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

• 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