Electromyography (EMG)
Electro – electrical
Myo – muscle
Graphy – record
Electromyography – involves recording the electrical activity of muscle
Electromyogram – electrical signal associated with the contraction of a muscle
Selected Historical Events Related to EMG
John Basmajian (1921- 1965) wrote the bible of electromyography entitled muscles .De Luca summarized the existing knowledge and research on muscle function as revealed by EMG studies.
ELECTRICAL CHARACTERITICS
The electrical source is the muscle membrane potential of about -70mV.
Measured EMG potentials range between
< 50 μV up to 20 to 30 mV, depending on the muscle under observation.
Typical repetition rate of muscle unit firing is about 7-20 Hz.
Damage to motor units can be expected at ranges between 450 and 780 mV
Recording of EMG:
Recording is done by the following three phase system.
Input
Amplifier
Display/ Audiovisual Output
Input:
Electrodes
All normal muscles contract, the muscle fiber in the motor unit depolarize and repolarize at the same time and a local disturbance is produce in the muscle. This disturbance can be detected either by the surface electrodes or needle electrodes (recording electrodes). There are two methods by which we can detect this disturbance by monopolar technique or bipolar technique. The number of electrodes are used for any study are three in number.
1) Recording electrodes
2) Reference electrode
3) Ground electrode
The ground electrode provides a mechanism for cancelling out the interference affect of external noise.
Monopolar technique:
In monopolar technique, the recording electrode should be placed over the muscle belly or inserted in the muscle fibers. The second electrode (reference electrode) is placed over the area where the muscle is inserted. The reference electrode should be surface electrode. The ground electrode should be placed near recording electrode.
Bipolar technique:
In bipolar technique, the two surface electrodes (recording as well as reference) are placed over the muscle belly in the longitudinal direction parallel to the muscle fibers and when the needle electrodes are used two wires are inserted through the cannula in the muscle belly. In this method ground electrode is not needed.
Amplifier:
The electrical activity derived from the body is very small i.e. mv or μv and contain undesired signals. The amplifier is conditioned to amplify that undesired signals and the useful signals derived from the motor
Display system:
After the signal is processed and amplified, it is displayed on the CRO which permits visual display of the motor unit. The CRO does not provide a permanent record, it only allows the signal to be displayed for a few seconds. However, if a photographic system is attached to the CRO a permanent record can be obtained. A CRO consists of the electric gun, screen, horizontal and vertical plates.
Techniques
Needle electromyogaphy
Skin preparation and Risk
Maximal voluntary contraction
EMG signal decomposition
EMG signal processing
ELECTRODE TYPES
Intramuscular –
Needle Electrodes
EMG PROCEDURE
Clean the site of application of electrode;
Insert needle/place surface electrodes at muscle belly;
Record muscle activity at rest;
Record muscle activity upon voluntary contraction of the muscle.
Needle Electromyography: Techniques
Needle electrode is inserted into the muscle
Needle is disposable, single use
Multiple muscles are accessible for examination
Combination of muscles tested
Dependent upon clinical question
Level of discomfort is mild
Needle Electromyography: Data
Insertional Activity
Spontaneous Activity
Motor Unit Configuration
Motor Unit Recruitment
Interference Pattern
Needle Electromyography: Data
Motor Unit Configuration
Single motor unit: A motor axon and all its muscle fibers
Motor Unit Configuration: Amplitude, Duration, Morphology
Muscle is volitionally activated at different force levels
Needle recording properties enable assessment of single MUs
Motor Unit Recruitment
Pattern of motor unit activation with increasing volitional activation
EMG: Spontaneous Activity
Fasciculation Potential
EMG: Spontaneous Activity
Positive sharp waves
Fibrillation Potentials
Skin preparation and Risks
The first step before insertion of the needle electrode is skin preparation. This typically involves simply cleaning the skin with an alcohol pad.
The actual placement of the needle electrode can be difficult and depends on a number of factors, such as specific muscle selection and the size of that muscle. Proper needle EMG placement is very important for accurate representation of the muscle of interest, although EMG is more effective on superficial muscles as it is unable to bypass the action potentials of superficial muscles and detect deeper muscles. Also, the more body fat an individual has, the weaker the EMG signal
Maximal voluntary contraction
One basic function of EMG is to see how well a muscle can be activated. The most common way that can be determined is by performing a maximum voluntary contraction (MVC) of the muscle that is being tested.
Muscle force, which is measured mechanically, typically correlates highly with measures of EMG activation of muscle. Most commonly this is assessed with surface electrodes, but it should be recognized that these typically only record from muscle fibers in close approximation to the surface.
EMG signal processing
Rectification is the translation of the raw EMG signal to a single polarity frequency (usually positive). The purpose of rectifying a signal is to ensure the raw signal does not average zero, due to the raw EMG signal having positive and negative components. It facilitates the signals and process and calculates the mean, integration and the fast fourier transform (FFT).
