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A cm Physiol Scant! 1997, 160, 175-183
Motor unit potential contribution to surface
electromyography
K. r o e l e v e l d ;
d
. f . st e g h m a n ;
h
. m . v in g e r h o e t s
and A . V A N O G S T E R O M 2
1 Department of C'Jimcat Netimphysiology, Institute of Neurology s University f i ospita! Nijmegen, the Netherlands
2 laboratory of Medical Physics and Biophysics, Faculty of Medical Sciences^ University of Nijmegen ) the Netherlands
ABSTRACT
The background of the bioelectric activity of muscle recorded from the surface of the skin (surface
electromyography) in terms of the representation of single motor units of the underlying muscle(s) is
not very weil documented or understood. An insight into the composition of an electromyogram is
essential for the proper interpretation of one of the most widely applied eJectrophysiological
techniques. In the present paper, a study of the contribution of single motor unit potentials to the
surface electromyogram is presented. To this end, the decline of different components of the motor
unit potential with depth of the motor unit is quantified. Experimentally, the action potentials from
motor units at several positions in the muscle were recorded by 30 skin surface electrodes.
Simultaneous use of scanning electromyography provided information about the actual position and
size of the motor unit. Observed linear log-log relationships between motor unit potential magnitudes
and distance indicated the usefulness of a power function to describe the motor unit potential's
dependence on recording distance. It is shown that different specific surface motor unit potential
characteristics fall off differently with depth. The magnitude-distance relationship is shown to be
dependent on the recording configuration (unipolar vs. bipolar recording, including the inter-electrode
distance) and the chosen motor unit potential parameter (negative peak amplitude, positive peak
amplitude and area).
Keywords electromyography, human, motor unit, surface EMG, uptake area.
Received 7 October 1996, accepted 15 January 1997
Surface electromyography (EMG) o f voluntary muscle
activity is often used in kinesiology to quantify muscle
activity and to study the timing o f muscle activation
(Van Ingen Schenau et al 1992), to estimate the available
number of motor units (McComas 1995) and to describe
the electrophysiological behaviour during fatiguing ex­
ercise (Linssen et ah 1993). One important, still unsolved,
problem in using the surface EMG is the uncertainty o f
what is actually measured. Motor units (MUs) are the
functional units of muscles, and their action potentials
are the basic contributors to surface EMG. Therefore,
systematic investigation o f the contribution o f identifi­
able MUs to surface EMG is necessary.
Motor unit po ten dais (MUPs) recorded with surface
electrodes unipolarly (the reference electrode being
placed relatively far from the muscle fibres) contain two
components with quite different volume conduction
characteristics (Gydikov & Kosarov 1972, Gootzcu el ah
1991, Dumitru & King 1991), The first one is a negative
component which propagates from the motor end plate
region over the muscle fibres to both tendons. The
second one is a positive wave which is generated when
the propagating potential reaches the muscle fibretendon transition. It is o f basic interest to know the
volume conduction o f the different components o f the
MUPs in
scie tissue.
Most frequently, bipolar leadings are used in surface
EMG. In a bipolar montage, both electrodes are placed
above the muscle tissue under study with a variety of
inter-electrode distances (IHDs). Common wave comI
Correspondence: 1C Roeleveld, Nciirmmisciilnr Rese:m:h ('entre Nijmegen, University Hospital Nijmegen, institute o f 'Neurology, lJ() Box lM01,
6500 MB Nijmegen, the Netherlands
© 1997 Scandinavian Physiological Society
175
Acta Physiol Scand 1997, 160, 175-183
MUP contribution to surface EMG • K Roelcveld et al.
