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EMG Theory and Facts
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What is electromyography (EMG)?
What is Action Potential?
What is Resting Potential?
What is Nerve Conduction Velocity?
What is Acoustic Myography (AMG)?
List of textbooks.
List of conferences.
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What is electromyography (EMG)?
Electromyography (EMG) is a medical technique for evaluating and recording physiologic properties of muscles at rest and while contracting. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest. Electrical Characteristics:
The electrical source is the muscle membrane potential, about −70mV. Due to the applied method the resulting measured potentials range between smaller than 50 μV and 20 to 30 mV. Typical repetition rate of muscle unit firing is about 7–20 Hz, depending on the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other factors. Damage to motor units can be expected at ranges between 450 and 780 mV.
Procedure:
To perform EMG, a needle electrode is inserted through the skin into the muscle tissue. A trained medical professional (most often a physiatrist, neurologist, or physical therapist) observes the electrical activity while inserting the electrode. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Normal muscles at rest make certain, normal electrical sounds when the needle is inserted into them. Then the electrical activity when the muscle is at rest is studied. Abnormal spontaneous activity might indicate some nerve and/or muscle damage. Then the patient is asked to contract the muscle smoothly. The shape, size and frequency of the resulting motor unit potentials is judged. Then the electrode is retracted a few millimeters, and again the activity is analyzed until at least 10-20 units have been collected. Each electrode track gives only a very local picture of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode has to be placed at various locations to obtain an accurate study.
A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate. After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibers of that particular motor unit. The sum of all this electrical activity is recorded as a motor unit potential. This electrophysiologic activity is evaluated during an EMG. The composition of the motor unit, the number of muscle fibers per motor unit, the metabolic type of muscle fibers and many other factors affect the shape of the motor unit potentials in the myogram.
Nerve conduction testing is also often done at the same time as an EMG. Because of the needle electrodes, EMG may be somewhat painful or extremely painful to the patient, and the muscle may feel tender for a few days. There also exists "needleless EMG"—an EMG performed using surface electrodes—though it gives much less accurate results with a higher level of disturbance from the surrounding environment.
Normal Results:
Muscle tissue at rest is normally electrically inactive. After the electrical activity caused by the irritation of needle insertion subsides, the electromyograph should detect no abnormal spontaneous activity (i.e. a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is normally electrically very spontaneously active)When the muscle is voluntarily contracted, action potentials begin to appear. As the strength of the muscle contraction is increased, more and more muscle fibers produce action potentials. When the muscle is fully contracted, there should appear a disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern).
Sources:
MedlinePlus entry on electromyography
http://www.wikipedia.org
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What is Action Potential?
An action potential is a wave of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They are also exhibited by some plants. Action potentials can be created by many types of cells, but are used most extensively by the nervous system for communication between neurons and to transmit information from neurons to other body tissues such as muscles and glands.
Action potentials are not the same in all cell types and can even vary in their properties at different locations in the same cell. For example, cardiac action potentials are significantly different from the action potentials in most neurons. This article is particularly concerned with the "typical" action potential of axons.
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What is Resting Potential?
The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. In most cells the resting potential has a negative value, which by convention means that there is excess negative charge inside compared to outside. The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. How the concentrations of ions and the membrane transport proteins influence the value of the resting potential is outlined below.
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What is Nerve Conduction Velocity?
Nerve conduction velocity (NCV) test is a measurement of the speed of conduction of an electrical impulse through a nerve. NCV can determine nerve damage and destruction.
During the test, the nerve is stimulated, usually with surface electrode patches attached to the skin. Two electrodes are placed on the skin over the nerve. One electrode stimulates the nerve with a very mild electrical impulse and the other electrode records it. This is repeated for each nerve being tested.
The nerve conduction velocity (speed) is then calculated by measuring the distance between electrodes and the time it takes for electrical impulses to travel between electrodes.
Nerve conduction studies are used mainly for evaluation of paresthesias (numbness, tingling, burning) and/or weakness of the arms and legs.
Reasons for the Procedure
Nerve conduction velocity is often used along with an EMG analysis to differentiate a nerve disorder from a muscle disorder. NCV detects a problem with the nerve whereas an EMG detects whether the muscle is functioning properly in response to the nerve's stimulus.
Diseases or conditions that may be evaluated with NCV include, but are not limited to, the following:
• Guillain-Barré syndrome - a condition in which the body's immune system attacks part of the peripheral nervous system. The first symptoms may include weakness or tingling sensations in the legs.
• Carpal tunnel syndrome - a condition in which the median nerve, which runs from the forearm into the hand, becomes pressed or squeezed at the wrist by enlarged tendons or ligaments. This results in pain and numbness in the fingers.
• Charcot-Marie-Tooth disease - a hereditary neurological condition that affects both the motor and sensory nerves. One characteristic is weakness of the foot and lower leg muscles.
• Herniated disc disease
• Chronic inflammatory polyneuropathy and neuropathy - conditions resulting from diabetes or alcoholism
• Sciatic nerve problems
• Pinched nerves
• Peripheral nerve injury
Nerve conduction studies may also be performed to identify the cause of symptoms such as numbness, tingling, and continuous pain.
