DESIGN OF A PORTABLE ELECTROMYOGRAPHY DEVICE

FOR BACK HERNIATED PATIENTS

Halil İbrahim Çakar, Sadık Kara and Onur Toker

Department of Electrical-Electronics Engineering, Fatih University, 34500, Buyukcekmece, Istanbul, Turkey

Keywords: Surface Electromyography, Back Herniation, Biomedical Instrumentation.

Abstract: Up to 80 percent of people are affected by back herniation or another low-back disease in their life. Some

studies have proven that there is a significant relation between paraspinal muscles and back herniation. In

this study, a portable surface electromyography device was designed for measuring the muscular activity of

paraspinal muscles from back herniated patients. The device’s gain value is adjusted to 1000, it includes a

band-pass filter and a 50 Hz notch filter to eliminate power source interference noise and other electrically

noises. The device designed as an electrically isolated single channel, and it is linked to Matlab software via

the serial port of Personal Computer.

1 INTRODUCTION

A superior cause of low-back pain is lumbar-disc

herniation (LDH). LDH pain influences 70%-85% of

people during their life and LDH is the most

common reason which limits the activity in people

younger than 45 years of age. (Hirose et al., 2008) In

the USA, LDH is the second most common reason

for visiting the physician, one of the most frequent

reason of entrance to the hospital. Furthermore, the

third most frequent reason of surgical interventions.

(Andersson, 1999) Magnetic Resonance Imaging

(MRI) is the commonly used imaging method in

diagnosis of LDH. Because, approximately 30% of

symptomless patients show a disc herniation on

MRI. Moreover, Computer Tomography (CT) and

myelography methods are rarely used for diagnosis.

(Skinner, 2003) Electromyography (EMG) is used in

LDH diagnosis to determine the neural injury in

herniated region and to measure the level of injury if

exists. (Çevikcan and Kara, 2007)

Electromyography (EMG) is a bio potential

signal which represents the neuromuscular activities

and occurs during the contraction and relaxation

activity of skeletal muscles. These activities of

muscles are controlled by the nervous system. (Reaz

et al., 2006) Surface electromyography (sEMG) is a

method used for appraisement of muscles’ activities

noninvasively. sEMG signals should be detected as

monopolar or bipolar. (Moore and Zouridakis, 2004)

Some studies have proven that sEMG should be

used in research of paraspinal muscle activities.

(Emley et al., 1992), (De Luca, 1993), (Kent, 1997),

(Kuriyama and Hito, 2005)

In this study, our goal is the measurement of the

paraspinal muscles’ activities and the determination

of the existence of back herniation with using the

sEMG bio potentials of these muscles. This portable

device offers a non-invasive, quicker, comfortable,

low-cost and alternative system for diagnosis of

back herniation.

2 GENERAL DESCRIPTIONS OF

CIRCUITS

In this section, the main components of the device

are described. Basically, whole device composed by

five parts. In the first stage, low bio potentials are

amplified and then for the safety of the patient

isolation circuit is used. In the stages, amplified

signals filtered and digitized with a high resolution

analog to digital converter (ADC).

Lastly, the digital data is transferred to the

personal computer (PC) by the microcontroller

PIC18F452. General block diagram of the device is

shown in Fig.1.

Raw sEMG data is collected with bipolar electrodes.

The disposable bipolar electrodes are recommended

by the European initiative, Surface

Electromyography for Noninvasive Assessment of

Çakar H., Kara S. and Toker O. (2010).

DESIGN OF A PORTABLE ELECTROMYOGRAPHY DEVICE FOR BACK HERNIATED PATIENTS.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 94-97

DOI: 10.5220/0002718100940097

 SciTePress

Figure 1: The block diagram of the device.

Muscles (SENIAM) who defines general agreements

about the basic criteria about sEMG since 1996.

(Merletti and Parker, 2004) Electrodes are connected

to amplification block by using shielded cables to

degrade the environmental noise effect.

2.1 Amplification and Isolation

Instrumentation amplifier (in-amp) amplifies the

difference of the sensed low amplitude signals by

bipolar electrodes. sEMG signal amplitude range

changes between 100 µV to 5 mV and frequency

range is between 10 Hz to 1000 Hz. For

amplification, Analog Devices’ (AD) low-power,

low-cost and high accuracy in-amp AD620 is used.

AD620 achieves a very high common mode

rejection ratio CMRR (100 dB min when the gain is

10) and its low noise level, low input bias current

and low power dissipation makes it suitable for

biomedical instrumentation applications. This high

accuracy in-amp’s gain setting is adjusted by using

just a single external resistor and its gain range

changes between 1 to 10000. The gain of AD620

should be adjusted by using the equation (1). We

chose G as 1000. (AD620 datasheet, 2004)

(49.4 / ) 1

GkR=+

(1)

AC-coupling is required in high-gain front-end

amplifiers for biopotential measurements. A

differential high-pass filter is designed without any

grounded resistor to reduce baseline wander and to

obtain the higher CMRR in front of the

instrumentation amplifier. (Spinelli et al., 2003) The

cut-off frequency of ac-coupling filter is 10 Hz.

Figure 2: The circuit diagram of amplification and

isolation block.

