Week 9 - Electromyography

Question 1

Define Electromyography.

Answer 1

technique for detecting and recording the electrical activity of the muscles

Question 2

EMG allows us to measure what aspects of muscle function?

Answer 2

• muscle activity, –> % MVC

Question 3

An electromyography signal is the composite electrical sum of what?

Answer 3

• all of the active motor units
  
  • Including positive and negative

Question 4

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

Answer 4

• Surface electrodes: Limited at recording deeper or small muscles, less precise
• Fine wire electrodes: poor reproducibility of output, invasive, wire can break off

Question 5

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

Answer 5

• Shaving area, removing dead skin cells and skin oils with alcohol and increasing local skin blood flow
• Use conducting gel

Question 6

Explain the use of amplifiers in EMG recording.

Answer 6

• Amplifier to obtain a ‘clean’ EMG signal
• Amplifier rejects unwanted signals by common mode rejection ratio (C.M.R.R.)

Question 7

What is meant by “full-wave rectification”?

Answer 7

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

Question 8

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

Answer 8

• Time Domain approach is to examine variation of EMG amplitude over time
• Whereas the Frequency Domain the EMG signal is thought to be many sine waves with different frequencies

Question 9

• What is meant by noise in an EMG signal?

Answer 9

• Signal interference

Question 10

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

Answer 10

• One electrode on bony landmark (reference electrode) to minimise unwanted noise - neutral site

Question 11

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

Answer 11

• At each moment the positive signal shows more motor units were positively charged, and each negative signal shows more motor units were negatively charged = doesn’t change activity

Question 12

• What does EMG actually measure?

Answer 12

• muscle electrical activity (NOT directly related to force ouput)