ponents cancel and it is mainly the propagating com­
ponents that are recorded, Since the propagating source
is extended in space, a change in the IED will cause a
substantial change in recorded MUP magnitude and in
the effects o f volume conduction,
In this paper, the decline in surface MUP magnitude
with increasing distance between electrode and MU is
studied experimentally. The action potentials from
different MUs located at different depths are studied by
recording each MUP at several places on the skin sur­
face with 30 surface recording electrodes and one ref­
erence electrode. Simultaneously, the same MU is
studied with intramuscular scanning EMG (Stalberg &
Antoni 1980) to provide information on the location
and the size o f the MU, From the unipolarly recorded
surface po ten dais, bipolar montages with inter-elec­
trode distances varying between 6 and 84 mm are
composed, The magn.itude~distan.ce relationship of
different components of the MUPs in muscle tissue is
Z
>
MNi
wMMI
o
o
+
M A T E R IA L S A N D M E T H O D S
Subject selection
Fifty-five MUs from nine healthy volunteers witii no
known signs o f neuromuscular disorders were investi­
gated in this study. In all subjects (six men, diree
women, aged 20-30 years) the MUs from the m. biceps
brachii were investigated, since that muscle has a rela­
tively simple morphology in which the fibres run nearly
parallel to the skin surface from tendon to tendon. All
subjects gave their informed consent, The Committee
on Experiments in Humans of the Faculty of Medicine
approved the experimental protocol,
Electrodes'
For skin surface recordings, 30 gold-coated screws,
with a diameter o f 1.2 mm and a length of 6 mm, were
used. They were screwed into an electrode holder with
inter-electrode distances (lEDs) of 6 mm. The holder
had circular holes to enable the simultaneous applica­
tion o f needles for recording needle EMG, After
cleaning the skin with alcohol, the holder with the 30
surface electrodes was placed over the muscle (Fig. 1),
such that two columns o f 15 electrodes (^ = 1—15)
were placed in alignment with the muscle fibres,
Double electrode columns were used to check for
consistency.
Additionally, a conventional silver-silver-chloride
EEG electrode with a diameter o f 1 cm was attached to
the elbow to serve as common reference for unipolar
surface EMG. Simultaneously, two needle electrodes
were used. A standard (bipolar) single fibre needle (SF)
electrode (Stalberg & Trontelj 1994) and a standard
176
50 ms
E
E
CD
6 mm
F ig u re 1 Typical example o f an unipolar: MUP distribution over the
skin surface resulting from a superficial MU, The MUPs are recorded
by two rows o f 15 electrodes placed over the m. biceps brachii parallel
to the muscle fibre direction (^direction). The upper part o f the figure
represents the proximal part of the muscle. The position o f the SF
trigger electrode and the CN scanning electrode are indicated as SF
and CN, respectively. The parametri'/ation o f one o f the unipolar
MUPs is shown,
(bipolar) concentric needle (CN) electrode were placed
in the muscle for triggering purposes and for recording
the scanning EMG profiles, respectively. A ground
electrode was placed on the wrist.
Data acquisition
The myoelectric signals leaded from the skin were
amplified (Neurotop®; Nihon Kohclen, input resistance 100 Milt) over a frequency range of 5-800 Hz
with a gain o f 50 000. For amplifying the needle elec­
trode signals, a Medelec MS-20 (Mystro) was used.
For the single fibre electrode and the concentric nee­
dle electrode, the filter setdngs were 500-5000 Hz and
8-5000 Hz, respectively. Depending on the amplitude
of the signals, the gain was set to 5000 or 10 000 for
these signals. A 50 ms delayed trigger pulse was obtained from the SF electrode signal with the threshold
trigger op don o f the Mystro (Aswip). This trigger pulse
was connected to a unit which controlled the scanning
© 1997 Scandinavian Physiological Socinty
Acia Physiol Scuncl 1997, 160, 175—183
motor steps. Each second trigger pulse made the motor
pull the needle 50 ¡Lim in the direction o f the skin.
The electrical signals from the surface electrodes and
those from both needle electrodes and the motor pulse
were A /D converted (DT2839 & DT2896, Data
Translation; 12 bits, total sample rate 224 ksamples s~l)
and continuously stored on a computer’s hard disk. The
surface EMG signals and die motor pulse were stored
ij
'—1
at a frequency o f 2 ksamples s channel"" . By selecting the needle EMG channels eight times as often for
the multiplexer input (a facility o f the A /D cards used),
the sample rate o f the needle signals was set to
”l each.
