EMG signals reveal the electrical activity of muscles during contraction. These signals originate from motor unit action potentials, which are the sum of action potentials from muscle fibers controlled by a single motor neuron.
Acquiring EMG signals involves , amplification, , and . Factors like electrode type, placement, and affect . provides a global view of muscle activity, while offers localized, specific information about individual motor units.
Electromyography (EMG) Signal Characteristics and Acquisition
Physiological origins of EMG signals
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Electrical activity of muscle fibers during contraction generates EMG signals
form the basic units of EMG signals
Summation of action potentials from muscle fibers innervated by a single motor neuron results in MUAPs
Motor units consist of a motor neuron and the muscle fibers it innervates (motor unit territory)
Recruitment of motor units and their firing rates determine the force of muscle contraction (size principle)
Principles of EMG signal acquisition
Surface electrodes placed on the skin over the muscle of interest provide non-invasive, global assessment of muscle activity (rectus femoris, biceps brachii)
Intramuscular electrodes inserted directly into the muscle tissue offer localized, specific information about individual motor units (single fiber EMG)
Electrode placement
with two active electrodes placed along the length of the muscle fibers reduces common mode noise through
placed on an electrically neutral site (bony prominence, tendon) minimizes noise and improves signal quality
Amplification and filtering
EMG signals are amplified with a of 500 to 2000 using to minimize signal attenuation
removes and (10-20 Hz high-pass, 500-1000 Hz low-pass)
Analog-to-digital conversion
Sampling EMG signals at a rate of at least 1000 Hz avoids aliasing, with higher rates (2000-4000 Hz) used for detailed analysis
of 12-16 bits captures the full range of EMG amplitudes
Factors affecting EMG signal quality
Skin preparation (cleaning, abrasion) reduces skin impedance and improves signal quality
ensures good electrical contact between the electrode and skin
from nearby muscles can contaminate the EMG signal
Proper electrode placement and smaller electrode size minimize crosstalk
caused by relative movement between the electrodes and the skin
Securing electrodes with adhesive tape or bandages reduces motion artifacts
from power lines, electrical devices, and fluorescent lights
and proper grounding reduce electromagnetic interference
Methods to minimize noise and artifacts: