EFFECT OF SURFACE ELECTRODE ORIENTATION ON

INDEPENDENT COMPONENT ANALYSIS FOR FEATURE

EXTRACTION OF SURFACE MOTOR UNIT ACTION

POTENTIAL

Jun Akazawa

, Takaharu Ikeuchi

, Takemasa Okamoto

, Ryuhei Okuno

Masaki Yoshida

, Tetsuo Sato

and Kotaro Minato

School of Health Sciences and Medical Care, Meiji University of Integrative Medicine

Honoda, Hiyoshi-cho, Nantan-shi,6290392 Kyoto, Japan

Department of Electrical and Electronics Engineering, Setsunan University

17-8 Ikedanaka-machi, Neyagawa-shi, 5728508 Osaka, Japan

Department of Biomedical Engineering, Osaka Electro-Communication University

1130-70 Kiyotaki, Shijonawate-shi. 5750063 Osaka, Japan

Graduate School of Information Science, Nara Institute of Science and Technology

8916-5 Takayama, Ikom-shi, 6300192 Nara, Japan

Keywords: Electromyogram, Motor Unit, Model, Independent Component Analysis.

Abstract: Recently, application of Independent Component Analysis (ICA) has been reported for effective decomposi-

tion of surface electromyogram (SEMG) signals into a train of surface motor unit action potentials

(SMUAPs) of a single motor unit (MU). Results of ICA were not always sufficient as the feature extraction

of SMUAP at first dorsal interosseous muscle (FDI). The purpose of this study is to propose an effective

method for feature extraction of SMUAP by simulation study of focusing on the effects of electrode orienta-

tion. SEMG signals were created with the model and application of ICA was applied to the signals. The

present study showed that the useful and actual method of ICA application was to repeat measurement of

SEMG signals with varying the electrode orientation, and then to select the better signals for the feature ex-

traction by executing ICA algorithm.

1 INTRODUCTION

In the field of sports science and rehabilitation, elec-

tromyogram (EMG) observed with the surface elec-

trode, rather than the invasive needle electrode, is

often used to investigate behaviors of the motor

units (MUs). Then the surface EMG (SEMG) could

be decomposed into a train of surface motor unit

action potential (SMUAP) of a single motor unit.

Subsequently, such factors as waveform of the

SMUAP, firing rates, recruitment, de-recruitment,

territory of the MUs could be examined.

Recently a few researches have been reported: Xu,

Xiao, and Chi (Xu et al., 2001) were proposed the

method using the artificial neural network. Bonato,

Erim, and Gonzalez-Cueto (Bonato et al., 2001)

were proposed the method using the method in the

area of time frequency. Recently, Independent Com-

ponent Analysis (ICA) (Bonato et al., 2001) algo-

rithm has been applied to the decomposition method

for large muscles such as biceps brachii muscle by

Maekawa, Arimoto, Kotani, and Fujiwara (Maekawa

et al., 2002), Nakamura, Yoshida, Kotani, Akazawa,

and Moritani (Nakamura et al., 2004), and Gonzalo,

Okuno, and Akazawa (Gonzalo et al., 2005). While

we applied to first dorsal interosseous muscle (FDI),

results of ICA were not always sufficient as the

feature extraction of SMUAP. It was difficult to

separate SMUAPs because several types of

SMUAPs appeared in the single ICA component.

421

Akazawa J., Ikeuchi T., Okamoto T., Okuno R., Yoshida M., Sato T. and Minato K. (2009).

EFFECT OF SURFACE ELECTRODE ORIENTATION ON INDEPENDENT COMPONENT ANALYSIS FOR FEATURE EXTRACTION OF SURFACE

MOTOR UNIT ACTION POTENTIAL.

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing, pages 421-425

DOI: 10.5220/0001777704210425

 SciTePress

The purpose of this study is to propose an effec-

tive method for feature extraction of SMUAP by

simulation study of focusing on the effects of elec-

trode orientation.

2 METHOD

2.1 Experimental Set-up

The subject put his hand on the desk horizontally

where the thumb and the fingers were loosely fixed

except the index finger. The isometric adductor

torque of the index finger was measured with strain

gauge. SEMG signals were obtained with eight-

channel bipolar surface electrodes array shown in

Fig. 1. The electrode was placed over the FDI. Each

electrode was stainless wire of 1 mm diameter, a

pair of electrodes was placed with inter-electrode

spacing of 2.54 mm and each pair was placed in

parallel with spacing 2.54 mm. The SEMG signal

was amplified with the gain 60 -80 dB and the cut

off frequency 800 Hz.