The experiments were performed on the m. biceps
brachii during voluntary, low force (up to 10% o f the
maximal voluntary contraction), isometric contraction,
During the whole experiment, the forearm was
supinated and the elbow angle was kept at 100°. After
positioning the surface electrodes, the SF electrode was
inserted. Once the action potential o f a stable firing
single muscle fibre (SFAP) was detected, an amplitude
threshold was set for the generation o f a trigger signal.
The CN then was placed «2.5 cm distal from the SF
electrode (see Fig. 1). This CN electrode was moved
until a signal appeared time-locked to the SFAP. Then
the CN electrode was pushed deeper into the muscle
until this time-locked potential completely disappeared.
Next, the scanning procedure was started, whereby the
scanning motor pulls the electrode towards the surface.
For more details on the scanning procedure, the reader
is referred to Stalberg & Antoni (1980), Hilton-Brown
& Stalberg (1983) and Goo teen et al (1992). The reg­
istrations took 40-120 s, the time needed to make the
scan. The measurement protocol o f one such needle
scan through the muscle will be referred to as a ‘session5, Before and after each session, the position o f the
CN electrode in the muscle was determined. Thereafter,
another session was started by repositioning the SF
electrode to a different depth. Between four and five
sessions were performed in each subject, which resulted
in a total of 42 sessions.
The recordings o f the SF electrode were used to find
triggers. From each session, between one and three
different SFAPs (belonging to different MUs) were
characterized by an amplitude window on the positive or
negative peak and with an upper limit value for duration.
From the 42 sessions, 55 triggers from different MUs
were obtained. After detecting the triggers, the surface
<0 1997 Scandinavian Physiological Society
K Roclo veld
et ai
• MUP contribution to surface EMG
MUPs were obtained by averaging around these triggers
with a window o f 128 ms (50 ms pre- and 78 ms posttrigger time), elevating the MUP distribution in that time
window from the background surface EMG, Simultaneously, the CN signal around each trigger and the occurrences of the motor steps were also stored,
Scanning E M G processing cmd parameterization
For each MU, 400-1400 triggers were detected, each
contributing to one CIS! EMG trace. A combination of
the begin and end positions o f the CN electrode and
the number o f motor steps taken at the moment o f
each trigger provided the location o f the CN electrode
at each trace. After DC correction, the traces obtained
at the same needle position were averaged and a median
filter over five points was used in the scanning direction
for elimination o f non-time-locked activity (Gootzen
et al. 1992), Figure 2 shows an example o f the result: o f
such a procedure. The traces with a peak-to-peak am­
plitude exceeding 50 /iV with the slope from positive to
negative were considered to be recorded from the MU
(Stalberg & Antoni 1980). The cross-section o f the MU
was defined as the distance between the deepest and the
most superficial trace belonging to the MU. The scan
was parameterized by two parameters:
(1) The integral over the peak-to-peak amplitude
over the cross-section o f the MU (.V) was taken to
represent the number o f fibres in the MU, often referred
to as ‘MU size’ in clinical neurophysiology. In this study,
S is used to normalize the surface MUPs to MU size.
(2) The position in the scan where 50% o f the
contribution to .Vwas obtained from the traces deeper
than that point in the cross-section o f the MU repre­
sented the depth (<7) o f the electrophysiological centre
of the MU relative to the skin surface.
Surface E M C param /eri^a/bn
Figure 1 shows a typical example o f the MUP distri­
bution on the skin surface generated by a rather su­
perficial MU. For all the averaged (unipolar) surface
MUPs, the following magnitude parameters were de­
termined (see inset to Fig. 1): peak values o f the neg­
ative wave (A///) and o f the positive wave (P m) and total
area (positive and negative) below the curve (/I//). Bi­
polar recordings with differen t 11'IDs were obtained
from the unipolar MUPs by subtracting successive
MUPs in the muscle fibre direction (top to bottom,
Fig. 1). By skipping one or more (up to 13) successive
MUPs, the!ED was increased from 6 to 84 mm in steps
of 6 mm. From all the bipolar MUPs, the peak*-to-peak
amplitude (PPb) and the area (Ab) were calculated. All
; 1. In kinare summarized in
parameters
esiology, where EMG is used to quantify muscle ac~
177
Acta Physiol Scand 1997,160, 175-183
IVIUP contribution to surface EMG ■ K Roclevekl et ni.
