EMG Physiology

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CHAPTER 2

PHYSIOLOGY OF EMG

Electromyography (EMG), also referred to as myoelectric activity, measures the

electrical impulses of muscles at rest and during contraction. As with other

electrophysiological signals, an EMG signal is small and needs to be amplified with an

amplifier that is specifically designed to measure physiological signals. This signal can be

recorded or measured with an electrode, and is then displayed on an oscilloscope, which

would then provide information about the ability of the muscle to respond to nerve stimuli

based upon the presence, size and shape of the wave the resulting action potential. While

the electrode could be inserted invasively into the muscle (needle electrodes), a skin surface

electrode is often the preferred instrument, because it is placed directly on the skin surface

above the muscle without employing the method of pinch insertion into the test subject.

When EMG is measured from electrodes, the electrical signal is composed of all the action

potentials occurring in the muscles underlying the electrode. This signal could either be of

positive or negative voltage since it is generated before muscle force is produced and occurs

at random intervals.The EMG signal is first picked up by electrode and amplified. Frequently more than

one amplification stages are needed, since before the signal could be displayed or recorded, it

must be processed to eliminate low or high frequency noise, or any other factors that may

affect the outcome of the data. The point of interest of the signal is the amplitude, which can

range between 0 to 10 millivolts (peak-to-peak) or 0 to 1.5 millivolts (rms). The frequency of

an EMG signal is between 0 to 500 Hz. However, the usable energy of EMG signal is

dominant between 50-150 Hz [7]. The extended study of EMG signal characteristics can be

found at [6] and [8].

In order to obtain a signal that yields the maximum information, the method

employed and the implementation device has to be considered. There are many dependent

factors that could affect a surface EMG since the signal is susceptible to noise interference

such as hum, signal acquisition such as clipping and baseline drift, skin artifacts, processing


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errors, and interpretation problems. For example, the contact of electrode to the skin could

distort a recording signal. The inadequate amplification of the signal could cause a recorder

detection problem. A wrong filter could efface some of desirable information of a signal.

Moreover, there are other factors such as the distance between electrodes as well as the

recording times used in the experiment. The device utilized in the measuring of the signal

must also be considered since low-level input into a recording device could also affect data

and yield inaccurate results.

2.1EXCITABLE TISSUE AND ACTION POTENTIAL

There are two main types of tissue in the nervous system: excitable tissue and non-

excitable tissue. The excitable tissue, which is composed of neurons, responds to and

transmits nerve stimuli. The non-excitable tissue, composed of glial cells, does not responseto voltage or any other conventional stimulus, since glial cells are non-conducting, and

function only as support cells in the nervous system.

Excitable tissue can be divided into four components: sensory receptors, neuron cell

bodies, axons, and muscle fibers [45]. In a situation involving a harmful stimulus such as

contact with a sharp pebble or a hot surface, the resulting pain and pressure are transmitted

by sensory receptors. The pain is by a receptor potential, which is the transmembrane

potential difference of a sensory cell. Produced by sensory transduction, a receptor potential

results from inward current flow, which will bring the membrane potential of the sensory

receptor toward the threshold to trigger the neuron into generating a rapid burst of voltage

pulses called the action potential(APs).

As shown in Figure 2.1, triggered by a constant high-pressured stimulus, the sensory

receptor generates an initially high receptor potential that rapidly decreases to a much lower,

steady level. This decrease in the receptor potential is called adaptation. The action potential

produced by the neuron has a magnitude of 0.1 volts [45], which is a value that is shared by

every animal, from a squid to a human.

