Electromyography

Question 1

Define Electromyography

Answer 1

• Measures muscle electrical activity

- Provides information about the control and execution of voluntary (and reflexive) movements

Question 2

EMG allows us to measure what aspects of muscle function?

Answer 2

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

Question 3

What does ENG actually measure?

Answer 3

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.)

Question 4

An electromyography signal is the composite electrical sum of what?

Answer 4

Electrical sum of all of the active motor units (sum of all positive and negative units)

Question 5

Why does an EMG signal have both positive and negative components?

Answer 5

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

Question 6

Describe the differences in EMG electrodes and any limitations in their use?

Answer 6

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

Question 7

Describe the correct placement of surface electrodes (including site preparation) for recording a
clean EMG signal.

Answer 7

• 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

Question 8

List several factors that may influence the quality of EMG signal.

Answer 8

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

Question 9

Explain the use of amplifiers in EMG recording.

Answer 9

• 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

Question 10

What is meant by “full-wave rectification”?

Answer 10

The full wave rectification process converts +ve and –ve EMG into +ve signals

Question 11

List and explain the 2 main classes of processing raw EMG signals.

Answer 11

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

Question 12

Discuss the relationship between EMG and biomechanical variables.

Answer 12

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
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Question 13

List three sources of noise in an EMG signal.

Answer 13

• Distance between electrode detection surface
• Length of cables
• Area of detection surface

Question 14

Explain the purpose of using a reference electrode and where it should be placed

Answer 14

• ## Electrode which has a stable and well-known electrode potential

Question 15

Explain the purpose of using a reference electrode and where it should be placed

Answer 15

• 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)