50 uV
10 0
pV
Time (ms)
Figure 2 Bipolar «cunning EM G (a) and one unipolar surface EM G channel (b), The scanning EM G parameters’ cross-sectional area ((7)
and positions relative to the skin surface (d ) are indicated,
t
tivity and to study the timing o f muscle activation, the
root mean square (RMS) value o f bipolar leaded EMG
is usually obtained This RMS value will behave ap­
proximately like A k
Surface M U P magmtude—M U depth relationship
The magnitude o f a MUP is related to the distance
between the active MU and the recording electrode. To
quantify this dependence, a power function was fitted
Table 1 Parameter definition
Vai’iahles
Units
Description
Mt
/tv
Pn
An
PPb 6 84
;«v
A b 6-H4
/¿V ms
ft
mm
,V
/tV mm
fiNit
nPn
-i
mm
-i
mm
Peak amplitude ol: the negative phase of the unipolar surface MUP
Peak amplitutle of the positive phase of the unipolar surface MUP
Area o f total unipolar surface MUP
Peak—peak amplitude o f the bipolar surface MUP with IED
of 6-84 mm
Area o f the bipolar surface MUPs with 1ED o f 6-84 mm
Location o f the elect rophy sit »logical centre o f MU
from the skin surface
Integral over the peak—peak amplitudes over the cross-section
in the scan
Nn normalized to MU size as N h/ S
Ph normalized to MU size as Pit/S
An normalized to MU size as A n /S
PPb normalized to MLJ size as PPb/S
A h normalized to MU size as A b /S
Power by which the MUP decreases with increasing d at position %
Power by which the normalized MUP decreases with increasing d
at position %
nAu
nPPb
nAb
QfL
tiQ(L
178
/(V ms
/<V
ms mm
m m -1
-i
ms mm
© 1997 Scandinavian Physiological Society
I\ Ruclevdd et a i • IVIUP contribution to surface EMG
Acta Physiol Scant! 1997, 160, J 75—183
through the observed values of the above-defined
i parameters (the unipolar as
well as the bipolar ones) as a function o f d. For this, the
electrodes from the column with the largest average
peak amplitude were used (for the MU of Fig. 1, the
left column). Because the electrode array position in the
re direction can be related to the motor end plate
region, this relationship was fitted for each electrode (or
pair) in the column separately.
For Nu:
4 i 'r \
(Jd.
(1)
jQrL is obtained for each o f the 15 positions on the
muscle fibre direction (^).
is a constant and is equal
to the amplitude estimate o f N u K at the distance ¿4,
whereby <4 is set to 1 mm* The power f)d K then de­
scribes, for a specific position ^ how fast the MUP
magnitude decreases with increasing d .
is estimated
as the slope o f a linear regression of the double loga­
rithmic relationship between d and N /u of the 55 dif­
ferent MUs (/ = 1 , . . . >55):
log(Afy») =
- j2 4 x Iog(r/)
(2 )
Theoretically, linear fitting o f the double logarithmic
relationship can give a solution that is different from
the one resulting from fitting a power function to the
original data. However, for the MUP parameters ob­
tained from the most proximal electrode set, Qd^ values
never differed more than 7%. Because o f the advan­
tages o f the linearity, the Qdy_ values listed are those
obtained by a least-squares linear fit to the log—log re­
lationship of the data. The standard error o f Qd^ and
the correlation coefficient from the linear regression
were used to validate the use o f the power model of
equation (1). In estimating these Qd^ values, the surface
MUPs o f 55 different MUs are used. To correct for the
natural differences in MU size, this procedure was re­
peated after using normalized values (//M/J) obtained by
dividing the surface MUP parameters by .V.
R ESU LTS
Suturing E M C
Figure 2 shows a typical example o f a scanning
EMG result. The distance from the electrical centre
o f the MUs to the skin surface (d) ranged between
3,6 and 25.5 mm. The size parameter S ranged be­
tween 231 /¿V mm and 2228 /¿V mm. S and d did
not correlate.