To step off that pebble, the neuron sends a message along a nerve axon to the base of

the spinal cord. The axon, or nerve fiber, is the slender projection of a neuron that conducts

electrical impulses away from the soma, or the nerve cell body. There are two types of axons:

the afferentaxon and efferentaxon. The afferent axon, or sensory axon, leads to the central


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Figure 2.1: Typical time variations associated with a sudden, steady stimulus.

nervous system, and carries messages from sensory receptors at the peripheral endings to the

spinal cord or brain. The efferent axon, or motor axon, originates at the spinal cord and

carries information through the body parts, synapse with muscle fibers to stimulate muscular

contraction as well as the muscle spindles to alter proprioceptive sensitivity, which is a key

factor in muscle memory and hand-eye coordination. Because these two types of axons are

designed to relay high-speed messages, their diameter is between 0.001 and 0.022

millimeters, which is longer than ordinary axons, which have a diameter between 0.0003 and

0.0013 millimeters [45]. When compared with the ordinary axons, the efferent and afferent

axons also have a thicker layer of myelin, an electrically insulating fatty layer that increases

the speed of impulses by means of saltatory conduction. Therefore, by inhibiting charge

leakage, myelinated axons propagate action potentials that recur at successive nodes rather

than waves, and thus hop along the axon, thereby increasing the speed of the impulse. With

a large diameter and thick of myelin sheaths, all signals can thus travel through the afferent

and efferent axons at speeds as high as 120 meters per second, or 270 miles per hour [45]. On

the other hand, the ordinary axons, which are solely responsible for simple activities such as


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reporting pain and temperature changes, have small diameters and unmyelinated fibers,

which are adequate to carry slow-speed signals.

As shown in Figure 2.2, the afferent axon carries the action potential burst from the

neuron to the interneuron, a neuron that communicates only to other neurons, or to the motor

neuron. This causes a chemical transmitter to be released across a narrow fluid gap called

synapses. The latter are specialized junctions that allow neurons to signal to their target cell,

which could be another neuron or a non-neuronal cell such as a muscle or gland. The action

potential crosses this junction to either another interneuron or a motoneuron, triggering

another action potential burst as the process repeats until the message reaches the efferent

axon, which then carries the action signal back down to the leg muscle. Once the signal

reaches the muscle tissue, the message instructs the muscle to contract, resulting in lifting the

foot off the pebble.

Figure 2.2: Excitable tissue called into play when a person steps on a sharp pebble.


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2.2GENERATION OF ACTION POTENTIAL

As stated earlier, after a sensory receptor generates information, this electric signal is

transmitted to its intended target by traveling through an axon. However, an axon is a

relatively poor conductor because it rapidly attenuates the electrical signal. The potential can

decrease to 37 % of its original value after traveling a distance of only 0.15 millimeters along

an axon, resulting in an unusable potential value [46]. This distance in which the potential

becomes unusable is called the length constant. The length constant is dependent upon the

size of the axon, as it is proportional to the square root of an axon diameter.

To overcome this tendency of signal attenuation, the nervous system uses a method to

increase the strength of the electric signal. When the potential decreases to a threshold level,

such as eight millivolts, the neuron will fire another 100 millivolts action potential [46].

However, the action potential will keep decreasing after travel through the axon, which in

effect will stimulate the neuron to fire one burst of action potential after action potential, a

process that is referred to asfrequency modulation. For example, in order to make a potential

increase to 10 millivolts, the neuron might fire ten times per second, although the neuron is

also able to extinguish voltage in order to end action potential.

To get more insight in this process, one must understand the structure of the axon.

There are ions arranged in constant random thermal motion inside an axon, with protein

molecules being one of the main components of the axon membrane. Under normalconditions, sodium (Na

+) and calcium (Ca

2+) are more concentrated in the extracellular fluid,

while potassium (K+) is more concentrated within the cell. In effect, K

+is the key

determinant of the resting membrane potential, since the resting cell membrane is more

permeable to K+

than to the Ca2+

and Na+

molecules [14]. However, while it plays a small

part in the resting membrane potential, Na+

is a key player in the generation of electric

signals. When a cell goes from a resting to an excited state, orfiring level, the cell increases

its Na+

permeability. This causes Na+

molecules to enter the cell through voltage-gated

channels, thus moving down its chemical gradient. This addition of the positive charge of

Na+

to the intracellular fluid causes the cell to become depolarizedand initiates an action

potential. (M.S. Gordon, 1972). The extinguishinglevel that marks the falling phase of the

action potential is the result of an increase in K+

permeability in the cell. However, the

closing and opening of the voltage-gated channels is regulated by the jostling of the atoms


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within the cell, which results in randomness in the train of the generated action potentials.

