1. Movement control and non-invasive electromyography: development of models
and web clinical teaching tools
LISiN, Politecnico di Torino, 2018
Module N. 5
Basic neuromuscular electrophysiology and mechanisms of sEMG generation
Roberto Merletti
LISIN, Politecnico di Torino

2. Why is it necessary to know the concepts described in this module
Why is it necessary to know the concepts described in this module ? (1 of 2)
The concepts of single fiber action potential and of motor unit action potential, and the concept of interferential surface EMG are fundamental for sEMG interpretation and for the recognition of external disturbances and artifacts:
the action potential is a transient of the membrane voltage in excitable cells. This transient generates an electric field and electric currents in the surrounding conductive tissue and produces a voltage distribution on the skin surface which is called surface electromyogram (sEMG). We are immersed in electric fields produced by electric devices and power line wirings. These fields are a source of interference for all bioelectric signals. Understanding the signal and recognising disturbances and interferences is a key point for the correct interpre- tation and clinical application of any bioelectric signal and of sEMG in particular.

3. Why is it necessary to know the concepts described in this material
Why is it necessary to know the concepts described in this material ? (2 of 2)
To understand the sEMG information content, to correctly interpret this information for clinical applications, the following concepts are necessary:
the generation, propagation and extinction of the muscle fiber and of the motor unit action potentials, the mechanism of muscle force control (recruitment and discharge rate), the relationship between the variables controlled by the CNS and force. One-dimensional and two-dimensional (imaging) sEMG are fundamental windows through which the operation of the peripheral and central nervous system can be observed and understood in different conditions.

4. Learning achievements, objectives and concepts
The basic electrophysiology of the neuromuscular system (the single fiber action potential, the motor unit, the motor unit action potential)
The electrical field generated in the volume conductor and the resulting surface potential distribution (the sEMG as an image evolving in time, that is a movie)
The basic properties of this image in space and time.
The concepts of generation, propagation and extinction of the motor unit action potential (MUAP).
The surface rapresentation of the MUAP.
The sEMG as the sum of the surface contributions of many MUAPs.
The information contained in the sEMG.
The concept of muscle activation and the relationship between sEMG and force produced by the muscle.

5. What sEMG is and what is not.
As for ECG and EEG, the sEMG signal provides information about the organ (muscle) that generated it and about its control strategy by the CNS.
Its therapeutic value is limited (as for ECG and EEG). Surface EMG is NOT a therapy but is a useful ally in assessement and some treatments (e.g. biofeedback) .
Surface EMG provides an important window of observation and documentation of events involving the central and peripheral nervous system and the muscles.
Surface EMG is mostly a technique for understanding and documenting specific conditions (fatigue, denervation and reinnervation, muscle coordination, load sharing, etc) for prevention of disorders, for monitoring and assessing the effectiveness of treatments, interventions and training procedures.

6. Applications of sEMG
To understand the physiology of the neuromuscular system (how does it work? What has changed in the system with respect to a month ago? How do I explain what I see?).
To provide a quantitative description of movement and muscle coordination through biomechanical and electrophysiological measurements. To reduce and replace subjective evaluations.
To provide tools and means for studying sport physiology and medicine.
To provide tools and means for quantitative rehabilitation medicine.
To provide tools and means for quantitative occupational and work medicine.
To provide a quantitative approach to ergonomics and prevention of work-related neuromuscular disorders.
Other applications: Space madicine, prevention of obstetric lesions, control of robots, artificial limbs and external devices.

7. Electrophysiologial signals: ECG, EEG, EMG, EGG, EOG and others
The electrocardiogram (ECG) is the electrical signal produced by the heart .
The electroencephalogram (EEG) is the electrical signal produced by the brain.
The electromyogram (EMG) is the electrical signal produced by a muscle.
The electrogastrogram (EGG) and electrooculogram (EOG): from the gastrontestinal system and the retina.
Each of these signals is detected from a number of "derivations" (channels) that are usually monopolar (with respect to a common reference) or bipolar (one with respect to another), double differential or Laplacian (2D double differential).
Example of ECG: the 12 standard derivations.
Precordial electrodes

8. Example of EEG : the 20 standard derivations:

9. The electromyogram can be detected with intra-
muscular needles, intramuscular thin wires and
surface electrodes. This module deals with the detection of surface EMG (sEMG).
Example of sEMG : 15 bipolar (SD) derivations along the biceps brachii.
Electrode array (IED = 10mm)
time scale (6,25 ms/div)

10. Needle, surface and thin wire EMG
Thin insulated wires with exposed tips
needle
surface electrodes
skin
needle
Detection volumes
Fibers of MU 1
Fibers of MU 2
The EMG needle might be inserted at different depths. It “sees” a few fibers of a few motor units within a volume of 1-2 mm3. Surface electrodes “see” motor units in several cm3, but not at great depths.
Detection volume of a coaxial needle (Adrian needle): spherical portion with ~1 mm radius. Shifts as small as 1 mm change the signal detected.