Surface M U P magnitude—M U depth relationship (Qdv)
Lire 3(a),(b) & (c) for the surface MUPs recorded by the
most: proximal electrode in the electrode column with
the highest average amplitude (^ = 1, upper left electrode, Fig. 1). The solid line is the power Fit through the
data with the inserted power
standard error o f
the Q d\ values is also inserted (se) and varied between
0.11 and 0.17. This analysis was performed for each o f
the 15 positions along die ^-axis. The resulting OtL did
not have a large variation (Table 2), In general, QcL was
slightly (±6%) lower in the middle o f the fibre than at:
the outer sides. At all ^ positions, N tt decreased faster
with increasing d than A n and P m. The power model
fitted the changes in N u (r = 0*84) bel ter than those o f
Att (/* = 0,75) and Pit (r = 0.72). Normalizing the
surface parameters (Fig. 3d, e & f, Table 2) hardly al­
tered Q(L. The 0<L values o f the normalized relationships {uQrL) were about 0.07 lower, but the standard
errors, their dependence on ^ and the differences be­
tween the different magnitude parameters were similar
to the relationships o f the original data,
The same analysis was also performed on the bipolar
MUPs. Figure 4 shows the relationship between the
parameters PPb\ and A b \ obtained from the bipolar
MUPs taken with an inter-electrode distance o f 6 mm
and d . As in Figure 3, the solid line is the power fit
through the data with the inserted power Qd\< The
power function fitted PPb almost perfectly (r = 0.91),
while Ah and d correlated slightly less well (r = 0.84)
on the double log-1 og scale. ThejQd-y values and the 13
other Qd** values estimated with the peak amplitude
as well as with the area o f these (ll '.D 6 mm) bipolar
MUPs are higher than their unipolar equivalents
(Table 2). Figure 5(a) .shows the results o f the same
analysis for all different HiDs available, The Q d\ ()f N m
is also shown on the plot (*). Increasing the II vD in the
bipolar montages causes the MUP magnitude to de­
crease less steeply with increasing d (lower Qri), until the
ifference between PPb and N u disappears for very
large IT'IDs. Furthermore, the absolute difference be­
tween PPb and A b hardly changes with a change in
IED. In Figure 5(h), the MUP magnitude dependence
on IKD is presented for the same two bipolar param­
eters. "Hie average A b and PPb o f the bipolar Ml.JPs
increase a bit more and a bit less than a factor o f 2,
respectively, with increasing inter-electrode distance
from 6 mm up to a value o f 4 cm. When the JED is
further increased, the magnitudes decrease again. The
PPb determined from MUPs with I ED.r o f over 4 cm
approaches twice the value o f Nu. N ote that the indi­
vidual tlata points in Figure 5(n)&(b) present the mean
of Q d and magnitude values obtained at different po­
sitions in the % direction. For the small IKDs, the
standard deviations o f the Q d values were around 0.2
(for IEDs of 6 mm, see Table 2), while the standard
deviations of the magnitudes were about 20% o f their
i
The relationship between the different magnitude parameters of the surface MUPs and d is shown in Mg(s') 1997 Scandinavian Physiological Socioty
»
179
Acta Physiol Sc and 1997, 160, 175-183
MUP contribution to surface EMG * K Roelcveld et al
(a)
(d)
Qd, 1.85
se 0.17
100
>
0.3
0.2
E
a.
E
rs
50
0.1
c
0
0
30
0
(b)
(e)
60
£ 20
<| 0.05
0
0
0
10d(mm)20
30
nQd, 1.09
se 0.14
o
IX
40
1000
o
0.15
I
>
30
0
0-1
o
0
1°d (m m )^
30
(c)
1
f/mm
■M M
E
C/5
E
D
<
C
Figure 3 Surface MUP magnimde~MU depth (d) relationship for three different unipolar surface MUP parameters, in absolute terms (a, bv c)
and normalized to the size of the MU (d, e, Q. ‘O ' represents the data, and the solid line represents the power function fitted to the data. The
power
see equations 1 and 2) and its standard error (se) obtained from the linear regression o f the log-log data are shown on the plots. All
parameters were obtained from MUPs recorded by the most proximal electrode
— 1).
mean (for IEDs o f 6 mm, see Fig. 4). For the large
IE D s, this was much smaller,
DISCUSSION
The dependence o f the experimentally observed surface
of die MU
MUP magnitude parameters on the
can be described by an inverse power function. The
présent; study shows that this magnitude-distance rclationship is dependent on the recording configuration
(unipolar vs. bipolar recordings and inter-electrode
distance in bipolar recordings) and on the chosen MUP
parameter (negative, positive and peak-to-peak ampli­
tudes and area).
