Electromyography
Electromyography (from ancient Greek: ἠλέκτρου μυός γραφία [ēlectrou myós graphia]) is the graphic recording technique of electrical activity produced by skeletal muscles. electrical activity is known as the electromyogram or “EMG”.
EMG can be monitored through electrodes inserted into the muscles (intramuscular electrodes) or through electrodes on the surface of the skin over the muscle (superficial electrodes).
EMG is used by scientists to study the neuromuscular system, by physicians to diagnose neuromuscular diseases, and by physical therapists to monitor a patient's muscle activation.
Electrical characteristics
The electrical source is the muscle membrane potential of around -70 mV, measuring EMG potentials range from low to high ranges between 50 μV up to 20 or 30 mV, depending on the muscle under observation.
The typical repetition rate of a muscular motor unit is around 7–20 Hz depending on the size of the muscle. Damage to expected units can be between 450 and 780 mV ranges.
History
The first material in which the EMG appears was in the work of Francesco Redi in 1666. Redi discovered a highly specialized muscle in the electric eel Electrophorus electricus that generated electricity. In 1773, Walsh was able to show that the muscle tissue of the Electric Stingray had the ability to generate a spark of electricity. In 1792, in a publication entitled De Viribus Electricitatis in Motu Musculari Commentarius written by Luigi Galvani, it appeared that the author demonstrated that electricity could initiate muscular contractions. Six decades later, in 1849, Dubois-Raymond discovered that it was also possible to record electrical activity during the activity of muscle contraction. The first real record was made by Marey in 1890, who also introduced the term electromyography. In 1922 Gasser and Erlanger used an oscilloscope to display electrical signals from muscles. Between the 1930s and 1950s, scientists began to use improved and more sophisticated electrodes for muscle studies. The clinical use of surface EMG (sEMG) for the treatment of more specific disorders began in the 1960s. Hardyck and his collaborators were the first (1966) to use sEMG. In the early 1980's, Cram and Steger introduced a clinical method for scanning a variety of muscles using an EMG sensing device.
It wasn't until the mid-1980s that electrode integration techniques were advanced enough to allow batch production of the small, lightweight amplifiers and instrumentation required. At present, there are a large number of commercially available amplifiers. Recent research has resulted in a better understanding of the properties of sEMG. Surface electromyography is increasingly used for recording superficial muscles in clinical or kinesiological protocols, while intramuscular electrodes are used to investigate deep muscle or localized muscle activity.
There are many applications for the use of EMG. EMG is used clinically for the diagnosis of neurological and neuromuscular problems. It is used diagnostically by gait laboratories and by clinicians trained in the use of biofeedback or ergonomic belaying. EMG is also used in many types of research laboratories, including those involved in the fields of biomechanics, motor control, neuromuscular physiology, movement disorders, postural control, and physical therapy.
Procedure
There are two methods to use the EMG, one is the superficial, and the other is the intramuscular method. To perform an intramuscular EMG, a needle electrode is used, inserted through the skin until it enters the muscle tissue. A trained professional (such as a neurophysiologist, neurologist, or physiatrist) watches the electrical activity as you insert the electrode. While the electrode is being inserted, it provides valuable information regarding muscle activity as well as the nerve that innervates that muscle. Muscles at rest show normal electrical signals when the electrode is inserted, therefore electrical activity is studied when the muscle is at rest. Spontaneous abnormal activity indicates nerve or muscle damage. The patient is then asked to gently contract the muscle for further analysis. The size, frequency, and resulting shape of the potential motor unit are analyzed. Subsequently, the electrode is withdrawn a few millimeters and inserted again to analyze the activity, which must have units of at least 10–20. Each electrode trace gives a very local picture of the activity of the whole muscle. Since skeletal muscle differs in its internal structure, the electrode must be placed in various locations to obtain reliable study results.
The Intramuscular EMG method may be considered too invasive or unnecessary in some cases. Instead, the surface method employs a surface on which the electrode can be used to monitor the overall picture of muscle activation, as opposed to the activity of only a few fibers as seen using intramuscular EMG. This technique is used in a number of settings, for example in physiotherapy, muscle activation will be monitored using surface EMG and patients are given an auditory or visual stimulus to help them know when the muscle is being activated (feedback).
A motor unit is defined as a motor neuron and all the muscle fibers it innervates. When a motor unit is activated, an impulse called an action potential travels from the motor neuron to the muscle. The area where the nerve makes contact with the muscle is called the neuromuscular junction. After the action potential is transmitted across the neuromuscular junction, a potential is obtained in all muscle fibers innervated by the particular motor unit. The sum of all this electrical activity is known as a motor unit action potential (MUAP). The electrophysiological activity of the multiple motor units is the signal that is normally evaluated during an EMG. Motor unit composition, number of muscle fibers per motor unit, metabolic type of muscle fibers, and many other factors affect the shape of motor unit potentials in the myogram.