Consequently, any undesired departure from a perfectly ordered system may give rise to so-

called noise. Higher receptor potential will initiate less noisy action potentials. However, a

noisy system is not always bad, as it enables living things to be able to adjust themselves to

changing environment [46].

The action potential is not only in the shape of narrow spike. Alan L. Hodgkin and

Andrew F. Huxley (1952) also suggest another model of action potential as illustrated at the

top of Figure 3-6 [46]. They applied a +20 millivolts trigger at zero time to the giant axon of

the squid, and found that during an action potential, an ion moves to the axon membrane by

using its protein molecules to create bridge to the membrane. The protein molecules are

unique for each ion species, with the Na+

ions trying to diffuse into the axon while the K+

ions are trying to diffuse out of the axon membrane.

Figure 2.3: Action potential model described by Alan L. Hodgkin and Andrew F.

Huxley.


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From Figure 2.3, the sodium in curve is negative because the current that flows into

the axonplasm is defined as negative. On the other hand, the current that flows out of the

axonplasm is defined as positive. The sodium carrier proteins convey Na+

ions into the axon

in accordance with the sodium in curve. When crossing the membrane, there is only low

voltage left to drive the sodium ions into the bridges of the transport proteins. Consequently,

the dip of the sodium curve exists at the peak of action potential curve. On the opposite side

of the sodium curve, the potassium carrier proteins convey K+

ions out of the axon in

accordance with the potassium out curve. To the left of the sodium dip, the Na+

current in

is much greater than the K+

current. As a result, the voltage rapidly rises to 100 millivolts

above the resting potential. To the right of the dip, the potassium ions are small excess to the

sodium ions, which marks the slow drop in voltage.

2.3PROPAGATION OF ACTION POTENTIAL

In this section, the focus will be on how unmyelinated and myelinated axons generate

their action potentials. Regenerating nerve fibers could either be unmyelinate or myelinated

[47]. As stated earlier, unmyelinated fibers have thin membranes to carry slow-speed signals.

On the other hand, myelinated axons have thick membrane allowing them to carry high-

speed signals.

In unmyelinated fibers, the AP propagates in the form of an ocean wave. When the

voltage across the membrane rises above the threshold level of eight millivolts, a

regenerating axon starts to generate an action potential. The thin membrane of the

unmyelinated axon allows ions to easily move across the membrane. Figure 2.4 shows the

AP waveform initiated by an unmyelinated axon.

Please note that the net ion current of Figure 2.4 is based on the model of the impulse

of an RC cable, which is a simpler and less accurate model than the Hodgkin and Huxley

model. Therefore, the net ion current curve in this section is different from that in section 2.2.

Figure 2.4 (a) depicts the net ion current that generates the action potential. The

voltage rises to its peak at 100 millivolts before decreasing in value. Figure 2.4 (b) shows the

curve of voltage and current versus distance. It is interesting to see the distance and time

curve are mirror image of each other. While generally the distance curve can differ from the


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time curve, the myelinated axon can generate the distance curve and time curve of the action

potential that has the same shape and amplitude [47].

Figure 2.4: Net ion current curve and action potential curve of an unmyelinated axon.

In comparison to unmyelinated axons, myelinated axons have a thicker wall. This

makes it impossible for sodium and potassium ions to move across the axon membrane. As a

result, regeneration of the action potential cannot occur. However, the thick membrane also

enables the axon to carry high voltage messages without breaking down. (P. Morell and W.T.

Norton, 1980)

The thick, non-regenerating myelin membrane is offset by periodic nodes, which are

also known as nodes ofRanvier[47]. The nodes are 100 outside diameter apart, as illustrated

at the top of Figure 2.5.