A MUP is the spatial and temporal summation of
action potentials o f the muscle fibres of that MU. The
Table 2 Mean and standard deviations of the powers found with least-scjuares fitting o f equation (2) through the MU depth-surfnee MUP
magnitude relationship on the original parameters
and the normalized parameters (¿¡Qty- The variation in Q(L and njQrL is caused by
different electrode positions along the muscle fibre ($.
Q4
nQiL
180
TV//
Pu
Au
PPb 6 mm
A b 6 mm
1.60 ± 0.13
1.53 ± 0.13
1.04 ± 0.09
0.97 i 0.09
0.99 ± 0.12
0.92 ± 0.12
2.08 ± 0.22
2.00 ± 0.22
1.29 ± 0.16
1.21 ± 0.16
© 1997 Scandinavian Physiological Socinty
Acta Physiol Samel 1997, 160, 175-183
K Roelevdd vt td.
(a)
(C)
200
Qd1 2.44
0.15
150
E
E
n
Q.
CL
c
3
XI
100
C^m
CL
50
0
0
• MUP contribution to surface EMG
10
20
30
d (mm)
d (mm)
(d)
600
cc
E
CO
E
n
<
PM
0
0
10
20
d (mm)
30
0
10
20
30
d (mm)
Figure 4 Surface MUP—MU depth relationship for bipolar recordings with an inter-electrode distance o f 6 m m , shown us the absolute and sizenormalized peak-peak amplitude (a, c) and area (b, d). *0* represents the data, and the solid line represents the power function fitting the data. The
power used (Qdy see equations 1 and 2) and the standard error o f this power (se) obtained from the log—linear data arc shown on the plots. All
parameters were obtained from MUPs recorded by the most proximal electrode pairs.
single fibre action potentials (SFAPs) can be seen as a
mathematical convolution o f the source and a transfer
function describing the volume conduction properties
of the resistive medium and the distance between the
active fibre and the recording site (e.g. Plonsey 1974,
Dimitrov el al. 1988, Gootzen el cil 1991, Van Veen vt <tl
1992). The bioelectric source o f a SFAP can be approximated by an electric current tripole (Rosenfalck
1969, Griep et al 1982). When this propagating tripole
reaches the muscle fibre—tendon transition, its leading
part is suppressed. This suppression causes the generation o f the terminal positive wave complex caused by a
dipolar source component (Stegeman vt ai. 1987, 1996,
Gc)c)17,en el al. 1991, Dumitru & ICing 1991). Thct)retically, in a homogeneous, infinite volume conductor,
the amplitude of a trip ole is expected to have an inverse
relationship with the third power o f the recording
distance, while a dipole then has an inverse quadratic
relationship with radial clistanee. ( ) ur measurements
indicate a much milder decrease in amplitude with increasing depth of the tripole {N n ~ 1.6) and the dipole
[Pti w 1.0). This lower decrease can be explained by
the finiteness o f the volume surrounding the MU (the
limb), the difference in conductivity between muscle
tissue and fat tissue, and the duration o f the action
potential (Gootzen et al, 1991). As expected from the
above-stated theoretical knowledge about the two dif
© 1D97 Scandinavian Physiological Society
ferent volume conducted electric sources o f the MUP
(Goot7.cn etol 1991), N n decreased faster with increasing
MU depth than Pu. Furthermore, A h decreased less
quickly than the peak amplitudes, which is explained by
the increasing duration for surface MUPs with increasing
distance. The decrease in MUP magnitude with MU depth
was faster than the previously reported dependence o f the
MUP magnitude of single MUs on electrode-to-MU
distance (O N n ~ 1: Buchthill el a!, 1957, Gydikov el at.
