Neuromuscular activity detection technology - surface electromyography
Compared with needle electromyography, surface electromyography is widely used in hand and upper limb function assessment due to its significant advantages such as quantification, non-invasiveness, real-time, continuity and ease of operation.
Muscles generate bioelectricity during exercise or contraction. The voltage value of the bioelectricity is measured by two measuring electrodes on the skin surface. The pattern obtained after amplification and recording by an amplifier is called surface electromyography. There is a correlation between it and the activity and functional status of muscles, so it can reflect neuromuscular activity to a certain extent.
The most common method of surface electromyography collection is dual-electrode surface electrode collection. In addition, there is also array surface electrode collection (not covered in this article).
surface electromyography sEMG
principle
Muscle fibers (cells), like nerve cells, have high excitability and are excitable cells. Their first response when excited is the action potential, which is the conductive potential that appears on both sides of the cell membrane where the excitement occurs. The action potential conducts along the cell membrane to the depth of the cell, further causing muscle contraction activity through excitation-contraction coupling.
Under normal circumstances, muscle fibers always generate excitement and contractile activity under the control of the nervous system. The motor neurons innervating the muscle fibers generate excitement, release nerve impulses and conduct them along the axons to the terminals, releasing acetylcholine as a transmitter to achieve excitation transmission at the motor nerve-muscle junction.
Electromyography measurement is based on the above bioelectric phenomena. Extracellular recording electrodes are used to guide the compound action potentials of muscle excitatory activities in the body to the electromyograph. After appropriate filtering and amplification, the amplitude, frequency and waveform of the potential changes can be Displayed on the recorder or on the oscilloscope.
surface electromyography sEMG
Measurement steps
(1) Set the environment (temperature, etc.) and position the subject;
(2) Select test muscles according to requirements and process the skin on the surface of the muscles (scrub peeling, alcohol cleaning, hair removal, etc.);
(3) Paste the two test electrodes on the most trochanter of the test muscle along the direction of the muscle fibers, with a distance of 2cm;
(4) Set the sampling frequency and other EMG collection modes and start testing.
surface electromyography sEMG
EMG signal processing and analysis
Preprocessing:
Although the electromyographic signal obtained by the electromyographic acquisition system has undergone a series of processing such as hardware filtering and denoising, there are still interferences such as power frequency noise and peak amplitude. The original electromyographic signal needs to be processed by software filtering. .
Perform fast Fourier transform (FFT) on the original electromyographic signal to obtain the single-sided amplitude spectrum of the signal. At this time, you can see from the spectrum how many Hertz power frequency interference is relatively large, and then use the IIR digital notch filter (recursive filter, with a recursive function), and then the unilateral amplitude spectrum of the signal after the notch can be obtained, eliminating power frequency interference. At this time, the frequency distribution of the electromyographic signal can also be observed.
Then perform software digital filtering on the EMG signal to remove high-frequency spike interference. Commonly used digital filters include Butterworth filter and Chebyshev filter.
Waveform analysis:
The analysis of surface electromyography includes time domain analysis, frequency domain analysis, and time-frequency analysis (not commonly used):
Time domain analysis is a graph with time as a function, the abscissa is time and the ordinate is amplitude;
Frequency domain analysis is a graph with frequency as a function. The abscissa is frequency and the ordinate is amplitude or power.
Time domain analysis:
Peak: The highest value in which the EMG signal amplitude occurs.
Waveform average: Average electromyography (AEMG) is the average of the instantaneous electromyogram amplitude over a period of time. Its changes mainly reflect the number of motor units activated during muscle activity, the types of motor units participating in the activity, and their degree of synchronization. , related to the central control function under different muscle load intensity conditions.
Waveform area: Integrated electromyography (IEMG) is the sum of the area under the curve of the electromyographic signal per unit time; it reflects the total discharge of the motor units participating in the muscle activity within a certain period of time, that is, it remains constant over time. Under the premise, its value reflects to a certain extent the number of motor units participating in the work and the discharge amount of each motor unit, and reflects the contraction characteristics of the muscle in unit time.
Root mean square (RMS): Root mean square processing of surface electromyographic raw signals is often used to reflect muscle activity status in real time and without damage. Its numerical changes are usually related to muscle contraction force.
Frequency domain analysis:
The original EMG signal in the time domain is subjected to Fast Fourier Transform (FFT) and converted into a graph in which the abscissa is frequency and the ordinate is amplitude or power; the spectrum or power spectrum of the surface EMG signal is obtained, reflecting the surface EMG signal at different frequencies. The changes in components better reflect the changes in surface electromyography in the frequency dimension.
Median frequency: The frequency corresponding to the point that divides the power spectrum area into equal parts.
Average frequency: The average frequency.
Zero-crossing rate: It is the number of times the signal crosses the baseline in unit time.
The average power frequency and median frequency are the most commonly used indicators for clinically judging fatigue during muscle activity.
surface electromyography sEMG
Application in assessment of hand and upper limb function
Assessing hand and upper extremity spasticity
Muscle endurance test
Muscle fatigue assessment
Assessment of muscle contraction strength and muscle activity coordination
pain assessment
Assess the neuromuscular function status of the affected side and the difference between it and the healthy side
Rehabilitation effect evaluation: observe the progress of neuromuscular function of the affected side before and after treatment and formulate and adjust the next treatment plan accordingly.
Others include applications in swallowing disorders, gait analysis and kinematics.
