The recording of electrical activity from muscles is called electromyography. It helps to determine whether the muscles contract or other. The depolarization of muscles causes muscular contractions. Recording the action potential of peripheral nerve is known as Electro neurography. Two types of electrodes are used for recording EMG. Surface electrodes are used when signals are recorded on the surface and it cannot detect deep potential from within the cell. Needle electrodes are inserted deep into the tissue, which records the muscle potential.
EMG Recording System
Electrodes record EMG potentials from the tissue. Disc shaped surface electrodes made of Ag/AgCl are used to pick the signals. The skin surface is cleaned and electrode gel is applied on the skin surface. Elastic bands are used to attach the electrodes to the skin. When electrodes are affixed tight on the skin, it reduces the skin contact impedance to below 10kΩ. By convention, two types of electrodes are used. In Bipolar Electrodes, the electric potential is calculated between two surface electrodes attached on the skin surface.
However, in Unipolar Electrodes two electrodes are used. Reference surface electrode is attached on the skin and one needle electrode is penetrated deep into the muscle. Needle electrode acts as active electrode. The potential difference between these two electrodes is measured in unipolar electrode system. Another type is coaxial electrodes, in which needle electrodes consists of insulated wire with a tip that can penetrate easily into the muscle. Here the reference is covering steel jacket and metal wire acts as exploring electrode. Microelectrode is used to record action potential from single nerve.
The amplitude of EMG signal depends on various factors like type of electrode used, placement of electrode on the muscle and the degree of muscular exertions. When surface electrodes are used, it picks signals from nearby spikes. Therefore, it produces average voltage value from the muscles ad motor units. Whereas needle electrodes pick signal from single nerve fiber and it produces one voltage value. The voltage value of EMG signals varies between 0.1 to 0.5 mV. The frequency of the EMG signal is the range of 20 Hz to 10 KHz, which lies in the audio range. The normal frequency of EMG is 60 Hz. The recorded EMG potentials are sent to the differential amplifiers. This amplifier has high gain and frequency range between 10 Hz to 10 KHz. In addition, the amplifier has CMRR between 80 to 100 dB and input impedance of 10 MΩ. Since we have only two electrodes, there is no necessity for lead selector switch. The amplified output is given to the power amplifier that helps to amplify the signal to a higher level. Then the amplified signal is given to the loudspeaker, tape recorder and CRO. The final output is given to CRO that displays the signal. The tape recorder is used sometime to record the signals from the differential amplifier.
Applications of Electromyography
- To study Neuromuscular functions.
- To know the neuromuscular condition.
- To study reflex responses.
- To know about the extent of nerve lesions.
- Helps to diagnose the muscular diseases.
Measurement of Conduction Velocity in Motor Nerves
In the motor nerves, to indicate the location and type of nerve lesions, conduction velocity is measured. At various segments of nerve, nerve function is examined. An electric shock of duration 0.2 – 0.5 milliseconds is stimulated on the nerves. Conduction velocity along the peripheral nerve is measured to calculate the latency. Hence, latency is defined as difference in time between stimulating impulse and action potential of the muscle.
On the skin at two points separated at a length of l1, EMG electrode and stimulating electrodes are placed. Via the stimulating electrodes electric shock is applied. A short twitch occurs, when the excitation reaches the muscle. All the nerve fibers get stimulated at the same. Therefore, the conduction velocity is same and muscle fibers are activated in a synchronous manner. The EMG electrode picks up the action potential generated and displays the output on the CRO. Finally the time t1(latency time) is measured. After this, the electrodes are repositioned on the skin and placed at a distance of l2. The elapsed time t2 is calculated. The distance should be such that l21.
The conduction velocity is calculated using the below formula.
Normally the value of conduction velocity is 50 m/s.