Electromyography (EMG) and Applications

Electromyography (EMG) and Its Application

• Surface EMG (sEMG) is a painless technique using stickers to measure muscle activity and is useful in rehabilitation
• sEMG provides useful information that helps rehabilitation professionals make better decisions and track progress
• Real-time sEMG helps patients understand muscle activity and relearn proper movements
• Wearable technology allows data collection outside clinics

EMG Generation

• EMG signals reveal how muscles work electrically during contraction through electrodes placed on the skin
• Muscle activation causes charged particles to move, creating electric current
• Changes in electrical potential are measured in Volts, with the voltage on the skin surface representing the sEMG signal, giving insight into muscle activity
• Brain/spinal cord signals travel to muscles via motoneurons, causing a quick change in electrical potential or an action potential
• sEMG represents a combination of action potentials from motor units within the electrode's detection area

Motor Units and Muscle Force

• Each motor unit's action potential has a distinct shape based on the number of muscle fibers it controls
• Individual motor units can be distinguished at low muscle activity, but action potentials merge into a complex sEMG signal at higher levels
• Increasing muscle force requires more motor units or increased firing rates
• Distinguishing individual motor units is possible at low activity levels, but the sEMG signal becomes more complex at higher levels
• Specialized algorithms analyze sEMG signals to provide insights into individual motoneuron firing times, offering information not easily accessible through invasive methods

EMG Signal Examples

• An example of a surface EMG signal at a low force level is seen at 10% of maximum voluntary contraction (MVC)
• An example of a surface EMG signal at a higher force level is seen at 40% MVC
• Single motor unit action potential trains have different inter-spike intervals (ISIs), that is, different motor unit firing rates
• There is an action potential duration increase correlated with a decline in muscle fiber conduction velocity
• A motor unit representation, shows how a surface EMG signal could be recorded from a muscle using a bipolar electrode, that is, two electrode contacts

EMG Detection and Acquisition

• Electromyographic (EMG) signals necessitate careful consideration of factors such as electrode distance, size, and placement
• Standard sEMG detection involves using a pair of electrodes on the skin, and the signal is processed using a differential amplifier
• Signal characteristics are greatly affected by electrode placement, spacing (interelectrode distance or IED), and electrode pair size
• Multichannel detection uses one-dimensional (1-D) or two-dimensional (2-D) electrode arrays which can create spatially sampled analog potential distribution images
• The electrode arrays can map spectral characteristics of the EMG signal, like its amplitude

Key Challenges of EMG Signal Acquisition

• Single Electrode-Skin Interface, Impedance, and Noise: Skin contact and electrode interface introduces impedance and noise based on factors such as electrode metal, size, gel, and skin condition
• Sensitivity to Power Line Interference: Parasitic capacitances from power lines can create interference, thus a robust detection system is needed to minimize interference
• Balancing Contact Impedances: Achieving similar electrode-skin contact impedances compared to the common mode amplifier input impedance is essential to reduce common mode voltages
• Transfer Function of the Electrode System: Spatial filtering affects the signal by introducing smoothing that should be carefully considered

Electrodes in Electromyography (EMG)

• Two main types of electrodes are used: surface and needle
• Surface electrodes are placed on the skin while needle electrodes are inserted into muscle tissue
• Both types are made of conductive metals and convert the electric potential from muscle activity into electronic signals, then transmitted to an amplifier
• Surface electrodes typically need electrolyte gel before application
• Action potentials from muscle fibers generate extracellular currents that travel to the skin surface electrode

Strategic Electrode Placement

• Electrodes are placed on or within the muscle for recording
• EMG signals undergo amplification, filtering, and magnitude increase before being sent to an analog-to-digital conversion board
• The system includes a ground electrode and a data acquisition processor (DAP)
• EMG amplifier detects and magnifies currents from potential gradients for analysis
• Electrodes function to convert ionic potentials from muscle activity into electronic potentials