The nerve conduction study (NCS) consists of the following four components:
1. Motor NCS
2. Sensory NCS
3. F-wave study
4. H-reflex study
Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a muscle supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the recording site is measured. This value is called latency and is measured in milliseconds (ms). The size of the response - called amplitude - is also measured. Motor amplitudes are measured in millivolts (mV). By stimulating in two or more different locations along the same nerve, the NCV across different segments can be determined. Calculations are performed using the distance between the different stimulating electrodes and the difference in latencies.
Sensory NCS are performed by electrical stimulation of a peripheral nerve and recording from a purely-sensory portion of the nerve, such as on a finger. Like the motor studies, sensory latencies are also measured in ms. Sensory amplitudes are measures in microvolts (ųV). The sensory NCV is calculated based upon the latency and the distance between the stimulating and recording electrode.
F-wave study uses stimulation of a motor nerve and recording of action potentials from a muscle supplied by the nerve. This is not a reflex, per se, in that the nerve potential travels from the site of the stimulating electrode in the limb to the spinal cord and back to the limb in the same nerve that was stimulated. The F-wave study evaluates conduction velocity of nerves between the limb and spine, whereas the motor and sensory nerve conduction studies evaluate conduction in the limb itself.
H-reflex study uses stimulation of a nerve and recording the reflexive electrical discharge from a muscle in the limb. This also evaluates conduction between the limb and the spinal cord, but in this case, the afferent impulses (those going towards the spinal cord) are in sensory nerves while the efferent impulses (those coming from the spinal cord) are in motor nerves.
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What is Acoustic Myography (AMG)?
Acoustic myography is the recording of sounds produced by contracting muscle. Acoustic signals can be amplified using a standard phonocardiograph, recorded, and digitally analyzed.
With fatigue, the acoustic amplitude decays, but the surface EMG amplitude doesn't. With decreased effort, however, the acoustic and the surface EMG amplitudes decline simultaneously. Analysis of acoustic signals from the muscle provides a noninvasive method for monitoring the motor unit fatigue in vivo. It may also be useful in distinguishing muscle fatigue from decreased volition.
The main frequency of the muscle sound is 25 Hertz, which is at the lower limit of human hearing. The mechanical stethoscope, used to listen to sounds in the body, is not suitable to listen to muscle sounds. In fact, it filters out most sounds below 50 Hertz. Oster has conducted experiments in which the subject supports a lead weight in the palm of his hand, while the muscle sounds are recorded from the biceps. When the weight is held steady to maintain constant contraction, the amplitude of the muscle sound is directly proportional to the weight. This fact implies that the measurement of muscle sounds can be used to determine how hard a muscle is working. With a weight held in the hand, the least sound comes from the biceps when the angle between the forearm and upper arm is 115 degrees.
Origin of Muscle Sounds
It appears that the rumbling sounds come from resonant frequency vibrations of muscle fibers. When the muscle contracts, the continuous rumbling tone does not always start immediately. With slight contractions, clicks or sharp pulse tones are sometimes heard. With increasing contraction, the time between clicks shortens until the sound merges into the low rumble. The clicks come from the muscle motor units.
Muscle Sounds in Physical Medicine and Rehabilitation
Barry et al. (1985) found that human muscle sounds were intrinsically tied to contraction and that sound amplitude declined during muscle fatigue while the surface EMG signals did not decline. Therefore, the ratio of acoustic amplitude to EMG amplitude can be used as a measure of the loss of electromechanical coupling that accompanies muscle fatigue. The data shows that the acoustic signal RMS amplitude increases with increasing force of contraction. The relationship of RMS amplitude to load is approximately linear in the mid range, with non-linearities appearing in both low load and high load conditions. The acoustic myograph signal, produced with maximal isometric effort, parallels declining force with fatigue. When a subject chooses to reduce effort, the EMG, AMG, and force amplitudes decline simultaneously. This could be a valuable parameter in distinguishing motor unit fatigue from lack of effort.
Acoustic myography, electromyography and bite force in the masseter muscle
Acoustic myography (AMG) offers some advantages over electromyography (EMG) in certain circumstances, but the use of AMG on the jaw-closing muscles has not been fully tested.
Tortopidis tested the AMG, which was recorded using a piezoelectric crystal microphone and the EMG recorded simultaneously with surface electrodes. They also record the force between the anterior teeth with a strain-gauge transducer. Analysis showed that Pearson's correlation coefficient was 0•913 for force/AMG and 0•973 for force/EMG in all subjects, indicating a linear relationship between force, AMG and EMG at the four different force levels tested (25–75% of maximum). It is apparent that AMG may be used as an accurate monitor of masseter muscle force production, although some care is required in the technique.
Sources:
Acoustic Myography (AMG), A Live Demonstration by ®GRASS
Barry DT, Geiringer SR, Ball RD, Acoustic myography: a noninvasive monitor of motor unit fatigue.
D. Tortopidis, M. F. Lyons & R. H. Baxendale, Acoustic myography, electromyography and bite force in the masseter muscle.
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List of
Biomedical and EMG textbooks
Biomedical Signal Analysis by Prof. Rangaraj M. Rangayyan
Electrodiagnosis in Diseases of Nerve and Muscle by Prof. Jun Kimura
Electromyography: Physiology, Engineering and Non-Invasive Applications by Prof. Roberto Merletti and Prof. Philip Parker
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List of conferences
Synaptic Inhibition in Health and
Disease conference-
October15-16, 2009,
Chicago, Illinois, USA
Neuroscience 2009-
October17-21, 2009,
Chicago, Illinois, USA
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