Analog VS Digital Signals
Analog
- Occurs continuously with time
Digital
- Discrete signals (either on or off)
- Non continuous signal
What is the source of an EMG?
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
How does an EMG work?
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
Factors influencing EMG signals
(6 Common types)
- Adipose tissue
- Skin conductivity
- Electrophysiological cross talk (putting electrode near the heart)
- Change in muscle belly size during the activity
- External Noise
- Amplifier Quality
Frequency range of an EMG amplifier
6Hz high pass filter
500Hz Low Pass filter
What is Bi-Polar EMG and what does it do
Allows removal of a signal that is common to both electrodes
Known as “differential recording”
Advantages of Surface Electrodes
- Easy to apply
- No medical supervision required
- Minimal discomfort
- Relatively stable signal
- Can sample from a relatively large volume of muscle
Disadvantages of Surface Electrodes
- More prone to “cross talk”
- Can only be used for superficial muscles (cant isolate the signal)
- Relative movement between skin and muscle
Advantages of Indwelling Electrodes
- Samples from a relatively small volume
- less cross talk concern
- Can access deep muscles
Disadvantages of Indwelling Electrodes
- Signal more likely to vary over time
- Medical supervision required
- Pain Involved
Extraneous Factors for EMG signals
- Electrical resistance between fibre and electrode (e.g skin, tissues etc)
- Distance between the electrodes
- Amount of noise
How to analyse EMG in the time domain
Measure the average of the absolute value of all the data (Mean EMG)
Also known as ‘signal rectification’
Difference between Raw EMG, Rectified EMG, Linear Envelope
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
Why is normalising important?
Removes noise from the data
Amplitude VS Frequency of an EMG. What do they measure?
Amplitude measures the level of recruitment
Frequency measures the level of fatigue
True or False?
Activation level of a muscle = Force of the muscle being measured
False
There are extraneous factors that impact on the force of the muscle beside the recruitment
What does Fatigue do when looking at an EMG?
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
What is the formula for data normalisation
EMG Norm =
EMG Value - EMG Resting Value
——————————————— X 100
EMG MVC - EMG Resting Value
