Additional material for the book:
Atlas of Muscle Innervation Zones:
Understanding Surface
Electromyography
and Its Applications
M. Barbero, R. Merletti, A. Rainoldi
ISBN 978-88-470-2462-5
Material provided by:
Laboratory for Engineering of the Neuromuscular
System (LISiN),
Politecnico di Torino, www.lisin.polito.it
This material is made available under the Creative
Commons Attribution-NonCommercial-NoDerivs 3.0
Unported License
Description of the additional material:
1. A set of Power Point animations showing the concepts of generation, propagation and extinction of
motor unit action potentials, the identification of innervation zones, the concept of spatial filtering,
EMG signals at different time scales, and the effect of pinnation angle
2. Eleven movies showing time-evolving instantaneous or averaged EMG intensity maps of trapezius,
gastrocnemius and tibialis anterior muscles during isometric contractions
Use space bar and arrows to control slide show
See also “Multimedia and educational tools” at www.lisin.polito.it
Generation and propagation of motor unit action potentials
Slides 1-3
Animation n. 1 shows a motor neuron (a) innervating a motor unit (b) consisting of three
fibers. When the motor neuron discharges, its action potential reaches the three
neuromuscular junctions (NMJ) and triggers the muscle fiber action potentials that propagate
to the right and to the left, towards the fiber terminations, where they extinguish. One singlefiber action potential (transmembrane voltage) is depicted in (c). Each depolarized zone
behaves as an electric field source in the surrounding conducting volume.
Animation n. 2 shows the same motor unit depicted in animation n.1. The motor unit is under
the skin and the surface potential profile is generated in space by the depolarized regions
which move to the right and to the left. The skin potential profile moving to the right passes
under the electrodes A and B, causing the generation of a potential evolving in time at the
amplifier output. The lower the conduction velocity of the motor unit, the wider is the potential
at the amplifier output, and the narrower its amplitude or power spectra.
Animation n. 3 shows a sequence of potential distributions on the skin above a muscle, 0.6 ms
apart from each other, detected while a motor unit action potential propagates under a 2-D
electrode array placed on one side of the IZ.
LISiN, Torino, Italy
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The Motor Unit (MU)
(electrical activity)
Inputs from
other neurons
motoneuron
c)
Axon
a)
0
Schwan cells and
Action potential
(90-100 mVpp)
Ranvier nodes
- 70
Muscle fibers
1 ms o 4 mm
NMJ
b)
One muscle: 10-1000 MU
4 m/s = 4 mm/ms
4 m/s = 4 mm/ms
One MU: 50-1000 fibers of the same type (I o II)
Slide 1 LISiN, Torino
V(t)
Potential distribution on the skin
t
x
CV
CV
Innervation zone
( 3 NMJs )
Subcutaneous tissue
Depolarized
Zone
V(x)
0 mV
- 70 mV
A
B
Skin
Muscle-tendon
junctions
x
CV
Action potentials travelling towards the tendons
Slide 2 LISiN, Torino
Animation n.3 – LISiN, Torino, IT
2D and 3D maps of a
single MUAP in time
each frame corresponds to one sample
(sampling interval = 0.6 ms)
Generation and propagation of motor unit action potentials
Slides 4-6
Animation n.4 shows the discharge of a motor neuron and the generation of depolarized
regions along the fibers controlled by the corresponding motor unit. These regions propagate
towards the tendons and generate a sequence of time shifted differential EMG potentials
between adjacent pairs of electrode arranged in a linear array. From these potentials it is
possible to estimate the conduction velocity of the fibers of the motor unit.
Animation n.5 is like animation n.4 but two motor units (and therefore two motor unit action
potentials) discharge at different times t1 and t2.
Slide n.6 shows a real multichannel recording from a biceps brachii muscle, showing more
than 20 firings of different motor units during 0.25 s.
LISiN, Torino, Italy
www.lisin.polito.it
Propagation of a single motor unit action potential (MUAP) under a
linear electrode array aligned with the fiber direction
Animation n.4 - LISiN, Torino, IT
60 mm
time
15 ms
e= 10 mm
Conduction velocity = CV = 60mm/15 ms = 4 mm/ms = 4 m/s
Propagation of the motor unit action potentials (MUAP) of two motor units under a linear
electrode array aligned with the fiber direction. MU 1 discharges at time t1 and MU 2
discharges at time t2
IZ1
time
IZ 2
MU 1 MU 2
e= 10 mm
t1
t2
Animation n.5 – LISiN, Torino, IT
Isometric contraction of biceps brachii at 70% MVC
Multichannel recording
Electrode array
1
7
1 mV
15
10 mm
50 ms
Depolarization
area
Differential EMG signals recorded using a linear
array of 16 electrodes. The innervation zone and
tendineous terminations can be clearly seen
Slide n. 6 - LISiN, Torino, IT
Generation and propagation of motor unit action potentials
Slides 7 - 9
Slide n.7 shows a multichannel recording from a biceps brachii that presents two well
identifiable innervazion zones. The motor units innervated in IZ 1 are marked in orange, while
those innervated in IZ 2 are marked in green. The muscle tendon regions are outlined with red
squares.