11. International publications on sEMG/year
1960
1970
1980
1990
2000
2010
2020
100
200
300
400
500
600
700
800
In the last 30 years the articles published on international journals increased from less than 70/year to about 700/year. Thirtheen textbooks marked this
progression, from Muscle Alive (1985) to
Surface Electromyography (2016).
Number of international articles published/year
From Pubmed
31 years

12. Summary (1 of 2)
The surface EMG (sEMG) is for the muscle (and for the physio- therapist) what the ECG is for the heart (and for the cardiologist) and what the EEG is for the brain (and for the neurologist).
The information content of these three signals has been investigated over an entire century ( ) with great progress in the last 40 years. ECG and EEG have extensive clinical applications and their detection and interpretation modalities are standardized at the international level.
Efforts for standardization of sEMG detection and interpretation have been promoted at the EU level (Project SENIAM, , but have not yet been successful and are still under way.

13. Summary (2 of 2)
The sEMG tells us more about a muscle than ECG tells us about the heart or EEG about the brain.
The information it provides has relevant value in the assessment of the muscle condition and of its role in movement and exercise performance. In addition it provides data about muscle coordi- nation, for prevention of disorders, and for the assessment of effectiveness of treatments, training and interventions.
A muscle is driven by the electrical activity (trains of action potentials) of the motorneurons and produces:
1. force (of biomechanical relevance)
2. electrical activity (EMG, of electrophysiological relevance)
3. mechancal vibrations (MMG or mechanomyogram or vibromyogram, not considered in this material)

14. What is the surface EMG ? triceps biceps
Since it is not possible to “enter” the system to place sensors of electrical and mechanical activity along nerves, muscles and tendons,
it is necessary to rely on non-invasive measurements that reflect the “events” taking place inside the system (as it happens for ECG and EEG).
Signal processing
triceps
biceps
Information on the drive, the activity and the condition of the muscle
Subcutaneous tissue
surface
elettrodes
Such “events” can be quantified by estimating a number of variables that describe the features of the signals (e.g. amplitude, power spectrum, single motor unit action potentials, their propagation velocity, etc.) versus time and versus the effort produced.

15. Basic muscle anatomy
A muscle is made of motor units (MU) (a few dozens to a few hundreds).
A MU is made by a motorneuron and by the muscle fibers it innervates.
A motorneuron is made by a soma (in the spinal cord) and a long myelinated axon that reaches the muscle where it fans out into terminal branches.
Each terminal branch forms a neuromuscular junction (NMJ) with a single muscle fiber.
Each muscle fiber has only one NMJ.
Each muscle fiber is an excitable cell able to generate and conduct an action potential under the effect of acetylcholine at the NMJ.
The NMJ is a special synapse that releases acetylcholine each time the motorneuron “fires" (that is each time an axonal action potential reaches the NMJs of the MU).
Acetilcholine changes the permeability of the muscle fiber membrane to sodium ions (Na+) bringing the resting membrane voltage (-70 mV) closer to zero and triggering a local action potential described in the next slides.

16. The single muscle fiber (SF) and the motor unit (MU)
Inputs from other neurons
motorneuron
potential
Action potential generated at the NMJ of the SF.
Axon
Schwann cells and
Ranvier nodes
(saltatory conduction)
action
- 70
Action potential
( mVpp)
Vm (mV) ms
axonal
Axonal branches
Single muscle fibers (SF)
NMJ
EoF IZ
The end of fiber zone (EoF) is the region containing the fiber-tendon junctions.
The Innervation zone (IZ) is the regione containing the NMJs of the MU