1972, Mans ter & Chan 1980, Hcrmens 1991). With model
simulations this different distance dependency o f MUP
magnitude on MU depth and electrode position can lie
explained (Roelevdd vt nLy 1997).
The MUP magnitude obtained from the bipolar
configuration with lEDs o f 6 mm decreased faster with
increasing MU depth than the unipolarly recorded
surface MUP. increasing the IKD up to 40 mm caused
an increase in MIJP magnitude and a decrease in the Qd
va 1ue o 1nained from themagiiil:ude—MU depth relation.
This can be explained in terms o f spatial overlap o f the
MIJP recorded by the two ctectrodes. When the IIvD is
smaller than the length o f the MUP, there is an overlap
between opposite profiles under both electrodes. Surface MUPs o f deeper MUs have longer durations and
relatively greater overlap. An increase in overlap de­
creases the magnitude. Bipolarly recorded MUPs with
IED s 1arger than the unipolar M U P 1e ng th have double
181
A m Physiol Scand 1997, 160, 175-183
MUP contribution to surface EMG • K Rodeveld et at.
2.5
(a)
2
O 1.5
(3
0 ~ ” 0 - -0
■
1
©
©
Q- CK
-O'
-o - e
0 - 0 - -o
1
0.5
0
20
60
40
80
IED (mm)
>
100
jQ
CL
CL
I
IED (mm)
Figure 5
Effect o f bipolar inter-elec{.rode distance (IED) on the ,Qd (a) and the magnitude (b) o f the two bipolar M lIP parameters PPb ( x ) and
Ah (O ), presented as their mean values over the £ direction. The Qd value fitting the unipolar negative peak amplitude (Ntt) and twice the Nti
amplitude are included (shown as *), to he compared with the Qd and the magnitude o f PPb, respectively.
the magnitude find similar volume conduction characteristies as the propagating part of the unipolarJy re­
corded MUP. An increase in IED above 4 cm caused a
decrease in the magnitude o f the bipolar MUP, A PPb
amplitude lower than two times N tt can only be ob­
tained when the two subtracted propagating action
potentials partly overlap. For larger lED s this can be
the case when the motor end plate region is in between
the two recording electrodes. In the electrode place­
ment used on the biceps brachii, lED s larger than 4 cm
m u s t viit ually always h ave l:he mo tore nd pi ate regi o n i n
between the two electrodes.
The variation in the %direction was small, whereby
the MUP was less dependent on the depth of the MU
in the middle o f the fibre than at the outer sides. The
interpretation o f this result is not easy, because the
exact fibre direction, fibre length and MU end plate
region can only he taken into account from the su­
perficial MUs. That some data scatter is present even
after normalization o f the surface MUP to MU size can
be explained by differences in the shape of the action
potential (Buchthal et aL 1957) between different MUs,
and a variation in the presence o f connective tissue and
182
active fibres in different subjects and different: contractions (Buchthal et al. 1955, Van Veen 1992),
The investigations were supported by the Life Sciences Foundation
(SIAV), which is subsidised by the Netherlands ( )rgauization for
Scientific Research (NWO). K.R and DUS participate in the National
Institute for Fundamental and Clinical Human Movement Sciences of
Nijmegen University and the Vrije Universiteit Amsterdam.
REFERENCES
Buchthal, P., Guld, Chr. & Roscnfalck, P. 1955. Innervation
zone and propagation velocity in human musclc, Acta
Physio! Samd S5, 174-190,
Buchthal, P., Guld, Chr, & Roscnfalck, P. 1957. Volume
conduction o f the spike o f the MU potential investigated
w ith a new type of tnultielectrode. A cla Physiol Sea >td 3 8,
331-354.
Dimitrov, G.V., Latcva, Z.C. & Dimitrova, N.A. 1988.
Effects o f changes in asymmetry, duration and propagation
velocity o f the intracellular potential on the power
spectrum o f extracellular potentials produced by an
excitable fibre. Rkctmuyagr Ctin Neurophysiol 28, 93-100.