The subject was instructed to keep the constant

force of 5% maximal voluntary contraction (MVC)

by watching the force output displayed with bright

lines on the oscilloscope. Both the isometric force

and the eight-channel SEMGs were A/D converted

at the 10 kHz sampling frequency. Informed consent

was given to each subject.

Figure 1: Eight-channel bipolar surface electrodes.

2.2 SEMG Model

Spatial information such as MU territory, muscle

fiber and electrode, is shown in Fig. 2 (a). In the

present study, Griep’s tripole model (Griep et al.,

1982) was used to calculate the action potential

generated on the skin surface by the excitation of a

single muscle fiber.

As shown in Fig. 2, the axis x is defined to be

perpendicular to the skin surface, the axis z is the

moving direction of excitation of the muscle fiber

and the axis y is direction orthogonal to x and z. The

distance between the electrode and the axis of mus-

cle fiber is x

in the x-axis, and y

in the y-axis. The

distance between the electrode and each point cur-

rent source is z

(i = 1, MU, 3) in the z-axis at t = 0

(the time beginning of excitation of the muscle fiber)

and z

+ v t at the time t, where v is the conduction

velocity of excitation. The action potential of a sin-

gle muscle fiber monitored at the electrode is given

∑

+++

=Φ

)(

),,,(

ninn

vtzyx

tzyx

πσ

(1)

where

is conductivity of the volume conduc-

tor and I

the strength of the point current source.

Assume that individual muscle fibers within the

MU are all identical in their characteristics and dif-

ferent in their locations and the number of muscle

fibers is N, the potential at the electrode is given by

∑

Φ=Φ

ninnnMU

tzyxx

),,,()(

(2)

Figure 2: Illustration for the SEMG generation model.

(a) Spatial relation between the electrode and the muscle

fiber; (b) Electrode angle θ.

3 RESULTS

3.1 Simulation

<Parameters for tripole model> As to the tripole

model, following parameters were used； conduc-

tivity of the volume conductor was

＝

0.16

)(

11 −−

Ω m

(Disselhorst-klug et al., 1998), point

current was I

)(4.0 A

, I

)(5.0 A

−=

, and

)(1.0 A

(Griep et al., 1982). The distance be-

tween I

and I

was 0.45 mm, and that between I

and I

was 1.8 mm (Griep et al., 1982). The conduc-

tion velocity v was 3.5 (m/s) (Disselhorst-klug et al.,

1998).

<Location of MU> Because we estimated that

territories of single MU of FDI were mostly square-

BIOSIGNALS 2009 - International Conference on Bio-inspired Systems and Signal Processing

422

type (Akazawa et al., 2005), squared territory model

is used here. As shown in Fig. 2 (a), W is the width,

T is the thickness, xmuc is the distance from skin

surface to the top of the MU, and ymuc is the center

of the MU. For the simplicity only two groups of

MU were used; the number of small-sized group was

hundred and that of large-sized group was ten. Dis-

tribution of the size was Gaussian. Mean values of

W and T were 10 mm, and 10 mm respectively for

the large-sized MU, and 1 mm and 1 mm for the

small-sized MU. The thickness of skin surface/fat

tissues was assumed to be 2.0 mm and the width of

FDI was 2.0 cm.

<Firing Rates> The firing rates of MUs in isome-

tric contraction were examined statistically by Cla-

mann (Clamann, 1969). His finding that the distribu-

tion was Gaussian was applied to the present model.

Because the isometric contraction to be studied in

the present study was approximately 5% MVC, the

mean firing rate was assumed to be 7 Hz for all the

MUs. It was showed that at the low force level of

isometric contraction, firing of individual MUs is

statistically independent (Kanosue et al., 1979). This

results was applied to the present model concerning

to the firing time of MUs.

<Electrode > The position of each electrode is

fixed with the actually used electrode in Fig. 1. Si-

mulation was executed with changing the electrode

angle θ up to 40 degree. Orientation of only the one

large-sized MU was changed from zero to certain

value for understanding clearly the effect of elec-

trode angle, while electrode angle of other MUs is

zero.

<Effects of Electrode Orientation on ICA > Ef-

fects of changing the electrode angle were examined

by simulation study; firstly eight-channel SEMG of

1 (s) duration were created by the model of SEMG

generation, and then ICA was applied to the SEMG.

Eight channel SEMG of the model with the elec-

trode angle of 0 degree is shown in Fig. 3 (a). The

output signals which are obtained by applying the

ICA algorithm to the SEMG signals in Fig. 3 (a) are

shown in Fig. 3 (b).

