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Electromyogram
spherical geometry as it moves away from the source. Muscle potentials usually
range from 2 to 6 mV and last for 5 to 8 ms.
The raw EMG (before filtering) is often considered as the most noise-like signal among almost all biomedical signals. This noise-like form makes the processing of EMG relatively different from ECG and EEG, which are described in other
chapters. The specific processing methods suitable for analysis of EMG will be
described later in this chapter.
11.3.1 SIGNIFICANCE OF EMG
In this section, we focus on the characteristics of muscle that can be determined by
EMG. In general, two major characteristics of muscle are evaluated using EMG.
These characteristics are conductivity and excitability.
In the first type of EMG analysis, the conductivity of the muscle is assessed by
spatially mapping the pattern and speed of muscle conductivity. In this type of analysis, specific stimuli (e.g., weak electric or nonelectric shocks) are applied to a particular position on the muscle while the EMG at several points along the muscle is
measured and analyzed. The spatial spread and timing of the spikes observed on the
EMG is expected to reveal any damage to the muscle fibers.
Another typical experiment based on EMG measures the excitability of a muscle.
In such experiments, typically, electric or nonelectric (e.g., mechanical) stimuli with
different amplitudes are applied to a point on the muscle, and the EMG of neighboring positions is measured. The relative response of different points to different levels
of stimulation is then used to analyze the excitability of the muscle at different points.
Other types of EMG experiments include identification of the muscle strength.
The total force of a muscular contraction and the number of motor units activated in
a muscle activity are directly reflected in the amplitude of EMG. As a result, EMG
is sometimes used to measure the force exerted by a muscle. In doing so, one would
need to consider factors such as the size of the muscle, the position of the muscle (i.e.,
distance between muscle and electrode), and the thickness of the subcutaneous fat
(i.e., electric insulation between muscle and electrode).
In every one of the aforementioned EMG-based experiments, certain technical and practical issues must be considered. These issues that can affect the EMG
readings include skin preparation, orientation of the electrodes with respect to the
muscle fiber direction, and the exact type of electrodes used for measurements.
One particularly significant application of the EMG signal is the opportunity to operate artificial prostheses with the electric signal of other muscles still functional. This
means that when a person loses an extremity and starts using an artificial prosthesis,
there is a need to allow the person to initiate the commands for certain types of motion
in the artificial limb. In such cases, often the EMG of some other still functional muscles is used to create the command. This is done by measuring the EMG of the healthy
muscles, and based on the type of motion of the healthy muscle and therefore the shape
of the EMG signal, the desirable command is detected and sent to the prosthetic limb.
Most often in prosthesis, the EMG-based command drives servomotors that are
battery operated. These servomotors, for instance, control the motion of prosthetic limbs.