From Figure 2.5, the upper-wave form is an action potential, initiated by the first

node on the left. Then, theoretically, the action potential would fall to the dashed curve as

showed in the bottom of the picture. Instead, when the action potential reaches eight

millivolts, as indicated by the . (dots), the second node fires to generate a new action

potential. as shown in the lower wave form. This generation at the node is the same as that of

the unmyelinate axon.


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Figure 2.5: Nodes in the myelinated axon and action potential that generated.

2.4EMGELECTRODES

The EMG electrode could be explained by a receiving antenna concept. A receiving

antenna is an electrical device that detects oscillating magnetic fields, which are generated

from various sources. Then the signal is transmitted through the air from source to the

receiving antenna, a concept that is used to engineer the design of electrode. In terms of

recording the EMG signal, the muscle fiber is a biological signal generator, spreading out

over voltage fields to the volume-conductivity surrounded by fluid [18]. This fluid serves to

convey an EMG signal to an electrode, like air carry signals to an antenna.

The EMG recording starts from the beginning of the bioelectrical events as shown in

Figure 2.6. The changing conductivities in the membranes will make action currents flow

across the membranes as well as into the extracellular fluids around active cells. The

extracellular currents will then generate potential gradients as they flow through the resistive

extracellular fluids. The changing potential gradients, subsequently, will produce electrical

currents in the electrode leads by capacitive conductance across the metal/electrolyte


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interface of the electrode contacts. These weak currents will then flow through the high-

impedance circuits of the amplifier input stages, which will then convert these currents into

large output voltages.

Figure 2.6: The series of bioelectrical events.

The EMG electrodes can be classified by using its geometry. There are six classes of

EMG electrodes: monopolar electrode, bipolar electrode, tripolar electrode, multipolar

electrode, barrier or patch electrode, and belly tendon electrode.

A monopolar electrode takes potential from electrode and ground as the inputs to the

differential amplifier. When measuring, only a bare electrode is placed, without utilizing

other electrical connection. Because the ground yields a negative input to differential

amplifier, the potential from electrode is always based on ground.


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A bipolar electrode is used to measure the voltage different between two specific

points. It generally must be used with a differential amplifier. A bipolar electrode has two

contacts that are not connected to each other. Therefore, one node will be used for positive

input, and the other will be used for a negative input for the differential amplifier. Because

the differential amplifier treats both inputs equally, it will yield an accurate output. However

the distance between the electrodes could affect the measurement result. Placing the

electrodes too far from one another could yield a weak signal. On the other hand, placing

them too close may also result in unusable data, since the amplifier preprocesses each inputs

signal separately before subtracting those signals for output. In addition, there must be

another contact used as a reference point for these two inputs.

A tripolar electrode has three electrodes that are placed at equal intervals along a

straight line. The central electrode is usually connected to the positive input of a differential

amplifier, while the electrodes on the sides are usually connected to the negative input of a

differential amplifier. This configuration also requires another electrode to serve as a

reference. The tripolar electrode is often used to record nerve potentials, as its configuration

holds the advantage of being able to reject some forms of biological noise.

A multipolar electrode consists of rows of bipolar electrodes where an equal lead is

connected each side of bipolar electrodes to serve as a positive and negative input for a

differential amplifier. Besides, another electrode must be applied as a reference point. The

multipolar electrode is often used to record the activity of certain motor units based on

idiosyncrasies in their fiber locations.

The barrier or patch electrodes are typical bipolar electrodes that are closely

connected to a dielectrical barrier. The dielectrical is a non conductive substance that is

placed between the electrodes. This configuration redirects currents in extracellular flowing

around the tissue nearby. The patch also keeps the currents that are generated from tissues on

each side to prevent them from spreading into each other. Consequently, the potential

gradient of a desired action is larger, and the potential gradient of an undesired action is

smaller.