Scanning electromyography (EMG scanning) consists of recording the activity of a single motor unit through a series of points located linearly along the path of a micro-motorized needle electrode. The innovation of this technique is that it allows observing the motor unit not only from one point, but from a series of points through a corridor located in the cross section of the territory of the motor unit. Observing a motor unit from more than one point yields an enormous amount of extra information about its electrophysiological activity. The analysis of the scanning-EMG signal allows access, in a more direct way, to the estimation of the anatomical and physiological parameters of the motor unit.
Some patients may find the EMG procedure painful, others experience a small level of discomfort when the needle is inserted. The muscles on which the procedure is performed may be sore for a day or two after the procedure.
Instrumental
A basic electromyography kit consists of the following elements:
- Electrodes. They collect the electrical activity within the muscle, either by insertion into it or through the skin that covers it.
- Surface electrodes. They are small metallic discs of highly conductive material that adhere to the skin. To reduce the impedance between the electrode and the skin, a special conductive pasta is applied. With these electrodes you get a general vision of the functioning of the muscle.
- Insertion electrodes or deep, with needle shape. There are several types.
- Monopolar. It consists of a current needle that has been isolated in all its length, except at the tip.
- Coaxial. It consists of a needle in whose interior very thin metal conductors have been inserted, isolated from each other and with respect to the needle. Only at the tip drivers do not present isolation and at that point the signal from muscle tissue is captured.
- Amplifier. They are necessary for analog electrical signals from the muscle to be visualized on a monitor. The amplification ratio can exceed 60 dB. The bandwidth is 40 to 10 kHz. In general, the electronic characteristics of the amplifier vary according to the type of study to be performed, the main ones being: Number of channels: 2, 4, 8. Sensitivity: 1 pV/div. to 10 mV/div. Input impedance: 100 MΩ // 47 pF. CMRR to 50 Hz 100 dB. High-pass filter: between 0.5 Hz and 3 kHz (6 dB/octava). Low-pass filter: between 0.1 and 15 kHz (12 dB/octava). Noise: 1 pV effective between 2 Hz and 10 kHz with shortcut input.
- Registration system. The signals obtained from the muscle can be recorded in a visual screen, and in a sound form through a speaker. You can also register on a permanent support, such as paper.
Normal results
Muscle tissue at rest is electrically inactive. Following the electrical activity caused by the insertion of the needles, the electromyograph should not detect any spontaneous abnormal activity (i.e., a resting muscle should be electrically silent, with the exception of the area of the neuromuscular junction, which under normal circumstances, activates very spontaneously). When the muscle contracts voluntarily, action potentials begin to appear. As the force of muscle contraction increases, more and more muscle fibers produce action potentials. When the muscle fully contracts, a disordered array of action potentials of varying rates and amplitudes should appear.
Abnormal results
EMG is used to diagnose diseases that generally fall into one of the following categories: neuropathies, neuromuscular junction diseases, and myopathies.
Neuropathies are defined from the following EMG:
- An action potential that is twice normal due to a growing number of fibers per motor unit due to the re-inervation of denerved fibers.
- An increase in the duration of the action potential.
- A decrease in motor units in the muscle (using numerical estimation techniques of motor units).
Myopathies defining EMG characteristics:
- Decrease in the duration of the action potential.
- A reduction in the area and the radius of the action potential.
- A decrease in the number of motor units in the muscle.
Abnormal results are caused by the following medical conditions:
- Alcoholic Neuropathy
- Amiotrophic lateral sclerosis
- Previous compartment syndrome
- Axillary nerve dysfunction
- Becker muscle dystrophy
- Brachial Plexopathy
- Carpal tunnel syndrome
- Myopathy
- Cervical spondylosis
- Charcot-Marie-Tooth Disease
- Disfunction of the common butine nerve
- Denervation
- Dermatomyositis
- Dysfunction of medium distal nerve
- Duchenne muscle dystrophy
- Femoral nerve dysfunction
- Ataxia de Friedreich
- Guillain-Barré syndrome
- Eaton-Lambert Myasthenic Syndrome
- Multiple Mononeuritis
- Mononeuropathy
- Motorbike disease
- Multiple system atrophy
- Miastenia grave
- Myopathy
- Neuromyopathy
- Peripheral neuropathy
- Poliomyelitis
- Poliomyositis
- Sensory-motriz Polineuropathy
- Spinal stenosis
- Thybial nerve dysfunction
- Radial nerve dysfunction
EMG signal breakdown
EMG signals are composed primarily of action potentials from overlapping motor units. The measured EMG signals can be broken down into the constituent motor unit action potentials (PAUMs). PAUMs from different motor units may have different shapes, while PAUMs recorded by the same motor unit electrode are typically similar. The shape and size of the PAUM strongly depend on where the electrode is located with respect to or to the fibers.
EMG Applications
EMG signals are used in many clinical and biomedical applications. EMG is used as a tool to diagnose neuromuscular diseases, and motor control disorders. EMG signals are also used for the development of hand, arm and lower extremity prostheses.
EMG is also used to detect muscle activity in places where there is no movement. The speech of a person with a speech impairment can be recognized by observing EMG activity in the muscles associated with speech.