Clear signals (SMUAP-like signals) with almost

the same waveform are apparently found on the first

component IC 1, while very small amplitudes of

signals are found in IC 1 component. Judging from

the time of appearance, these SMUAP-like signals in

IC 1 correspond to the SMUAP marked with B in

Fig. 3 (a); MU corresponds to this SMUAP is re-

ferred to as MU (B). Furthermore, no SMUAP-like

signals are found at the same time in other compo-

nents IC 2, IC 3, and IC 4. These results mean that

feature of MU (B) is extracted by ICA explicitly.

Similarly the same type of SMUAP-like signals are

also found in IC 2 in Fig. 3 (b), which correspond to

MU (A) in Fig. 3 (a).

The effect of electrode angle was examined. Fig.

4 shows thus obtained component of ICA. SMUAP-

like signals are found at the same time in IC 1 and

IC 2. The angle between the electrode and the MU is

10 degree. On the other hand, clear SMUAP-like

signal is found in IC 3, which corresponds to the

MU of the electrode angle of 0 degree. When the

electrode angle changes to 30 degrees, SMUAP-like

signals appear in all the ICA components from IC 1

to IC 8. This result implies that SMUAP amplitude

of the MU (A) in SEMG signal is so large that the

SMUAP-like signal appears in two or more compo-

nents of ICA. Executing these simulations, we found

that as the electrode angle increased, the number of

components in which SMUAP-like signals of the

same MU appeared was increased, and the number

of MUs the SMUAP-like signal of which appeared

in one component was also increased.

(a)

(b)

Figure 3: SEMG and ICA. (a) SEMG signals. (b) The

output signals obtained by applying ICA algorithm to

SEMG signals.

Figure 4: Results of ICA with electrode angle of 10 degree.

EFFECT OF SURFACE ELECTRODE ORIENTATION ON INDEPENDENT COMPONENT ANALYSIS FOR

FEATURE EXTRACTION OF SURFACE MOTOR UNIT ACTION POTENTIAL

423

3.2 Result with Consideration of

Electrode Orientation

Apart from insufficient, we tried to obtain better

results with repeating measurement three times with

changing the electrode orientation. Isometric con-

traction was at 5% MVC and the duration 15 (s).

The result of ICA is shown in Fig. 5 (a) and the

corresponding measured SEMG in Fig. 5 (b). In IC

1, SMUAP-like signals marked with × could be

found clearly; briefly the corresponding MU is

called MU 2. The larger amplitude signal marked

with open circle ○ was also found in IC 1; the cor-

responding MU is called MU 1. In IC 3, a large

amplitude SMUAP-like signal marked by ● was

found. This MU might correspond to MU 1 because

of appearance at the same time in both IC 1 and IC 3.

Consequently the feature extraction of MU 2 was

effectively sufficient in IC 1, and small amplitude

signal corresponding to MU 3 was found in IC 2 as

shown with triangle △. Focusing attention on IC 1

and IC 3, MUs marked by ● and ○ are the same

MU 1. This conclusion could be supported by com-

paring the SEMG signal in Fig. 5 (b) with ICA in

Fig. 5 (a). SMUAPs of MU 1 are clearly seen from

CH 1 to CH 6 in Fig. (b) and those of MU 2 from

CH 3 to CH 5. It should be noted that both SMUAPs

of MU 1 and MU 2 in CH 5 are very similar in the

shape, which means that decomposition of SMUAP

is difficult in judging from only the SEMG signal.

4 CONCLUSIONS

In this study, effects of electrode orientation on the

result of ICA was analyzed with simulation study

and actual voluntary isometric contraction of FDI.

Obtained results were as follows. When the long

axis of the eight-channel electrode was perpendicu-

lar to the long axis of muscle fiber, the result of ICA

was best in terms of the feature extraction of

SMUAP; large amplitude of SMUAP-like signals of

the single MUs appeared in one component of ICA.

As the orientation of the electrode changed apart

from this direction, unexpected results of ICA were

obtained; i.e., large amplitude of SMUAP-like sig-

nals of the single MUs appeared almost at the same

time in other components of ICA, and SMUAP-like

signals of different MUs appeared in one component.

Figure 5: Result of ICA (a) and measured SEMG (b) at

5% MVC of isometric contraction.

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EFFECT OF SURFACE ELECTRODE ORIENTATION ON INDEPENDENT COMPONENT ANALYSIS FOR

FEATURE EXTRACTION OF SURFACE MOTOR UNIT ACTION POTENTIAL

425