17. flows due to the concentr.
Active and passive ion flows in a patch of an excitable membrane: the “resting membrane potential”.
The basic “motor“ generating a voltage between the inside and the outside of an exitable cell is the active transport of Na+ and K+ ions against their concentration gradients.
Small membrane patch
Na+ channels
K+ channels a)
The opening and closing of the passive Na+ and K+ channels depend on the transmembrane voltage. Cl- channels are not shown.
outside
inside
Na+ K+
active pumps
At the equilibrium the net flow of EACH ion is zero and the resting membrane voltage is -70 mV. b)
Na+ K+ +
outside
Resting voltage: -70 mV _
inside
Na+ K+ Na+ K+ Na+ K+ Cl_ Cl_
active
pumps
flows due to the concentr.
gradient
Many other ions are present but their relevance is negligeble in determining the membrane potential.
flows due to
the electric
gradient
flow due
to the concentr.
gradient
“ "

18. membrane conductivities
Time course of a single fiber local action potential
The release of acethycholine at the neuromuscular junction (NMJ) increases the permeability to Na+ (gNa) causing an increase of the membrane resting potential above the exitation threshold. This further increases the conductivity to Na+ which flows into the cell and further increase the membrane potential with further increase of the conductivity to Na until the Na+ pores are completely open (phase 1). The change of transmembrane potential causes an increase of the conductivity to K+ ions (gK) which begin to flow out. This flow causes the membrane potential to decrease (phase 2). As a consequence, Na+ conducti-vity will decrease and the transmembrane potential will also decrease until the process is reversed and the resting potential is reached again (phase 3), with or without an undershoot. The time interval RF is the refractory period during which further chemical or electrical excitations do not elicit another action potential.
muscle fiber 3 6
Excitation threshold
- 70
Transmembrane voltage
gNa gK
Na+ and K+ onductivities
(qualitative patterns)
Vm (mV)
Resting voltage 1 2
membrane conductivities
time (ms) RF
gNa = sodium conductance
gK = potassium conductance

19. Two dipoles merge into the tripole model of action potential currents
Approximation of a local action potential
battery - + I1 I2
I1 + I2 = I3 I3
membrane
muscle fiber I1 I2
-I2
Excited region (action potential )
Outside
Membrane
Inside + -
Current lines across the membrane out in out α β γ
-I1
dipole dipole 2 I1 I2
α = region of inward current (negative)
β and γ = regions of outward current (positive)
I3 = - (I1 + I2)
The current distribution has cylindrical symmetry but is assumed to enter the cell in one point and exit it in two points.
Two dipoles merge into the tripole model of action potential currents
Tripole current model: Rosenfalck, 1969.

20. - Generation of a local action potential at the NMJ a) + _ b)
Intracellular
conductances
Extracellular
Cell membrane
Inward Na+ current lines - +
internal
current lines
Depolarizing area
(causes widening of the depolarized region)
Resting membrane potential: -70 mV
OUTSIDE
INSIDE 3 1 2 a) _
Outward K+ current lines
The inward and outward current lines in regions α, β and γ are approximated as being concentrated in three points.
The outward currents cross the membrane impedance and increase (towards zero) the membrane voltage,
Region of outward current
Region of inward current
space (mm) α β γ b)
Mostrare accensione sequenziale delle celle HH animare i filetti di corrente.
The process has circular symmetry.
causing a widening of the depolarized area of the cell.

21. The propagating action potential
NMJ
The propagating action potential
At time t1 the muscle fiber membrane depolarizes under the NMJ because of the release of acetylcholine and inflow of Na+. The depolarized zone begins to widen because of the outward currents.
At time t2 the central part of the depolarized region begins to ripolarize and two depolarized regions begin to form.
At time t3 two depolarized regions are formed and are moving one right and one left. The region in between is in the refractory perod. t1
out
membrane in
membrane t2
out
membrane in
membrane t3
out
membrane in
Depol. region moving left
Depol. region moving right
Repolarized region in refractory condition

22. The propagating action potential
Depolarization front _ + t1
NMJ
axonal branch
Depolarization front
At time t1 the muscle fiber membrane depolarizes under the NMJ because of the release of acetylcholine.
The depolarized zone begins to widen because of the outward currents at the depolarization fronts.
At time t2 the central part of the depolarized region begins to ripolarize and two depolarized regions begin to form.
At time t3 two depolarized regions are formed and are moving one right and one left. The region in between is in the refractory perod.
Membrane currents _ + t2
Repolarization front
Repolarization front _ + t3
Left tripole
Membrane currents
Right tripole

23. Depolarized region terminating at the end of the fiber
Depolarized region terminating at the end of the fiber. The end-of-fiber effect. _ _ + a) b) c) d) t1 t2 e) f) t3 g) h) t4 D1 D2
end of fiber
t1: the tripole (double dipole) approaches the end of the fiber
t2: the tripole reaches the end of the fiber and dipole 1 beacomes narrower
t3: dipole 1 cancels out and disappears. Only dipole 2 remains. The electric field is stronger.
t4: dipole 2 narrows and cancels out.

