Introduction to the device:
It is a technique that helps to record the electrical currents resulting from the contraction and relaxation of the muscles, the currents that pass the muscles during their contraction are called action currents. Muscle signals (EMG) are recorded by placing surface electrodes on the surface of the muscle to be examined or by implanting two electrically conductive electrodes isolated in the muscle to be planned.
This technique is an important diagnostic tool in cases of trauma and paralysis, because it determines the presence or absence of nerve injury or interruption, thus giving the possibility of differential diagnosis between functional cases of obstruction of muscle movement and between organic cases of interruption of intolerance and this technique also allows the possibility of monitoring the development of the patient's condition.
Before starting to explain the EMG, it is necessary to recall the composition and physiology of the muscles and the principle of their functioning.
muscles:
The muscle consists of a number of fibers and cells, which in turn form muscle tissue, there are three main types of muscles that differ from each other in tissue structure, location, physiological function and type of nerve fibers connected to them:
Types of muscle contraction:
There are two types of inotropic:
Contraction of equal length:
In which there is no change in the length of the muscle, but the pressure or tension inside it increases, such contraction occurs when the muscle fails to lift a certain weight, in this case there is no external work exerted, because the weight of the body is heavier than the muscle can move, and therefore the length of the muscle remains the same while the rate of tension inside it rises.
isotonic contraction:
In it, there is a change in the length of the muscle while the pressure or tension inside it remains the same. Such contraction occurs when it is possible for the muscle to lift a certain weight.
The relationship between stimulus and response:
- Simple muscle twitching:
Muscle jerk occurs, as mentioned earlier, in response to an electrical tremor, and the jerk goes through three different successive stages:
- Latent Period:
- Contraction Phase:
This phase dedicated to actual contraction lasts up to 40 milliseconds and coincides with a contraction in muscle length or increased tension.- Relaxation Phase:
It is the period required for the muscle to return from contraction to latency The relaxation phase lasts about 50 milliseconds, during which the muscle returns to its length or tension when it was resting.
Figure( 1 )represents the phase of muscle contraction if exposed to a single stimulus.
Figure( 2 )represents the phase of muscle contraction for a number of consecutive stimuli in which case the muscle remains in a contraction state until the stimulus ends or when it is exhausted.
Figure (1) Contractile phase if exposed to a single stimulus
Isotonic contraction and isometric contraction:
When talking about muscle contraction usually emphasizes the cases of muscle contraction and increased tension in it, and this is a special case that combines two types of contractions, the first type is called isotonic contraction, in which the muscle shrinks without any change in its tension, whether loaded or unloaded, to take into account the weights that do not prevent contraction. All that happens when the muscle is stimulated is a noticeable increase in its tension.
In our bodies occur contractions equal in tension or length in varying proportions depending on the task carried out by the muscle contractions of equal length maintain in humans the erection of the body and resistance to gravity through reflexes Postrural Reflexes and include the muscles of the back of the neck and muscles of the back and muscles of the lower extensor limbs. Other movements, such as walking and running, are carried out through isotonic contractions.
Stair Contraction Gradient Case:
The nerve that supplies skeletal muscles consists of motor fibers and sensory fibers in approximately equal numbers. The motor neuron bodies are located in the anterior horn of squirrel matter, in the spinal cord. Since the muscle fibers far exceed the number of motor nerves, one neuron is connected through its branches to a number of muscle fibers amounting to 200 fibers in large muscles and only 5 in the small muscle motor of the eyeball and called the system that includes the motor neuron and muscle fibers that equip the motor unit Motor unit. The smaller the motor unit, the more accurate the muscle work compared to larger muscles. This determines the neural output of motor neurons, muscle contraction force and voluntary movement force. In normal activities, motor units alternate in their work and thus perform work without fatigue. As the work required becomes more stressful, the number of units employed increases.
