EMG Block Diagram Explanation

Electromyography is the technique for calculating and recording the action potential of muscles. EMG is taken using a device called electromyography and the record obtained is known as electromyogram. The electrical activity of muscle cells when they are active and at rest can be analysed using an EMG. The measured EMG potentials range from 50 µ Volt to 30 millivolts.

Mainly there are two kinds of EMG measurements. The first method is using surface electrodes and the second one is using needle electrodes. The Surface EMG electrodes are used to monitor the electrical activity of muscles generally whereas the needle electrodes are used to observe the electrical activity of only few fibers.

A trained expert can observe the electrical activity of muscles when the needle is inserted. There is a normal electrical activity for the muscle fibers at rest. The physician or concerned expert examines the normal activity of muscles when the needle is inserted. There is a normal electrical activity for the muscle fibers at rest. The physician or concerned expert examines the normal activity of muscles. The abnormal spontaneous activity indicates that some nerve or muscle cells are damaged. At a time potentials from different electrodes are taken. So the needle electrodes have to be placed at different locations to obtain an accurate EMG. So the intramuscular EMG is considered to be too invasive.

So in order to obtain the general activity of the muscle cells we use surface electrodes which need to be placed only on the concerned area. So no insertion is required. This technique is commonly used in many applications such as in a physiotherapy clinic where the muscle activity is monitored by the surface EMG electrodes and the patients can have visual stimulus when they activating the muscles.

So when the motor neuron or muscle fiber is stimulated, the action potential is transmitted across the muscle it is passed to the connected nerve fibers. Actually during EMG we are evaluating this bioelectric potential from different cells. This potential is collectively called motor unit action potential (MUAP). EMG signals are made up of superimposed MUAPs. Hence the shape of the electromyogram is affected by factors such as number of muscle fibers under consideration, the metabolic type of muscle fibers etc.

EMG Block Diagram

EMG Block Diagram Explanation:

1. EMG electrode: As told earlier, the electrode used for EMG recording can be of surface type or needle type depending on the area from which the EMG is to be obtained and the type of measurement. If we need to have EMGs from many individual muscle cells rather than from the surface as a group, needle electrodes are the best choice. But if the general activity of a muscle is to be analysed the surface electrodes can give the accurate values. 

2. Bioelectric amplifier: As the name implies, the bioelectric amplifiers are used to amplify the bioelectric signals obtained from EMG electrodes.

3. AF amplifier: At rest under normal condition the sound does not undergo large variations. But abnormal and spontaneous activity may be distinguished by the sudden change in sound and this can be analyzed by the physician. The abnormal activity usually indicates muscle damage and they can easily find out the nerve or muscle damage. So physicians normally use AF amplifiers during EMG measurement so as to distinguish there sounds clearly.

4. Oscilloscope: The measured EMG can be connected to the oscilloscope to visualize the EMG. The abnormalities in the working of nerves and muscle  cells can be identified by a physician by analyzing the EMG waveform. The EMG can also be stored using special oscilloscopes such as DSO (Digital Storage Oscilloscope) for future analysis.

5. EMG recorder: Unlike ECG, EMG cannot be recorded in a low speed chart paper recorders because of its extreme low frequencies. So it will be less useful. Normally we use the photographic recording of EMG. For this a light sensitive paper is moved over the recording CRT.

Frequency Limitations:

1. The normal EMG is in the range of 60 – 70 Hz. The EMG appear as a random noise signal the shape of the waveform may vary based on the part of the body from which EMG is taken.

2. Due to the low frequency limitation, EMG cannot be recorded on a strip chart recorder because it cannot give a clear idea of the waveform.

Applications of EMG:

1. EMG can be used to diagnose two main categories of diseases. They are neuropathies and myopathies. In EMG representation, an increase in duration of action potential is an indication of neuropathic disease and a decrease in duration of action potential is an indication of myopathic disease.

2. EMG is utilized as a diagnostic instrument for detecting neuromuscular diseases.

3. EMG can be utilized for silent speech detection which identifying the speech by examining the EMG activity of muscles related with speech.