When to order EMG
Mononeuropathy
Mononeuropathy Multiplex
Radiculopathy
Plexopathy (Brachial or Lumbosacral)
Anterior Horn Cell Disorders
Diffuse neuropathies
Cranial neuropathies
Neuromuscular Junction Disorders
Myopathy
When Not to order EMG
Central Nervous System Disorders (Stroke, TIA, Encephalopathy, spinal cord injury)
Multiple Sclerosis
Total body fatigue, fibromyalgia
Joint pain
Unexplained weakness (without a neurologic consultation)
Failed back, S/P multiple neck and low back surgeries
In place of a neurologic consultation
Uses
EMG testing has a variety of clinical and biomedical applications.
EMG is used as a diagnostics tool for identifying neuromuscular diseases, or as a research tool for studying kinesiology, and disorders of motor control.
EMG signals are sometimes used to guide botulinum toxin or phenol injections into muscles.
EMG signals are also used as a control signal for prosthetic devices such as prosthetic hands, arms, and lower limbs.
Needle EMG may aid with the diagnosis of nerve compression or injury (such as carpal tunnel syndrome, nerve root injury (such as sciatica), and with other problems of the muscles or nerves.
Less common medical conditions includeamyotrophic lateral sclerosis, myasthenia gravis, and muscular dystrophy.
EMG then acceleromyograph may be used for neuromuscular monitoring in general anesthesia with neuromuscular-blocking drugs, in order to avoid postoperative residual curarization
What to Expect From an
EMG Report
A clinically and physiologically relevant interpretation/diagnosis
An outline of the localization, severity, and acuity of the process
Notation of other diagnoses that are detected/excluded
Explanation of any technical problems
Utility of EMG
Highly sensitive indicator of early nerve injury
Detects dynamic and functional injury missed by MRI
Provides information regarding chronicity of nerve injury
Provides prognostic data
Highly localizing
Clarifies clinical scenarios when one disorder mimics another
Identifies combined multi-site injury, avoiding missed diagnoses
Identifies more global neuromuscular injury with focal onset
Provides longitudinal data for charting course, response to therapy
Identifies more global neuromuscular injury with focal onset
Provides longitudinal data for charting course, response to therapy
Electromyography and Temprature Feedback techniques
EMG feedback as a muscle reeducation technique
In an effort to evaluate the efficacy and function of EMG feedback in muscle reeducation, improvement of the abductor function of the abductor hallucis muscle was studied under three training conditions involving
1) EMG feedback,
2) sensory stimulation or
3) equal time for unassisted practice;
4) control condition involving testing without training.
Active range of motion was measured before and after training to assess ability to use the muscle as an abductor. EMG activity was quantified for a 1-minute test contraction to evaluate ability to maintain and maximize a voluntary contraction of the target muscle.
The results indicated that EMG feedback was highly effective when subjects had little initial use of the target muscle.
EMG feedback improved the ability of these subjects to maintain and maximize voluntary muscle contractions, as demonstrated on the EMG measure.
EMG feedback did not add to the learning situation when only a relatively brief, phasic contraction was required, as on the range-of-motion measure; similar gains were made with equivalent practive without EMG feedback.
When subjects already had considerable use of the target muscle prior to training, EMG feedback may have actually interfered with training; in this case unassisted practice was more effective.
Skin Temperature Biofeedback
Skin temperature biofeedback, also called Thermal Biofeedback, is the most common of all biofeedback techniques. Temperature biofeedback focuses on teaching you to alter your hand temperature.
A thermistor is attached to one finger of your dominant hand. Changes in temperature as small as one tenth of a degree are registered and fed back to you through a digital display. Your job is to increase or decrease the temperature of your hand.
Using thermal biofeedback relaxation to alter temperature of the body was one of the first biofeedback techniques to be used for healing.
Researchers found that this particular method was useful in treating Raynaud’s phenomena and migraine headaches.
EMG Muscle Tension Biofeedback
EMG biofeedback technique gives feedback about what is happening in a particular group of muscles, for example in the forehead or forearm. This feedback is usually both visual (digital display) and auditory (clicking sounds). With this feedback, you can learn to voluntarily relax or tense particular muscle groups.
When muscles tighten, a series of electrical impulses travel to the muscle fibres. With decrease of electrical activity, relaxation of the muscles occurs. With EMG biofeedback, the electrical activity of the muscle is detected by the used of electrodes placed on the skin directly over the muscle that is being measured. The information is then fed back to you.
Your goal is to decrease (or increase) this electrical activity, thus learning to control your muscle tension.
EMG biofeedback was found to be particularly useful for tension headaches, anxiety, phobias, and insomnia.
Electroencephalogram (EEG) Biofeedback
EEG biofeedback, also called neurofeedback, is a learning strategy that allows you to alter your brain waves. As you watch your brainwave pattern on a monitor, you learn that you can change your brainwaves.
Why would you want to change your brainwaves? Simple. There are 4 brainwave patterns (beta, alpha, theta, and delta), each associated with a different state. If you are looking for stress relief, then your goal is to learn to induce alpha brainwave patterns, associated with relaxation and calmness.
In a typical EEG session, one or more electrodes are placed on your scalp, and one on each ear. Your brainwaves are monitored and displayed on a monitor. Through a computer game you learn how to change your brainwaves to a more desired frequency.
EEG biofeedback is used for anxiety, depression, insomnia, chronic pain, addictions, chronic fatigue syndrome, and autoimmune disorders.
