L07. Electromyogram: Mechanisms, Measurement, and Signal Processing Flashcards
What is EMG
A technique focused on the recording and analysis of myoelectric signals
How are myoelectric signals formed?
Formed by physiological variations in the state of muscle fiber membranes
What industries are EMGs used?
Medical Research, Rehabilitation, Ergonomics, and Sports Science
Where does an EMG signal originate?
A motor unit
What is a motor unit?
A motor neuron and all the muscle fibers it innervates
Depolarization triggers the release of ________ into the intra-cellular space
Calcium ions
What does MUAP stand for?
Motor unit action potential
How is a MUAP formed?
By the combination of several hundred muscle fibers that are all activated at about the same time
How does a monopolar signal work?
EMG signals are recorded from one electrode in or on a muscle and another remote electrode
How does a bipolar signal work?
EMG signals are recorded between 2 electrodes that pick up signals from the muscle of interest
Advantages and disadvantages to surface EMGs
Advantages: non-invasive and easy to use
Disadvantages: potential for distortion of signal due to fat; potential for contamination by signals from nearby muscles
Advantages and disadvantages to percutaneous EMGs
Advantages: More selective
Disadvantages: More invasive
What frequency is a surface EMG typically at?
6-500 Hz
(but most between 20-150Hz)
What is the surface EMG Vpp?
5000 microvolts
What factors affect EMG Recording?
- Amplifier gain
- A/D Resolution and Sampling
- Skin and amplifier input impedance
- Frequency response
- Common-mode rejection ratio
- Filtering
What is the purpose of amplifier gain?
To amplify the signal without amplifying the ambient/background noise
(usually 2k-10k)
What is differential amplification?
Amplification of the difference between the 2 input signals while rejecting any common input signals
Common mode rejection ratio: rejection of noise common to both electrode inputs (want higher value)
What do filters of EMG signals do?
Filters remove specific frequency components from a signal
What do high pass filters do?
Remove low frequencies
- due to relative movement
- changes in electrical coupling
What do low pass filters do?
Remove higher frequencies
- anti-aliasing filtering and smoothing the EMG signal
How are EMG amplitude and level of contraction related?
EMG amplitude increases with the level of contraction
What are the 2 steps to demodulate an EMG signal?
- Rectification - take absolute value
- Smoothing - usually apply a filter
