There are over 600 muscles arranged around the frame of a human skeleton and inside the body. These muscles are responsible for various movements of the body. When these muscles are active, as happens in the case of muscle contraction and stimulation, they produce electric impulses. Electromyogram or EMG is a record of such impulses.
Electromyogram provides valuable information regarding the disorders of the muscles. The electric signals from the muscles are obtained during voluntary contraction. For this purpose, an instrument is used whose electrodes are connected to the particular muscle. These electrodes carry the electrical impulses through the wires to a pen recorder which marks these in the form of a graph on a paper.
Now the question arises, how do muscles produce electric impulses? A muscle is made up of many motor units. A motor unit consists of a single branching neuron from the spinal cord to the muscle fibres. This gives rise to a potential difference. In fact, this potential difference is recorded in the form of electromyogram.
Single muscle cells are usually not monitored in an EMG examination because it is difficult to isolate a single fibre. Instead, EMG electrodes record the electric impulses from several fibres. EMG gives valuable diagnostic information about muscles. With the help of an EMG the contraction of a muscle can be known.
EMG can also be recorded with electric stimulation. A typical stimulating pulse may have amplitude of 100V and lasts from 0.1 to 0.5 m-secs. With the help of an EMG we can study the reflex responses of the muscles and can know their working level. For instance our heart muscles go on pumping blood. If there is any disorder in pumping system, the device can identify heart muscles which may not be in its normal state. The major muscles in humans can be stimulated at a rate between 5 and 15 hertz. Normal nerves and muscles show some changes during prolonged restimulation. On the basis of EMG we can gather information about any malfunctioning of the muscles.