Surface EMG versus Needle EMG in Applications

• Needle EMG is used in neuromuscular disorder assessment to isolate single motor unit activity and detect abnormalities in motor unit firing patterns
• Needle EMG reflects activity from a small number of motor units, making it suitable for low levels of isometric contraction
• Surface EMG (sEMG) provides a non-invasive, global muscle activity measurement, useful for extracting temporal muscle activity patterns during movements
• Surface EMG monitors symmetry and relative activation of muscles used for assessment and treatment

Surface EMG Electrodes

• Placement: on the skin over the target muscle
• Procedure: non-invasive and painless
• Applications: studying overall muscle activity, muscle fatigue, and rehabilitation progress
• Limitations: records signals from a larger area, including activity from nearby muscles (crosstalk), and unable to reach deeper muscles

Needle EMG Electrodes

• Placement: inserted directly into the muscle using a thin needle
• Procedure: more invasive and cause discomfort, requires a trained healthcare professional to perform
• Applications: used for detailed evaluation of individual muscle fibers and nerve conduction studies
• Limitations: invasive, uncomfortable, and carries the risk of infection or bleeding

Advantages and Applications of Surface EMG (sEMG)

• Non-invasive: sEMG electrodes are placed on the skin, making it more comfortable for patients compared to needle EMG
• Global muscle activity measurement: sEMG provides a broader picture of muscle activation compared to needle EMG's focus on individual motor units
• Applications include assessment of muscle activation patterns, biofeedback with real-time feedback for muscle control and rehabilitation, fatigue assessment indicated by changes in sEMG signal, and indirect assessment of muscle force and fiber conduction velocity
• Research: Can study movement coordination and motor control

Types and Limitations of sEMG Recordings

• Traditional bipolar sEMG: The most common and suitable for many applications
• sEMG electrode arrays: Offer more detailed information but require complex analysis
• Limitations: Limited information on individual motor units compared to needle EMG, and interpretation requires some signal processing knowledge

EMG Electrode Types

• Monopolar EMG Electrode: A monopolar EMG electrode has a single recording electrode that measures the voltage difference between itself and a distant reference electrode, typically placed on the wrist or ankle
• Bipolar EMG Electrode: A bipolar EMG electrode has two recording electrodes placed close together on the skin, and measures the voltage difference to be specific to muscle fibers directly below the electrodes

Conventional vs. Capacitive Surface Electrodes

• Conventional electrodes, commonly wet or dry, function as transducers, and convert ionic current in tissue and gel into a flow of electrons in the metal
• These sensors need careful skin preparation to reduce impedance and noise
• Capacitive electrodes have a conductive plate covered by a dielectric layer avoiding direct galvanic contact and detect electrical displacement rather than charge currents
• This eliminates the need for skin preparation and conductive gels, with notable advantage for long-term monitoring, such as home health monitoring or telemedicine

Electrode types compared

• Conventional Electrodes: Require skin preparation, use conductive gel to improve conductivity, prone to movement artifacts, and not ideal for long-term monitoring
• Capacitive Electrodes: Do not require skin prep or conductive gel, are comfortable for long-term use, and are easy to apply
• Capacitive Electrodes require special amplifier design: high impedance, need for DC path for bias current and sensitivity to movement changes in electrode-skin distance
• Capacitive Electrodes are limited in miniaturization and are unsuitable for high-density EMG
• Both conventional and capacitive electrodes are used for acquiring electromyography (EMG) signals on the surface of the skin

Conventional Surface Electrodes

• Technology: they utilize a conductive gel interface between the electrode and skin
• Mechanism: The gel medium facilitates transfer of electrical signals from muscle fibers to the electrode surface
• Advantages: well-understood, inexpensive and comfortable for short-term recordings
• Disadvantages: require skin preparation, get dries out, and unsuitable for long-term recordings

Capacitive Surface Electrodes

• Technology: does not require a conductive gel
• Mechanism: the electrode acts as one plate of a capacitor, with the skin forming the other plate, and changes in muscle activity cause changes in the dielectric permittivity detected as electrical signals
• Advantages: eliminates the need for gels and comfortable for long-term recordings
• Disadvantages: new tech, May be expensive, and signal quality sensitive to skin properties and environmental factors
• Conventional electrodes are preferred for high-density EMG due to smaller size and simpler amplifier requirements