Using right-leg driver (RDL) circuit is another

way of reducing the noise. Basically, RDL circuit

inverts the common mode voltage which occurs

during the recording of sEMG signals with respect

to amplifier’s common. Then, the inverted common

mode voltage is applied to the body by attaching a

second monopolar electrode to the patient’s right

leg. (Winter and Webster, 1983) AD’s low power

and high precision operational amplifier OP97 is

used in the right-leg driver circuit.

Another important issue in biomedical

instrumentation is the isolation of the patient from

non-biomedical equipments such as computers,

which could be connected to EMG device and have

high power rates. Maximum accepted leakage

current for biomedical equipments in Europe is 100

µA

RMS

during instrument’s normal working

procedure. (Moore and Zouridakis, 2004) Galvanic

isolation and optoisolation are commonly used types

of isolation. For the isolation of our device, Burr-

Brown’s (BB) monolithic precision lowest cost

isolation amplifier is used. Basically, ISO122

includes a galvanic isolation system and matched

1pF of isolating capacitors. ISO122 uses a novel

duty cycle modulation-demodulation technique. The

signal is transmitted to the other side of isolation

DESIGN OF A PORTABLE ELECTROMYOGRAPHY DEVICE FOR BACK HERNIATED PATIENTS

barrier by a 500 kHz duty cycle modulation

technique. 0.020 % maximum nonlinearity ratio, 140

dB of isolation mode rejection (IMR) value and

maximum 0.5 µA

RMS

leakage current make ISO122

suitable for our system. (ISO122 datasheet, 1993)

The general block diagram of amplification and

isolation stage should be seen in Fig. 2.

2.2 Filtering

To eliminate the unwanted signals such as, DC

offset generated by the half-cell potential of the

electrodes that DC-coupled in the input stage and

motion artefacts, a band-pass filter (BPF) should be

after the input stage of the device. (Moore and

Zouridakis, 2004) According to the recommended

sEMG amplifier bandwidth, an active high-pass

filter (HPF) having 10 Hz cut-off frequency and an

active low-pass filter (LPF) with a 500 Hz of cut-off

frequency are designed. The circuit diagram of BPF

is in Fig. 3.

In design of both filters, Linear Technology’s

(LT) Quad Low Cost Precision Op Amp LTC1053 is

used. HPF and LPF both have 4 poles and the types

of filters are Sallen-Key. The responses of filters are

Butterworth. We chose Butterworth because of its

flattest response in the passband.

In the second stage of filtering, 50 Hz of notch

filter is designed. LT’s Dual Precision Op Amp

LT1013 is used for making notch filter. Fig. 4 shows

the circuit diagram of notch filter.

Figure 3: The circuit diagram of high-pass filter and low-

pass filter.

Figure 4: The circuit diagram of 50 Hz notch filter.

2.3 Analog to Digital Conversion and

Microcontroller

After amplification, isolation and filtering

procedures, the analog raw sEMG signal should be

digitized with an ADC. BB’s low-power, serial 16-

bit sampling ADC ADS7813 is used for digitizing.

ADS7813 contains the 16-bit capacitor based

successive approximation (SAR) ADC with a

sample/hold, clock, reference and easy to use serial

interface. The throughout rate of ADS7813 is

minimum 40 kHz. The conversion time lengths

maximum 20μs. Its integral linearity error is

maximum +3 LSB and differential linearity error is

+3, -2 LSB. ADS7813 includes single channel. But,

it is suitable for multichannel applications.

(ADS7813 datasheet, 2005)

Figure 5: Relation of ADC, microcontroller and PC.

When transferring the 16 bit digital signals to

PC, Microchip’s microcontroller PIC18F452 is used.

PIC18F452 has the master synchronous serial port

(MSSP) module including serial peripheral interface

(SPI) suitable for interfacing ADS7813. PIC18F452

is chosen as master and ADS7813 as a slave.

PIC18F452’s addressable universal synchronous

asynchronous receiver transmitter (USART) is

another serial I/O module. (PIC18FXX2 datasheet,

2006) UART module is used to send the signals to

PC. Fig. 5 shows the relation of ADC,

microcontroller and PC. PIC18F452 sends the

BIODEVICES 2010 - International Conference on Biomedical Electronics and Devices

digitized signal via serial cable. Matlab’s serial port

I/O feature lets us to communicate with

microcontroller. The sEMG signals are buffered to

Matlab during the measurement and then other

signal processing methods are applied. An example

of raw unprocessed sample of sEMG signal should

be seen in Fig. 6. This sample is taken from

Thoracolumbar Fascia muscle of a volunteer healthy

male. In the next stages of this study, we aim to

evaluate sEMG signals collected with our device

belong to patients with various signal processing

techniques.

Figure 6: A sample of raw sEMG signal.

3 CONCLUSIONS

In this study, we described a non-invasive, low-cost

and portable electromyography device for back

herniated patients. Device includes a single channel

and 16-bit resolution ADC. It has serial

communication with PC and has a direct connection

with Matlab software. In future, we aim to add a

Bluetooth® module to make our device wireless and

more portable.

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DESIGN OF A PORTABLE ELECTROMYOGRAPHY DEVICE FOR BACK HERNIATED PATIENTS