Slide n.8 shows the discharge of three motor units in the biceps brachii. Two of them are
innervated in the same location. The third one is innervated in a different location. They
discharge at times t1, t2 and t3, respectively. Signals are propagating in opposite directions at
different times under electrode pairs 5 to 8. The electrode array, in this case, is too short to
show the muscle-tendon junctions.
Slide n.9 shows a progressive increase of voluntary contraction level and the recruitment of
motor units with progressively larger MUAPs and progressively increasing firing frequency.
Individual MUAPs can be identified and classified even at high levels of voluntary contraction
(80% MVC). Conduction velocity of individual MUAPs and their statistical distributions can be
estimated up to mid contraction levels. Surface EMG signals can be decomposed into their
constituent MUAP potential trains.
LISiN, Torino, Italy
www.lisin.polito.it
Identification of innervation zones, tendon
junctions and fiber length
I.Z. 1
I.Z. 2
Tendon junctions
Slide n.7 - LISiN, Torino, IT
25 ms
Biceps brachii: 50% MVC
from: Merletti, Farina, Granata, 1999
Discharge of three motor units in the biceps brachii. Two of them are
innervated in the same location. They discharge at times t1, t2 and t3
t1
t2
MU1
MU2
t3
MU3
30
40
11
10
9
e
8
7
6
5
4
3
2
1
e = 5 mm
0
10
20
time (ms)
Slide n.8 - LISiN, Torino, IT
HEALTHY BICEPS BRACHII SD DETECTION
0%
MVC
Force
100 %
MVC
0
1
25 % MVC
2
50 % MVC
3
75 % MVC
time (s)
4
90 % MVC
5
e =10mm
250 ms
Slide n.9 - LISiN, Torino, IT
Spatial filtering by a single electrode pair
Slides 10 - 12
A sinusoidal waveform, representing one harmonic of the EMG signal, propagates under a
cardboard carrying two slots that simulates two electrodes.The height of the mark appearing
through each slot corresponds to the monopolar signal level. The difference between two mark
levels corresponds to the differential EMG signal. The distance in space between two
corresponding points of the sinusoid is called wavelength (λ, measured in meters) and the
corresponding time interval is the period (T, measured in seconds). Period and wavelength are
related by T=λ/v, where v is the propagation velocity.
Animation n. 10 shows that if the distance d between the two slots (electrodes) is equal to λ, the
output of the differential amplifier is zero.
Animation n. 11 shows that if the distance d between the two slots (electrodes) is different from
λ, the output of the differential amplifier is different from zero. In particular, if d = λ/2, the output is
maximum.
Animation n. 12 shows that if the distance d between the two slots (electrodes) is much smaller
than λ, the output of the differential amplifier is small (with respect to the previous case) and
proportional to the spatial derivative of the propagating signal.
The differential detection system behaves differently with respect to different spatial frequencies
and different inter-electrode distances, and therefore acts as a spatial filter.
LISiN, Torino, Italy
www.lisin.polito.it
Vo
+
_
t
d=λ
Animation n. 10 - LISiN, Torino, IT
Vo
+
_
t
T=λ/v
d≠λ
Animation n. 11- LISiN, Torino, IT
Vo
f ’(x)
+
_
t
d << λ
Animation n. 12 - LISiN, Torino, IT
Array of EMG signals at different time scales
Slides 13 - 16
Slide n.13 shows seven differential channels of surface EMG detected along the fiber direction
during an isometric contraction, with force first increasing and then decreasing over a 40s
interval (red plot and scale).
Slide n.14 shows a time interval of about 3 s “zoomed in” from the previous slide. MUAPs
discharges and the recruitment of individual motor units begin to appear.
Slide n.15 shows a time interval of about 200 ms “zoomed in” from the previous slide. At this
time resolution, discharges of individual motor units can be clearly seen. The muscle innervation
zone is at the top of the slide and the bottom channels (6 and 7) correspond to the tendon area.
Slide n.16 shows a time interval of about 25 ms “zoomed in” from the previous slide. The
discharge of a single motor unit is clearly seen. The propagation velocity of this motor unit action
potential (MUAP) can be estimated. Hundreds of MUAPs like this one form the signal shown in
slide n.13.