24. The end-of-fiber effect
Transmembrane current
End-of-fiber effect
fiber end t1
Tripole model of the current t2 t3
0 mV
Transmembrane voltage t4
- 70 mV t5
Tripole model of the transmembrane voltage
The tripole stops propagating at the end of the fiber, becomes a dipole (with a momentary stronger field) and then cancels out.
0 mV
- 70 mV
The electric field generated by the dipole at t3 is stronger that that generated by the tripole at t1 and t2 because an opposing dipole disappeared.

25. Propagation and extinction of the current tripole
EXTINCTION (end effect)
Fiber end effect
(non prop.)
muscle fiber
Propagation CV
NMJ
Generation
(non prop.)
space
Propagation CV
Fiber end effect
(non prop.) t1 t2 t3 t4 t5 t6
time
The NMJ is taken here at the middle of the fiber. This is not always the case.

26. Generation, propagation and extinction of a single fiber action potential
The tripole model of the transmembrane currents implies a triangular model of the action potential (AP) shape.
NMJ CV
Fiber end 1 11
tendon 2 3 4 5 6 7 8 9 10
The action potential of a single fiber is depicted over the NMJ (1-3), during propagation (4, 6, 7), and the tendon termination (8-11). Numbers from 1 to 11 represent 11 sequential time instants, from the generation to the extinction of the AP approximated by a triangle (current tripole).
LISiN, Torino

27. Transmembrane current Transmembrane current
Generation, propagation and extinction of a single fiber action potential.
End of fiber
End of fiber
NMJ
Transmembrane current
(tripoles)
Transmembrane current
(tripoles)
Transmembrane voltage
Click to start.

28. Some important warnings
The concepts described in the previous slides apply only to single muscle fibers that are parallel to the skin surface. The situation is different if the fiber is not parallel to the skin (pinnate muscles). This situation is described later.
The length of the depolarized zone is about 5-10 mm. In short muscle fibers (L<20mm) the AP extinction phase (end-of-fiber effect) begins immediately after the generation phase and propagation is not easily detectable..
The single fiber AP can be detected only with special needles (single fiber needles) and NOT from the skin surface. Only the motor unit action potential (MUAP), generated by many fibers activated (almost) at the same time can be detected from the surface of the skin.
The NMJ of the fibers of a motor unit are not aligned. They are scattered in space within the MU innervation zone. Therefore, the propagating action potentials are not aligned in space and may be scattered by a few millimeters. They add to form the MUAP.
The fibers of a motor unit do not terminate at the same point in space.
The fiber-tendon junctions may be scattered in space over many mm..

29. From the source to the surface.
The volume conductor.
The muscle and the subcutaneous tissue are electrical conductors. This means that they have a conductivity σ (inverse of the resistivity ρ).
The muscle is electrically anisotropic. This means that its conductivity σ in the longitudinal (along the fiber) direction is higher than the conductivity transversal to the fiber direction. The electrical current flows more easily in the longitudinal direction than in the transversal direction.
The voltage present on a point of the skin is inversely proportional to the distance between such point and the point-like source. Deeper sources provide smaller surface contributions than more superficial sources
A pont-like source is one pole of the two dipoles (tripole) representing the action potential.
The surface potential distribution is a blurred version of the source. That is, it is a filtered version of the source (through a diffusion filter).
k is a
proportionality constant R1 R2
k/R1
k/R2 S1 S2
For an isotropic medium the surface potential distribution has circular symmetry around
the vertical axis.
For an anisotropic medium the surface potential distribution is elliptical around
the vertical axis (see module on models).

