EMG is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed by using an instrument called electromyography, to produce a record called electromyogram.

EMG involves detecting, amplifying/ analyzing, imaging/ auditory sound/ audiovisual sounds. These processes occur when the muscle contract. It provides a powerful tool for documenting the role of muscle in physical activity and for assessing neuromuscular system (nerve + muscle + motor unit + neuromuscular junction).

EMG was first accomplished by Helmholtz in 1850 but its clinical efficacy was not recognized until animal experiment showed its method and observations. In 1926 Adrian and Brank introduce the concentric needle for a single motor unit and a single muscle fiber.

The development of CRO (Cathode Ray Oscilloscope) and electronic regarding machine made it possible as one of the vital role and clinical significance in the electrodiagnosis.

The stimulus which causes depolarization is called Action potential. The influx of Na ions is called depolarization. The action potential of motor unit is called motor unit action potential.

Recording is done by the following three phase system.

As it is known that when all the 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.

            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.

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 units.

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. The electron gun projects the electric beam towards the screen through the two sets of plates. As the electron beam passes, there is deflection at the vertical plates and sweep at the horizontal plate. The deflection at the vertical plate is shown as vertical plate signal voltage in micro volts and the sweep at the horizontal plate shows the duration of signals in milliseconds. A tape recorder may be used to store the EMG information so that it can be redisplayed.

Muscles at rest are inactive. During the insertion of the needle electrodes some spontaneous activities will be heard. When the needle comes to rest there will be no spontaneous activity at normal circumstances. When the muscle is voluntarily contracted action potential begins to appear. As the strength of contraction is increased more and more muscle fibers produce action potentials. These action potentials appear on screen of EMG.

As a normal muscle at rest shows electrical silence, any activity during the relaxed state is known as spontaneous activity. These spontaneous activities are not produced by the voluntarily muscle contraction. Four types of spontaneous action potentials have been identified.

These spontaneous potentials are arises from spontaneous depolarization of a single muscle fiber. They may have up to three phases with amplitude between 20 -300 μv and duration is 2 ms. They produce high pitch click sound having frequency 30 pulses/ s. they are indicative of lower motor neuron disorders such as peripheral nerve lesion (peripheral neuropathy), AHC diseases, rediculopathy, polyneuropathy but found in lesser extent in myopathic diseases such as muscular dystrophy, poliomyositis and myasthenia gravis.

There are spontaneous potentials seen with irritation and degeneration of AHCs, compression of the nerve roots as well as muscle spasm or muscle cramps (muscle fibers overlap each other) or muscle guarding. They are found in Mylopathies, Myopathies, benign myokemia, motor poliomyelitis, spinal muscular dystrophy. These fasciculating potential may be 5 -200 μv, duration is 5 – 25 ms, frequency is 50 pulses per second they produce low pitch thump sound.

There are descriptive because they produce initial positive deflection. Usually they are monophasic but can be diphasic. The negative phase is off much lower amplitude and longer duration (sometimes up to 100 ms) than positive phase. The peak to peak amplitude is variable but may be up to 1 mv. Their duration may vary greatly 2 – 100 ms. But usually the duration is in between 10 – 20 ms. Frequency is 10 pulses/ second. They produce characteristic dull sounds. They are found in denervated muscle along with fibrillation potential. Primary muscular disorders such as muscular dystrophy, polymyositis, positive sharp waves and fibrillation potential are rarely found in UMN lesions.