A belly tendon electrode is one of the fields of interest in the clinical EMG. Its

geometry is an interesting hybrid of the monopolar and bipolar approaches. In this technique,

the first electrode is placed in or over the middle point of the muscle of the belly, which


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serves as the positive input to the amplifier. The second electrode is placed over the tendon

of the same muscle, which is usually about the end of contractile elements, and serves as the

negative input to the amplifier. This arrangement gives a clean leading negative waveform,

since there is no virtual active contribution from tendon electrode. A belly tendon electrode is

employed specifically for tendon applications although it is not used for measuring a

selective muscle EMG recording during physiological activity.

All of electrode geometries discussed above could be considered as a dipole antenna

in term of electrical behavior. Monopolar electrodes are used to measure the EMG signal of

very small muscle. This is a good approach for sampling a signal that occurs near the surface

of an active single fiber. On the other hand, tripolar electrodes and multipolar electrodes are

used for sampling some large muscles.

2.5AMPLIFIER OF EMGSIGNALS

The amplifier is an electronic device that serves to boost low power signal to higher

power signal that is usable to perform work. There are two reasons to amplify the signal [19].

First, amplification increases the level of signal enough to protect an electrical interference

during transmission. Second, the signal is amplified so that it could be stored in a storage

device, or displayed by a measurement device like oscilloscope. In case of an EMG signal, an

amplifier is necessary. There are no such devices that can measure EMG signal without

amplification.

A differential amplifier is used to amplify an EMG signal, as it has the ability to

eliminate the noise from the signal [7]. As shown in Figure 2.7 the differential amplifier

takes two inputs, subtracts them and amplifies the different. In this case, if there is noise

interference through the input wires, the noise could be circuitry canceled out so long the

transmission of the two inputs is completely symmetrical manner.

It is difficult to make an amplifier with perfect subtraction. The Common Mode

Rejection Ratio (CMRR) could measure the accuracy of subtraction in each amplifier. It is

suggested to have a CMRR value at 90dB in order to sufficiently discard a contaminated

noise [7]. Yet with modern technology, the differential amplifier could make a CMRR value

of 120dB. However, even though a differential amplifier has the ability to reduce unwanted

noise signals that occur from both sides of the input wires, contaminated noises could still


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exist. This noise could have been injected into the signal by a stray capacitance that has been

amplified, and thus, degrading the signal [19].

Figure 2.7 A schematic of the differential amplifier configuration. The EMG signal is

represented by m and the noise signal by n.

Every electronic component or even an amplifier itself behaves as an effective filter,

since there are no such electronic devices that can transfer all frequency range. The electrode

itself tends to have lower impedance for a higher frequency and have higher impedance for a

lower frequency [19]. The connection of electrode, cable and amplifier creates an implicit

filter effect. The electrode contacts are connected in series to an amplifier; they function

similar to that of a capacitor, while an impedance of amplifier is similar to that of a resistor.

This connection visualizes a High-Pass filter circuit. The low frequency voltage tends to be

attenuated and drop the highest voltage across the electrode contacts rather than the

amplifier. On the other hand, the cables that connect electrodes to an amplifier have a stray

capacitor behavior. It is considered that this capacitor is connected to ground, which

simulates a Low-Pass filter circuit. The stray capacitor will provide low impedance, at which

the high frequency picked up by electrodes tend to drop their voltage here. Therefore an

amplifier will see an attenuation of high frequency.

The implicit filter could cause signal problems if it is not considered carefully in the

design of an amplifier. The explicit filter with real components (resistor and capacitor)

functions by using the same concept of an implicit filter, and could help in increasing the

signal-to-noise ratio. Since a signal is desired to be within in some frequency range, it is good


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idea to have an explicit filter for that particular band. Therefore, noise with the frequency

outside a desired frequency band will be distorted. (see Figure 2.8)

Figure 2.8 Implicit RC filter.

The implicit filter is important in designing a differential amplifier. To reduce an

implicit capacitance effect, the electrode contact should be placed close to an amplifier. In

other word, an amplifier should be located as close to the signal source as possible.

