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Instrumentation

Dynamics

MotionTracking

Rigid

Link

ModelsBone on

Bone Forces

Joint

Modeling

Optimiza-

tion

Single

muscle

equivalent

EMG

Driven

Muscle

Mechanics

EMG

Tissue

Mechanics

Injury

Mechanics

Viscoelas-

tic tissue

models

Elastic

tissue

models

Material

properties

Geometricproperties

Work,

Energy,

Power

Module D

Electromyography(Collection, signal, processing/analysis

and interpretation)

Chapter 8

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Module D Objectives

Understand what EMG can (or cant) tell us

Describe the characteristics / specifications

of an EMG system

Learn how to use EMG (Lab #3)

Electrode placement

Detecting and avoiding signal contamination/

error

Learn how to process and interpret EMGsignals

Why use EMG? Detect phasic activity

Is the muscle on or off?

State of activation (effort)

Prediction of muscle force

Biofeedback (Clinical/Training)

Health diagnoses (Neuropathies/

Myopathies) Fatigue

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YouTube Video

(Rehab Institute of Chicago)

http://www.youtube.com/watch?

v=T6R5bm6qx2E

Source: www.thalmic.com

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Biological Signal Source

Motor Cortex

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Motor Unit

(Silverthorn, 2004)

Neuromuscular Junction

(Silverthorn, 2004)

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Release of Acetylcholine

(Silverthorn, 2004)

Ion Exchange

(Silverthorn, 2004)

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Action Potential

(Silverthorn, 2004)

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Release of Calcium

(Silverthorn, 2004)

The Muscle Twitch

Winter (2005)

The smallest increment of muscle tension

Force produced by all of the muscle fibres in

a motor unit

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Instrumentation

Dynamics

MotionTracking

Rigid

Link

ModelsBone on

Bone Forces

Joint

Modeling

Optimiza-

tion

Single

muscle

equivalent

EMG

Driven

Muscle

Mechanics

EMG

Tissue

Mechanics

Injury

Mechanics

Viscoelas-

tic tissue

models

Elastic

tissue

models

Material

properties

Geometricproperties

Work,

Energy,

Power

Why use EMG? Detect phasic activity

Is the muscle on or off?

State of activation (effort)

Prediction of muscle force

Biofeedback (Clinical/Training)

Health diagnoses (Neuropathies/

Myopathies) Fatigue

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Recording EMG signals

We must consider!

Electrode type

Electrode placement

Electrode geometry

Electrode arrangement

Common mode rejection

Amplifier gain

Input impedance

Frequency response

Recording EMG System

and Setup Characteristics

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Electrode Type - IndwellingNeedle electrode Fine wire

Electrode Type - Surface

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Electrode placement -

Location Where would the best location for EMGelectrodes be?

Near the tendon?

Over the motor point?

Somewhere in between?

Electrode placement -

Orientation What is the best orientation for electrodes

on the muscle?

along length of muscle fibre?

across many muscle fibres?

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Inter-electrode distance

Small distance = low

amplitude

Typical distances are

1 2 cm.

May alter magnitude

BE CONSISTENT

Fuglevand et al. (1992), Biol Cybern 67:143-1

Electrode Geometry

(size & shape) Determines pickup volume

Typically 1-2 cm. in diameter

Too large = potential for crosstalk

Picking up the MUAPs from a muscle you do

not intend to measure.

Too small = reduced pickup volume

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Cross-talk

To identify cross-talk:

Stimulate a muscle that is

not the target

To minimize cross-talk

Use smaller electrodes

Use a double differential

electrode.

Target muscle

Electrode arrangement

Monopolar = 1electrode + reference

Bipolar = 2 electrodes

+ reference

Double differential = 3electrodes +

reference

Muscle

Neutral site

Out

Muscle

Neutral site

OutMuscle

Muscle Out

Muscle

Muscle

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Common Mode Rejection

The amplifier attenuates (removes) the

portion of the signal that is common

between the two electrodes

A+B

Neutralsite

Out = (A+B) (C+B) = A-CC+B

Differential amplifier

Common Mode Rejection

Ratio

The extent to which common signals are attenuated

(removed)

Not all noise is common

E

cm= signals common to both electrodes

E

e= error voltage

The common signal that remains in the output

We want at least 80dB (Ecm

:Ee=10,000:1)

!!

"

#$$

%

&=

e

cm

E

ECMRR 10log20

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Winter (2005), Fig. 9.11, pp. 244

With hum

Without

hum

Amplifier Gain Maximum EMG signal amplitude

Surface EMG: 5mV peak-to-peak

Indwelling EMG: 10mV peak-to-peak

Amplitude of a single MUAP: 10V

Gain = output voltage / input voltage

Question: if the input voltage is 2.5mV

and the gain is 1000, what is the outputvoltage?

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Amplifier Gain

Magnifies the signal

Typically, we report output as it appears at

the electrodes (i.e. in mV)

Bioamplifier gain range: 100 = 10000

Avoid amp saturation!

-2048

-1024

0

1024

2048

0 1 2 3 4 5

Time (s)

A/D

Units

-2048

-1024

0

1024

2048

0 1 2 3 4 5

Time (s)

A/D

Units

Clipped data!

Impedance: Electrode-skin

Impedance = resistance

Electrode-skin impedance = bad

Due to: thickness of skin

cleanliness of skin

size of electrodes

temperature of paste/gel

Can reduce electrode-skin impedance through skin

preparation

remove oils with alcohol

shave and abrade skin

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Amplifier Input Impedance

A measure of amplifier sensitivity

Input impedance prevents attenuation of

the signal when it is connected to the input

terminals of the amplifier

Should be > 1M"

Impedance

We want:

LOW Electrode-skin impedance

HIGH Amplifier input impedance

Winter (2005), Fig. 9.5, pp. 238

Muscle +

tissueSkin Elec-

trode

Amp

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Amplifier Frequency Response

f1 f2

400

600

1000

800

Gain

Frequency, Hz

Gain,db

60

57

54

Amplifier Frequency Response

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2

EMG Signal Contamination

or Errors

EMG Signal Contamination Clipping

No ground electrode

DC offset

Movement artifact

ECG

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Clipping

0 1 2 3 4 5

Time (s)

0 1 2 3 4 5

Time (s)

Amplitude(mV)

0

1

-1 A

mplitude(mV)

0

1

-1

No ground electrode

Robertson et al. (2004), Fig 8.4, pp. 167

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DC offset

Robertson et al. (2004), Fig 8.4, pp. 167

Movement Artifact

Robertson et al. (2004), Fig 8.4, pp. 167

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2

ECG Contamination

Robertson et al. (2004), Fig 8.4, pp. 167