Electromyography (EMG) is a specialized diagnostic technique used to study the muscle function by analyzing the electrical signals emanating from the nerves and the muscles. EMG can distinguish the abnormal or diseased muscle from the normal muscle. The electromyography can detect muscular abnormalities such as fasciculation or chronic denervation in clinically normal muscle. The neurogenic abnormalities detected by electromyography are then used to differentiate the focal nerve, plexus, or radicular pathology. It can also help the researchers to diagnose peripheral neuropathy such as axonal degeneration or demyelination and distinguish between the small myopathic or myositic potentials and the surviving motor units in chronic partial denervation (Mills, 2005). Furthermore, electromyography is used to study the response latency (jitter) at the neuromuscular junction. The electromyographic findings are quite specific and have obvious diagnostic value.
Electromyography has become a compelling and advanced methodology in biomedical engineering and clinical diagnosis. The waveforms and firing rates of Motor Unit Action Potentials (MUAPs) in electromyography signals give an insight into the diagnosis of neuromuscular disorders (Raez., Hussain., & Mohd-Yasin, 2006). The electromyography serves as a new tool for the assessment of peripheral nerve function in neuromuscular disease animal models in which long-term, repeated, and minimally invasive monitoring is needed.
The history of the electromyography dates back to 1666 when Francesco Redi published a document. It reported that there is a highly specialized muscle in the electric ray fish which generates electricity. In 1773, Walsh demonstrated that the Eel fish’s muscle tissue could generate electricity. Then after six decades, in 1849, Dubios-Raymond proposed that the electrical activity during a voluntary muscle contraction can be recorded. Marey coined the term electromyography in 1890. In 1922, the oscilloscope was used by Gasser and Erlanger to display the electrical signals emanating from muscles. In the 1930s, the use of electromyographic signals further advanced, and in 1950s the researchers started to use improved electrodes for the study of muscles. The EMG was then used clinically for the treatment of more specific disorders. In 1966, Hardyck and his researchers introduced surface electromyography (sEMG). In the early 1980s, Cram and Steger were the first ones to introduce a clinical method using EMG for scanning the muscles (Raez., Hussain., & Mohd-Yasin, 2006). In recent years, sEMG has been increasingly used to record electrical signals from the superficial muscles using intramuscular electrodes in clinical protocols.
Apparatus and Equipment
The basic electromyography equipment consists of electrodes, attachment cables that link the electrodes to the signal amplifier, a speaker, an oscilloscope, and a recorder. Surface electrodes (made up of silver or platinum) or needle electrodes can also be used because they are easy to apply and are painless. The needle electrodes accurately record small amplitude signals. A fine wire is attached at the axis of the needle. The surface area of the needle axis determines the volume of the muscle to which the electrode is attached. Usually, the EMG needles can record from the hemisphere with a radius of 1 mm. This 1 mm can cover around 100 muscle fibers. The recorded nerve and muscle potentials are relayed to the amplifiers to increase the size up to one million times. The signal is then sent to the oscilloscope, from which a visual and auditory display is obtained for the analysis.
- Anesthetize the animals with an intraperitoneal injection of 50 mg/kg pentobarbital sodium.
- Fix the animal in a supine position on a wooden table.
- Maintain the body temperature at 37oC using a heater.
- Make an incision from the right inguinal area to the ankle through the medial aspect of the thigh.
- Excise the dorsal surface of the leg and the abductor muscles of the thigh.
- Expose the gastrocnemius muscle and the inguinal part of the sciatic nerve.
- To minimize the limb movement during the electric stimulation, excise the branches of the sciatic nerve except the one supplied to the gastrocnemius muscle.
- To prevent dryness, rinse the exposed tissues with normal saline.
- Insert a bare-tip, insulated monopolar needle electrode in the sciatic nerve; this serves as a stimulating cathode.
- Place a non-insulated needle electrode in the abdominal wall; this serves as an anode.
- At the firing rate of 1 Hz, administer the electric stimulation in constant current square waves of 50-μs duration. Adjust the stimulation intensity to the level that it elicits only small twitching of the gastrocnemius muscle. Adequate current flow typically ranges from 0.05 to 2 mA. Note: the duration and current flow rate can be adjusted as per the experimental needs.
- Introduce the single fiber electromyography electrode into the gastrocnemius muscles. Careful and precise adjustment of its position is necessary to pick up the potentials.
- Record the signals and observe the muscular activity.
Factors Affecting the Electromyographic Signals
The EMG signals can be affected by the following factors;
Causative factors have a direct effect on the EMG signals. The causative factors can be broadly divided into two categories:
Extrinsic factors include electrode structure and placement. In these, the surface area, electrode shape, distance between the electrode detection surface, location of the electrode, and the location of the muscle are included.
The factors like the physiological, anatomical and biochemical factors are included in the intrinsic factors. The active motor units, the composition of the fiber, blood flow, diameter of the fiber, the depth and the locat