LISiN, Torino, Italy
www.lisin.polito.it
One array with 8 contacts. Right biceps brachii short head - 80% MVC ramp 40s
zoom
1 mV
EMG
ch.
Torque
%MVC
80
70
1
60
2
50
3
4
40
5
30
6
20
7
10
0
5
10
15
20
Time (s)
25
30
35
40
0
Slide n.13 - LISiN, Torino, IT
Torque
%MVC
80
Right biceps brachii short head - 80% MVC - ramp 40s
zoom
1 mV
EMG
ch.
70
1
60
2
50
3
4
40
5
30
6
20
7
10
4
4.5
5
5.5
Time (s)
6
6.5
0
Slide n. 14 - LISiN, Torino, IT
Torque
%MVC
80
Right biceps brachii, short head - 80% MVC - ramp 40s
zoom
EMG
ch.
1 mV
70
1
60
2
50
3
4
40
5
30
6
20
7
10
5.58
5.6
5.62
5.64
5.66
5.68
Time (s)
5.7
5.72
5.74
5.76
5.78
0
Slide n.15 - LISiN, Torino, IT
Right biceps brachii, short head - 80% MVC - ramp 40s
EMG
ch.
Torque
%MVC
80
1 mV
70
1
60
2
50
3
4
40
5
30
6
20
7
10
5.690
5.695
5.700
Time (s)
5.705
0
Slide n.16 - LISiN, Torino, IT
Effect of the innervation zone sliding under an electrode pair
Slides 17 - 18
Two pairs of electrodes, collecting two single differential EMG signals SD1 and SD2 are
positioned on the biceps brachii muscle. When the elbow is flexed, the muscle shortens under
the two electrodes.
In animation n.17, the innervation zone is distal with respect to SD2, and progressively slides
under it, causing a decrement of the SD2 signal and its envelope.
In animation n.18, the innervation zone is near SD2 and progressively slides under SD2 and
SD1, causing first the SD2 signal to be smaller than SD1, and then a decrement of SD1
amplitude.
These amplitude changes are due exclusively to geometrical changes and not to changes in
the muscle activation level.
LISiN, Torino, Italy
www.lisin.polito.it
Effect of Innervation Zone (IZ)
shift under the electrodes
EMG
Prox.
SD1
1 mV
1
SD2
Dist.
Norm. envelope
25 mm
IZ
Animation n.17 - LISiN, Torino, IT
Elbow
angle: 0o
Elbow
angle: 90o
0.8
0.6
SD1
SD2
0.4
0.2
0
2
4
6
8
10
Elbow flexion from 0° to 90°: biceps brachii muscle
Electrodes: Ø = 15 mm, d = 25 mm IZ near electrode pair 2
12
14
16
Time (s)
Effect of Innervation Zone (IZ)
shift under the electrodes
Slide n.18 - LISiN, Torino, IT
EMG
SD1
Prox.
SD1
1 mV
SD2
25 mm
1.0
ZI
Dist.
Norm. envelope
SD2
SD2
0.8
0.6
SD1
0.4
Elbow
angle: 0o
0.2
0
2
4
6
Elbow
angle: 90o
8
10
Elbow flexion from 0° to 90°: biceps brachii muscle
Electrodes: Ø = 15 mm, d = 25 mm
IZ under electrode pair 2
12
14
16
Time (s)
Local representation of motor unit action potentials in
pennate muscles
Slide 19 - 20
Animation n.19 A shows a multichannel action potential generated by fibers parallel to the
skin. Animation n.19 B shows the multichannel action potential generated by fibers inclined
with respect to the skin.
Slide n.20 A shows the distribution of the motor unit action potential on the skin above a motor
unit with a large territory. Slide n.20 B shows the distribution of the motor unit action potential
on the skin above a motor unit with a small territory.
LISiN, Torino, Italy
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Action potentials in pennate fibres as
they appear locally on the skin
A)
B)
Pennation angle = 0°
Animation n.19 - LISiN, Torino, IT
Mesin, Merletti, Vieira (2011)
J Biomech 44: 1096–103
Pennation angle = 20°
+
–
+
–
Fat tissue
Skin
+
–
Conduction velocity: 4 m/s
+
–
A.U.
5 ms
Skin thickness: 1 mm
Fat thickness: 3 mm
A.U.
16 electrodes with 5 mm IED
Testing for the localisation of motor
units in the human MG muscle
Non-localised motor unit
Vieira, Loram, Muceli, Merletti, Farina
(2011) J Physiol 589:431-43
Localised motor unit
B)
A)
End-plate
Action
potential
Tibial nerve
Muscle fibres
Skin
Action potential
propagation
Medial gastrocnemius
(longitudinal section)
Slide n.20 - LISiN, Torino, IT
Fat tissue