor Nerves

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Voluntary Muscle System (Normal Muscles-under our conscious control)

Automatic Muscle System (Smooth Muscles-not under our conscious control)

Electromyogram Greek words MYOS-Muscle GRAM-Picture Picture of Electrical Activities of Muscles

Voluntary (under willful action of brain) Not good for diagnosis of muscle

disorders which has to be diagnosed early Evoked (on artificial stimulation)

Measurement of Potetial Difference How do we get a potential difference

between two points outside a muscle fiber (or a nerve fiber??)

When Fully Polarized!! Partially Depolarized!! Fully Depolarized!!

When there is partially depolarization, ionic current start flowing which gives rise to voltage

In case of fully polarized or fully depolarized, no current flows and hence we don’t get any voltage out

Voluntary EMG Measurement Using Skin Surface electrodes Using Needle electrodes

Monopolar Bipolar

Skin Surface Electrodes Compound or composite of Muscle Action Potential

from individual muscle fibers is recorded Sometime called Interference Pattern Contribution from muscle fibers will depend on the

closeness and proximity to the electrodes We cannot make out much on the origin of these

signals We can only use it to find gross muscular disorders

Which can already be felt by muscle weakness and can be visually seen as wasted muscle

Skin Surface Electrodes Surface electrodes are not very much used for

the diagnosis of muscle disorders They are used majorly for evoked potential

study in Nerve Conduction Velocity (NCV)

measurement Bio feedback study or exercise (kind of

mitigation or relaxing) Another application is Bio-feedback for stroke

recovery

Needle Electrodes Monopolar Similar to a coaxial We use instrumentation (differential)

amplifiers Requires 3 probes Active, Reference, Common Common is taken from a skin surface

electrode

Needle Electrodes Bipolar In contrast to monopolar electrodes,

bipolar have two electrodes inside and one outside

Instrument amplifiers are used All three probes are taken from the

bipolar needle electrode itself Mostly used for research purpose

Needle EMG Used for diagnosis of muscle disorders Helps in localizing a focus of disorder

As injecting a pin (needle) inside skin is painful and to diagnose properly multiple points are needed, the whole process becomes very painful

To reduce pain, insertion points are reduced and in each points the angle of pin is changed without bringing needle outside the skin (mostly 3 angles)

Analysis of EMG Analysis is done empirically by doctors

(clinical experiences) Looks for EMG patterns when the needle

is being inserted Listens to the sound produced by

feeding the muscle signal into a loud speaker

Also looks at the pattern and listens to the sound on mild voluntary contraction

Analysis of EMG Signal Processing in EMG For automated diagnosis, pattern

recognition techniques are being investigated

Old instruments used to have integrators

Analysis of EMG Simple Block Diagram of EMG

EMG Amplifiers Filter Display Integrator (signal processing unit) Audio amplifier

Measurement of NCV Using evoked potential Through artificial stimulation of nerve For example by giving a voltage of 100

volts for very short time approximately 2 msec, hand movements must be observed

--fig. evoking an action potential using surface electrodes

Nothing happens under anode (+ve electrode)

Reversal of transmembrane potential occurs under cathode (-ve electrode)

This causes generation of an action potential

Generated action potential travels along the nerves

Similar to a sprint race where a stopwatch is pressed on when runner starts and time is recorded untill he reaches the finish line and velocity is calculated from the distance travelled and time, NCV is recoded by measuring the time for nerve action potential to travel a distance “d” from stimulation point to recording point

Sensory NCV Nerve stimulator applies stimulation

through ring electrodes at fingers Median Nerve contains both sensory

and motor nerves Recording site is selected near middle of

arm

Conventions Cathode of the stimulation electrodes is kept

near the recording side, so that action potential is not perturbed by anode)

Recording electrode which is towards the stimulation side is connected to the inverting input of the amplifier

Common electrode is placed ideally at an equidistant point from both electrodes (to have min common mode voltage)

--fig. stimulation pulse --fig. recording side, stimulation artifacts

and compounded action potential Latency of the pulse is recorded SNVC=d/∆t

Motor NCV In contrast to SNCV measurement, MNCV

measurement involves stimulating at two sites and recording at one

For median nerve Stimulation sites

Wrist Elbow

Recording site Thenar Muscle

Why we stimulate on two sites? Neuromuscular junction has unknown delay Record latencies of proximal and distal

stimulation sites individually (let t1 and t2 be the latencies of both respectively)

Distance between both stimulation sites is taken

--fig. MNVC signals MNCV=d/(t2-t1)

Diagnosis and Diseases If either SNCV or MNCV is significantly

less then normal values? Is the distal latency prolonged?

Causes of low NCV Demyelination Conduction block Axonopathy

Disorders Peripheral Neurotherapy Carpel Tunnel Syndrome GB Syndrome

Nerve Stimulator For a single pulse: Monostable Multi-vibrator For repetitive pulses: Astable Multi-vibrator

Amplitude required: 100-200 volts Pulse duration: less then 2msec

Peak current requirement near to 20 mA (max 50 mA)

Power requirement (for peak power 300x50mA)

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Commonly measured Upper limb, Median, Ulnar, Radial, Lower

Limb, Common Peroneal, Tibial

Class Activity

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Thank You!