Dumitru, D. & KingjJ.C. 1991. Far-field potentials in muscle.
Muscle N e rve 14, 981-989.
© 1997 Scandinavian Physiological Soeioiy
Acta Physiol Scant! 1997, 160, 175-183
Gootzen, T.H.J.M,, Stegeman, D.F, & van Oosterom, A.
1991. Finite limb dimensions and Unite muscle length in a
model for the generation o f electromyographic signals.
Electroeuceph Clin Nenropbyswl 8 '1, 152-162.
Gootzen» T.H.J.M., Vingerhoets, D.J,M. & Stegeman, D.F.
1992. A study o f motor units structure by means o f
scanning EMG. Muscle Nerve 15, 349—357.
Griep, P.A.M., Gielen, F.L.H., Boom, H.B.K., Boon, K.L.,
Hoogstraten, L.L.W., Pool, C.W. & Wallinga-de Jonge, W.
1982. Calculation and registration o f the same MU action
potential. Eleclmncepb Clin Neurophysiol 53, 388-404.
Gydikov, A, & Kosarov, D. 1972. Volume conduction o f the
potentials from separate motor units in human muscle.
lilet 11ynnyogtupby
12, 127—147.
Gydikov, A., Kosarov, D. & Tankov, N. 1^72. Studying the
alpha motoneuron activity by investigating motor units o f
various sizes, I ilectromyogr 12, 99—117.
Mermens, 11. 1991. Surface E M C . P h D thesis, University
Twente, Enschede, The Netherlands.
Hilton-Brown, P & Stalberg, E. 1983, Motor unit size in
muscular dystrophy, a macro E M G and scanning EM G
study. J Neurol Neurosurg & Psych 46, 996-1005.
Ltnssen, W.H., Stegeman, D.F., Joosten, H.M.G., van't-Tlof,
M.A., Binkhofst, R.A. &c Notermans, S.L.H. 1993.
Variability and interrelationships of surface E M G
parameters during local muscle fatigue. Muscle Nerve 16,
849-856.
McComas, AJ. 1995. Motor unit estimation: anxieties and
achievements, Muscle Nerve 18, 369—379.
Monster, AAV. & Chan, H. 1980, Surface electro myogram
potentials o f m otor units; relationship between potential
<ò 1997 Scandinavian Physiological Society
K Roclcveld d at. • MUP contribution to surface EMG
si/e and unit location in a large human skeletal muscle. Exp
Neurol 67, 2H()-2(J7.
Plonsey, R. 1974. The active liber in a volume conductor,
IE E E Trans Bmmd Eng B M E -21, 371-381.
Roeleveld, K,, Blok, J.M., Stegeman, D.F. &: Van Oosterom,
A. 1997. Volume conduction models for surface EMG;
confrontadon with measurcments. J Ekctmmjogr Kineshl,
accepted.
Rosenfalck, P. 1969, Intra- and extracellular potential fields o f
active nerve and muscle fibres. Acta Physiol Scaud 321,
S1—S168.
Stalberg, E & Antoni, L. 1980, Electrophysiological
cross section o f the MU, J Neural Nettras/ng & Psych 43,
469-474.
Stalberg, E & Trontelj, J.V. 1994. Single fibre electromyography
studies in healthy and diseased wusch\ 2nd edn. Raven Press,
New York.
Stegeman, D.F., Roeleveld, K., Dumitrn, D. & Vingerhoets,
1LM. 1996. Far-field potentials in surface EMG. Excerpta
Medtea International Congress Series 1101, 271-275
Stegeman, D.F., van Oosterom, A. & Colon, E.J. 1987,
Far-field evoked potential com ponents induced by a
propagating generator: computational evidence.
Electroeucephalogr Clin Nenrophysiol 67, 176-187.
van Ingen Schenau, G.J., Boots, P.J., D e G root, G., Snackers,
R,j, & van Woensel, W.W. 1992. T h e constrained control
o f force and position in multi-joint movements. Neuroscience
46, 197-207.
van Veen, B.K. 1992. Single fiber action potentials in
inhomogeneously conducting skeletal muscle. P hD thesis,
University Twente, Enschede, the Netherlands.