30. From the source to the surface.
The volume conductor.
A source of electric field in a conductive medium produces a two-dimensional (2D) distribution of electric potential on the surface of the medium just like a point-like source of light in the water would produce a halo of light on the surface of the water. x y z
Point source S
Volume conductor z
surface y x
Isotropic volume conductor (same
electrical conductivity in all directions). x
Halo of source S
on the surface
Spatial support of the halo along the x axis

31. Spatial support of the potential along the x axis
A point-source of electric field in a conductive medium produces a two-Dimensional (2D) distribution of electric potential on the surface of the medium. If the medium is not isortropic, with different conductivities along x and y a point source generates an elliptical potential distribution.
Plane x-y: skin surface.
z-axis: electric potential on the surface.
Anisotropy: a muscle with fibers along x has conductivity along x greater than along y. z
x = longitudinal (fiber) direction
y = transversal direction
z = intensity of the electric potential on the surface x y
Halo of source S
on the surface
Spatial support of the potential along the x axis
Spatial support of the potential along the y axis y z

32. From a single pole to a dipole and to a double dipole with two overlapping sources (tripole)1 2 a)
space x (mm)
space y (mm) 1 2 b)
space x (mm) 1 2 3 4 c)
2 + 3
space x (mm)
space y (mm)
space x (mm)
space x (mm) d)
Surface potential profile of a tripole

33. The effect of conduction velocity on the time signal.VP
time P
Voltage distribution on the skin surface
muscle fiber VP
time P
Voltage distribution on the skin surface
The same spatial distribution results in scaled time signals at point P, depending on CV. The time signal is a scaled version of the spatial signal. The scale factor is the CV.

34. Conduction velocity is a scaling factor between the AP rapresentation in time and space1 2 3 D
Sensors
(cameras)
L/V1
Sensor 1
Sensor 2
D/V1 L
Sensor 3 v1
time 1 2 3 D
L/V2
v2 > v1
Sensor 1
Sensor 2
D/V2 L
Sensor 3 v2
time
The greater is v, the shorter are the delays d and the time duration L/V of the signal detected by each sensor. The scaling factor between space and time is 1/v.
The velocity v can be estimated as v = D/d where D is the intersensor distance
and d is the delay between the signals in time (there is a better way).

35. (electrical activity)
The Motor Unit (MU)
(electrical activity)
inputs from other neurons
motoneuron
60 m/s
Axon
action potential
( mVpp)
Schwan cells and
Ranvier nodes
Vm (mV)
- 70
muscle fibers
1 ms or 4 mm
4 m/s = 4 mm/ms
4 m/s = 4 mm/ms
One muscle: MU One MU: fibers of the same type (I o II)

36. Contribution of fibers of MU1 and of MU2 at point P
The motor unit action potential (MUAP)
Contribution of fibers of MU1 and of MU2 at point P P
skin
MUAP MUAP 2
axon 2
axon 1
Fibers of MU 1
Fibers of MU 2
IZ of MU 2
IZ of MU 1
IZ : innervation zone
Propagation of MUAP 2
time
time
Propagation of MUAP 1
The time duration of a MUAP depends on the width of the IZ, on the width of the single fibre depolarized regions and on the MUAP conduction velocity. A MUAP lasts 5-15 ms.

37. The concept of MUAP propagation in space and time on the skin
(monopolar signal)
VP(t)
time x P CV CV
subcutaneous tissue
innervation zone
skin
depolarized
Zone
muscle-tendon
junctions
 motoneuron
V(x)
0 mV x CV CV
- 70 mV
Action potentials travelling towards the tendons

38. The concept of MUAP propagation in space and time on the skin. x
(monopolar signal)
VP1
time
VP(t)
VP2
time
VP3
time x P1 P2 P3 CV CV
subcutaneous tissue
innervation zone
skin
depolarized
Zone
muscle-tendon
junctions
 motoneuron

39. Example of end-of-fiber effect in a sequence of 16 mono-polar signals detected by 16 electrodes placed distally with respect to the innervation zone of a MU of a biceps brachii muscle. Weak contraction.
The amplitude of the e.o.f
components depends on the spread of the fiber
terminations.
10 ms 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Monopolar MUAP signal
0.5 mV
End of fiber effect (e.o.f)
Muscle: biceps brachii
Interelectrode distance: 5 mm
Weak e.o.f. effect
Strong e.o.f. effect
The end-of-fiber effect is part of the monopolar MUAP.

