EMG

Question 1

Analog VS Digital Signals

Answer 1

Analog
- Occurs continuously with time

Digital
- Discrete signals (either on or off)
- Non continuous signal

Question 2

What is the source of an EMG?

Answer 2

Nerve impulses cross the neuromuscular junction and stimulate muscle fibre depolarisation
EMG measures the signals resulting from the movement of ions in and out of the fibres

The sum of many units that are underlying the electrodes

Question 3

How does an EMG work?

Answer 3

Uses two electrodes outside the muscle fibres (optimal distance = 2cm)
Measures the difference between the two electrodes (potential difference)

Measures the voltage at 2 different points across the muscle fibre

Question 4

Factors influencing EMG signals
(6 Common types)

Answer 4

• Adipose tissue
• Skin conductivity
• Electrophysiological cross talk (putting electrode near the heart)
• Change in muscle belly size during the activity
• External Noise
• Amplifier Quality

Question 5

Frequency range of an EMG amplifier

Answer 5

6Hz high pass filter
500Hz Low Pass filter

Question 6

What is Bi-Polar EMG and what does it do

Answer 6

Allows removal of a signal that is common to both electrodes
Known as “differential recording”

Question 7

Advantages of Surface Electrodes

Answer 7

• Easy to apply
• No medical supervision required
• Minimal discomfort
• Relatively stable signal
• Can sample from a relatively large volume of muscle

Question 8

Disadvantages of Surface Electrodes

Answer 8

• More prone to “cross talk”
• Can only be used for superficial muscles (cant isolate the signal)
• Relative movement between skin and muscle

Question 9

Advantages of Indwelling Electrodes

Answer 9

• Samples from a relatively small volume
• less cross talk concern
• Can access deep muscles

Question 10

Disadvantages of Indwelling Electrodes

Answer 10

• Signal more likely to vary over time
• Medical supervision required
• Pain Involved

Question 11

Extraneous Factors for EMG signals

Answer 11

• Electrical resistance between fibre and electrode (e.g skin, tissues etc)
• Distance between the electrodes
• Amount of noise

Question 12

How to analyse EMG in the time domain

Answer 12

Measure the average of the absolute value of all the data (Mean EMG)
Also known as ‘signal rectification’

Question 13

Difference between Raw EMG, Rectified EMG, Linear Envelope

Answer 13

Raw EMG - unmodified signal (has positive and negative components)
Rectified EMG - Flipping values to make absolute values
Linear Envelope - Root Mean Square (smoothing the data)
–> Allows for a value at a single point in time

Question 14

Why is normalising important?

Answer 14

Removes noise from the data

Question 15

Amplitude VS Frequency of an EMG. What do they measure?

Answer 15

Amplitude measures the level of recruitment
Frequency measures the level of fatigue

Question 16

True or False?
Activation level of a muscle = Force of the muscle being measured

Answer 16

False
There are extraneous factors that impact on the force of the muscle beside the recruitment

Question 17

What does Fatigue do when looking at an EMG?

Answer 17

There is a lower frequency within each depolarisation

Due to summation of the action potentials, the EMG shows high amplitude at low frequencies and Low amplitude at high frequencies

IMPORTANT DOES NOT MEAN THE FIRING RATE OF FIBRES DECLINES

Question 18

What is the formula for data normalisation

Answer 18

EMG Norm =

EMG Value - EMG Resting Value
——————————————— X 100
EMG MVC - EMG Resting Value