14.1 EMG signal characteristics and acquisition

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 electrode placement, amplification, filtering, and analog-to-digital conversion. Factors like electrode type, placement, and skin preparation affect signal quality. Surface EMG provides a global view of muscle activity, while intramuscular EMG 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
• Motor unit action potentials (MUAPs) 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

• Electrode types
  • 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
  • Bipolar configuration with two active electrodes placed along the length of the muscle fibers reduces common mode noise through differential amplification
  • Reference electrode placed on an electrically neutral site (bony prominence, tendon) minimizes noise and improves signal quality
• Amplification and filtering
  1. EMG signals are amplified with a gain of 500 to 2000 using high input impedance amplifiers to minimize signal attenuation
  2. Bandpass filtering removes low-frequency motion artifacts and high-frequency noise (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
  • Signal resolution of 12-16 bits captures the full range of EMG amplitudes

Factors affecting EMG signal quality

• Electrode-skin interface
  • Skin preparation (cleaning, abrasion) reduces skin impedance and improves signal quality
  • Conductive gel ensures good electrical contact between the electrode and skin
• Crosstalk from nearby muscles can contaminate the EMG signal
  • Proper electrode placement and smaller electrode size minimize crosstalk
• Motion artifacts caused by relative movement between the electrodes and the skin
  • Securing electrodes with adhesive tape or bandages reduces motion artifacts
• Electromagnetic interference from power lines, electrical devices, and fluorescent lights
  • Shielded cables and proper grounding reduce electromagnetic interference Methods to minimize noise and artifacts:

1. Skin preparation and proper electrode placement
2. Use of shielded cables and proper grounding
3. Bandpass filtering (low-frequency motion artifacts, high-frequency noise)
4. Notch filtering at power line frequency (50/60 Hz) to reduce electromagnetic interference
5. Signal averaging and template matching techniques to enhance signal-to-noise ratio

Surface vs intramuscular EMG recording

• Surface EMG
  • Non-invasive and easy to apply
  • Provides a global view of muscle activity
  • Suitable for studying superficial muscles (gastrocnemius, tibialis anterior)
  • Limited by crosstalk from nearby muscles and subcutaneous tissue
  • Lower signal-to-noise ratio compared to intramuscular EMG
  • Cannot isolate individual motor unit activity
• Intramuscular EMG
  • Invasive, requires needle or fine-wire electrode insertion
  • Provides a localized view of muscle activity
  • Suitable for studying deep muscles or individual motor units (multifidus, lumbar paraspinals)
  • Minimal crosstalk from nearby muscles
  • Higher signal-to-noise ratio compared to surface EMG
  • Can isolate individual motor unit activity
  • Requires skilled personnel for electrode insertion
  • May cause discomfort or pain during the procedure