40. Force and sEMG produced by a MU whose motorneuron discharges at increasing frequency.
The CNS controls muscle force by:
Changing the discharge frequency of each motor unit
Changing the number of the recruited motor units
The modality of regulation of these variables defines the control strategy.
Motorneuron discharges (CNS command signal)
Tetanic contraction
time s
Single twitch response
Force produced by the motor unit
time
Motor unit action potential train
time
50 ms

41. Motor unit action potential trains and interference sEMG
Consider N motor units discharging f1 …. fN times per second. The resulting sEMG signal is the algebraic sum of their contributions. Signal cancellation takes place and some information is lost. For N large (>10) the resulting signal is random with an approximately Gaussian probability distribution. Its spectrum spans tha bandwidth from 10 to about Hz (95% of the power in this band).
MUAP train 1 s
Approximately Gaussian probability distribution s
MUAP train N
stand. dev.
Monopolar interferential sEMG signal
RMS
Sum of N MUAP trains.
(N>10)
pdf s
The root mean square value (RMS) of the signal is the standard deviation of its probability distribution function (pdf). RMS2 is the power of the signal.

42. Summation of N motor unit force contributions
The CNS regulates muscle force by recruiting/derecruiting motor units and by modulating the discharge frequency of each of them according to a particular strategy. This strategy is described by the Henneman’s principle and by the Onion Skin principle. These principles have been investigated through sEMG +
MU 2
Total force produced by N motor units during a constant force isometric contraction s =
MU 1
MU N
time
Force
Progressively larger and stronger MUs
Henneman E., The size-principle: a deterministic output emerges from
a set of probabilistic connections. J. Exp. Biol Mar;115:

43. The Onion Skin motor unit scheme for force control
In many muscles MUs are recruited up to 50-60% of the max. contraction force: force in then further increased by increasing MU discharge rate. Early recruited MU have the highest firing rate. MUs recruited at higher force levels start with lower discharge rates
and discharge at fre-quencies lower than those recruited earlier.
100% MVC
50% 30 20 10
small MU
large MU
Recruitment tresholds
MU discharge rate (pps)
MU 1
MU N
force
recruitment order
Tetanic condition
force
slow twitch MU
time
fast twitch MU
force
time
MVC = maximal voluntary contraction.
De Luca C.J., Contessa P., Hierarchical control of motor units in voluntary contractions. Journal of Neurophysiology. 2012; 107:178–195
De Luca C. J., Contessa P., Biomechanical benefits of the Onion-Skin motor unit control scheme , J Biomech January 21; 48(2): 195–203.

44. EMG amplitude increases with increasing force level.
More and more motor units are recruited, according to Henneman’s Principle, starting from the smallest ones.
force
Single channel differential EMG
(limited information, see Module 6)
muscle
Force or torque
motor units
EMG amplitude is NOT necessarily proportional to muscle force.

45. Right biceps brachii short head - 80% MVC – ramp-up-down in 40s.5 10 15 20 25 30 35 40 1 2 3 4 6 7
time (s)
EMG SD channel
1 mV
zoom 80 60 70 50
Torque %MVC
Next slide
Differential recording
Linear electrode array of 8 contacts (7 single differential channels)

46. Right biceps brachii short head - 80% MVC – zoom of ramp-up.
4.0
4.5
5.0
5.5
6.0
6.5
Time (s)
1 mV
zoom 1 2 3 4 5 6 7 80 60 40 20 70 50 30 10
Torque %MVC
Differential recording
Next slide
EMG SD channel
Linear electrode array of 8 contacts (7 single differential channels)

47. Right biceps brachii short head - 80% MVC – zoom of ramp-up.80 60 40 20 70 50 30 10
Torque %MVC
EMG SD channel
1 mV 1 2 3 4 5 6 7
5.58
5.60
5.62
5.64
5.66
5.68
5.70
5.72
5.74
5.76
5.78
time (s)
Linear electrode array of 8 contacts (7 single differential channels)

48. sEMG-Force relationship
The amplitude of sEMG reflects the degree of activation of skeletal muscles and is highly correlated to the muscle force. The relationship is generally monotonic (if the force iproduces by a muscle ncreases its sEMG increases) but is affected by many confounding factors. In general, many muscles contribute to the measured force but the detected sEMG may be associated to only e few of them. Excellent reviews on this topic are:
Vigotsky AD, Halperin I, Lehman GJ, Trajano GS, Vieira TM. Interpreting Signal Amplitudes in Surface Electromyography Studies in Sport and Rehabilitation Sciences. Front Physiol Jan 4;8:985. doi: /fphys
Disselhorst-Klug C, Schmitz-Rode T, Rau G. Surface electromyography and muscle force: limits in sEMG-force relationship and new approaches for applications.
Clin Biomech (Bristol, Avon) Mar;24(3): doi: /j.clinbiomech
MU 7
MU 5
MU 4
MU 3
MU 2
MU 1
MU 6
Order of recuitment

49. sEMG-Force relationship
A monotonic relationship between the sEMG amplitude ( RMS or ARV) and muscle force (for a single muscle) has been reported in the literature. This does NOT mean that muscle force can be estimated by means of sEMG.
As a muscle force progressively increases in a voluntary isometric contraction, motor units are recruited in order of increasing size (Henneman’s Principle). Small and large motor units may be distributed randomly in the muscle cross-section or may have a geometrical distribution (smaller units more or less superficial than the larger ones).
The sEMG-Force relationship is strongly affected by this pattern. An extreme example is shown here.
electrodes
Normalization values
100, 100 A
Normalized sEMG ampliude B
Normalized Force A B

50. + A B T The problem of force estimation and
load sharing among muscles.
Muscles acting on the same joint 1
MUSCLE A
Force/Torque A 2 +
Total force/torque NA
EMG A
Alpha-Motoneuron pulse trrains
Observable quantities (EMG may be affected by crosstalk and depth). 1
MUSCLE B
Force/Torque B 2 NB
EMG B
An open question: Can some feature of the sEMG provide estimation of force contributed by the single muscles in either isometric or dynamic conditions?
Other muscles
Many muscles are producing the torque T.
The sEMG of some
can be detected.
The torque measured with an isometric brace or a load cell is the sum of the contributions from many muscles. The sEMG detected on the surface does not necessarily reflect the activity of all of them and is dominated by the most superficial ones. T

51. Crosstalk is the signal detected on one muscle but generated by another one.V1 V2
Muscle A
Muscle B
Sources
Sensor 1
Sensor 2 a)
The signal detected by an EMG sensor is usually a mixture of contributions from the muscle of interest and neraby muscles. The separation of these contributions is an open problem. b) b c d + a
Source A SA
Source B SB
Mixture 1
Mixture 2 V1 V2
In general the coefficients a, b, c, d, are filters and nor just numbers.
A suitable spatial filter (or a “blind source separation technique” ) may be used to attempt the separation process. No consensus exists yet about reliable solutions.
V1 = a SA + c SB
V2 = b SA + d SB
Linear algebraic mixture:

52. Pinnate muscles (not parallel to the skin)
The surface potential may be VERY different in fusiform and pinnate muscles. You MUST know your muscles!
skin
Propagation is seen typically in fusiform muscles (biceps brachii, trapezius, vasti, etc)
The sEMGs from pinnate muscles (gastrocnemius, biceps femoris) with high pinnation angle do NOT show propagating signals. They show bursts of localized activity (end of fiber effects) and the innervation zone cannot always be detected.
skin
aponeurosis1
aponeurosis2

53. Pinnate muscles (not parallel to the skin)
Regions of non propagating sEMG activity associated to discharges of
MUs A, B, C, and D.
MU A: fibers 1, 2, 3, 5, MU B: fibers 4, 6, 7, 9
MU C: fibers 8, 10, 11, MU D: fibers 12, 14, 15, 16, 17 B C D A
electrodes
a b c d e f g h i l m n o p
skin
Subcutaneous tissue
Aponeurosi 1
proximal
muscle fibers d1
propagation
distal
NMJ
axonal branch
Aponeurosi 2 d2
A B C D
motoneurons
Muscle-tendon junctions (end of fiber effects) α
α = pinnation angle
Single fiber arrangement

54. The sEMG recorded with an electrode array is often a mixture of traveling and non travelling components.
travelling component
non-travelling component ms
time
This mixture makes the estimation of conduction velocity difficult and often impossible.

55. Additional sEMG topics of interest in neuromuscular physiology
(not adressed in this material)
Response of the neuromuscular system to electrical stimulation.
Concept of Common Drive of MUAPs.
Concept of MUAPs synchronization.
Decomposition of sEMG into the constituent MUAP trains.
Mechanomyogram and its applications.
Tensiomyogram and its applications.
sEMG in spasticity and cramps.
sEMG in denervated and reinnervating muscles.
sEMG for the control of prosthesis and external devices.