Electromyography Flashcards
Define Electromyography
- Measures muscle electrical activity
- Provides information about the control and execution of voluntary (and reflexive) movements
EMG allows us to measure what aspects of muscle function?
Allows us to:
- Identify which muscles are active
- Identify when they begin or cease their activity
- Quantify the magnitude of muscle involvement in motor activity
What does ENG actually measure?
Measures the sum of all motor unit action potential transmitted along mm fibre at a given point in time
- Electrical signal generated in mm fibres as a result of motor unit recruitment is known as motor unit action potential (m.u.a.p.)
An electromyography signal is the composite electrical sum of what?
Electrical sum of all of the active motor units (sum of all positive and negative units)
Why does an EMG signal have both positive and negative components?
When the signal crosses the baseline, a positive phase of one motor unit action potential is likely balanced by the negative phase of other motor unit potentials
Describe the differences in EMG electrodes and any limitations in their use?
Surface Electrodes
- Metal electrodes composed of silver, gold, stainless steel or even tin
- Common for clinical application
- Prep the skin (shaving, conducting gel) to reduce electrical impedance
- Collect data from a much greater area
Surface Electrodes Limitations
- Surface electrodes have limited use in recording activity from deeper muscles or deeper portions of large muscles
- Another difficulty is their use in recording from small muscles
- Less localisation of signal noise (can collect electrical activity from other places/muscles)
Fine Wire Electrodes
- Facilitates recording from deeper or smaller muscles is the use of fine wire electrodes
- Consists of two fine-diameter insulated wires (about 0.75 mm diameter) that are inserted with a hypodermic needle (Remain in position due to hooks at the ends)
- The tip of the wires constitute the recording surfaces and are cut flush or scraped bare of any insulation
Fine Wire Limitation
- Invasive
- Leaves scare tissue
- Wire may fracture (remain in muscle)
- Limits movement
- Poor in reproducibility of output
Describe the correct placement of surface electrodes (including site preparation) for recording a
clean EMG signal.
- Placing the electrodes away from highly tendinous areas
- The motor point (the area where the nerve enters the muscle) is not a good location to place surface electrodes
- Usually placed longitudinally over the mm of interest (2 – 5 cm apart, centre to centre)
- Parallel to muscle fibres (if not amplitude of signal reduced by 50%)
- Bipolar arrangement is preferred – both electrodes are placed on mm of interest allowing for a localised effect
List several factors that may influence the quality of EMG signal.
Electrode
- Electrode characteristics can affect the frequency and amplitude content of the EMG signal
- These electrode features include the type of electrode (surface, metal plate, silver-silver chloride, indwelling, etc), electrode size and the interelectrode distance
Blood flow and tissue influences:
- The tissue between the muscle and surface electrodes have a dramatic low-pass filtering effect
- Even 100 micrometres from the muscle fibre - about 80% of the signal strength is lost
- Low pass filtering can also potentially be induced by increases in muscle blood flow, which increases dramatically during contraction
Muscle Length and Depth
- This is demonstrated as a decrease in muscle fibre conduction velocity with muscle lengthening and the amplitude of the AP declines with increasing length (we cannot determine the force produced using the electrical activity a muscle produces)
- Deeper muscle fibres are considerably more difficult to record from the surface
- One estimate is that surface electrodes may only record muscle fibres within 10-20 mm of the skin surface
Explain the use of amplifiers in EMG recording.
- Amplifier to obtain a ‘clean’ EMG signal
- Undistorted = signal has been amplified linearly (over the amplifier and recording system)
- Amplification or gain and filtering are adjustable on the amplifier
- The gain is the ratio of output voltage to the input voltage, for example for a 2mV input and a gain of 1000 the output will be 2V
What is meant by “full-wave rectification”?
The full wave rectification process converts +ve and –ve EMG into +ve signals
List and explain the 2 main classes of processing raw EMG signals.
Time Domain Processing:
- Examine variation of EMG amplitude over time
1. Mean value - Series of mean EMG values are computed (smoothing data)
2. Linear envelop - Passes through a low-pass filter (cuts off signals above a particular frequency)
3. Integrated/reset - 1 st calculates area under wave rectified signal, reset the area measurement to zero when target is reached
Frequency domain processing
- Frequency domain: The EMG signal is thought to be many sine waves with different frequencies
- Perform a Fourier analysis - Fast Fourier Transformation technique to obtain power density spectrum
- Frequency bandwidth to amplify all frequencies present in the EMG
- Median frequency – divides the spectrum into two equal areas
- Peak power frequency – greatest power contribution
Discuss the relationship between EMG and biomechanical variables.
Isometric Contraction
- Both linear and non-linear studies reported
- Fixed relationship between EMG and relative force, i.e. the % of max force or absolute force determined by the length-tension relationship
Concentric and Eccentric contraction
- Maximum contractions - EMG reflects activation in contractile component due to rate of movement
- Does not show muscle tension
- Both positive and negative work
- Findings reported less metabolic cost with negative work
During Fatigue
- Constant activation – muscle produces tension that will decrease -> fatigue
MORE
List three sources of noise in an EMG signal.
- Distance between electrode detection surface
- Length of cables
- Area of detection surface
Explain the purpose of using a reference electrode and where it should be placed
- ## Electrode which has a stable and well-known electrode potential
Explain the purpose of using a reference electrode and where it should be placed
- Electrode which has a stable and well-known electrode potential
- It should be placed far from the EMG detecting surfaces, on an electrically neutral tissue (Boney landmark - to minimise noise)