A raw EMG signal is an AC signal. Its bandwidth could occur anywhere from a few

tens of cycles per second to 3000 cycles per second. Therefore, sometime a large amount of

DC voltage appeared at the output of preamplifier [19]. A DC offset can be removed by

adding a series capacitor to the output, which also allows the AC signal to pass through. The

DC offset can also happen between the recording electrodes themselves, especially if they are

made from different materials. A battery-powered source sometime could overpower a

sensitive preamplifier, which could also disgrace the contact of electrodes or damage

surrounding tissue. Placing a capacitor between inputs of AC coupling could prevent this


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problem. (In chapter 3, Figure 3.6, a small capacitor is placed at the inputs to the preamplifier

to prevent this problem.)

A DC-offset-adjust potentiometer is often added to sensitive equipment, since it is

easy to adjust the DC-offset. However, the DC offset also dependents on temperature [19].

Therefore, it is better to power up the equipment for sometime before adjusting the DC

offset.

2.6PROBLEM WITH NOISE AND ARTIFACTS

In the process of recording an EMG signal, the source of the generated signal is not

only from bioelectrical generator or active cell, but also from any electrical fields that occur

around an electrode and lead cables. These electrical fields produce some signals that could

also be added to an EMG signal, causing a form of interference that is called noise.Interference noise can be produced from anything that has an electrical field such as power

lines, computer monitors, transformers, or EMG amplifier itself. Once noise has occurred, it

could cause problem in recording an EMG signal. Therefore while planning out the design of

the amplifier device and recording the EMG signal, noise factors should be taken into

consideration, since noise could come from a variety of sources such as electronic

components, recording devices, ambient noise, motion artifacts, or inherent instability of the

signal [7].

Any electronic devices can produce noise. The noise frequency is range between 0 Hz

to a 1000 Hz [7]. This kind of noise cannot be eliminated. Using an intelligent circuit design

and a good quality of electronic components to construct the device can only reduce the

noise.

Ambient noise could be generated from any electronic device that created an

electromagnetic field such as televisions, computer monitors, motors, electrical power lines,

fluorescent lamps or light bulbs. In fact there are radio waves and magnetic fields floating all

over our body. It is virtually impossible to drain these radiators to ground (earth surface). The

ambient noise also cannot be avoided. The ambient noise frequency occurs primarily within

the range 50 Hz or 60 Hz [7] [44], while the amplitude of an ambient noise is about one to

three times grater than that of an EMG signal.


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Motor artifacts come from two sources [7]; first, from the contact of an electrode to

skin; second, from the connection of cable from electrode to the amplifier. When performing

a grasping experiment, a movement of the wire alone could cause a noise problem. This

electrical noise from both sources has the frequency range between zero to twenty hertz [7].

However, a proper design of circuitry with a good connector and stable electrode contact

could reduce this motion artifact problem.

Inherent instability of the signal is caused by a nature of EMG signal. The amplitude

of the EMG signal frequency range between zero hertz to twenty hertz is particularly

unstable due to the quasi-random nature of the firing rate of motor units. It is suggested to

consider an EMG signal frequency in this range as an unwanted noise signal [7].

There is no boundary in what amplitude of an EMG signal is good for yielding

accurate recordings. While a certain amount of noise could be tolerated, the question is

exactly how much noise could be allowed. In other words, there is a question about the

tolerable levels of thesignal-to-noise ratio. The signal-to-noise ratio is determined by taking

the ratio of the amplitude of desire signal over the amplitude of added noise signal. The main

concern now is in the lever of the signal-to-noise ratio that could degrade an analysis result.

If the noise is produced from thermal motion, then twice of desired signal amplitude over the

noise amplitude is good enough. However, if the noise occurred periodically (pause like) and

forms a pattern similar to the desired signal, it is suggested that ten times grater of the desire

signal amplitude than the noise amplitude would be clarified a confusing event [20]. In order

to determine signal-to-noise ratio, the study of noise behavior alone may be required to see

how noise could affect the real signal.

EMG Physiology

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