When the subtraction is suitable for the neurotransmitter of the motor neurons slow by -1 -5 beats per second you get simple pulses alone and at the same frequency and by increasing the subtraction from 10 to 30 beats per second we get an incomplete fusion between the jerks as a result of muscle combination accompanied by a noticeable rise in tension and the contraction appears in the form of a mechanical tremor called a spear.
If the subtraction frequency is 50 to 200 jerks per second, the muscle responds with a stronger continuous contraction than a muscular spear known as tetanus.
muscle planning:
Electromyography (EMG) is a technique that helps record the electrical currents resulting from the contraction and relaxation of the muscles. The currents that pass through the muscles during their contraction are called action currents. Muscle signals (EMG) are recorded by placing surface electrodes on the surface of the muscle to be examined or by implanting two electrodes connected to electricity and isolated in the muscle to be planned activity.
The MSG technique is an important diagnostic tool in cases of trauma and paralysis, because it determines the presence or absence of nerve injury or interruption, thus giving the possibility of differential diagnosis between functional cases of obstruction of muscle movement and organic cases such as nerve interruption and this technique allows the possibility of monitoring the development of the patient's condition.
The muscle planning signal, as shown in Figure 5.5, is characterized as a weak signal, so it is not permissible to rise in most cases 5mV, so when recording that signal, you need to enlarge it with the operation amplifier, and the frequency range of these signals Muscle planning ranges from 10 to 2500 Muscle signals are affected by multiple types of jamming, the most important of which are:
- Noise caused by a movement of electrode (motion artifact) and this noise has low frequencies ranging from 0-20 Hz and is eliminated by a filter that passes high frequencies with a cut-off frequency of 10-20 Hz.
- Interference from the electrical noise surrounding the MEG device and this noise has high frequencies with a range greater than 200-500 Hz and is eliminated by a filter that passes low frequencies with a cut-off frequency of 200-500 Hz Figure (4) shows the shape of the MSG signal after passing through that filter.
Components of the EMG device:
The EMG signal is a weak signal that does not exceed 5mV in height, so when recording this signal, we need to magnify it with the operation amplifier. As the frequency range of the ECG signals ranges from 10 to 500 Hz from the above, we note that the difference between the ECG signal and the MSG signal varies in the frequency range and strength of the signal. Therefore, the main components of both devices are similar in design with different values such as signal magnification and cut-off frequency values for filters. In general, as shown in Figure 5, the device consists of the following basic parts:
- Electrodes:
The device consists of three electrodes, two active electrodes and a third (joint) pole for grounding. If the muscle to be examined is far from the surface of the skin, the electrodes are inserted into that muscle by the needle electrodes (see Figure 6).
To learn more about the electrodes and how to get their signals, click here.
- Recording and Magnification Unit:
The first stage of the muscle planning process begins with connecting the electrodes to the device, specifically to the operational amplifier, the process of magnifying the EMG signal is necessary because the height of these waves is small, ranging from 5mv to 10mv. So we need to zoom in as much as 500 times (ACL=500).
- Filters:
Their work is limited to filtering the wave from external influences that can affect muscle planning, and the magnifier circuit and filters represent the basic electronic circuit of the muscle planning device, as it achieves a magnification of 500 through the operational process amplifier, and that magnification can be calculated as follows:
The capacitor (48 nf) and resistance (52 MΩ) represent the high frequency pass filter with a cut-off frequency (10 Hz) while the capacitor (68 nf) and resistance (74) KΩ) represent the low frequency pass filter with cut-off frequencies of 500 Hz and both frequencies can be calculated from the following relationship:
- dScreen:
When the doctor dispenses with paper or is not needed to obtain a continuous reading. In modern devices, the signal is entered and displayed by the computer. This means converting the signal from an analogue signal to a digital signal with a cutting frequency (fs=1000Hz Sampling frequency(fs) by A\D Analog to digital converter and because the ECG signal does not take a specific shape, easy to read and diagnose, as is the case in the ECG signal, the modern diagnosis depends on the study and analysis of Spectral analysis signal spectrum. Therefore, most of the EMG machines work within a computerized system as shown in Figure 9.
